New collective and singgple particle phenomena at oxide...

New collective and single particle g pphenomena at oxide interfaces and

digital superlatticesdigital superlattices

James N Eckstein 1,2James N. Eckstein, ,

1Department of Physics, Univ. of Illinois, Urbana, IL2Fredrick Seitz Materials Research Laboratory, Univ. of Illinois, Urbana, IL

Xiaofang Zhai,1 Maitri Warusawithana,2 Bruce Davidson,3Jian-Min Zuo,4 Amish Shah,4

11University of Science and Technology of China, Hefei, China2Department of Physics, Florida State Univ.

3TASC INFM, Trieste, Italy4Department of Materials Science and Engineering, Univ. of Illinois, Urbana, IL

University of Illinois

Supported by Dept of Energy - Basic Energy Sciences

New phenomena at oxide interfaces (3 topics)

What are the new results?• global symmetry can be controlled by architecture Warusawithana

d i SL t bt i ti l t iti ( ) Zh• design SL to obtain new optical transitions, σ(ω) Zhai• superconductivity with e-h antisymmetry Davidson

Global symmetry:Global symmetry:Make a digital superlattice out of 3 dielectric phases and use SL architecture that breaks inversion symmetry

dielectric has permanent polarization and finite χ(2) dielectric has permanent polarization and finite χ( ) .

New electronic transitionsDigital superlattice to mix bands at interface

ti l t iti b t it t t new optical transitions between composite states.

Modified superconductivity at interface (unexpected)a-axis interface between YBCO and CaTiO3 (NIS tunnel jct)f ( j )

crystalline interface causes DoS measured by tunneling to have broken e-h symmetry at interface (not like BCS)

N d i di l itopic 1

summary

Nanostructured asymmetric dielectrics

summary

Lattice strain distorts bonds at heterointerfacesasymmetric supercell asymmetric strain “field”asymmetric supercell asymmetric strain field

Supercells with broken inversion symmetry lead to self-poled materials (similar to ferroelectricity but )self poled materials (similar to ferroelectricity, but…)

asymm. strain field acts like effective bias field, Eeffpermanent polarization, with no two state switching

Strain “proximity effect” extends about 2 uc


Electro-Optic Frequency Shifter

index of refraction wave

n(y,t)-n0

z

x

Eopt(y,t)

x

optical pulse

direction of propagation

)sin(0 wtkyEE

Light pulsemicrowave

0

Index of refraction experiences a dynamic change given by

Ernn 31 Ernn c02

D. A. Farías and J. N. Eckstein. “Coupled-Mode Analysis of an Electro-optic Frequency Shifter,” IEEE J. of Quant. Elect. Vol 39. No 2. , pp. 358-363, Feb. 2003.

Experimental Results EOFSExperimental Results EOFS

323.55

Frequency Shift vs. Microwave Powerpower vs frequency

323.4

323.45

323.5

323.25

323.3

323.35

323.2-3 -2 -1 0 1 2 3

sign*(power)1/2

Frequency shifting proportional to P1/2

Experiments done with 927 nm., 20 ps. pulses at a 75.7 MHz repetition rate. Microwave was chosen at the 80th harmonic (6.056 GHz), and powers of 0 0 3 1 2 2 7 4 8 and 7 5 Watts were applied to the

P1/2

powers of 0, 0.3, 1.2, 2.7, 4.8, and 7.5 Watts were applied to the modulator. Freq shifting range = 1 THz limited by rc

D. A. Farías and J. N. Eckstein. IEEE J. of Quant. Elect. Vol 41. No 1. , pp. 94-99, Jan 2005.

What you would like for non-linear modulators, etc…

P For non-linear optical and other field tuning applications would prefer a applications would prefer a characteristic like this.

St bl i l l tiE

Stable, single solutioncan’t de-pole

Permanently polarizeddielectric with big (2)

Use molecular nanostructuring to make s h t i l (MBE)such a material (MBE)(once you figure out what matters!)


