Phase diagram of q1D cuprates Sr 14-x Ca x Cu 24 O 41 Tomislav Vuletić Zagreb, 2003 Naslov.

Phase diagram of q1D cuprates Sr14-xCaxCu24O41

Tomislav VuletićZagreb, 2003

www.ifs.hr/real_science Naslov

T. Vuletić, B. Hamzić, S. Tomić Institut za fiziku, Zagreb

Phase diagram of q1D cuprates Sr14-xCaxCu24O41

www.ifs.hr/real_science

J. Akimitsu, T. Sasaki Dept. of Physics, Aoyama-Gakuin

University, Tokyo, Japan

T. Nagata Dept. of Physics, Ochanomizu

University, Tokyo, Japan

B. Gorshunov, P. Haas, M. Dressel 1. Physikalisches Institut,

Universität Stuttgart

Naslov

Sr14-xCaxCu24O41 (x = 0, 3, 9, 11.5)Ca-doped q1D cuprates

Outline

₪ q1D cuprates, importance, motivation₪ crystallographic structure₪ distribution of holes

₪ dielectric spectroscopy, electrical transport

₪ we identified a low-temp. phase – charge density wave (CDW)₪ CDW phase in the phase diagram of Sr14-xCaxCu24O41

₪ CONCLUSION

www.ifs.hr/real_science Slijed

Motivation

Superconductivity under pressure (30-45 kbar) in Sr0.4Ca13.6Cu24O41 Uehara et al., 1996.

₪ q1D cuprates – realization of hole-doped spin-ladders


₪ q1D cuprates – only superconducting cuprates without square-lattice layers

₪ spin-ladders: spin gap, short-range correlations

₪ doping spin-ladders with holes

₪ pairing of the holes superconducting or CDW correlations

Dagotto et al., 1992.

Motivacija

b=12

.9 Å

a=11.4 ÅcCChains: Ladders: cC=2.75 Å cL=3.9 Å 10·cC≈7·cL≈27.5 Å

cL

Crystallographic structure Sr14-xCaxCu24O41


A14 Cu2O3 laddersCuO2 chains

Kristal

Struktur

a

CuO2 layerHTSC

2D cuprate

s90o - exchange, FM, J<0180o - superexchange, AF, J>0

Cu-O-Cu interaction on the ladders

Holes distribution...

7 Sr2+: 14+6 Cu3+: 18+4 Cu2+: 8+20 O2-: 40-

______________Ø

7 Sr2+: 14+14 Cu2+: 28+21 O2-: 42-

____________Ø

Cu2O3 laddersCuO2 chains

₪ Formal valence for copper +2.25₪ 6 holes per f.u. all on chains

holes O2p orbitals Cu2+ spin ½


Sr14Cu24O41, x=0

Stehiometrija

stoichiometry no dependence on Ca-doping

Calculation of Madelung energy of the crystal₪ x=0 has an energy minimum when all 6 holes are on chains. ₪ on Ca-doping hole count on chains decreases Mizuno et al., 1997.

chains ladders

₪ NEXAFS (T=300K) near edge x-ray absorption fine structure

Nücker et al., 2000.

Holes distribution, experimentally...

www.ifs.hr/real_science Nucker

₪ hole count on the ladders increases slightly (0.81.1) on Ca – doping Ca-doping

ladders

chains

₪ quantitative analysis – hole counts on different O2p sites₪ different O2p orbital

orientations different polarized x-ray absorption


₪ quantitative analysis – hole counts on different O2p sites

ladders

chains

Ca-doping Nucker

₪ different O2p orbital orientations different polarized x-ray absorption

Holes distribution, experimentally...₪ NEXAFS (T=300K) near edge x-ray

absorption fine structureNücker et al., 2000.

₪ hole count on the ladders increases slightly (0.81.1) on Ca – doping


₪ Optical conductivity (T=300K) Osafune et al., 1997.

Cu3d↔O2p

x=0x=3x=6x=10x=11

HTSC cuprates (2D):₪ analogous spectral weight transfer on hole doping

q1D cuprates:₪ Cu3d↔O2p peak spectral weight transferred to lower energies on Ca-doping

ladders

chains

Cu3d↔O2p peak is related to holes localized on chains

Osafune

₪ hole count on the ladders increases (12.8) on Ca – doping

Kumagai et al., 1997.63Cu NMR (T<300K)


Spin ordering

₪ spin gaps induce the activated temperature dependence of the spin-lattice relaxation rate 1/T1

₪ spin gap appears on ladders below 250 K, on chains, below 70 K

1/T1aktiva

cija4

chains

ladders

Kumagai et al., 1997.

chains

ladders

63Cu NMR (T<300K)

₪ Ca-doping decreases spin gap on ladders, but not on chains


Spin ordering

Spinski procjep

Interacting antiferomagnetic dimers model:


Spin ordering & charge localization on chains x=0...

AF dimera

2cC 3cC

₪ T=5-20K Eccleston et al. 1998. Regnault et al., 1999.

Spin excitations were measured by inelastic neutron scattering

₪ T=8.5KMatsuda et al. 1996. 6 holes/10 sites

4cC2cC2cC 2cC

₪ T=50K X-ray diffraction directly points to structural change related to charge-order Cox et al. 1998.

Spin ordering related to charge-order – holes are localized on chains

www.ifs.hr/real_science AF dimera

2cC 2cC5 šupljina/10 mjesta

Spin ordering & charge localization on chains x=0...

Motoyama et al., 1997.

₪ x=0: insulating behavior₪ : 2200K (300 K 80 K)₪ c(300 K): 500 (cm)-1.

