Thesis - 東京大学wyvern.phys.s.u-tokyo.ac.jp/f/Research/arch/Thesis...Thesis Photoemission and...

Thesis

Photoemission and inverse-photoemission study of

late 3d transition-metal chalcogenides

Kazutoshi Mamiya

Department of Physics, Graduate School of Science,

University of Tokyo

December, 1996

論文の内容の要旨論文題目:Photoemissionand inverse-photoemission study of

late 3d transition-metal chalcogenides

光電子・逆光電子分光法による重い3d遷移金属

カルコゲナイドの研究

氏名:間宮一敏

はじめに

強相関電子系は今日の固体物理の中心的課題の一つである。 3d遷移金属化合物では電気的

磁気的性質に 3d電子が大きな役割を担っており、これらの電子では電子相関が無視できるほ

ど小さくはない。 3d遷移金属化合物は強磁性、反強磁性、金属絶縁体転移、最近発見された高

温超伝導などのように広範な性質を示す。 3d遷移金属化合物において電子相聞が無視できな

い有名な例として一電子的なバンド計算が ~iO のバンドギャップを導けないことが挙げられ

る。本論文では 3d遷移金属硫化物のうち金属絶縁体転移を示す物質とその関連物質の中から

パイライト型化合物である FeS2，CoS2-xSex，NiS2-xSexとNiAs型 Ni1-xVxSおよび二次元的

な結晶構造を持つ BaNiS2をとりあげる。パイライト型化合物の FeS2，CoS2-xSex，NiS2-xSex

は同ーの結晶構造を持ちながらさまざまな電気的磁気的性質を示す物質群でその多様性が単

一の 3degバンドの電子によっていることから 60年台から 70年台に掛けて精力的に研究が

行なわれた。しかしながら、 NiS2_xSe..，の金属非金属転移のメカニズムなどいくつかの間題

は今日においても活発な研究の対象として残っている。 NiAs型 NiSは金属非金属転移を起

こす物質で、非金属相のキャリアは軽い遷移金属や Ni空孔のドーピングによって制御でき

る。本論文ではドープしていない p型の NiSとVをドープして n型にした NiSの比較を行

なっている。我々は上記の物質群に対して光電子-逆光電子分光実験を行ない、その電子構造

を研究した。また、 BaNiS2については、 X線吸収スベクトルの測定から、 Niイオン上の 3d

電子の配置について研究を行なった。

1

実験方法

試料として、パイライト型化合物のFeSz，CoSz， COSI.7ZSeo.28， NiS2， NiSl.ii5S句.45，NiS1.34S匂.66

とVをドープした NiAs型 NiSの Nio.97V 0.03S， Nio.94 VO.06、そして、 Ba.1'IliS2を用意した。

FeS2， CoS2， COS1.72SeO.28， NiS2， 1.34SeO・66については He放電管を光源とした紫外線光電

子分光と Mg の特性 X 線である ~lg・ Ka線を光源とした X線光電子分光、さらに X線逆光

電子分光実験を行なった。試料のうち FeS2，CoS2， NiS2については遷移金属のめ 3d共鳴

を用いた共鳴光電子分光実験を行なった。さらに、 NiS2，NiSl品 Se0.45，NiS1.34SeO.66につい

ては He放電管を光源とした高エネルギー分解能光電子分光実験も行なった。 Nio.97V O.o3S，

Nio・94Vo.068については He放電管を光源とした高エネルギ一分解能光電子分光実験を行なっ

た。 BaNiS2については NiL2.3 X線吸収の計測を行なった。

パイライト型遷移金属力ルコゲナイド

価電子体の紫外線光電子分光、 X線光電子分光スベクトルは FeS2，CoS2， Ni82のEいにつ

いてよく似ていて、 1"，2eVに選移金属の狭い 3dバンドがあり、それより深い側で約lOeV

位までの範囲に比較的広い S3pバンドが広がっている。 FeS2の3d主ピークがきれいな単一

ピークなのに対して CoS2，NiS2では、 3d電子が増えるのに伴って遷移金属の 3degバンドを

電子がそれぞれ 1個 2個占有するため主ピークの浅い側に肩構造が認められる。紫外線共鳴

光電子分光実験では FeS2ヲ CoS2，NiSzのすべてにおいて、 6~7eV辺りに共鳴増大するサテ

ライト構造がみられ、これらの物質では電子相関が強いことが分かつった。

NiS2の光電子スベクトルをクラスターモデルを用いて解析した結果、 NiS2 は 3d~3d クー

ロン相互作用 U より 3p~3d 電荷移動エネルギームの方が小さい電荷移動型絶縁体であるこ

とが分かった。この解析で得られたパラメータからおらのパラメータを演緯し FeS2の低ス

ピンの安定性の起源を調べた結果、短い Fe~S 間距離による大きな移動積分 (pd，σ) がおらの

低スピン電子配置を安定化させていることが分かった。

強磁性相の CoS2と常磁性相の CoS2およびCoS1.34SeO.66の紫外線光電子分光スベクトルを

比較すると実験の精度以内で変化が見られないが、局所スピン密度汎関数法からスベクトル

を予測すると本実験の分解能で観測可能な変化が認められるはずであった。現実の CoS2は

わずかなバンド構造の変化しか伴わずに強磁性転移を起こしていることが分る。

高分解能紫外線光電子分光実験によって NiS1.55Se0.45の金属絶縁体転移にともなう電子状態

の変化が観測できた。低温の金属相ではlOOmeV付近の強度に増大が見られる一方、 200"，500

meVにおいては強度の減少が見られた。高温の絶縁体相でもスベクトルの形状は金属的で

ギャップは観測されなかった。我々は、金属相と絶縁体相のスベクトルの変化を半金属的な

バンドの重なりが起こったからであると解釈した。絶縁体の?むらでも光電子スベクトルは

金属的でフェルミエネルギー直下で、も大きな光電子放出強度を示し、室温では熱的に大量の

ホールが励起されていると考えられる。大量のキャリアがあるにも関わらず絶縁体であるた

めにはキャリアの移動度が非常に小さくなければならない。

2

Vドープした NiS

ドープをしていない p型の NiSでは高分解能紫外線光電子分光実験の結果、低温の非金属

相で約lOmeVのギャップが開くことが確認されているが、光学的なギャップ (140meV)より

小さい。これは p型の半導体ではフェルミエネルギーが価電子帯の直上にあり、ギャップの

大半がフェルミエネルギーより上にあるということで説明されていた。一方 Vをドープし

てn型にした NiSの光電子分光スベクトルを観測すれば、この大きなギャップが観測される

ことになるが、実際のスベクトルでは大きなギャップは聞かず、かえって p型の NiSで開い

ていた小さなlOmeVのギャップも閉じていた。 n型の NiSでギャップがなくなることは半金

属的なバンド構造を考えることによって説明できる。 NiSにおける金属非金属転移は大きな

フェルミ面を持つ通常の金属から小さなフェルミ面を持つ半金属への転移と考えられる。

BaNiS2

2次元的な結品構造を持つ BaNiS2では Niが五つの S原子によってピラミッド型に配位さ

れており、 Ni3d電子は対称性の低い結品場中に存在している。立方対称場などの高い対称性

の結晶場中の Ni+2価イオンは高スピン電子配置をとるが、低い対称性の結晶場中では、フ

ントカップリングと結晶場分裂の大小で高スピン電子配置をとるか、低スピン電子配置をと

るかが決まる。 BaNiS2の NiL2.3 X線吸収スベクトルを、ピラミッド型 NiS58ークラスター

モデルによる計算で解析した結果、 BaNiS:1が高スピン電子配置をとることが分かった。

3

Contents

1 Introduction 1

2 Experimental methods

2.1 introduction.

2.2 Bremsstrahlung isochromat measurement system. .

2.3 High-resolution ultraviolet photoemission measurement system

2.4 Ultraviolet and x-ray photoemission measurement system . .

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2.5 Ultraviolet photoemission spectroscopy using synchrotron radiation .. 11

2.6 X-ray absorption measurement. . . . . . . . . . . . . . . . . . . . . .. 11

3 Pyrite-type transition-metal dichalcogenides

3.1 Introduction....

3.2 Experimental

3.3 Valence and conduction band 'spectra of FeS2ヲ CoS2and NiS2・

3.4 Cluster-model analysis of the photoemission spectra of NiS2

3.5 Origin of the low-spin state in FeS2 ・・

3.6 Ferromagnetic transition in CoS2-xSex

3.7 Metal-Insulator transition in NiS2-xSex・

3.8 Concluding remarks. .

3

3

5

8

9

1

3

4

4

1

1

1

2

2

4

4

4

5

4 V-substituted NiS

4.1 Introduction..

4.2 Experimental

4.3 Experimental results

4.4 Discussion and concluding remarks

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5.1 Introduction.

5.2 Experimental



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CONTENTS

89

Chapter 1

Introduction

Correlated electron systems are one of the most important and attractive subjects in

current condensed matter physics. 3d transition-metal compounds shows a wide vari-

ety of electronic and magnetic properties: ferromagnetism， anti-ferromagnetism， metal-insulator transition， and recently high-Tc superconductivity. In the 3d transition-metal

compounds， 3d electrons play an important role in the electronic and magnetic prop-

erties， and electron correlation is generally strong. Band theories， of which treat the system as a collection of electrons in the averaged potential of the rest of electrons， are a fundamental basis of material science and have succeeded to clarify and understand

the electronic structure of many metals and semiconductors. However， they often fails to predict the physical properties of transition metal compounds because electronic

correlation in these systems is not negligible. For example， one-electron band theory fails to predict the band gap of NiO. In order to include electron correlation more

precisely in band theory， LDA+U or unrestricted Hartree-Fock calculations have been

done recently for some transition metal compounds. From the view point of experi-

mental methods， photoemission is a powerful probe to study the electronic structure of

correlated systems， because the photoemission process changes the number of electrons in the system and the spectra reflect electron correlation， by which the emitted electron W邸 a宜ectedin the system.

In the vast field of condensed matter physics， metal-insulator transition is one of the most interesting subjects. Hubbard has suggested a simple Hamiltonian known

部 theHubbard model and pointed out if the ratio of the electron-electron correla-

tion repulsion U to the band width W becomes larger than a critical value of order

1， the system undergoes a transition from a metallic to an insulating phase [1]. Many

theoretical works for the metal-insulator transition in correlated electron system were

done and several theoretical phase diagrams for these systems were proposed. Moriya

and Hasegawa have applied spin fluctuation theory to the tight-binding model and

suggested a phase diagram shown in Fig. 1.1 [2]. According to this ph錨 ediagram，

1

2 Chapter 1. Introduction

there is three phases， namely， a paramagnetic metallic phase， a paramagnetic insulat-

ing phase and an antiferromagnetic insulating phase. The Neel temperature increases

as the repulsion becomes strong in the weak correlation regime while it decre剖 esin

the strong correlation regime. Rozenberg et al. have calculated the optical and pho-

toemission spectra based on the dynamical mean field theory and suggested a phase

diagram for metal-insulator transition (Figure 1.2 [3]). The characteristic feature of

this diagram is that there is an antiferromagnetic metallic phase on the weak correla-

tion side of the antiferromagnetic insulating phase for a certain parameter set. Another

characteristic feature is that the phase boundary between the paramagnetic metallic

phase and paramagnetic insulating phase at higher temperature is blurred and be-

comes a crossover line. This thesis is intended to understand the electronic structures

of some metal-insulator transition systems and related systems by photoemission and

inverse photoemission experiments and their analysis. Systems to be investigated are

pyrite-type 3d transition metal dichalcogenides， NiAs-type NiS and BaNiS2・

Pyrite-type NiS2-xSex undergoes a metal-insulator transition as the composition is

varied (Figure 1.3 [4]). At the composition of metal-insulator phase boundary (x rv

0.5)， this system undergoes a metal-insulator transition as temperature changes. There

is an antiferromagnetic metallic phase on low temperature side of the transition similar

to the Rρzenberg's phase diagram. Inc1uding NiS2-xSex， pyrite-type 3d transition metal dichalcogenides show a wide variety of magnetic and electronic properties without

changing the crystal structures[5]， and were extensively investigated in 1960s and 1970s，

but many of the problems remain to be solved ti1l now. The magnetic transition in

CoS2-xSex and metal-insulator transition in NiS2-xSex are still controversial issues

nowadays. In this work， photoemission and inverse photoemission experiments for FeS2， CoS2 and NiS2 were done in order to get insight of the electronic structure of these

compounds. Furthermore， high-resolution photoemission experiments were performed for NiS2-xSex in order to investigate the electronic structure and their change through

the metal-insulator transition in this system.