Artificial structuresusing ALL-MBE to synthesize materials and heterostructures not found in natureg y

Controlling material properties via epitaxial strain

anisotropy energy surface(La0.7Ca0.3MnO3 on SrTiO3 substrate)

p

Tensile strain-induced magnetic anisotropy in magnetic oxide

0

10

20

20

-5

0

5

Z

0

10

20

-5

0

5

Z

lattice propertyProducing new “materials” by modulated heterostructure growth

-20

-10

0X

-20

-10

0

10

Y

-20

-10

0X

Grow crystal using ferroelectric and related phases:stack with structurally broken c-axis inversion

lattice property, e.g. strain, Tc

ariz

atio

n

ariz

atio

n

uper

cell

heterostructure growth

• broken symmetry throughout film favors one polarization

stack with structurally broken c axis inversion symmetry

favo

red

pola

unfa

vore

d po

la

su

broken symmetry throughout film favors one polarization• permits stable operation nearer to Curie temperature• obtain larger response at zero bias


Electrodynamic properties of atomically layered “meta-materials”

A t i di l t i l ttiAsymmetric dielectric superlatticesbroken inversion symmetry

A B C D EA B C D E

222+

LSMO111+

STO (1)BTO (1)CTO (1)

111-

BTO (1)STO (1)CTO (1)

222-222+

LSMO111+


111-


222-

1212

STO (1)BTO (2)


622+

STO (2)BTO (6)

DSLBTO (2)STO (2)CTO (2)

00lBa

Sr1212

STO (1)BTO (2)


622+

STO (2)BTO (6)

DSLBTO (2)STO (2)CTO (2)

00lBa

Sr


BTO (6)CTO (2)LSMO

0k0CaBaBTO (2)

STO (1)CTO (2)

BTO (6)CTO (2)LSMO

0k0CaBa

Calculation by Mike O’Keefe ASUMicroscopy by J. Zuo and H. Chen UIUC


bond valence sum calculationbond valence sum calculationMike O’Keefe (ASU)

bondpolarization

Z-contrast STEM image Hao Chen UIUCJim Zuo UIUC

Atomic Layer-by-Layer Molecular Beam Epitaxy

electrongun

hollowcathode

lamp

photomultipliertube quadrupole

massspectrometerMolecular Beam Epitaxy

Ca

Sr

Ba

shutters• atomic absorption spectroscopy for feedback

Al

Larotating

substrate positioner

quartzcrystalmonitor

loadlock

spectroscopy for feedback control (better than 10-3)

• ozone oxidation: Sb

Ti

Fe

positioner

substrateholder

(introduced by Allen Goldman, U. MN.)

• in-situ RHEED w/ Cu

Bi RHEED

turbopump

hollowcathode photomultiplier

tube

digital video

• 12 sources, incl. Ge and Ce

OzonegeneratorO

xyge

n

Pump

lamp tube

RHEED revealsgeneratorO Pump

ozonestill

RHEED reveals surface crystal structure

IntensityStart of Super Cell

End of Super Cell After 1 ML BTO After 1 ML STO

CTO Surface CTO Surface

Specular Spot1 ML BTO

2 ML BTO

1 ML STO

2 ML STO

1 ML CTO

2 ML CTO

a

a

aa a a

Oscillation from 1 Super Cell

1.0 34.0 67.0 100.0 133.0 time (s)1.0 34.0 67.0 100.0 133.0 time (s)

RHEED images at diff t i t f thdifferent points of the

super cell growthAfter 0.5 ML STO

digital superlatticeBTO-STO-CTO Behaves as though

1500111- at 250K

111- subject to effective bias field ~150 kV/cm

P(E=0) = 0.17 C/m2

1000

111- at 180K111- at 100K111- at 20K

( )

Samples without broken inv. sym. have sym. Curves

r

Sign of P depends on + or –superlattice architecture

500

Vbiasdigital superlattice

0-600 -300 0 300 600

biasdigital superlattice


E field (kV/cm)

700

' at 20K' at 120K

422+ proximity effect

1400

1737 (222+) Device B12' vs E Field (kV/cm)

222+

400

500

600

700 ' at 180K' at 260K' at 370K

'

1000

1200

1400 ' at 100K' at 200K' at 280K' at 340K

200

300

400'

400

600

800'

0

100

-600 -400 -200 0 200 400 600

E Field (kV/cm)

0

200

-600 -400 -200 0 200 400 600

E Field (kV/cm)

422+ appears to be made up of a superposition of a larger (+) curve plus a smaller (-) curve

( ) f /B CTO

(+) response comes from stronger CTO/BTO heterointerface, and….

