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Naslov

Naslov. www.ifs.hr/real_science. Phase diagram of q1D cuprates Sr 14-x Ca x C u 24 O 41 Tomislav Vuletić Zagreb, 2003. Naslov. Phase diagram of q1D cuprates Sr 14-x Ca x C u 24 O 41. dielectric spectroscopy, electrical transport.

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Naslov

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  1. Naslov www.ifs.hr/real_science Phase diagram of q1D cuprates Sr14-xCaxCu24O41 Tomislav Vuletić Zagreb, 2003

  2. Naslov Phase diagram of q1D cuprates Sr14-xCaxCu24O41 dielectric spectroscopy, electrical transport T. Vuletić,B. Hamzić, S. TomićInstitut za fiziku, Zagreb optical measurements B. Gorshunov, P. Haas, M. Dressel 1. Physikalisches Institut, Universität Stuttgart J. Akimitsu, T. Sasaki Dept. of Physics, Aoyama-Gakuin University, Tokyo, Japan T. Nagata Dept. of Physics, Ochanomizu University, Tokyo, Japan samples www.ifs.hr/real_science

  3. Sr14-xCaxCu24O41 (x = 0, 3, 9, 11.5) Ca-doped q1D cuprates Slijed www.ifs.hr/real_science 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

  4. Motivation Motivacija q1D cuprates – realization of hole-doped spin-ladders spin-ladders: spin gap, short-range correlations doping spin-ladders with holes pairing of the holes superconducting or CDW correlations Dagotto et al., 1992. www.ifs.hr/real_science q1D cuprates – only superconducting cuprates without square-lattice layers Superconductivity under pressure (30-45 kbar) in Sr0.4Ca13.6Cu24O41 Uehara et al., 1996.

  5. Crystallographic structure Sr14-xCaxCu24O41 Kristal Struktura CuO2 chains A14 Cu2O3 ladders b=12.9 Å cC cL a=11.4 Å Chains: Ladders:cC=2.75 ÅcL=3.9 Å 10·cC≈7·cL≈27.5 Å CuO2layer HTSC 2D cuprates Cu-O-Cu interaction on the ladders 180o -superexchange, AF, J>0 90o- exchange, FM, J<0 www.ifs.hr/real_science

  6. Holes distribution... Stehiometrija www.ifs.hr/real_science Cu2+ spin ½ holes O2p orbitals Sr14Cu24O41, x=0 stoichiometry  no dependence on Ca-doping CuO2chains Cu2O3ladders 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. 7 Sr2+: 14+ 6 Cu3+: 18+ 4 Cu2+: 8+ 20 O2-: 40- ______________ Ø 7 Sr2+: 14+ 14 Cu2+: 28+ 21 O2-: 42- ____________ Ø Formal valence for copper +2.25 6 holes per f.u.  all on chains

  7. Holes distribution, experimentally... Nucker NEXAFS (T=300K) near edge x-ray absorption fine structure Nücker et al., 2000. www.ifs.hr/real_science ladders chains chains quantitative analysis – hole counts on different O2psites different O2p orbital orientations  different polarized x-ray absorption ladders hole count on the ladders increases slightly (0.81.1) on Ca – doping Ca-doping

  8. Holes distribution, experimentally... Nucker NEXAFS (T=300K) near edge x-ray absorption fine structure Nücker et al., 2000. www.ifs.hr/real_science quantitative analysis – hole counts on different O2psites chains different O2p orbital orientations  different polarized x-ray absorption ladders hole count on the ladders increases slightly (0.81.1) on Ca – doping Ca-doping

  9. Osafune www.ifs.hr/real_science Optical conductivity (T=300K) Osafune et al., 1997. q1D cuprates: • Cu3d↔O2p peak spectral weight transferred to lower energies on Ca-doping Cu3d↔O2p Cu3d↔O2p peak is related to holes localized on chains x=11 x=10 x=6 x=3 HTSC cuprates (2D): • analogous spectral weight transfer on hole doping x=0 chains hole count on the ladders increases (12.8) on Ca – doping ladders

  10. Spin ordering 1/T1aktivacija4 www.ifs.hr/real_science 63Cu NMR (T<300K) Kumagai et al., 1997. chains spin gaps induce the activated temperature dependence of the spin-lattice relaxation rate 1/T1 ladders spin gap appears on ladders below 250 K, on chains, below 70 K

  11. Spin ordering Spinski procjep www.ifs.hr/real_science 63Cu NMR (T<300K) Kumagai et al., 1997. ladders chains Ca-doping decreases spin gap on ladders, but not on chains

  12. Spin ordering & charge localization on chains x=0... AF dimera 2cC 3cC 2cC 2cC 4cC 2cC T=5-20KEccleston et al. 1998. Regnault et al., 1999. www.ifs.hr/real_science Interacting antiferomagnetic dimers model: Spin excitations were measured by inelastic neutron scattering T=8.5KMatsuda et al. 1996. 6 holes/10 sites

  13. Spin ordering & charge localization on chains x=0... AF dimera 2cC 2cC www.ifs.hr/real_science T=50KX-ray diffraction directly points to structural change related to charge-order Cox et al. 1998. 5 šupljina/10 mjesta Spin ordering related to charge-order – holes are localized on chains

  14. Longitudinal dc resistivity Transport Nagata et al., 1997. x=11.5 www.ifs.hr/real_science x≠0: Ca–doped materials D: decreases sc(300 K): increases x=0: insulating behavior D: 2200K (300 K  80 K) sc(300 K): 500 (Wcm)-1. x≥11.5 i T<12K, p=30-80 kbar : superconductivity x≥11 i T>50K : metallic conductivity Motoyama et al., 1997.