NiAs-type NiS was discovered to undergo a metal-nonmetal transiもionwith tem-

perature or under pressure in 1967加 d1968， respectively， and a lot of studies have

been carried out for understanding the mechanism of the metal-nonmetal transition

(Figure 1.4) [6， 7， 8]. There are only two phases， namely， the paramagnetic metal-lic phase and the antiferromagnetic nonmeta11ic phase， similar to the weak electronic correlation region of Moriya-Hasegawa phase diagram. The nature of the nonmetallic

phase， however has not been clear unti1 now. It has been considered as a semimetal or a

degenerate semiconductors. According to the optical absorption study， the optical gap of NiS in nonmetallic phase is rvO.14 eV[9]. On the other hand， high-resolution phかtoemission study has found that a portion of the gap below the Fermi level is rv 10 me V

3

[10]. This is interpreted that the Fermi level is located near the top of the valence band

because the NiS in the nonmeta11ic phase is a p-type conductor. Hence， if one takes the high-resolution photoemission experiments for n-type NiS， the change of the electronic structure across the metal-insulator transition is expected to be observed more clear1y.

N-type NiS can be obtained by the substitution of Ni with early transition metals

[11]. In order to investigate the change of the electronic structure through the metal-

nonmetal transition in NiS， we have done high resolution photoemission spectroscopy experiment on n-type V-substituted NiS.

BaNiS2 is a quasi two dimensional compound and consists of Ni-S and Ba-S layers

[12]. It shows meta11ic conductivity with large anisotropy [13]. Isomorphic BaCoS2

is a Mott insulator and the solid solution BaNh-xCoxS2 undergoes a metal-insulator

transition at x rv 0.2 [14]. In this work the photoemission and inverse photoemission

spectra and Ni L2，3 X-ray absorption spectra of BaNiS2 were taken as a first step to

understand the electronic structure of the BaNi1-xCOxS2 system.

The rest of this thesis consists of five chapters as follows. In chapter 2 we describe

the principle of photoemission and the experimental apparatus briefly. The resu1ts

of the photoemission and inverse photoemission study of the pyrite叩 typecompounds

(FeS2， CoS2-xSex and NiS2-xSex) are described in chapter 3. The photoemission study

ofV-substituted NiS is described in chapter 4. The X-ray absorption and photoemission

study of BaNiS2 is described in chapter 5. In chapter 6 we summarize the results about

these compounds and give conclusions.

Introduction

6

20

5

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n=1.0

S U

Chapter 1.

z

Figure 1.1: Moriya-Hasegawぜsphase diagrarn. [2].

INSULATOR

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Figure 1.2: Phase diagrarn of NiS1_xSex・[3].

15

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paramagnetic metal

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Chapter 1. Introduction

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Figure 1企 Phasediagram of NiS1-xSex・[6].

Chapter 2

Experimental methods

2.1 introduction

In this chapter we describe the experimental methods of photoemission and inverse-

photoemission spectroscopy and X-ray absorption spectroscopy. Photoemission， inverse photoemission and x-ray absorption processes are shown in Fig.2.1.

Photoemission spectroscopy is a method to investigate the valence and core elec-

trons by measuring the energy distribution of photoemitted electrons. Because the

number of electrons in the measured system is decreased by one through the photoe-

mission process， the kinetic energy of the photoemitted electron is a旺ectedby the cor-

relation energy by which the initial and final state electrons are a百ected.Consequently，

photoemission spectroscopy is a powerful method to investigate highly correlated sys-

tems. The binding energy of the initial state is obtained as

EB = hv-争 -Ekin，

where EB' hv，争 andEkin denotes the binding energy， the incident photon energy， the work function of the sample and the kinetic energy of the photoemitted electrons.

Inverse-photoemission spectroscopy is a method similar to photoemission spec-

troscopy. It uses the inverse process of photoemission， namely， phot.ons which are emitted when the incident electrons are absorbed in the sample are detected. This is

a comp1imentary method of photoemission spectroscopy and is used to investigate the

empty states. The energy of the initial state is obtained as

Ee = Ekin +争-hv，

where Ee， Ekin， <t and hv denotes the energy of final state， the incident electron energy， the work function of the sample and the energy of the detected photon.

X-ray absorption spectroscopy is a method related to the photoemission and inverse

photoemission spectroscopy. This method is usually used to investigate the conduction

7

8 Chapter 2. Experimental methods

band. The spectrum is obtained either by me制 lfingthe total photoelectron current

or counting photoemitted electrons as a function of the energy of incident soft x-rays.

final state

initial state

E

E F

(a) photoem ission

final state

E

E F

e

‘一一一一

。コ)invere photoem ission

E

E F

hV

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(c) x-ray absorption

Figure 2.1: Processes of (a) photoemission， (b) inverse-photoemission and (c) x-ray absorption.

2.2 Bremsstrahlung isochromat measurement sys-

tem

The Bremsstrahlung isochromat spectroscopy (BIS) measurement system is拙 emati-

cally shown in Fig.2.2. BIS is one mode of inverse-photoemission spectroscopy， where the energy of emitted photon is fixed and incident electrons are scanned. The electron

source is a Pierce-type electron gun with a BaO coated cathode. Emitted photons were

monochromatized with the crystal monochromator using Si02 [1101] surface， which is tuned for the photon energy of hv = 1486.6 eV， photons is focused on a thin CsI film

evaporated on a stainless steel plate. Photcトemittedelectrons from the CsI film were

collected with a Ceratron and pulse-counted. Much e百6rtwas needed to setting up

and aligning the geometry of the monochromator， electrongun and samples. Samples could be cooled down to liquid-nitrogen temperature. The base pressure of the system

2.3. High-resolution ultraviolet photoemission measurement system 9

was '" 2 X 10-10 Torr. Incident electron energies were calibrated with the Fermi edge

of Au evaporated on the samples. The total energy resolution was "，0.9 eV.

electron

gun

crystal

ceratron

Figure 2.2: Schematic description of BIS measurement system.

2.3 High-resolution ultraviolet photoemission mea-

surement system

The schematic figure of the high energy resolution UPS measurement system is shown

in Fig.2.3. This measurement were performed using a hemispherical analyzer (VSW

CLASS 150). The ultraviolet source was a He discharge lamp. Samples could be cooled

down to 15 K with a closed-cycle He-gas refrigerator. The base pressure of the system

was '" 5 X 10-11 Torr. Binding energies were calibrated with the Fermi edge of Au

evaporated on the samples. The total energy resolution is listed in Table 2.1.

Light source

He 1: 21.2 eV

He Ii: 40.8 eV

V一

e-

U一

ぽ一

1一5

n

E

a

p

Bnergy resolution (meV)

"，25

"，70

Table 2.1: Total energy resolution of UPS and XPS

10

light source

channel tron

Chapter 2. Experimental methods

electron analyzer

e (m onochrom ized)

Figure 2.3: Schematic description of photoemission measurement system.

2.4 Ultraviolet and x-ray photoemission measure-

ment system

Ultraviolet photoemission spectroscopy (UPS) and x-ray photoemission spectroscopy

(XPS) measurements with conventional energy resolution were performed using a spec-

trometer equipped with an x-ray source with a Mg anode， a He discharge lamp and

a double-pass cy1indrical-mirror analyzer ( PHI 15-255). Samples were cooled down

to 1iquid-nitrogen temperature (LNT). The base pressu回 ofthe spectrometer was

rv 3 X 10-10 Torr. Binding energies were calibrated with the Fermi edge of Au evapo-

rated on the samples for UPS and with the Au 417/2 peak (84.0 eV) for XPS. The peak

of the Cu 2P3/2 co四 level(932.6 e V) was also used to calibrate the binding energy. The

total energy resolution for the XPS and UPS measurements is listed in Table 2.2.

Light source Pass energy (eV) I Energy resolution (eV)

XPS I Mg-Ka: 1253.6 e V 25 '""0.90

UPS I He 1: 21.2 eV， He II: 40.8 eV 15 '""0.35

Table 2.2: Total energy resolution of UPS and XPS

2.5. U1traviolet photoemission spectroscopy using synchrotron radiation 11

2.5 Ultraviolet photoemission spectroscopy using

synchrotron radiation

Photoemission measurements using synchrotron radiation were performed at beam-

line BL-2 of the Synchrotron Radiation Laboratory， lnstitute for Solid State Physics，

University of Tokyo. Measurements were done at room temperature. Binding energi邸

were calibrated with the Fermi edge of Au evaporated on the samples. The total energy

resolution was '" 0.5 eV.

2.6 X-ray absorption measurement

The schematic figure of X-ray absorption spectroscopy (XAS) measurements system is

shown in Fig.2.4. The measurements were carried out at the plane-grating monochro-

mator beam line BL・2Bof Photon Factory， N ational Laboratory for High Energy

Physics. The spectra were taken in the total electron yield method. lncident pho-

ton energies were calibrated with the 0 ls core-level absorption peak of Ti02 at 530.7

eV [15] and the 2p core level absorption edge of Cu metal at 932.5 eV [16]. The total

energy resolution w部"，0.2e V for Ni 2p core absorption region..

ky

sam ple channel tron

Figure 2.4: Schematic description of x-ray absorption system.

12 Chapter 2. Experimental methods

Chapter 3

Pyrite-type transition-metal

dichalcogenides

3.1 Introduction

In this chapter， we will study the electronic structure of NiS2-xSex， which under goes a rnetal-insulator transition， and related pyrite-type 3d transition rnetal chalcogenides

by rneans of photoernission and inverse photoemission experirnents. Pyrite-type 3d

transition-rnetal (TM) chalcogenides exhibit a variety of electrical and rnagnetic prop-

erties [5， 17， 18]. The crystal structure of pyrite is shown in Fig. 3.1. The characteristic

feature of this structure is that the anions are dirnerized and the dirners instead of each

anions are negatively divalent. The electronic and rnagnetic properties of pyrite-type

3d cha1cogenides are listed in Table 3.1. FeS2 is a nonrnagnetic serniconductor， CoS2

is a ferrornagnetic rnetal with Tc 120 K [19]， and NiS2 is an antiferromagnetic

serniconductor with TN = 40 K [21]. In order to explain the rnetallic or in凶凶su叫11a抗.ti阻n

conductivities of pyrite-type 3d transition rnetal cha叫.lc∞og伊en凶I泊id白es鳥， Wilson has suggested

a schernatic band rnodel for these cornpounds (Fig. 3.3) [22]. The valence and conduc-

tion bands of FeS2 consist of S 38， S 3p， Fe 3d t2g， Fe 3d eg， S 3pσヘFe48 and Fe

4p bands in the order of increasing energy. In FeS2， the Fe 3d t2g ba吋 iscompletely

filled and the Fe 3d eg band is completely ernpty， and hence there is a gap between Fe

3d t2g and Fe 3d eg bands. In CoS2， because the Co 3d eg band is quarter-filled， it is rnetallic. In NiS2， the Ni 3d eg is half-filled and split into empty and occupied bands

due to electron correlations， and therefore the systern is insulating. In NiSe2， the Ni 3d eg is half-filled but does not split into two bands because the band width of Ni 3d

eg hybridized with the Se 4p band is wider than that in NiS2・Thisrnodel can crudely

explain the transport properties of conduction in these cornpounds.

The optical absorption spectrurn of FeS2 shows a gap of rvO.95 e V as shown in Fig 3.6

[23]. The rnagnetic susceptibility of FeS2 (Fig. 3.7) is very srnall and the positive value of

13

14 Chapter 3. Pyrite-type transition-metal dichalcogenides

susceptibility originate from Van Vleck paramagnetism [24]. The electric conductivity

and the magnetic susceptibility of CoS2 are shown in Figs 3.8 and 3.9 [19， 20]. The

temperature dependence of electronic resistivity at low temperature is of the type of

AT2， indicating electron-electron scat旬ring.The kink in 1/χ-T curve at "，，300 K h槌

been explained by Moriyaゐspinfiuctuation theory. The optical absorption spectra of

NiS2 shows an indirect gap of "，，0.3 eV (Fig 3.10) [25]. The electrical conductivity of

NiS2 is shown in Fig. 3.11 [25]. The activation energy of the conductivity varies出

temperature varies. The activation energy is rv70 meV at room temperature and "，300

me V at higher temperature.

The electronic and magnetic phase diagram of the pyrite-type 3d transition metal

dicha1cogenides are shown in Fig. 3.2 [18]. There is a ferromagnetic phase around

CoS2 and a substitution of S with Se or Co with Ni rapidly lowers the Curie temper-

ature. Near the ferromagnetic-paramagnetic phase boundary， there are metamagnetic

phases. There are a Mott antiferromagnetic insulating phase and antiferromagnetic

metallic phase around NiS2・ Inthe Mott insulating phase， the Neel temperature is

lowered compared to that in the metallic phase. Figure 3.4 shows the phase diagram

of CoS2-xSex [19]. CoS2 becomes paramagnetic for 12% substitution of S by Se [19].

Figure 3.5 shows the phase diagram of NiS2-xSex・NiS2becomes an antiferromagnetic

metal for 23% substitution of Se for S [4]. The electrical resistivity and the activa-

tion energy in the insulating phase of NiS2-xSex are shown in Figs. 3.12 and 3.13 [26].