h t int f ci l st in b nd dist ti n p l i ti n

BTO

STO


heterointerfacial strain bond distortion polarizationThis says that the polarization extends ~ 2uc away from int

N d i di l itopic 1

summary

Nanostructured asymmetric dielectrics

summary

Lattice strain distorts bonds at heterointerfacesasymmetric supercell asymmetric strain “field”asymmetric supercell asymmetric strain field

Supercells with broken inversion symmetry lead to self-poled materials (paraelectric)self poled materials (paraelectric)

asymm. strain field acts like effective bias field, Eeffpermanent polarization, with no two state switching

Strain “proximity effect” extends about 2 uc


New optical absorption bands in digital topic 2

summary

New optical absorption bands in digital superlattices

summary

•Short period digital superlattices of perovskite phases having different TMs can be accurately grownhaving different TMs can be accurately grown.

•New optical transitions can be created by placing different molecular layers that electronically hybridize next to each y y yother

•Electron transitions from source state to destination stateS t t i i l f l l l•Source state is mainly from one molecular layer

•Destination state is mainly from other molecular layer

•Measure heterojunction band line-upj p

New optical absorption bands in atomically layered digital superlattices

Combine Jahn Teller - Mott Hubbard insulator with band insulatorband line-up

Epitaxial LaMnO3 thin film

band J-T Mott

2 105

2.5 105

3 105

3.5 105

4 105

4

5

6

7

8Epitaxial LaMnO3 thin film

nduc

tivity

LM

O(S

/m)

Im relative dielec

Optical conductivityof LaMnO3 (red)

~1 5 eV

0

5 104

1 105

1.5 105

0

1

2

3

1 2 3 4 5 6

Re

Opt

ical

Con

ctric constant

Energy (eV) 1.5 eVgy ( )

5 105

6 105

7 105

10

12

14SrTiO3

MO

(S/m

)

Im rel

Optical conductivityof SrTiO3 (red)

LMO is an A-type antiferromagnet1 105

2 105

3 105

4 105

5 105

2

4

6

8

10

e O

ptic

al C

ondu

ctiv

ity L

M ative dielectric constant yp gbelow 140 K, FM planes stacked AFM-ly


0 01 2 3 4 5 6

Re

Energy (eV)

New optical absorption bands in atomically layered digital superlattices

Combine Jahn Teller - Mott Hubbard insulator with band insulatorband line-up

band J-T Mott

~1 5 eV

SrTi

Sr1.5 eV

LaM

LMO is an A-type antiferromagnet

MnLa

JEOL 2200FSwith probe forming yp g

below 140 K, FM planes stacked AFM-ly


with probe formingCs Corrector J-M Zuo group

Amish Shah

New bandstructure in short period superlatticessuperlatticesKey: SrTiO3

LaMnO3

1 15 104 4.4 10

N=8 LaMnO /25.00 4.40

4

1.1 104

(2 134 V 2 47 V)1.10

1x1

2x2 3 104

4 104

3.3 10

N 8N=3N=2N=1

LaMnO3/2

S/m

) 1.8

2.13 2.47 2.73.00

4.0004

S/m

) 3.30

8500

9000

9500

1 104

1.05 104 (2.134 eV, 2.47 eV)

3 104 7 1040.90

1.05

1.00

0.95

3 00

eV/Ω

m)

7 00

3x3 1 104

2 104

1.1 10

2.2 10

E

(104

1.00

2.00

σ(e

V) (

10

1.10

2.208500

2 2 104

2.4 104

2.6 104

2.8 104

3 10

5.5 104

6 104

6.5 104

7 10(1.8 eV, 2.7 eV)(0.75 eV, 2.7 eV)

3.00

2 20

2.80

2.60

2.40

SW(1

04 7.00

6.50

6.00

5.50

8x80 0

0.5 1 1.5 2 2.5 3 3.5Photon Energy (eV)

0.75

sum rule2 104

2.2 10

5 104

0 0.2 0.4 0.6 0.8 11/N

2.20

5.00

5.50

Like GaAs/AlAs minibands, but here the quantum mechanically blended phases are very different: STO is band insulator while LMO is insulator due to e-ph and e-e interactons Jahn Teller and Mott Hubbard

1.5 eV peak (MH-gap) blue shifts in shorter period SL1.5 eV peak (MH gap) blue shifts in shorter period SL

New spectral weight emerges between 1 and 3 eV, and SW lost below 1.7 eV.