₪ x≠0: Ca–doped materials ₪ decreases₪ c(300 K): increases

₪ x≥11 i T>50K : metallic conductivity


Longitudinal dc resistivity

Transport

Nagata et al., 1997.

x=11.5

₪ x≥11.5 i T<12K, p=30-80 kbar : superconductivity

Sr14-xCaxCu24O41 –chains subsystem:

• T decreases – spin ordering according to AF dimer model• Spin gap (independent of Ca-doping)• Spin ordering ↔ charge order (localization of holes)• Localized holes, do not contribute to electrical transport

Sr14-xCaxCu24O41 – ladders subsystem:

• Singlet ground state – spins paired on rungs of the ladders• Spin gap (decreases on Ca-doping)• x=0: hole count on ladders different from zero• x≠0: hole transfer on ladders increases• Mobile holes, contribute to electrical transport

www.ifs.hr/real_science MedjuSazetak

c- l

ongi

tudi

nal r

esist

ivity

Ca-doping:₪ & Tc decrease ₪ transition widthTc/Tc increases

Holes transferred on ladders single-particle electrical transport


Insulator-insulator transition for x=0,3,9

Well defined:₪ transition temp. Tc

₪ activation energy

Exp:transport

HP 4284A

2 complimentary techniques:

Low frequencies (1mHz-100kHz)

Very large resistances (up to 1 T)

Lock-inV

V+V-

SR-570

resistances (0.1 k< 1 G), frequencies 20 Hz-1 MHz


Dielectric spectroscopy

Diel.tehnika

0

' BSl

0

0'' GGSl

GeneralizedDebye function


Complex dielectric function

1

01

1

iHF∞

Debye.fja


1

01

1

iHF∞

Debye.fja

Complex dielectric functionGeneralized

Debye function

₪ relaxation process strength = (0) - ∞ ₪ 0 – central relaxation time₪ symmetric broadening of the relaxation time distribution 1 -

0

' BSl

0

0'' GGSl

Eps im eps re

₪ We analyze real & imaginary part of the dielectric function ₪ We fit to the exp. data in the complex plane₪ We get the temp. dependence , 0, 1-


reim

Deps vs T

₪ x=0,3,9: on decreasing temp. dielectric response appears suddenly₪ response strength, , decreases gradually with temperature

CORRESPONDENCE:

Maximum in

Tc determined from DC measurements.


–

die

lect

ric re

spon

se st

reng

th

char

acte

rizat

ion

of th

e di

elec

tric

resp

onse

∞: relaxation time10-11 s » qp=10-15 s

=105 » qp=10 : strong dielectric response

Dielectric relaxation in low-temp. phase we correlate with collective excitations of the Charge Density Wave on ladders


1-: relaxation time distribution wider than Debye

Activation in0 = activation in DC resistivity z~0

Fukuyama, Lee, Rice

i

tiexii eErrQRrVK

dtd

dtdm

012

2

2

)(sin)(*

Phason: Elementary excitation associated with spatio-temporal variation of the CDW phase (x,t)

₪ Periodic modulation of charge density₪ Random distribution of pinning centers₪ Local elastic deformations (modulus K) of the phase (x,t) ₪ Damping ₪ Effective mass m*»1₪ External AC electric field Eex is applied


)(sin10 rrQ

)( ii RrV

Phason dielectric response governed by: free carrier screening, nonuniform pinning

Phason CDW dielectric response


www.ifs.hr/real_science Littlewood

Max. conductivity close to the pinning

frequency

pinned mode - transversal

0- weak damping

=*/0 mV


Longitudinal mode is not visible in diel. response since it exists only for =0!

Low frequency tail extends to 1/0=

strong damping»0

Screening:

200 / Vz

Max. conductivity close to the pinning

frequency

pinned mode - transversal

0- weak damping

=*/0 mV


plasmon peak longitudinal


Experiments detect two modes

=

Nonuniform pinning of CDW gives the true phason mode a mixed character!

*/0 mV

0= 200 / Vz

Longitudinal response mixes into the low-frequency conductivity


Low frequency mode:- Spectral weight mostly shifted to low-freq. mode

₪ standard CDW systems- Dielectric constant = 106-107, independent of T

₪ q1d cuprates: = 105 only holes on ladders condense into CDW₪ q1d cuprates: decreases with Thole transfer back from ladders to chains:0 changes

- Characteristic relaxation time of the low-freq. mode 0~1/z

₪ q1d cuprates: , activation energy equal for DC (z) & AC (0) measurements



& 0 are related: 0 & z – from our experiments

0 – carriers condensed in CDW (holes transferred to ladders = 1·1027 m-3 = 1/6 of the total)

m* - CDW condensate effective mass Sr14-xCaxCu24O41

Microwave conductivity measurements (cavity perturbation) a peak at =60 GHz CDW pinned mode Kitano et al., 2001.

CDW effective mass m*≈100

www.ifs.hr/real_science m*

20

20

0 *

mz

=*/0 mV

0= 020 / Vz

Phase diagram of Sr14-xCaxCu24O41


Conclusion

₪ localized holes on chains do not contribute to el. transport₪ mobile holes on ladders are responsible for el. transport

₪ x>9: CDW suppressed, HT insulating phase persists₪ x≥11.5: external pressure suppresses HT insulating phase

and establishes superconductivity


₪ phase transition from HT insulating to CDW phase (0≤x≤9)₪ CDW develops on ladders (mobile holes)

₪ Ca-doping: graduallly suppresses CDW phase (, Tc decrease), increases disorder (Tc/Tc increases), increases dimensionality (/Tc falls of to 3.5)

Phase diagram of q1D cuprates Sr 14-x Ca x Cu 24 O 41 Tomislav Vuletić Zagreb, 2003 Naslov.

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Transcript of Phase diagram of q1D cuprates Sr 14-x Ca x Cu 24 O 41 Tomislav Vuletić Zagreb, 2003 Naslov.