The activation energy decreases as the composition approaches the insulator-tか metal

transition. With further substitution， the Neel temperature TN is suppressed to 0 K

at around 50% Se substitution. For compositions near x ::: 0.45， NiS2-xSex undergoes

a transition from a semiconductor to an antiferromagnetic metal with decreasing tem-

perature. The Hall coe白cientof NiS2-xSex shown in Fig. 3.14 [27] indicates that the

carrier number in the metallic phase and the insulating phase above 200 K is nearly

identical. It a1so indicates that the carrier number decreases from the insulating phase

to the metallic phase in NiS1.5Seoか Figure.3.15 shows the magnetic susceptibility of

NiS2-xSex [18， 28]. The magnetic susceptibility is Curie-Weiss type in NiS2 and Pauli

paramagnetic in NiSe2・Themetal-insulator transitio

3.2. Experimental 15

and electronic structure of related pyrite-type 3s transition metal compounds are re-

ported. At first the electronic structure of the valence and the conduction bands are

studied in section 3.3 as a basis of further studies. Then the electronic structure of

NiS2 is discussed with a cluster model analysis in section 3.4 and subsequently the

stabi1ity of low spin configuration in FeS2 is discussed using the results of the cluster

model analysis of NiS2 in section 3.5. In the following section 3.6， the ferromagnetism

in CoS2 is studied from the view point of photoemission. Then， the metal-insulator

transition in NiS2-xSex is studied with high-resolution photoemission experiments in

section 3.7.

正)M Qs

Figure 3.1: Pyrite-type crystal structure. [30]

3.2 Experimental

In order to investigate the structure of the valence and conduction bands in the pyrite-

type 3d transition metal compounds， UPS， XPS and BIS measurements were performed

for FeS2， CoS2， CoSl.72SeO.28， NiS2 and NiS1.34SeO.66・ FeS2was a natural mineral pro・

vided by Prof. M. Suga in Osaka University. CoS2， COS1.72SeO.28 and NiS1.34SeO.66 were

single crystals supplied by Prof. T. Miyadai in Dohto University. NiS2 was also single

crystal and supplied by Prof. H. Takahashi in Nihon University and Prof. N. Mori in

University of Tokyo and Prof. T. Miyadai in Dohto University. All the spectra were

taken at liquid nitrogen temperature (LNT) except for the BIS spectrum of FeS2， which

W槌 takenat room temperature (RT) in order to avoid charging effect. UPS measure-

16 Chapter 3. Pyrite-type transition-metal dicha1cogenides

Table 3.1: Physical properties of pyrite-type 3d transition metal compounds.

conduction magnetism Curie Temperature (K) N eel Temperature (K) J FeS2 semicond uctor nonmagnetic

CoS2 metal ferromagnetic 110

CoSe2 metal paramagnetic

NiS2 semicond uctor antiferromagnetic

NiSe2 metal paramagnetic

magnetic moments (μB) optical gap (e V) activation energy (eV)

FeS2

CoS2

CoSe2

NiS2

NiSe2

F.Se;>

es

0.84

1.17

SEMICON臥JCTOAT.K ノ

200 a:;;コ/

0.95

0.3

METAし

100

CoSe2 eg

Pcw

NiSe;> 2 e g

0.3

MOTT INSULATOR

CuSe2

e~

METAL

Pp

SUPEA-α訓 D.

¥当h-ーヲ

CUS2

40

Figure 3.2: Electronic and magnetic phase diagram of pyrite-type 3d transition metal

dichalcogenides. [18].

3.2. Experimental

(p)

0-'5)

ー'，

l4〕

関繍

Fig. 12

し」x-x'I 「•••

一--r・・L

t" 叫が

(5)

F<<Sz s

d

S制帽ωnd.

Co~

J NISz e d

Mott i肉sula'町...tQI

17

」J. . 」・

一一--ーー

NiSe，

J m.tQI

Figure 3ふ Bandstructures of pyrite-type 3d transition metal dichalcogenides [22]

話

。‘・... 3包g 5 tO0

E ト也，

-g ‘-コι》

O! 0.0 1.0 2.0

COS2 x COS'2

Figure 3.4: Ph錨 ediagram of CoS2-xSex. [19]

18 Chapter 3. Pyrite-type transition-meta1 dicha1cogenides

T(K】

100

80

岡田magnetlcInsu恒tor

60

--/ 40 antlf.。πomagnetlc

加su句tor

、、EAVa

、、、、

¥ 。20

W回 wfeπ。mag剖 Ic

NiS_ Se

¥ ¥

mm¥ ヘ

pa阻町田gneticmetat

'I11

‘11

、、

。0.2 1.2

x 0.4 0.6 0.8

Figure 3.5: Phase diagram of NiS2-xSex. [4]

19 Experimenta1 3.2.

8←

6

4

2

(F'εuもとで22tgucozaL02《

1-1 0・9

energy (eV)

0・05

Photon

OD[.

Figure 3.6: Optical absorption spectra of FeS2 [23].

0.3

FeS2

3 SAMPLE

0.2

(εOLO¥コεω)

mwo--JR

0.1 600 400

T (K)

200 O

Figure 3.7: Magnetic susceptibility of FeS2 [24].

Jへ/.

ノ

~

PyTIte-type transition-metal dicha1cogenides Chapter 3. 20

一

一

COS2

I // < '00> ト

∞

1(EUC『『

}

Q、

ト

o

klv 〉ト一〉一ト

ω一ωωE

i 2α3

( K )

i 100

TEMPERATURE

O O

Figure 3.8: Electrical resistivity of CoS2 [20].

3.2. Experimental

"10'

ヨ

E ω h、

315 .:1 ーぜ

1.0

Q5

O~ o 200 4∞ 6∞ s∞ 1000

T '.K)

Figure 3.9: Inverse magnetic susceptibility of CoS2-xSex [19].

21

Pyrite-type transition-metal dichalcogenides Chapter 3. 22

1200

•

• •

• ' •

i 一寸一一一一T一一…下一一

1000‘・

ε ど 800C O

a. 0600 cn 」コo

S 4001-0.. O

200…

340 320 1

260 280 300 Energy (meV)

240 220 200 O

Figure 3.10: Optical absorption spectrum of NiS2 [25].

〆/-Ea=68meV

100

10

0.1

(--EU70)PE一ちコ百coυ

10 •

9 8 7 4 5 6 1000/T( oK・1)

3 2 O

Figure 3.11: Electrical conductivity of NiS2 [25].

3.2. Experimental

10.

100

~ 10.' ε &:a

.2 IO2

Q...

lσ'

200 40つT (K)

Figure 3.12: Electrical resistivity of NiS2-xSex [26].

23

600


Ni S2_xSex • 。<X< 0.55

250

• -E3 • -E2

A -E 1

¥.¥.¥・¥1

• •

200

150 (>ωε) LムJ

100

• 50

0.7

Figure 3.13: Activation energy of electrical transport NiS2-xSex in the insulating phase

[26].

0.6 0.5

X

0.1

3.2. Experimental

10~.... •

• • • • • TN v

O

2ヤヘ • 寸

O

F

)

。。。 • •

工庄 •

T (K)

NiS2-xSex

• x=O.50 o x=O.55 ・x=O.70ロ x=O.85<> x=1.00 A x=1.33 v x=1.67

300

Figure 3.14: Hall coe伍cientof NiS2-xSex [27].

25


.1♂ (a) 4

も}

♀口

ート司ー...-..・.40 4 1.40

. 2.00

，ーーー戸ーー

NiSZ...Se.

TIKI 創淘 JOO 01 。

100 TC則 o 2∞ 4ω eoo

Figure 3.15: Magnetic susceptibility of NiS2-xSex in (a) inSl山 tingregime [28] and (b)

metallic regime [18].

ments for CoS2 were done both at LNT and RT in order to compare the spectra of

the ferromagnetic ph槌 eand the paramagnetic phase. 3p-3d resonant photoemission

measurements using synchrotron radiation were performed for FeS2， CoS2 and NiS2・Incident photon energies were from 40 eV to 120 cV and the measurements were done

at room temperature.

High-resolution UPS spectra were mcasured for NiS2， NiS1.45SeO品 andNiS 1.34SeO.66

in order to get insight into changes in band structure through the metal-i1!sulator transi-

tion. NiS1.45SeO.55 was sintered polycrystal provided by Dr. Nirmala Chandrasekharan， Dr. S. R. Krishnakumar and Prof. D. D. Sarma in Indian Institute of Science. Re-

sistivities of these samples are plotted in Fig. 3.16. The metal-insulator transition

temperature of NiS1.55Se0.45 w槌 rv60 K. The me剖 urementswere done at !:::::: 15 K and

room temperature. Therefore， NiS2 w槌 insulatingand NiS1.34SeO.66 was metallic at

both 15 and 300K. NiS1.55SeO品 W剖 metallicat 15 K組 dinsulating at 300 K.

The sample surfaces were scraped in situ with a diamond file to obtain clean sur-

faces.

3.2. Experimental

ε O q

〉、土- 0.1

〉4圃 d

.包 0.01(J) 。江 0・001

0.0001 O

100

27

-. ..・司............................................................................

NiS2_xSex x=O 一一一 x=0.45_._. x=O.66

300

Figure 3.16: Resistivity curve of NiS2- x Sex • The resistivity data of NiS2 are quoted

from the work of Kautz etαl. [25].

• H

-•

-e

-•

-•

-F

• ，

• ，

• ，

• ，

50 100 150 200 250

Temperature (K)


3.3 Valence and conduction band spectra of FeS2ヲ

CoS2 and NiS2

Figure 3.17 shows the XPS and BIS spectra of FeS2 and calculated spectra derived

from the local-density-approximation (LDA) band-structure calculation [32]. As the

experimental and theoretical spectra show good agreement， we have部 signedfeatures

in the spectra as follows. The narrow peak at -1 e V in the XPS spectrum is derived

from Fe 3d (t2g) states. The broad peak from -2 eV to -9 eV is derived from S 3p states.

The peaks at around -15 eV are derived from S 3s states. In the BIS spectrum， the peak at 2 e V with an shoulder at 3 e V is a mixture of Fe 3d(匂)and S 3p (anti-bonding

pσ*) states. The plateau above 6 eV is derived from Fe 4sp and S 3d states.

Figure 3.18 shows the He 11 UPS spectra of FeS2， CoS2 and NiS2・Sincethe pho-

toionization cross-section of the TM 3d electrons for this photon energy (40.8 e V) is

more than four times larger than those of the S 3p or Se 4p electrons， the TM 3d

component dominates the spectra [33]. The main peaks at 1-2 eV for CoS2 and NiS2

have an additional feature on the lower binding energy side， due to the partially filled

eg band. Among the three spectra， only CoS2 shows a high intensity at the Fermi level

due to the metallic conductivity.

The He 1 UPS spectra of FeS2， CoS2 and NiS2・areshown in Fig. 3.19. The spectra

for FeS2 and CoS2 consists of a narrow TM 3d peak at 1-2 eV below EF and a broad

S 3p band from 2 e V to 9 e V. In the spectrum of NiS2 the two components could not

be clearly resolved， due to the smaller energy di百erencebetween these levels.

The XPS spectra of FeS2， CoS2 and NiS2 are shown in Fig. 3.20. As shown in the

case of the He 1 UPS spectra， the TM 3d and S 3p peaks were observed for FeS2 and

CoS2 also in the XPS spectra and these two features were not resolved for NiS2・On

the higher binding energy side of the S 3p feature the line shapes are similar for these

compounds. There lies a S 3s peak around 13 e V below EF in these spectra.

The 3p-3d resonant photoemission spectra of FeS2， CoS2 and NiS2 are shown in

Figs. 3.21-3.23. In going from FeS2 to CoS2 to NiS2ヲ themain peak becomes broader.

The broadening of the t2g peak is interpreted as due to its exchange splitting as the

spin polarization of the d (eg) band is increased. For such an interpretation to be

valid， however， because the me部 urementshave been done above Tc or TN， the spin polarization should persist above Tc or TN on the short time scale of photoemission

spectroscopy (rv 10-15 sec) even in the paramagnetic state. [34]

The appearance of the broad satellite feat

3.4. Cluster-model analysis of the photoemission spectra of NiS2 29

can be seen from the constant-initial-state spectra shown in Figs. 3.24-3.26.

We therefore interpret the satellite primarily due to d7 final states and the main

band as due to d8 L final states (L: a ligand hole) as in the case of NiO (Ref. [35， 36])

and NiS. [37， 38] That is， the highest occupied states in NiS2 are S 3p-like rather than

Ni 3d-like and the band gap is of the p-tcトdcharge-transfer type rather than the d-d

Mott-Hubbard type. This view is supported by the cluster-model analysis described

below in this section.