Oxygen EELS shows similar spectra in Ti and Mn layersFurther evidence that the metal wavefunctions appreciably hybridize with oxygen

Spatially indirect or hybridized interface band transition at

LMO STO 2x2 Superlatticeoxygen K-edge EELS

electron causes transition from 1s to hybridized 2P spectral weight

Spatially indirect or hybridized interface band transition at 2.4 eV (MnTi)

6000

8000

electron causes transition from 1s to hybridized 2P spectral weight(sensitive to electron state delocalization at pre-peak energy)

oxygen "pre-peak"

2000

4000

C54-56-Ti1C59-62-Ti2

-2000

0

535 540 545 550 555 560

C39-42-Mn1

C47-c50-Mn2

STO

Energy (eV)Ti EELS O EELS


Energy (eV)

microscopy by Zuo and ShahTi - EELS

energyO - EELS

energy

Band line-up and electronic structureSrTiO3

eg

4

6

8

gy (e

V)

LaMnO3

e 2

eg

SrTiO3

eg

4

6

8

4

6

8

4

6

8

gy (e

V)

LaMnO3

e 2

eg

a b

2

O 2

t2g

0

2

4

Ener

g eg

t2g eg1

t2g

∆E (JT+Mott)

O 2

t2g

0

2

4

0

2

4

0

2

4

Ener

g eg

t2g eg1

t2g

∆E (JT+Mott)

LaO

LaOMnO2

MnO2SrOTiO2

LaO

LaOMnO2

MnO2

LaO

LaO

LaO

LaOMnO2

MnO2

MnO2

MnO2SrOTiO2

SrOSrOTiO2TiO2

2.3 eV

5 104 4.4 10N=8 LaMnO

3/2

5.00 4.40

Oxygen 2p

DoS

Oxygen 2p

DoS

SrOTiO2

TiO2

SrOTiO2

TiO2

SrOSrOTiO2

TiO2

TiO2

TiO2

2.3 eV peak is Mn eg1 to Ti t2g

for example, Mn d(3x2-r2) to Ti d(xz)3 104

4 104

2.2 10

3.3 10

N=3N=2N=1

3

104 S

/m)

1.82.13 2.47 2.7

3.00

4.00

(104

S/m

)

2.20

3.30

p , ( ) ( )

oxygen bands nearly align1 104

2 104

1.1 10

E

(1

0.75

1.00

2.00σ(

eV) (

1.10


0 00.5 1 1.5 2 2.5 3 3.5

Photon Energy (eV)

New optical absorption bands in digital l i

summary

superlattices

summary

•Short period digital superlattices of perovskite phases having different TMs can be accurately grownhaving different TMs can be accurately grown.

•New optical transitions can be created by placing molecular layers that electronically hybridize next to each othery y y

•Electron transitions from source state to destination state•Source state is mainly from one molecular layerD ti ti t t i i l f th l l l•Destination state is mainly from other molecular layer

•Measure heterojunction band line-up

U l i ti l D S t topic 3

Unusual quasiparticle DoS at cuprate/insulator interfaces (NIS tunnel junctions)

(not c axis cuprate interfaces rather they are a axis)

• If the interface causes a k-state to scatter to sit si s f d th SC is h d

(not c-axis cuprate interfaces, rather they are a-axis)

opposite signs of dx2-y2 then SC is quenched– At the surface a normal 2-D band is formed confined by the

gap the Andreev bound stateTh l t t ll i i th t t d thi – These electrons eventually pair in some other state and this is unique (expt. L.H. Greene group, theory J Sauls group)


interface scattering at amorphous barriers!

Si l t l h b ia-axis NIS TJ with amorphous barrier

disordered interface

Single crystal vs amorphous barriers

• non-specular reflection at amorphous barriers 0.002

0.0025

57 K

47 K10 K

• amorphous barrier devices show ZBCP DOS at surface similar to a+b direction devics: ZBCP which splits at lower 0.001

0.0015

47 K20 K

4 K

temperatures (LH Greene)• G(V,T)/G(V,Tc) is particle/hole symmetric

here 0.00026

ZBCP with amorphous Al2O

3 barrier

splits at low temp --> like (110) surface

-80-60-40-20020406080voltage (mV)

