Phase Separation in Cuprate Superconductors, 1994
Proceedings of the second international workshop on “Phase Separation in Cuprate Superconductors” September 4 - 10, 1993, Cottbus, Germany

Phase separation has become a fascinating subject in the discussion of cuprate superconductors. All these materials have layered structures containing CU02 planes as the most important building blocks. They are coupled only weakly so that the electronic properties show a nearly two-dimensional behaviour. Due to correlations the undoped compounds are insulators of the Mott­ Hubbard type exhibiting long-range antiferromagnetic order. Upon doping a rich scenario of physical phenomena appears: Even at low hole concentra­ tions the antiferromagnetic ordering temperature is reduced drastically and spin-glass behaviour as well as a hopping type conductivity can be observed. Further doping leads to metallic-like conductivity and below Tc to super­ conductivity. In this doping regime antiferromagnetic fluctuations are still observed. At very high charge carrier densities superconductivity is lost and the systems show pure metallic conduction without ,magnetic correlations. One of the most interesting phenomena in high-T c research is the interplay between magnetism and conductivity or superconductivity. Especially the behaviour of charge carriers in the antiferromagnetic background raises a number of open questions. Two scenarios become possible: the carriers tend to delocalize over the whole crystal forming a homogeneous state with band-like structure or they separate into hole-rich (conducting, superconducting) and hole-poor (insulating, antiferromagnetic) phases leading to an inhomogeneous structure.

I General Properties.- Recipe for High-Tc Transition-Metal Chalcogenide Superconductors.- 1. Introduction.- 2. The Properties.- 3. Conclusions.- Phase Separation as a Possible Scenario for High Tc Superconductors: A Particular Overview.- 1. Introduction.- 2. Phase separation: a relevant issue.- 3. A specific example.- Point Contact Spectroscopy in the High Tc Oxides.- 1. Introduction.- 2. Energy gap and Fermi velocity determination. The Eliashberg interpretation.- 3. The electron phonon interaction in La1.85Sr0.15CuO4.- 4. Conjectures on the strong electron phonon and correlations interactions scenario - and some consequences regarding phase separation.- Cuprate Superconductors: Universal Properties and Trends; Evidence for Bose-Einstein Condensation.- 1. Universal Properties and Trends.- 2. Evidence for Bose-Einstein Condensation.- 3. Conclusion.- II Electronic Results I.- Polar on Formation and Percolative Phase Separation in HTSC.- 1. Introduction.- 2. Hole dynamics in CuO2 planes.- 2.1 Doped system.- 3. Two-particle states.- 4. Conditional hopping in one- and three band Hubbard models.- 4.1 One-band model for spin-conditioned hopping.- 5. Concluding remarks.- On the Role of Electronic and Chemical Phase Separation: Susceptibility and Conductivity Experiments on La2?xCuO4+?.- 1. Introduction.- 2. Electronic Phase Separation.- 2.1 Quasiparticles.- 2.2 Phase Separation and Metalization.- 2.3 Counterions; chemical versus electronic phase separation.- 3. Experimental.- 4. Results and Discussion.- 4.1 Conductivity experiments.- 4.2 Equilibration and annealing experiments.- 5. Summary and Conclusions.- The Phase Diagrams and Doped-Hole Segregation in La2CuO4+? and La2?xSrxCuO4+? (x ? 0.15, ? ? 0.12).- 1. Introduction.- 2. The La2CuO4+? System.- 2.1 Relationship Between Tps and TN.- 2.2 La2CuO4+? Phase Diagram and Structures.