The fact that the resonant enhancement in the satellite region is observed in every

compound [Figs. 3.21-3.23] indicates that electron correlation is important in every

compound including the non-magnetic insulator FeS2・ Thismeans that FeS2 is not

simply an ordinary band insulator but is a correlated insulator. Figures 3.21-3.23 show

that in going from NiS2 to CoS2 to FeS2， the resonance behavior of the satellite above

the 3p→3d threshold becomes less prominent relative to the (anti)resonance behavior

of the main band剖 in3d transition-metal oxides. [35， 36， 39， 40] The BIS spectra of FeS2， CoS2 and NiS2 are shown in Fig. 3.27. As we have stated

above， the BIS spectrum of FeS2 consists of a peak at 2 e V with shoulder at 3 e V and

plateau above 7 eV. In going from FeS2 to CoS2 to NiS2， the peak is shifted towards lower energy and the shoulder becomes a distinct peak without energy shift. The

plateau is shifted towards lower energy without changing its line shape. This tendency

is quite re出 onableif we consider that the low energy peak and the high energy shoulder

are mainly derived from the TM 3d and S 3p states， respectively， and that the plateau is

derived from TM 4sp states because the TM 3d level is lowered with increasing atomic

number.

3.4 Cluster-model analysis ofthe photoemission spec-

tra of NiS2

In order to gain more insight into the electronic structure of these compounds， we

have analyzed the photoemission spectra by a standard co凶 gurationinteraction (CI)

calculation on a [NiS6PO-cluster (Fig. 3.28).

The ground-state wave function of the cluster is given by a linear combination of

the d8， CJ9 L and d10 L 2 configurations and the photoemission final states by those of

the d7， d8 L and d9 L 2 configurations. The model contains a few adjustable parameters，

namely， the on-site d-d Coulomb energy U， the p-tcトdcharge-transfer energyム三

(♂LIHI♂L) -(d8IHld8)， and the d-p transfer integra1s (pdσ) and (pd付， where we

have assumed (pdσ)/(pdπ) = -2.2出 before[41]. Here，ム andU are defined with

respect to the center of gravity of each multiplet. Atomic values are used for Racah B，

C parameters.[35， 36， 37， 38] For simplicity， the S 3s orbitals have been neglected in


FeS2

Experiment

XPS

Theoretical Calculation

Folkers et. al

-15 -10 -5

BIS

Fe 3d

O

Energy (eV)

5 10 15

Figure 3.17: XPS and BIS spectra of FeS2 and the density of states deduced from the

LDA band-structure calculation [32].

31 Cluster-model analysis of the photoemission spectra of NiS2 3.4.

• • • •

hv =40.8eV

FeS2

••••••• CoS2

(∞右ロロ.モ〈)b羽田ω吉岡

-』----

he--・唖‘.4

4

・・・・・、p，g NiS2

O 2 4

Binding Energy (e V)

ー‘ー• • • ‘-e ・.‘ ，，， -Ittj

6 8 10 12 14

Figure 3.18: He 11 UPS spectra of FeS21 CoS2 and NiS2・


hv =21.2eV

FeS2

l-------

..... ‘、.1

〆'園、υョ..... ・F司

ロ~ I CoS2 s h

〈、-"〉、‘d

uヨロ4主吻d

ロー圃司

NiS2

12 6 10 8 4 2 O

Biding Energy (eV)

Figure 3.19: He 1 UPS spectra of FeS2， CoS2 and NiS2・


• •

hv =1253.6eV

• • 、• 、叫

(凶】何回口.宅〈)kp窃ロ

ω百円，、』"，. ."

L---

p

・‘.4

.

4

、~

。4

B inding Energy (e V)

8 12 16

Figure 3.20: XPS spectra of FeS2 CoS2 and NiS2・


FeS2 XPS

Experiment

Theoretical Calculation

Folkers et. al

-15 明 10 -5

BIS

Fe 3d

O

Energy (eV)

5 10 15

Figure 3.17: XPS and BIS spectra of FeS2 and the density of states deduced from the

LDA band-structure calculation [32].


• • • •

•••••••

~....・ー可

~ -• • • ‘-e ・.‘ ，，， -，N

hv =40.8eV

CoS2

FeS2

(的広何回口

.sh〈)hzgss

'L・・・・・

，‘‘ .. 4

'・e.. 4

AP--

、p，g NiS2

O 2 4


6 8 10 12 14

Figure 3.18: He II UPS spectra of FeS21 CoS2 and NiS2・


.， ...... .

‘

••• ‘、.ー

hv =21.2eV

FeS2

CoS2

(∞諸国

D.f〈

)

k

m

判明∞ロ

ω吉岡

ヘNiS2

r

。2 4

B iding Energy (e V)

6 8 10 12

Figure 3.19: He 1 UPS spectra of FeS2， CoS2 and NiS2・


.、・.日• •

hv =1253.6eV

• 、• ‘ 、~

(gED.ah〈)h判明白ロ

82 ，、』

"， - ."

L'

・.

• ，.‘.4

、"・。4


8 12 16

Figure 3.20: XPS spectra of FeS2 CoS2 and NiS2・

Pyrite-type transition-metal dicha1cogenides Chapter 3. 34

---・ rr ・-.#. ..'-

ー，..""'"時晴."，.・.九州内ヘ~AyIo"〆~..:，-.~~..，- : . .'，-

1:#...r~.Iム~…~... '."':; .，:".;ヘ:?.-L

岨-.. ‘・-.".J:';"鴫 ........ 内・品、戸、両州、':'.'・ 1p: .:帆'品、，拘置'

ムゅーーん~宅、-J 、M.10 .... 岬 d胸岡戸

lMm-d九九，J:.叫世.，.".，..，・・~，J.・，.".，.-寸，句、、.・ ¥-.

". . _.f .. ~--!“~~-:~怖#WO_ :'. '1Of.N

乙』圃."".~・-'..

，品、輸品.".... 岨~--一、f ・

--..，.，..;-.;・4w.zw~A-J1Lw…-………r-，-:''''- . -...; :'.'

.• 帆ー

，，.，.ρ句時九"・・.

・u・..，.'''~岬.，..，..，.， ... .""，岨由尚南V品ら~-λ..，.~

‘ .... ‘・_ .，，-A~、pd~・_.~.-

a品戸、必町胸‘、噛昌司--咽岬同町F

目E

FeS2

• '

hll (eV)

~戸--九一困層}= 100

58

50

40

56

60

55

54

57

64 (ω↑-zコ.国立《)

〉LF

一ωzωト

Z

一zo-ωω一三凶Oト

oza

. . . 、均時

i O

i

8 4 ENERGY (eV)

1 よ

12 BINDING

よ

16 i

Figure 3.21: 3p-3d resonant photoemission spectra of FeS2・


COS2

」

-h

••

」

.

」

••

」

，

.

」

」

」

l

・

}

.

」

・

h

F申噌司，ー‘・向-，.，.，

hν(eV)

=120

68

64

63

(ωト

-Zコ.田庄司)

〉↑-ωzω

」

FZ

一ZO一ωω一芝凶Oト

oza

司旬、-

，.‘4

62

61

60

59

58

55

45

o 16 J2 8 4 BINDING ENERGY (eV)

Figure 3.22: 3~3d resonant photoemission spectra of CoS2・

Pyrite-type transition-metal dicha1cogenides Chapter 3. 36

T τ

.

.

.

.

.

.

.

.

.

.

.

.

.

.

.

‘ ....•...

・・・、

.

.

.

.

.

.

.

.

.

.

・・

...

・・・・・、ht

・--・・・・・1.111

.

.

.

.

.

:・・.

1

・一日・・・

'

メ

ペ

ハ

〈

H

A

.

h

E

.

ヘ.町、・・P

・r

，df

f

d

t

，

"

'

T I T I I

NiS2

d・.&，

hν(eV)

73

69

68

=120

67

66

65

64

(のト一Zコ.田区〈)〉ト一

ω一Z凶トZ一ZO一的的一三

ωo↑oza 63

59

44

12 8 4 BINDING ENERGY (eV)

1 よ16

Figure 3.23: 3p-3d resonant photoemission spectra of NiS2・

3.4. Cluster-model anaJysis of the photoemission spectra of NiS2

ゐ

(ω↑一Zコ

司，-qd e

r・

3 TOTAL YIELO

内，

ι

.∞α4)〉」「一

ω一Zωト

Z

O 40 50 60 70

PHOTON ENERGY{eV) 80

Figure 3.24: Constant-initial-state spectra of FeS2・

37

38

今'-

{例ト一

Zコ.国庄司)〉」F

一的

Zω」「

Z

Chapter 3. Pyrite-type transition-metal dichalcogenides

3

C052

TOTAL YIELD

Ea{eV)

=1.4

3.6

7.3

o 50 60 70

PHOTON ENERGY (eV) 80

Figure 3.25: Constant-initial-state spectra of CoS2・

3.4. Cluster-model analysis of the photoemission spectra of NiS2

2

(的↑一Zコ.田区〈)〉ト一

ω一Z凶トZ一

39

3

NiS2 M3

TOTAL YIELD

Ea (eV) = t7

7.2

050 60 70 PHOTON ENERGY (eV)

80

Figure 3.26: Constant-initial-state spectra of NiS2・


FeS2

... 2F、2 、-.可-• -ー• • ，司 .. ." 、、YI'

• ，・・・'

CoS2

fh問、• • 、JF、r

• • • • NiS2 . ----. .

九三・d命ぜ・. ・.・.、A吋tpaJJ. --，.-. • • •

句ー

-. .・.・・里・.司，、，." -.. ・・ ¥ -• ・.・・#-.-

•••

(∞芯ロ

D.sh〈

)

k

n

判明白ロ

ω百円

ー-e •

12 10 8 4 6 Energy (eV)

2 O -2

Figure 3.27: BIS spectra of FeS2， CoS2 and NiS2・

3.5. Origin o[ the low-spin state in FeS2 41

the ba.sis set [37，38]; instead， e百'ectsof hybridization between the 8 3s and Ni 3d (eg)

orbitals are incorporated through a crystal-field parameter 10Dq '" [ゾヨ(sdσ)]2/(ム+ε3p - c3s) in the initial state and lODq rv [V3(sdσ)]2/(ムー U +ε3p一 ε3s)in the final

state of photoemission， where (sdσ)/(pdσ) 1.1 and the 8 3p-3s energy difference

ε3p ε3s = 10 eV [37， 38]. Figure 3.29 shows the best fit to the hv = 40.8 eV spectrum

obtained withム=1.8 eV， U = 3.3 eV and (pd，σ 1.5 eV， typical errors being 土0.2eV for U andムand土0.05e V for (pdσ). At this photon energy， the 8 3p cross-

section is negligibly small compared to Ni 3d and therefore ha.s been neglected in the

analysis. Using this parameter set， the d8 L-like main peak， which is broader than that of Ni8， ha.s been reproduced a.s shown in Fig. 3.29. However， the discrepancy between

theory and experiment at 3-6 e V could not be eliminated in the present calculation.

This indicates that a more realistic model which takes into account the characteristic

feature of the pyrite-type structure， namelyヲthepresence of the 82 molecules [42] would

be necessary. Also， the strong Ni-8 covalency will make the inter-cluster hybridization

important， possibly making it necessary to go beyond the single-impurity cluster model. In order to obtain the wider d8 L main peak in Ni82 than in Ni8， theム valuefor

Ni82 had to be chosen smaller and the (pdσ) value larger than those for Ni8， for which

we useム=2.2土0.2eV， U = 3.2士0.6e V and (z:肋)= 1.3土 0.05eV， a.s shown in

Fig.3.29.

o Ni

Os

Figure 3.28: Model cluster of Ni86 10-.

3.5 Origin of the low-spin state in FeS2

Considering the systematic decre部 eof .d and the increa.se of U with cation atomic

number (by "，0.5 e V forムandby "，0.3 eV for U， for a unit increa.se of the atomic

number)， [43] we estimate ム~ 2.3 eV and U ~ 3.0 eV for CoS2 and ム~ 2.8 eV and

42

戸-、、υョゆ4...司ロロ.AIH伺)

h判明∞ロωH口同

Chapter 3. Pyrite-type transition-metal dicha1cogenides

NiS2 e

• experiment 一一-cluster model calcuration

12 10 8 6 4 2 B inding Energy (e V)

。

Figure 3.29: He II UPS spectrum of NiS2 and the result of the cluster model calculation.

3.6. Ferromagnetic transition in CoS2-xSex 43

u ~ 2.8 eV for FeS2・ Thislocates CoS2 and FeS2 closer to the boundary between the

charge-transfer and Mott-Hubbard regimes.