806040200-20-40-60-80

• everything that follows now depends on single crystal heterostructure 0.00025

4.2 K

1.7 K

0.00023

0.00024Amorphous insulator

a

+


-40 -20 0 20 40Voltage (mV)

+-

+-

Physica C 335, 184 (2000)

U l i ti l D S t Unusual quasiparticle DoS at cuprate/insulator interfaces (NIS tunnel junctions)

(not c axis cuprate interfaces rather they are a axis)

• If the interface causes a k-state to scatter to opposite signs of d then SC is quenched

(not c-axis cuprate interfaces, rather they are a-axis)

opposite signs of dx2-y2 then SC is quenched– At the surface a normal 2-D band is formed confined by the

gap the Andreev bound state– These electrons eventually pair in some other state and this – These electrons eventually pair in some other state and this

is unique (expt. L.H. Greene group, theory J Sauls group)

• If the interface is specular, and reflections don’t mix f f p , f m+ with -, then we find that the superconductivity at the surface doesn’t have particle-hole symmetry (violates the core of BCS)( olates the core of B S)


new electronic structure emerges because f the specular interfacebecause of the specular interface

Nb SIS JJ

• In bulk, approximate e-h symmetry (BCS)– c.f. Renner, Davis, Yazdani, others (STM of 2212)

At l (100) f fi d thi i ll b k !• At specular (100) surface we find this is really broken!• Is it a superconducting state at interface?

It only appears below Tc– It only appears below Tc

• How deep does it extend?I it f l f i i l t i d • Is it useful for engineering new electronic ground states that may have interesting properties?

• No theory yetNo theory yet.University of Illinois

N-I-S tunnel junction fabricationN-I-S tunnel junction fabrication

Trilayer film growth Junction process

or amorphous AlOx

16-24 A CaTiO3

in-situ gold

V- I-V+ I+

asingle2000 A a-axis

YBCO or DBCO

V+ I+

c

singlecrystal

STO substrate


kincflat

1

inc

Rheed image of Rheed image ofRheed image of substrate before

growth

Rheed image of YBCO after growth of 2000 Angstroms


AFM Image of a-axis-YBCO

RMS Roughness = 0 489 nm= 0.489 nm


NIS tunnel junctionRHEED oscillations during crystalline barrier heteroepitaxy

0.6 ml CTO

1 ml CTO2 ml CTO

4ml CTO4ml CTO

50

55

60

y (a

rb.)

CTOmonolayer 1 2 3 4 5a-axis YBCO

35

40

45

lar I

nten

sity

5 ml CTO

20

25

30

0 80 160 240 320 400 480 560 640

Spe

cul

0 80 160 240 320 400 480 560 640Frame Number


spectroscopy of electronic structurebroken particle-hole symmetry

0.000587K a-axis YBCO NIS tunnel junction Offset parabola due

to different band line

broken particle hole symmetry

0 0004

0.00045

ms)

up.

goldybco cto

0.00035

0.0004

V (

1/O

hm

4.3K

goldybco cto

0.00025

0.0003

dI/d

V 10.019.829.949.559.7

0.0002-0.1 -0.05 0 0.05 0.1

YBCO voltage

73.186.7


YBCO voltage

Phys Rev Lett 93, 107004 (2004)

Normalize data to parabola above TcTc=87K

1 1

1.15device 10V G(V) normalized to parabola at T

c

4.3K

1

1.05

1.110.019.829.937.049.5

0.9

0.95

49.559.773.186.7extracting

electronsinsertingelectrons

0.8

0.85

-0.1-0.0500.050.1

Voltage


Magnetic field parallel to junction

0.0005

0.00055device 9V G(V) 2K parallel field

dI/dVdI/dVdI/dV

0 Tesla1.32 5

0.00032

0.00034device 9V G(V) 2K parallel field

0.00035

0.0004

0.00045dI/dVdI/dVdI/dVdI/dV

2.5467

0.00026

0.00028

0.0003

dI/dV0 Tesla

0.0002

0.00025

0.0003

-0.1-0.0500.050.1

Udc

0.0002

0.00022

0.00024

-0.02-0.015-0.01-0.00500.0050.010.0150.02

dI/dVdI/dVdI/dVdI/dVdI/dVdI/dV

Udc

0 Tesla1.32.5467

Udc Udc


New collective and singgple particle phenomena at oxide...

Documents

Transcript of New collective and singgple particle phenomena at oxide...