  15. MedjuSazetak www.ifs.hr/real_science 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

  16. Exp:transport Well defined: transition temp. Tc activation energy D www.ifs.hr/real_science Holes transferred on ladders single-particle electrical transport rc- longitudinal resistivity Insulator-insulator transition for x=0,3,9 Ca-doping: D & Tc decrease transition width dTc/Tc increases

  17. Dielectric spectroscopy Vout Lock-in V V- SR-570 V+ www.ifs.hr/real_science 2 complimentary techniques: Diel.tehnika resistances (0.1 kW < 1 GW), frequencies 20 Hz-1 MHz Very large resistances (up to 1 TW) Low frequencies (1mHz-100kHz) HP 4284A

  18. Complex dielectric function ∞ www.ifs.hr/real_science Debye.fja Generalized Debye function

  19. Complex dielectric function ∞ www.ifs.hr/real_science Generalized Debye function Debye.fja relaxation process strength = (0) - ∞ 0– central relaxation time symmetric broadening of the relaxation time distribution1 - 

  20. www.ifs.hr/real_science We analyze real & imaginary part of the dielectric function We fit to the exp. data in the complex plane We get the temp. dependence De, t0, 1-a Eps im eps re ere eim

  21. www.ifs.hr/real_science x=0,3,9: on decreasing temp. dielectric response appears suddenly response strength, De, decreases gradually with temperature Deps vs T De – dielectric response strength CORRESPONDENCE: Maximum in De Tc determined from DC measurements.

  22. www.ifs.hr/real_science De=105 »Deqp=10 : strong dielectric response characterization of the dielectric response 1-a:relaxation time distribution wider than Debye Activation in t0 = activation in DC resistivity rz~t0 t∞:relaxation time 10-11 s »tqp=10-15 s Dielectric relaxation in low-temp. phase we correlate with collective excitations of the Charge Density Wave on ladders

  23. Phason CDW dielectric response www.ifs.hr/real_science Fukuyama, Lee, Rice Periodic modulation of charge density Random distribution of pinning centers Local elastic deformations (modulus K) of the phase f(x,t) Damping g Effective mass m*»1 External AC electric field Eex is applied Phason: Elementary excitation associated with spatio-temporal variation of the CDW phase f(x,t) Phason dielectric response governed by: free carrier screening, nonuniform pinning

  24. Phason CDW dielectric response Littlewood W0= www.ifs.hr/real_science Max. conductivity close to the pinning frequency W0 pinned mode - transversal g0- weak damping

  25. Phason CDW dielectric response Littlewood W0= www.ifs.hr/real_science Max. conductivity close to the pinning frequency W0 pinned mode - transversal g0- weak damping plasmon peak longitudinal Screening: Low frequency tail extends to 1/t0= strong damping g»g0 Longitudinal mode is not visible in diel. response since it exists only for e=0!

  26. Phason CDW dielectric response Littlewood 1/t0= www.ifs.hr/real_science Nonuniform pinning of CDW gives the true phason mode a mixed character! W0= Longitudinal response mixes into the low-frequency conductivity Experiments detect two modes

  27. Phason CDW dielectric response Littlewood www.ifs.hr/real_science Low frequency mode: • Spectral weight mostly shifted to low-freq. mode • standard CDW systems • Dielectric constant De= 106-107, independent of T • q1d cuprates: De= 105 only holes on ladders condense into CDW • q1d cuprates: Dedecreases with T hole transfer back from ladders to chains:r0 changes - Characteristic relaxation time of the low-freq. mode t0~1/sz • q1d cuprates: D, activation energy equal for DC (sz) & AC (t0) measurements

  28. m* m* - CDW condensate effective mass Sr14-xCaxCu24O41 W0= t0= www.ifs.hr/real_science t0&sz– from our experiments r0– carriers condensed in CDW (holes transferred to ladders = 1·1027 m-3 = 1/6 of the total) W0&t0 are related: Microwave conductivity measurements (cavity perturbation)  a peak at W0=60 GHz  CDW pinned mode Kitano et al., 2001. CDW effective mass m*≈100

  29. www.ifs.hr/real_science Phase diagram of Sr14-xCaxCu24O41

  30. www.ifs.hr/real_science Conclusion localized holes on chains do not contribute to el. transport mobile holes on ladders are responsible for el. transport phase transition from HT insulating to CDW phase (0≤x≤9) CDW develops on ladders (mobile holes) Ca-doping: graduallly suppresses CDW phase (D, Tc decrease), increases disorder (dTc/Tc increases), increases dimensionality (D/Tc falls of to 3.5) x>9: CDW suppressed, HT insulating phase persists x≥11.5: external pressure suppresses HT insulating phase and establishes superconductivity

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