In order to explain the contr部 tingbehaviors of the low-spin FeS2 and the high-spin

FeS， we have calculated the lowest energies of the S = 0 and S = 2 states for the

(FeS6) 10-cluster model. Following the results of NiS2 and NiS， we have却 sumedthat

U andムaresmaller for FeS2 than for FeS by rvO.7 eV and rvO.5 eV， respectively. The

(Pdσ) of FeS2 h舗 beenestimated to be 2.2 e V from that of NiS2 using the relationship

(pdσ ) α T Y / dii s ， w袖he凹r陀erd 1おst山heぜ"a叫仰tωoml町ICra凶adius路ぽS

Ni泊S2and 0.80 A for FeS白2，and dM -8 is the metal-sulfur atomic distance， dM -8 = 2.40 A

for NiS2 and 2.26 A for FeS2・[44]Likewise， the (pdσ) of FeS has been estimated to be

1.4 eV from that of NiS using dM-8 = 2.38 A (NiS) and 2必 A(FeS). Thus in FeS2 the

low-spin state is calculated to be lower than the high-spin state by 1.5 e V while in FeS

the high-spin state is calculated to be lower than the low-spin state by 0.6 e V. Although

these absolute values may not be accurate due to the various uncertainties introduced

in the parameter estimates， it can be concluded that the cluster-model calculations

well explain the low-spin and high-spin behaviors in FeS2 and FeS primarily as due to

the larger (pd，σ) arising from the smaller Fe-S distance in FeS2 than in FeS.

3.6 Ferromagnetic transition in CoS2-xSex

CoS2 becomes paramagnetic for 12% Se-substitution for S. The magnetic properties of

CoS2-xSex have been studied theoretically by spin-自制uationtheory [45].

Figure 3.30 shows the He II UPS and BIS spectra of CoS2 and CoSl.72SeO.28・Because

measurements were done at LNT， below the Tc of CoS2， CoS2 was in the ferromag-

netic state， while CoSl.72SeO.28 was in the paramagnetic state. Although these two

compounds were in the different magnetic states， the UPS and BIS spectra do not

show detectable difference within the present energy resolution (rv 0.3 e V and rv 0.9

eV for UPS and BIS， respectively).

Figure 3.31 shows the He II UPS spectra of CoS2 taken at LNT and‘RT， i.e.， below and above the Curie temperature. The figure also shows theoretical spectra derived

from local幽 density-approximation(LDA) calculations for the ferromagnetic and para-

magnetic state [46]. Although CoS2 is ferromagnetic at LNT and paramagnetic at RT，

the experimental spectra for the different magnetic states do not show detectable dif-

ferences as in the case of the comparison between CoS2 and COS1.72SeO.28 while spectra

derived from the LDA calculations differ from each other drastically. This indicates

that the electronic structure does not change so much across the ferromagnetic ph剖 e

transition as the theoretical calculations predict，部 inthe case of the antiferromagnetic

ph錨 etransition in NiS.


The LDA calculation for the ferromagnetic state reproduces the main features of

the UPS spectrum of the ferromagnetic state including the Co 3d eg band shoulder

on the low binding energy side of the Co 3d t2g band although the calculated main

peak is broad due to the exchange splitting. Also， the energy of the calculated main peak is little lower than experiment. On the other hand， the LDA calculation for the

paramagnetic state reproduce the the sharp main peak in the UPS spectrum of the

paramagnetic state， whereas it produces another sharp peak at EF， in disagreement with experiment. The close similarity between the experimental spectra of the ferrか

magnetic and paramagnetic states indicates that the exchange splitting of the main 3d

t2g band is rather small (of order 0.1 e V or less) or a small exchange splitting persists

in the paramagnetic state.

A configuration interaction calculation on a [COS6]1l-cluster has been performed in

order to clarify the importance of electron correlation in the photoemission spectrum.

This is the same model as that used in the preceding section 3.4 to analyze the electronic

structure of NiS2・ Theratio between the two ιp transfer integrals were assumed

(pdσ)/(Pdπ) = -2.2 as in the case of [NiS6Pト Thecrystal-field parameter 10Dq in

the initial and final states is given as in the case of [NiS6po-. The S 3p-3s energy

differenceε3pε38 W加 10eV， the same value as that for [NiS6po-. Racah B， C

parameters are taken from the atomic value. Adjustable parameters areム，U and

(pd，σ). The best fit for the He II UPS spectrum (plotted in Fig 3.32) was obtained

withム=2.3 eV， U = 3.0 eV and (pdσ) = 1.5 e V. The calculated spectrum catches

the characteristic features of the experimental spectra，namely， the sharp peak rv 1 eV

below EF and the shoulder on the lower binding energy side of the peak錨 well錨

the presence of the satellite feature (although the satellite intensity has been a little

overestirr凶 ed).

3.7 1¥伍etal-Insulatortransition in NiS2-xSex

Pyrite-type NiS2-xSex undergoes an insulator-to-metal transition with increasing x.

These compounds show antiferromagnetic order for x < 1. The Neel temperature of

NiS2 is ，，-，40 K and it increases with x towards the insulator-to-metal transition. In the

metallic regime， the Neel temperature decreases with x and is suppressed at around x

= 1. Near the composition of the insulator-metal phase boundary (x 1"V0.5) the system

also undergoes an insulator-to-antiferromagnetic metal transition as the temperature

decreases. Because the metal-insulator transition in this system is not accompanied

by a change in the symmetry of the crystal structure， the system is suitable for a study

of electronically driven metal-insulator transitions. UPS and BIS measurements for

NiS2 and NiS1.34SeO.66 were performed at LNT. High-resolution UPS measurements for

3.7. Metal-Insulator transition in NiS2_xSex

UPS hv=40.8eV

BIS

(ωtcコ

77K hv=1486.6eV

.0」何)

『・-CoS2(ferromagnetic)

ーひ COS1.34SeO.66(paramagnetic)

hH一ωcoHC

-10 O 10 -5 5 Energy relative to EF (e V)

Figure 3.30: He 11 UPS and BIS spectra of CoS2 and CoSl.72SeO.28・

45


(ωHEP-f〈)kp冒

g吉岡

COS2 hv=40.8e V

(a) experiment

口 3∞K(paramagnetic) ・77K(ferromagnetic)

¥、

ヘ.JF /

(b) LDA calculation

口 paramagnettc・ferromagnetic。♂

\~\ロ5 4 2

E

、、、ノv

e

/・1vd

ob

r且

2m

E

ob

Au n

3B

O

Figure 3.31: (a) He II UPS spectra of ferromagnetic and paramagnetic CoS2・ (b)

Spectra derived from the LDA calculations for the ferromagnetic and paramagnetic

states [46].

NiS2， NiS1.55Se0.45 and NiSL34SeO.66 were performed at 300 K and 20 K. All the samples

were paramagnetic at 300 K and antiferromagnetic at 20K.

Figure 3.33 shows the combined He II UPS and BIS spectra of NiS2 and NiSL34SeO.6・

It can be seen that the BIS spectrum of NiS1.34SeO.6 is shifted toward lower energy by

"，0.3 eV compared to that of NiS2・Thismay correspond to the closing of the band

gap of NiS2 in going from the insulating NiS2 to the metallic NiSL34SeO.66・Themain

peak in the UPS spectrum of NiSL34SeO.6 is somewhat narrower than that in NiS2・

The high energy resolution photoemission spectra near the Fermi level for NiS2， NiSL45SeO.55 and NiS1.34SeO.66 taken at 15 K and 300 K are shown in Fig. 3.34. NiS2

is an antiferromagnetic insulator at 15 K and a paramagnetic insulator at 300 K .

Although the optical gap of NiS2 is 0.3 eV [25]， the UPS spectra do not show such

a large gap， and especially the spectrum taken at 300 K looks like that of a metal

showing a thermally broadened Fermi edge at Ep. NiSL45Se0.45 is an antiferromagnetic

metal at 15 K and a paramagnetic insulator at 300 K. The spectrum for the metallic

phase indicates a peak just below Ep， which distinct1y differs from the spectra of

insulating NiS2 taken at the same temperature. The spectrum taken at 300 K ag泊n

looks like a metallic one with a thermally broadened Fermi edge. NiS1.34SeO.66 is an

antiferromagnetic metal at 15 K， and a paramagnetic metal at 300 K. Both spectra are

3.7. Meta1-Insulator transition in NiS2-xSex

(323・A』

訂

)

COS2

k

門訪問.ロ

ω判同H

10 642

Binding Energy (eV)

。8

d6

d7L

L.J.

d~2

ll...t

1...1

Figure 3.32: He II UPS spectra of CoS2 compared with the cluster calculation.

47

-2


showing metallic line shapes around EF similar to those of NiS1.55Seo必・

The resolution of the measurement system (rv 25 me V) is much better than the ther-

mal broadening (rv 3.8 kT) at 300 K， while it is not so good as the thermal broadening at 15 K. Therefore， it is di伍cultto conclude whether the difference between the spectra

taken at the high and low temperatures is due to the thermal broadening or intrinsic

changes in the electronic structure. Then， we plot the high temperature spectra in raw data and the low temperature spectra which have been broadened with the thermal

broadening of 300 K in Fig. 3.35. This comparison for every compωition shows that

the changes of spectra with temperature is not simply due to the thermal broadening.

As for the Se-substituted metallic samples， the spectral weight near EF increases and

the region from 0.2 to 0.5 eV loses intensity in going from the high-temperature phase

to the antiferromagnetic metallic phase. The total intensity down to 0.6 e V below EF

is conserved across the phase transition if we normalize the spectra at rv 0.6 eV組 d

higher binding energies. NiS2 show di百erenttype of temperature-dependent changes in

the spectra. The low temperature spectrum loses spectral weight near EF over a rather

wide energy range down to rv 0.6 eV below EF， if we normalize the spectra in the same

way， or the spectrum is slightly shifted towards higher binding energy rigidly when it

goes from the paramagnetic insulating phase to the antiferromagnetic insulating phase.

This shows that in addition to the thermal broadening the line shape of the spectra

changes between the high-temperature paramagnetic ph舗 eand the metallic phase for

the Se-substituted samples while an overall intensity change or rigid shift occurs in the

insulating NiS2・Figure3.36 shows the composition dependence of the spectra both at

300 and 15 K， where the spectra have been normalized at rv 0.6 e V and higher binding

energies. The intensity just below EF become higher as the Se concentration increases

at both temperatures.

Now we discuss the implication of the present experimental results for the transport

properties， the metal-insulator transition and the low-energy electronic structure in the NiS2-xSex system. High-resolution photoemission spectra of paramagnetic insulating

NiS2 do not show any visible gap in spite of the gap of 300 meV found_by the optical

absorption study [25] and the rv 300 meV activation energ

3.7. Metal-Insulator transition in NiS2-xSex 49

300 meV. Indeed， a small mobility of < 0.5 cm2/Vs has been deduced from the Hall

coefficient measurement [4可.For s吋 1a small mobility， the conduction mechanism will be attributed to hopping of small polarons.

The angle resolved high-resolution photoemission study of NiS1.5SeO.5 by Matsuura

et α1. [49] has indicated a sharp peak just below Ep at a certain五pointin the

Brillouin zone and the peak intensity grows as the temperature decre錨 esfrom the

insulating phase to the metallic phase. On the other hand， we have found that the

angle integrated results for NiS1.5SSe0.45 shows the less dramatic change of the line shape

and the intensities and that the total spectral weight integrated from Ep to 600 me V

below conserves. Then， the growth of the peak in the angle resolved high-resolution

photoemission spectra means that spectral transfer is occurring between di百erentk points.

The high-resolution photoemission spectrum of NiS1.55Se0.45 in the antiferromag-

netic metallic phase shows an enhancement of the intensity within 100 meV of Ep

compared to the paramagnetic insulating phase. This may be interpreted as a semi-

metallic band overlapping as schematically shown in Fig. 3.38. In the insulating phase

at "，300 K， the top of the valence band almost reaches Ep and there must be a large

number of thermally activated holes with very low mobility. The bottom of the con-

duction band would lie far above Ep compared to the activation energy (26 meV for

300 K)， and electrons cannot be tl町 mallyactivated. In the metallic ph部 eat low

temperatures， the top of the valence band reaches Ep and the bottom of the conduc-

tion band overlap making the band structure semimetallic. Indeed， as the optical gap

seems to be an indirect one [25]， the bottom of the conduction band and the top ofthe

valence band should lie at different k points in the Brillouin zone. In the photoemission

spectra of the semimetallic phase， the bottom of the conduction band is observed as an enhancement of the intensity within "，100 meV of Ep. The semimetallic picture

gives a natural explanation to the spectral weight transfer between different k's with

varying temperature. The higher intensity just below Ep for NiS1.34SeO.66 than for

NiS1.55Se0.45 can be interpreted as due to an increased overlap of the valence band and

the conduction band in NiS1.34SeO.66・

It should be noted that the effect of the antiferromagnetic order on the spectra is

opposite between NiS


of antiferromagnetic ordering.

The semimetallic picture indeed consistent with the recent transport study of

NiS2-xSex by Miyωaka et al. [27]， where they found a decrease of the carrier con-

centration when the metal-insulator boundary x ~ 0.5 is approached from the metallic

side. Although the rather simple semimetallic picture is sensible for a transition be-

tween the antiferromagnetic phases since the band structure does not change qualita-

tively， the situation may be more complicated by the strong correlation inherent in the d-electron system， especially in the vicinity of the phase boundary. In order to c1arify this problem， one has to make more detailed studies by varying the compositions with

smaller intervals.

