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Sr 14-x Ca x Cu 24 O 41 (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 Sr 14-x Ca x Cu 24 O 41 ₪ CONCLUSION SlijedSlijed
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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
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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
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₪ 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
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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
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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
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₪ 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
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₪ 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)
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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
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Spin ordering
Spinski procjep
Interacting antiferomagnetic dimers model:
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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
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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
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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
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Dielectric spectroscopy
Diel.tehnika
0
' BSl
0
0'' GGSl
GeneralizedDebye function
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Complex dielectric function
1
01
1
iHF∞
Debye.fja
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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-
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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.
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–
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
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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
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)(sin10 rrQ
)( ii RrV
Phason dielectric response governed by: free carrier screening, nonuniform pinning
Phason CDW dielectric response
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
www.ifs.hr/real_science Littlewood
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
Phason CDW dielectric response
plasmon peak longitudinal
www.ifs.hr/real_science Littlewood
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
Phason CDW dielectric response
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
www.ifs.hr/real_science Littlewood
Phason CDW dielectric response
& 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
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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
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₪ 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)