UPS hv=40.8eV

BIS

(ωtcコ.2」何)と一

ωCOVC

hv=1486.6eV

--*-NiS2 (insulator)

-c:r NiS1.34SeO.66 (metal)

-10 -5 O 5 10

Energy relative to EF (e V)

Figure 3.33: He 11 UPS and BIS spectra of NiS2 and NiS1.34SeO.66・

3.7. Metal-Insulator transition in NiS2-xSex

ω

c コ4コL...

ω

hv = 21.2 eV

NiS1.34SeO.66

芸 INiS1.55Se0.45 ω c o c

NiS2

0.6 0.4 0.2 0.0

Binding Energy (eV)

300K 15K

ー0.2

51

Figure 3.34: High energy resolution He 1 UPS spectra of NiS21 NiS1.45SeO.55 and

NiS1.34 SeO.66・

52

，.圃h、cn 4・4c コ. ぷ'-コ-ro 、-〉、4・4

cn c Q) 4・4

c

Chapter 3. Pyrite-type transition-metal dichalcogenides

ロ 300K15K with thermal broadening

I NiS1.34SeO.66

|トJiS1.55Se0.45

NiS2

0.6 0.4 0.2

hv = 21.2 eV

0.0 ー0.2

Binding Energy (eV)

Figure 3.35: High energy resolution He 1 UPS spectra of NiS2l NiS1必 SeO.55and

NiS1.34SeO・66・Thespectra taken at 15 K are artificially broadened with thermal broad-

ening e旺ect.

3.7. Metal-Insulator transition in NiS2-xSex

(ωZCコ.2」何、'--'h-

一ωcφHF」

53

hv = 21.2 eV

企 NiS1.34SeO.66口 NiS1.55SeO.45・NiS2

15K

0.6 0.4 0.2 0.0 ー0.2

Binding Energy (eV)

Figure 3.36: Composition dependence of the high-resolution He 1 UPS spectra of NiS2，

NiS1品 SeO.55and NiS1.34SeO.66・


(ωtcコ

〉、4圃 d

ω 写|・ experimentw 四一 Hartree-Fockcalculation 4・4

c

hv = 21.2 eV NiS2

15K .0・・5、----'

0.6 0.4 0.2 0.0 ー0.2

Binding Energy (eV)

Figure 3.37: He 1 UPS spectra of NiS2 compared to the Hartree-Fock band structure

calculation.

3.8 Concluding remarks

We have studied the electronic structure of pyrite-type transition dichalcogenides FeS2，

CoS2 and NiS2 and their Se-substituted compounds by photoemission and inverse pho-

toemission experiments and analyzed them with cluster model calculations including

configuration interaction.

The satellite structure of resonant photoemission spectra of FeS2， COS2 and NiS2

indicate that electron correlation is important in every compounds including the non-

magnetic insulator FeS2， although the LDA ca1culation shows the good agreement with

the UPS and BIS spectra of FeS2・Thecluster calculation analysis clarifi~d that NiS2 is

a charge transfer type insulator and the low-spin configuration of FeS2 is stabilized by

the smaller Fe-S distance in FeS2 than FeS， which has the high-spin configuration. The

UPS spectra of the ferromagnetic and paramagnetic COS2 does not show a detectable

difference expected from the LDA calculations offerromagnetic and paramagnetic state.

It indicates that the exchange splitting of the 3d t2g band is rather small or a small

exchange splitting persists in the paramagnetic state.

High resolution photoemission spectra of insulating NiS2 show metallic edge at

Fermi energy. It indicates that the a large number of holes are thermally activated

but they can not move due to a large activation energy for mobi1ity. High resolution

3.8. Conc1uding remarks

ω ω ω ω o ;?:- I activation

~ I type 白

Ni弓(i剛 Iato r)

valence band

EFer同 Energy

ωOHSω』

ob一ωC@Q

Ni九Seo.sinsulating phase

High temperature

e e

valence band e e e e

--OQnduction band • • • E向 rm Energy

ωo-sω』

ob一ωcoo

Ni句sSeo.smetallic phase

Low temperature

EFermi Energy

Figure 3.38: Semimetallic band structure of NiS2 and NiS1.55Se0.45・

55


photoemission spectra show the intrinsic difference between the metallic and antifer-

romagnetic insulating NiS1.55Se0.45・ Theenhancement of the spectral weight within

f"V 100 me V of Fermi energy is observed in the spectrum for the metallic phase. The

enhancement of spectral weight for the same region are observed as the Se司 substitution

is increased. In order to explain the change of photoemission spectra with metal-

insulator transition we suggested the semimetallic band structure making a overlap of

the valence and conduction bands as going from the insulating phase to the metallic

ph錨 e.

Chapter 4

V-substituted NiS

4.1 Introduction

In this chapter we discuss another metal-nonmetal transition system of the hexagonal

NiAトtypeNiS (Figure 4.1). This system undergoes a first order metal-nop.metal tran-

sition at 1t=260 K (Figure 4.2) [7]. It is a Pauli-paramagnetic metal above Tt and

an antiferromagnetic nonmetal below Tt. The conductivity changes by a factor of rv

40 at the transition. The lattice constants a and c abruptly increases by 0.3% and

1 %， respectively， in going from the metallic state to the nonmetallic state (Figure 4.3)

[51]. The hexagonal NiAs-type NiS is thermodynamically a metastable state at room

temperature and stable at higher thanrv400・C，and therefor the samples can only be

obtained by quenching.

In this system， the conductivity in the lower temperature phase is nearly tempera-

ture independent， and therefor the low-temperature ph錨 eis called nonmetallic phase.

The non metallic phase is a semimetal [52， 53] or a degenerate semiconductor [54].

The Hall coe血cientincreases its absolute value by a factor of "，400 and changes its

sign from negative to positive in going from the metallic phase to the non metallic

phase (Figure 4.4) [6]. This suggests that the character of the Fermi surface changes

from a large electron-like Fermi surface to small hole pockets in going from the metallic

state to the non metallic state. NiAs-type NiS naturally contains a small concentration

of Ni vacancy， and the carrier number of holes obtained from the Hall coe血cientby

n = 1jeRH in the nonmetallic phase is nearly equals to 2x per formula unit cell for

Ni1_xS (Figure 4.5) [55]. This indicates that the Ni vacancy accepts two electrons in

the non metallic state. The mobility of the hole is small (= 3 cm2 /V /s) and nearly

temperature independent， indicating that the top of the valence band forms a narrow

band. A neutron diffraction study of NiS revealed that the magnetic moments on Ni

ions in the antiferromagnetic state are 1.5-1. 7μB The magnetic moments are coupled

ferromagnetically within the ab-plane and antiferromagnetically between neighboring

57

58 Chapter 4. V-substituted NiS

ab-planes [56]. The magnetic susceptibility below Tt is nearly temperature independent

and abruptly decreases across the transition (Figure 4.6) [6]. The susceptibility above

Tt has a weak positive temperature dependence. According to the optical absorption

study， NiS has an optical gap of "，0.14 eV [9].

In order to explain the transport and magnetic properties of NiS， band structure

calculations were performed. Mattheiss has performed non-self-consistent augmented

plane wave calculations and succeeded in opening the gap for the antiferromagnetic

state (Figure 4.8) [57]. In this calculation， the top of the valence band is at the A

point and the bottom of the conduction band is at the K point of the Brillouin zone

and the minimum optical transition is possible at a certain k point between A and

L. The value of the optical gap is " ，0.15 eV and in good agreement with the optical

absorption experiment ("'0.14 eV). Recently， Anisimov has succeeded in opening the

gap for the antiferromagnetic state with LDA+U calculation [58].

Se-substitution of S decreases the transition temperature. The ph剖 ediagram of

NiS1-xSex is shown in Fig. 4.9. This is much simpler than that of NiS2-xSex and

has the same topology as the weak correlation region. of the Moriya.・Hasegawaphase

diagram (Figure 1.1). S←substitution corresponds to the decrease of the correlation

strength U jt. It is interpreted that the Se-substitution e旺ectivelybroaden the Ni 3d

band through the hybridization with more expanded Se 4p orbitals than that of S 3p.

The temperature dependence of the electronic structure near the Fermi level was

investigated by a high-resolution photoemission study [10]. From the analysis of this

spectra， a the spectral weight is absent from the Fermi level to '" 10 me V in the non

metallic phase. The "，10 meV gap observed in the previous study is much smaller than

the 1'V0.14 eV gap deduced from the optical study， probably because the conductivity

is p-type and Fermi level is located near the top of the valence band. Then， the high-resolution photoemission study of n-type NiS is expected to clarify the change of the

electronic structure in the metal-nonmetal transition in this system， because if the

gap is加 large剖 0.14eV， a much larger gap would be observed. Several percent of

early transition metals (Ti， V， Cr) can be substituted for Ni and these substitutions

lower the transition temperature of the system (Figure 4.10，4.11) [59]. These systems

were found to be n-type in the non metallic phase by a thermか electronicpower study

(Figure 4.12) [11]. Substituted ions are trivalent in both non metallic and metallic

phases according the 3d core XPS of substituted ions (Figure 4.13).

In this work， high-resolution photoemission experiment for n-type V-doped NiS was undergone and the electronic structure of this system wi1l be discussed based on the

experimental results.


('¥-ベγ?

?

Figure 4.1: NiAs-type crystal structure [50].

4.2 Experimental

Quenched sintered polycrystals of Nio.97VO.03S and Nio.94 VO.06S were supplied from Mr.

M. Nishioka， Dr. M. Matoba and Prof. S. Anzai of Keio University. Both samples are n-

type nonmetals in the low-temperature phase. Due to the instability of the metastable

state， NiAs-type NiS undergoes a phase transition to the stable rhombohedral B15

structure in one or two weeks. Therefor， the experiment had to be finished within one

week after the quenching and the samples were kept from heat during the preparation

and the experiments. The high-resolution UPS measurements were done for these

samples at 20 K and 300 K. Because the metal-nonmetal transition in this system

is accompanied by the discontinuous change of lattice constants， the samples become

fragile and have small cracks whenever they undergoes a metal-nonmetal transition.

In order to minimize the this damages of samples， spectra were taken at 300 K at first，

and then the samples were cooled down to 20 K and spectra were taken. The filing of

the samples were done carefully， especially at 20 K. The basic experimental method is

described in chapter 2.


P (uc吋

10・2

レ...---'一一ずずーァτ一一宇一一→一一一τ一一可一一「ー←ー←も~吋

10・3

10 ・4

一一ー『ー--'-100

a 200

Figure 4.2: Electrical resistivity of NiS [7].

:刊-+-+-

下れ30可→

61 ExperimentaJ 4.2.

5・38

(《}。 5・34

C-OXIS

5・30

"↑FEO

↑ZWC00

a -OXIS @

υ 二 3・456・ーo J

320 280 240

(0 K)

200

Temperat ure

160 120 3・440

80

Figure 4.3: Lattice constants of NiS [51].

62

RH (<ml.c・1)

10・2

10・3

10・4

Chapter 4. V-substituted NiS

ー+ーーー-..-一一ー・4・ーー+ー-.. ・ーーー『ι

ρ'，p<: ~-\ヘ

100 200

Figure 4.4: Hall coe血cientof NiS [6].

+、、。

:r+-・司令 -

.' .. .yp<:

300

Tlぺ

4.2. Experimental

1022

63

，〆

10

(

「

'εU)LC

泊。

・1

，，，〆，，

，，， d

，， J ，

，，〆

/ ノ，

〆， " ，，

，， r ，，

，，，

2 x Ni vacancies (cm 3 )

Figure 4.5: Number of holes as a function of the concentration of Ni vacancies [55].

V-substituted NiS Chapter 4. 64

3.0

2.6

:E 3.2 4 g o 、妻2..8ω

司PO

(0) 2.4

350

(K)

Figure 4.6: Magnetic susceptibility of NiS [59].

おO150 50

FOR 制iS

KRAMERS-KRONIG ANALYSIS CONDUCTIVITY

Eム c-axis

〆'' ，，

J /

、w

!

、

リ

刷

、

1

8¥

T1寸111L

、、、、

X 10'

12

4

14

tO

8

6

2

Eど。εεhab

10.' tev 。105 10"

(cm-') FREQUENCY

'0.5

LOG

102

Figure 4.7: Real part of the optical conductivity of NiS [9].


0.6 . .・ANTIFERROωAGNETIC NLS

2+

L L¥ ~\ 1 I 1 ¥-' I I T3¥l 4 九l[ A' ... ‘ ，、

.a: 】

「ーでー「ーγ A..寸工 -~ζ::1-r---...i'?3+ 且"ト

2+ 3国2- l- ~3

。4L3+U《き辛子気掛笠ー汁ミL4-=当2・2て，.d'.'t M K r A L H A M L. t<糾

Figure 4.8: APW band structure of NiS [57].

3∞

Figure 4.9: Phase diagram of NiS1-xSex・[6].


300 細}

2SO

《

VS

ー・トー

200 0 2

I02X

4 6

IN NII-xVXS

8

Figure 4.10: Transition temperature of Ni1-x V xS.[59]. Solid and open circles indicate

transition temperatures determined in cooling and heating runs， respectively.)


Figure 4.14 shows the He 1 UPS spectra for Ni1-x V xS. There is no such a wide absence

of spectral weight just below the Fermi level in the spectra for both Nio.97 VO.03 and

Nio.94 VO•06 as implied by the optical conductivity of NiS. The main peak 1.3 eV below

the Fermi level， which is derived from the Ni 3d states， is shifted towards higher binding energy by 40 -50 me V as temperature goes down from 300 K to 20 K in the spectra

of NiS， Nio.97 VO.03 and Nio.94 V 0.06・FromFig. 4.14， one can see that the intensity of the edge at the Fermi level decreases with decreasing temperature if the spectra are

normalized to the intensity of the peak at 1.3 eV. The spectra near theFermi level are

shown in Fig. 4.15. Hence， the spectra are normalized at 0.3 eV to investigate the shift

of the leading edge of the spectra. NiS shows a shift of the leading edge just below

the Fermi level toward higher binding energy in going from 300 K to 20 K， indicating

the opening of a gap， while Nio.97 VO.03 and Nio.94 VO.06 show the leading edge at the

Fermi level even at 20 K. The shift of the leading edge just below the Fermi level for

the nonmetallic state was estimated by assuming a model DOS which is linear in the

binding energy but cut off just below the Fermi level as shown in Fig. 4.16. If the

system has this type of DOS， the binding energy of the leading edge is determined by

the crossing point of the spectrum and extrapolated line of the half height of the DOS.

67 Experimental results

10・2

4.3.

1σ3

A20c-

x=O.02

x=O.05

x=O.06 工++

q、

x=O.08 10-4

お0(K)

200 TEMPERATURE

100

Figure 4.11: Electrical resistivity of Ni1-x VxS.[59]. Solid symbols and open symbols

indicate cooling runs and heating runs， respectively.

V-substituted NiS Chapter 4. 68

Nil-xVxS x=o

nUAUAUAU

8

4

4

ー'ー・-~駈面

o

0.01

。UAUAυnυAU

2

8

4

4

1且

一

0.02

40 O

-40

ハUQUAU

A斗

A『

40 戸扇面画面d _~

1-40 -80

-・・句圃掴ijD

込S主

nunUAUAU

06a斗

a斗

0.035

0.04

(凶¥〉ミ

)ω

6

2

300 100 200 T(K)

40 O

-40

O -40

40 0

-40 -80

0

Fi伊 re4.12: Therrr附 lectroniccoefficient of Ni l-x V x S. [11].

69 4.3. Experimenta1 resu1ts

V2PXPS

可也 .，、知/、rU3K

(g酒田.aお)〉

km

530 520 510 BINDING ENERGY (eV)

Figure 4.13: V 2p core level XPS spectra of Ni1-x VxS.[l1].


The linear part of model DOS were determined by fitting the experimental spectra

taken at low temperature in the range of 0.1 to 0.3 eV. The shift of the leading edge of

DOS were estimated from Fig. 4.17 to be 91:2， 0土1，and 1土1me V for NiS， Nio.97 VO.03

and Nio.94 VO.06， respectively.


As NiS undergoes a metal-tcトnonmetaltransition， the opening of the gap ('" 10 me V)

was observed in the previous high energy resolution photoemission study [10]， whereas

the optical study has given a gap of ，，-，140 meV. The smallness of the gap observed

by photoemission was understood as a nature of the p--type semiconductor that the

Fermi energy lies near the top of the valence band. On the other hand， the present high-resolution photoemission spectra of n-type V-doped NiS in the non metallic phase

does not show the large gap， in spite of the expectation that the Fermi level would lie

near the bottom of the conduction band. The nonmetallic phase of NiS has been

considered to be either a degenerate semiconductor [54] or a semi-metal [52， 53]. The absence of the large gap in the photoemission spectra of the n-type V -doped NiS may

be understood from the view point of a semi-metallic band structure in the following

way.

The metallic NiS is a normal metal， but when it undergoes a transition to the

nonmetallic phase， the band structure changes by the antiferromagnetic order and opens a semi-metallic band gap at the Fermi level as shown in the ca1culation by

Mattheiss [57]. Then the direct optical gap can be much larger than the transport

gap. The valence band DOS has a sharp edge just below the Fermi level with a tail

extending beyond the Fermi level. The midpoint of the edge of the valence band is

"，10 meV below the Fermi level and the tail from the edge to the Fermi level could

not be identified in the photoemission spectra. For the undoped samples the dominant

carriers are holes because it is naturally doped with holes by Ni vacancies. When

V atoms are substituted for Ni， and electrons donated by the V3+ ions [11] fill the

overlapping valence and conduction bands. Beyond a certain level of V-substitution，

the number of electrons exceeds that of holes， and the system becomes an n-type

conductor. If the tail of the valence band becomes less extended， the leading edge of the top of the valence band would approach the Fermi level， although the system

becomes n-type. In this model， when the system changes from the p-type to the n-type

the Fermi energy does not show a jump but could even approach to the edge of the

valence band. If this semi-metallic picture is valid， the metal-to-nonmetal transition in NiS can be viewd as a transition from the paramagnetic metal with the large Fermi

surfaces to an antiferromagnetic metal with small Fermi surfaces.

4.4. Discussion and concluding remarks

〆-、υつ喝圃.・...・司ロロ

71

hv=21.2eV

、、』司、‘

，

‘ .

• 、、

‘

、•

、•

‘. --‘.

ー--300K NiS -40K

S H C司、ー〆〆、 l ・・・ 300K -ヨ |Ni09Ad-mロ4ふ~

ロ~

---、、、

• 、‘

、•

300K NiO.94VO.06S - 15K

1.5 0.5 1.0 ハVOU

Binding Energy (eV)

Figure 4.14: High-resolution He 1 UPS spectra of NiS [10]， Nio.97VO.03 and Nio.94 VO.06・

V-substitlited NiS Chapter 4. 72

hv = 21.2 eV

h .. ‘ ‘ ‘ ‘

‘ ‘ ・・・、‘'‘'‘

一-300K -30K

NiS /ーヘ∞ ~ .~

ロロ

，，

、、‘ 、

‘・‘ 、、‘ • .. . ‘ ・・

-300K Nio.97 VO.03S一一15K

‘ ‘ 、‘ ‘ ‘ 、‘ 、. 、--、

.S十四MW)

ヘハ右∞ロω一-ロ][

-300K・、一一 15K

NiO.94 VO.06S

Binding Energy (eV)

High-resolution He 1 UPS spectra near EF of NiS [10]， Nio.97 VO.03 and Figure 4.15:

Nio.94 V 0.06・

4.4. Discussion and concluding remarks 73

(∞判明ロロ

kn右目ロ

ωH口同

shift of the leading edge

一-modelDOS - O.5xDOS ---extrapolation of 0.5 xDOS 『・-experimental spectrum -nt〈

)

0.3 0.2 0.1 0.0


ー0.1

Figure 4.16: (a) Method of the determination of the leading edge. The cross point of

spectrum and extrapolated line of the half height of the DOS is equals to the leading

edge.

In the above model， it is considered that the band edge would develop a tail when

carriers are doped and that the Fermi levellies within the tail. As a possible mechanism

for the creation of the tail， disruption of antiferromagnetic order by carrier doping may

be considered. In order to confirm the evolution of the tail， more systematic doping dependence ( including p-type dopi時 byNi vacancies) would be required.

74

(凶苦言

.fsb刃包ω百四

Chapter 4. V-substituted NiS

10 0 -10 Binding Energy (me V)

10 0 -10 Binding Energy (me V)

10 0 -10 Binding Energy (meV)

Figure 4.17: (a) Determination of the leading edge position for NiS [10J.

(b ) Determination of the leading edge position for Nio.97 VO.03・ (c) Determination of

the leading edge position for Nio.97 V 0.03・

30K 〆'・、

2 .-略c s .0 L噌

C司、、，〆

b ・，司uヨロ4主.....

-0.2 ..s 0.2 0.0 Binding Energy (e V)

(gZ2.fmw)b窃ロ

ω百四 0.2 0.0 Binding Energy (eV)

Fヘυ。・.... ・F・4ロ2

.0 L司

C司、--'

〉、..... ..咽υラ

ロQ) ・....

-0.2五

(回苦言

.fω)bヨロω吉岡

15K 〆'ー、、v.> ..... c s .0 1-0 岡、--'

b ・，・4υぅロω .....

-02£

hv = 21.2 eV

0.2 0.0 Binding Energy (eV)


』

OKAH

一ωcoo

valence band

NiS ωωVMWHmw

conduction band

EFermi Energy

valence band

Ni1-ys ωOHdwvmw ちと一ωcoo

n>p

conduction band

n p

EFermi Energy

Figure 4.18: Proposed Schematic electronic band structure of NiS and V-doped NiS

near the Fermi level.

Chapter 5

BaNiS2

5.1 Introduction

BaCo1_xNix82 has a two dimensional structure which consists of TM・8(TM=Co， Ni)

sheets and Ba-S sheets. (Fig. 5.1) [12]. It has been discovered that BaCo1-xNixS2 un-

dergoes a metal insulator transition (MIT)剖 afunction of x [14] and that BaCol_xNix82_y

exhibit a MIT剖 afunction oftemperature [61]. The electrical resistivity and the mag-

netic susceptibility of BaCo1_xNix82 are shown in Fig.s 5.2 and 5.3. BaCo82 is a Mott

insulator and BaNiS2 is a metal. The MIT occurs around x rv 0.2. Because of the two

dimensional crystal structure， the anisotropy of the electrical resistivity in BaNi82 is as

large as 30 (Fig. 5.4)[13]. The magnetic susceptibility decreases in going from BaCoS2

to BaNiS2・ BaNiS2shows a very unusuallinear temperature dependence of the mag-

netic susceptibility above 100 K [14]. The small magnetic susceptibility of BaNi82

is explained by LDA band structure calculations， which indicate a DOS minimum at

the Fermi level Fig. 5.5 [62， 63]. The linear temperature dependence of the magnetic

susceptibility is also explained qualitatively if sharp edges in the density of states are

assumed on both sides of the Fermi level.

In this work we have measured the photoemission and inverse photoemission spec-

tra of BaNi82剖 afirst step to understand the electronic structure of these compounds.

Ni L2，3 X-ray absorption spectra were also measured to determine the electron config-

uration of the Ni ion in BaNiS2・ Theelectronic configuration reflects the strength of

the crystal field in this compound. Each Ni ion is coordinated to five 8 ions forming a

pyramid type structure. This coordination is rather rare for Ni compounds while often

found in high-Tc cuprates. The Ni ion in BaNiS2 has nominally 3d8 configuration，

which would cause the high spin configuration (S=1) if the Ni ion were octahedrally or

tetrahedrally coordinated. In this compound， it is not clear whether the Ni ions have

the low spin (8=0) co凶 gurationor the high spin (8=1) co凶 gurationbecause the Ni

ions are in the crystal field of lower symmetry.

77

78 Chapter 5. BaNiS2

Figure 5.1: Crystal structure of BaNiS2・[61]

79

x= 0.2

0.22 0.23 0.25 0.3 0.4 0.5 1.0

Introduction

ーーーー..，.一一ーーーー・、

BaCo1-xNixS2

10，

¥ 8

6

2

(εoαε)ミ

5.1.

300 200

T(附

O

Figure 5.2: Electrical resistivity of BaCo1_xNixS2・[14]

BaNiS2 Chapter 5.

8aCo1-xNixS2

80

10

旬、J

s

l

〉-

M

ニ

tパ

.

・

，

J

mxooo

， ar i

∞

司

4

笠)。

3∞1・

(03Er10F) -・.

.・・・ぬこ::::?"・・・…・・・・…・……………・4・空・ニ・.-・.・..

---・・・・・・・_....__.・-------_....._...，・ι回・‘'ー・・・・・・・・側....................iII闘.........聞・~・v ・v

------ ・・....・・"・・……1.0一-....竺句"旦旦J:tW.!!竺竺f 主主笠・?さ竺竺竺コ一一一一」一一一一ーーl一一日一-

o 100 200 300 T (K)

><

Figure 5.3: Magnetic susceptibi1ity of BaCo1-xNixS2・[14]

81 Introduction 5.1.

一寸一寸一「， O~

pムplane

stngle crystal BaNiS2 (NS611)

104

(EUC4a p 11 plane

3∞ l aー」一一ムー

100 却 OT emperature (K)

A

10:1

102 l_-L

0

Figure 5ιElectrical resistivity of single-crystal BaNiS2・[13]

82

200

的g 100 剣o・

注的、

GE 2

o 50

2.2 0.0

I 1111" 1111

I 11" 11 11 ~II

0.2

Energy (Ry)

Chapter 5. BaNiS2

BaNiS2

ー一一 to抱l

一一 Ni3d• Ef

Figure 5.5: LDA band-structure structure calculation of BaNiS2・[63]

5.2. Experimenta.l 83

5.2 Experimental

Sintered polycrystals of BaNiS2 were provided by Dr. N. Shirakawa of Electrotechinical

Laboratory and Prof. M. Sato of Nagoya University. The photoemission and inverse

photoemission experiments were done using laboratory equipments. These measure-

ments were done at liquid nitrogen temperature. Ni L2，3 x-ray absorption experiment

was done at beam line BL-2B of Photon Factory， National Laboratory for High Energy

Physics. The measurements were done at rv40 K. The surfaces of samples were scraped

with diamond filer in order to obtain clean surfaces in all measurements.


Figure 5.6 shows the UPS spectrum of BaNiS2・ TheNi 3d derived peak at 1 e V

is dominant and structures derived from S 3p at ，，-，3 eV are suppressed due to the

relatively small photoionization cross-section of S 3p at this incident photon energy.

Small peaks from 5 to 9 e V may be from contaminations on the sample surface. The

BIS spectrum of BaNiS2 is shown in Fig. 5.7. A flat structure just above EF is derived

from Ni 3d states， and a broad peak at 6 e V above EF is derived from Ba 5p and S 3d

stat田.An intense narrow peak at 14 eV above EF is derived from Ba 4f states.

Figure 5.8 shows the Ni L2，3 XAS spectrum of BaNiS2・ Inorder to determine

whether the Ni ion in BaNiS2 has the low-spin or high-spin electron configuration， a

cluster model analysis w凶 performed.Figure 5.10 shows the model cluster NiS58一.Ni

3d and ligand S 3p level were considered. The initial state is obtained by picking up

the lowest energy state 企omthe diagonalization of the initial state Hamiltonian. The

initial state Hamiltonian is spanned by the configuration of d8， CP L and dlO L 2， where L

denotes a ligand hole in the S 3p state. The final state was obtained by diagonalization

of the final state Hamiltonian spanned by the configuration of d8f::， CP Lc and dlO L2f::，

where f:: denotes a core hole in the Ni 2p state. Adjustable parameters were the on-site

d-d Coulomb energy U， the p-to-d charge-transfer energyム三(♂LIHI♂L)-(d8IHld8)， and the d-p transfer integrals (pdσ) and (pd7r)， where we have assumed (pd，σ)/(pd7r) =

-2.2出 before[41]. The cluster calculation were done for both high-spin configu凶 ion

and low-spin configuration. The results of cluster calculations are shown in Fig. 5.9.

The best fit was obtained by chooseng the parameters as U = 5.0 e V，ム=1.0 eV and

(pd，σ) = -1.6 eV for the high-spin configuration. The calculated spectra for the low-

spin configuration could not be fit well the experimental one although a wide variety

of parameters were tried. A characteristic feature of the low-spin spectra is a small

satellite separated from the main peak. This satellite could not be moved to ne町 the

main peak nor enhanced within the low-spin configuration. Hence， we have concluded


that the Ni ion in BaNi82 is in the high-spin configuration.

I 、.S I BaNiS今と・..1"""'4 '"

ロlI d -.

~ I~~ ~ ~、、Ah二 JJ・'"8 ，--、AJu-、•. r:.J ~ I ・、、-.-/

• -• hz∞ロω一守口]{

• • hv=40.8eV • LL-

12 8 40

Binding Energy (eV)

崎弘前... ~

Figure 5.6: He II ultraviolet photoemission spectrum of BaNi82・


Now， we discuss the electron configuration of the Ni ion in BaNi82・ 3dstates in the

octahedral coordination are sp1it into the lower triply degenerate t2g states and higher

doubly degenerate eg states. 80， the Ni2+ ion has the t2g 6eg 2 configuration and the high-

spin configuration is favored by the Hund coupling. In the pyramidal coordination， the degeneracy of the eg states is lifted and the eg band is split into the lo，，::-lying 3z2 - r2

band and high-lying x2_y2 band， each ofwhich is further split by exchange interaction.

The low-spin or high-spin configuration is realized as a result of competition between

the Hund coup1ing and the crystal-field sp1itting. Because the XA8 spectrum has

been well reproduced by the c1uster-model ca1culation for the high-spin configuration，

we have concluded that the high-spin configuration is realized and the crystal field is

considered to be smaller than the Hund coupling.

The present result appears to contradict with the absence of magnetic long-range

order. In order to reconcile the local high-spin configuration with the non-magnetic

ground state， one has to consider inter site Ni-Ni coupling. Presumably， spin fluctua-

5.4. Discussion and conc1uding remarks

(SED.f〈)knZ882

t E J、-、 1.-・・..・.. _. . -(∞沼口門戸.モ〈)

BaNiS2

.d

・

、ー叫

ヘhv = 1486.6 eV •

6・.々、旬、ペミ~ザ、，

4・， ‘ ‘F

4・，

凶納予伊曜、-

1--，，1'

Ba4f

k内判明∞ロ

ωH口同

Ni3d sa4d

+ S 3d + Ni4sp

。 5 10

Energy above EF

よ

15

Figure 5.7: Bremsstrahlung isochromat spectrum of BaNiS2・

BaNiS2 NiL2，3 XAS L 3

L 2

880 890 900 910

Photon Energy (e V) 920

Figure 5.8: Ni L2，3 X-ray absorption spectrum of BaNiS2・

85


(gED.f〈)b-Egg-

BaNiS2

NiL2，3 XAS • exp町 iment

一一-c1uster calcuralion

high spin U=5.0eV d=1.0eV

(pdσ)=-1.6 eV

-・・・

880 890 鈎o 910

Photon Energy (eV) 920

BaNiS2

NiL2•3 XAS • experiment

一一-c1usler calcuration

(凶泊四

HD.f〈)bgcscH

low spin U=3.0eV d=O.2eV

(pdσ)=ー2.0eV

岡田・・

880 890 鈎o 910

Photon Energy (eV) 920

Figure 5.9: Ni L2，3 X-ray absorption spectra compared to the cluster model calculations

for the high-spin configuration (upper panel) and the low-spin configuration (lower

panel).


o Ni

Os Figure 5.10: Model cluster of NiS5

8一.

tions caused by the intersite spin-spin coupling has lead to the non~magnetic ground

state， in spite of the high spin configuration. This situation is analogous to the para-

magnetic metallic state of NiS and further theoretical and experimental studies are

necessary to elucidate these metallic states.

Chapter 6

Conclusion

In the preceding chapters， we have studied the electronic structures of several late transition-metal chalcogenides including pyrite-type and NiAs-type compounds， which

undergo metal-insulator transition or exhibit other interesting physical properties.

These properties are strongly infiuenced by electron correlation.

Pyrite-type compounds were extensively studied from 1960's to 1970's， but many problems remain to be resolved till now. The present study has revealed several new

aspects of the electronic structure of the pyrite-type compounds. Resonant photoemis-

sion spectra of FeS2， CoS2 and NiS2 have shown satellite structures， indicating strong electron-electron correlation. The cluster-model analysis of NiS2 has clarified that NiS2

is a charge transfer type insulator. Subsequent analysis has shown that the low-spin

state of FeS2 is stabilized by a large transfer energy (pdσ) owing to the short Fe-S dis-

tance compared to the high-spin FeS. The photoemission spectra of ferromagnetic and

paramagnetic CoS2 did not show detectable differences while LDA calculations predict

more dr部 ticchanges. This phenomena imply that electron correlation strongly influ-

ences not only the ferromagnetic state but also the paramagnetic state. Changes in

the electronic structure across the metal-insulator transition in NiS1.55Se0.45 have been

studied by high-resolution photoemission measurements and careful analysis has been

made. We have explained these changes from the view point of a semimetallic band

structure model. The insulating state of NiS2 is also unusual in that the high intensity

just below EF indicates a large number of hole carrier density. The semiconducting

transport in NiS2 was thus attributed to a very low carrier mobi1ity of activation type.

The metal-nonmetal transition in NiS h出 alsobeen the subject of a number of

studies since 1960's. In order to study the nature the nonmetallic state in more detail， we have studied V-substituted samples， which are n-type nonmetals， expecting to see a

large (140 me V) gap by high-resolution photoemission measurements. On the contrary， the spectra did not even show the small gap (10 meV)， which was observed for the p-

type NiS. In order to explain the absence of the gap in the n-type NiS， we have proposed

89

90 Chapter 6. Conc1usion

a semi-metallic band structure model for NiS.

Finally， we have studied the electronic structure of metallic compound BaNiS2・The

Ni ion in BaNiS2 has a 3d8 configuration. A1though the Ni2+ ion in a cubic crystal

field has a high-spin configuration， either the high-spin or low-spin configuration of the

Ni ions can be realized in a low symmetry crystal field as a result of the competition

between the Hund coupling and the crystal-field splitting. A Ni L2，3 XAS study of

BaNiS2 have clarified that the high-spin configuration is realized on the Ni ions in

BaNiS2 due to small crystal field.

After the present work， many questions have not yet been answered concerning the electronic and magnetic properties of these compounds. For instance， as for pho-

toemission studies of NiS2-xSex， measurements of other compositions including the

paramagnetic metallic region (x > 1) remain to be done. Control of carrier doping

in NiS in the photoemission studies would allow us to understand microscopic mech-

anisms for the transport properties of this system. More systematic high-resolution

photoemission studies of p-type and n-type doped NiS will give us further information.

These experiments would be of great help in understanding the electronic structure of

these interesting systems.

91

Acknowledgment

1 would like to express my gratitude to the following people giving me supportヲ advice

and useful discussions.

First of all， 1 would like to thank Prof. Atsushi Fujimori who hωsuggested this

work and given me many enlightening discussions. 1 would like to express my gratitude

to Dr. Takashi Mizokawa. 1 am deeply indebted to him for analysis of photoemission

spectra with the cluster model ca1culations. 1 am grateful to Prof. Hirohumi Namatame，

of Hiroshima University， the previous research associate of Fujimori group， who have

introduced me to the experimental techniques.

1 am grateful to Prof. Tomonao Miyadai and Mr. Tadashi Sekiguchi for supply-

ing the samples of CoS2-xSex and NiS2-xSex・1wish to thank Prof. N obuo Mori and

Prof. Hiroki Takahashi for supplying the samples of NiS2・ 1acknowledge Prof. Shige-

m部 aSuga for supplying the samples of FeS2 and colaboration on resonant photoemis-

sion mersurements. 1 am grateful to Prof. Dipanker D. Sarma， Dr. Nirmala Chan-

drωekharan and Dr. S. R. Krishnakumar for providing me with samples of NiS2-xSex.

1 acknowledge Prof. Shuichiro Anzai， Dr. Masanori Matoba and Mr. Masaya Nishioka

for providing me with samples of Ni1-x V xS. 1 am grateful to Dr. Naoki Shirakawa

and Prof. Masatoshi Sato for supplying samplcs of BaNi1-xCOxS2・ 1wish to thank

Prof. Hideji Yamada for providing me with results of band structure ca1culations prior

to publication. 1 thank Prof. Hidenori Takagi for enlightening discussion and providing

me a preprint prior to publication.

1 would like to express my gratitude to Dr. Keiji Morikawa of Tohoku University

and Mr. Izumi Hase of Electrotechnical Laboratory who have set up the BIS measure-

ment system together with me and gave me a lot of advice for improvement of this

system. 1 thank Dr. Shin'ichi Nohara for porting the measurement program to the

BIS me部 urementsystem. 1 would like to thank Mr. Takehisa Konishi， Mr. Akihiro

Ino and Mr. Toshiyuki Tsujioka and Mr. Jobu Matsuno， who have maintained and

improved the BIS measurement system with me. In addition， 1 am deeply indebted to

Mr. Ino for the very convenient data processing programs. 1 acknowledge Mr. Akira

Sekiyama， Mr. Kensuke Kobayashi， Mr. Tomohumi Susaki and Mr. Keiji Otomo for

giving me a lot of help during high-resolution UPS measurements. 1 thank Mr. Jin-

Young Son and Mr. Jun Okamoto for providing me a great support during XPS and

UPS measurements. 1 am grateful

92 Chapter 6. Conc1usion

Finally， I am very grateful to all whose name are not mentioned here but give

various kinds of support to me.

Part of the calculations in this work was performed on V AX computers in Meson

Science Laboratory， University of Tokyo. This work is supported by a Grant-in-Aid for

Scientific Research from the Ministry of Education， Science and Culture， Japan， and

the New Energy and Industrial Technology Development Organization (NEDO).

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