- 2.3 The Oxidation State(s) of the Excess Oxygen and the Doped- Hole Concentration in the CuO2 Plane.- 2.4 Single Crystal Synthesis and Characterization.- 2.5 Dependence of Superconducting Properties on Temperature and Pressure History.- 3. The La2?xCuO4+? System.- 3.1 The La2?xSrxCuO4 System (? = 0).- 4. Concluding Remarks.- Neutron Spectroscopy in RBa2Cu3Ox (R=Rare Barth, 6?x?7) Compounds: Charge Transfer, Phase Separation, Spin Fluctuations.- 1. Introduction.- 2. CEF excitations in ErBa2Cu3Ox (6 ? x ? 7).- 3. Analysis of the CEF results obtained for ErBa2Cu3Ox.- 3.1 Charge transfer.- 3.2 Phase separation.- 3.3 Percolative superconductivity.- 4. Fractal sizes of the clusters.- 5. Low-energy Cu2+ spin fluctuations.- 5.1 Introductory remarks.- 5.2 Asymmetric line shape of CEF transitions in Ho0.1Y0.9Ba2Cu3O7.- 5.3 Neutron scattering from spin-polarized clusters.- 6. Conclusion.- III Electronic Results II.- Phase Separation in Cuprate Superconductors: Magnetic Resonance Studies.- 1. Introduction.- 2. Experimental details.- 3. Results and discussion.- 4. Summary.- Paramagnetic Resonance Studies of Phase Separation in Cuprate Superconductors.- 1. Introduction.- 2. Investigated materials and experimental details.- 3. Experimental results.- 3.1 La2CuO4+? ceramics.- 3.2 La2CuO4+? single crystal.- 3.3 La2?xSrxCuO4+? ceramics.- 4. Discussion.- Fermi Glass State in Partially Doped Insulating Cuprates.- 1. Introduction.- 2. Dark transport in partially doped insulating cuprates.- 3. Conductivity under photoexcitation.- 4. Spectral response of the photoconductivity in partially doped insulating cuprates.- 5. Supporting evidence of the Fermi-glass state from ARPES, NQR and other experiments.- 6. Discussions and Remarks.- Two-Magnon Raman Scattering in Doped YBa2Cu3Ox with Respect to Confined Hole Clusters.- 1. Introduction.- 2. Light scattering of spin fluctuations.- 3. Results.- IV NMR and Mössbauer Studies.- Electron Hole Ordering in La2CuO4+?.- 1. Introduction.- 2. Spin dynamics in as-prepared and Sr-doped La2?xSrxCuO4.- 3. Spin dynamics in electrochemically oxidized La2CuO4+?.- 4. Comparison of 1/T1*, LFS and dc susceptibility measurements.- 5. The percolation model.- 6. A Ginsburg-Landau-Wilson model for phase separation.- 7. Summary.- Microscopic Study of Local Structure and Charge Distribution in Metallic La2CuO4+?.- 1. Introduction.- 2. Macroscopic phase separation.- 3. Cooling rate dependence of Tc.- 4. Determination of local structure and local charge environments.- 4.1 La NQR.- 4.2 La NMR.- 4.3 Cu NQR.- 5. Interpretation and discussion.- 6. Conclusions.- Mössbauer Probe Measurements in La2?xSrxCuO4 and YBa2Cu3Ox.- 1. Mössbauer emission measurements on highly dilute 57Fe impurities in La2?xSrxCuO4.- 1.1 57Co doping process.- 1.2 57Fe room temperature emission measurements.- 1.3 57Fe low temperature emission measurements.- 1.4 Discussion and conclusions.- 2. Local properties of substituted YBa2Cu3Ox examined by Mössbauer Spectroscopy on Yb3+ ion probes.- 2.1 Influence of the spatial distribution of oxygen on the magnetic properties of YBa2Cu3O6.55.- 2.2 Influence of cationic substitutions.- 2.3 Zn2+ substitution.- 2.4 Pr3+ substitution.- 2.5 Summary and Discussion.- V Structural Phase Separation.- Indications for a Phase Separation in YBa2Cu3O7?x.- 1. Introduction.- 2. Synthesis of the samples.- 3. X-ray lattice constants.- 4. Neutron diffraction.- 4.1 Experimental.- 4.2 Anomalies in the Cu-O bond lengths.- 5. Magnetization measurements.- 6. Oxygen diffusion and Tc splitting.- 7. Conclusions.- Superconductivity Versus Oxygen Concentration in 123 Compounds: Influence of RE Ionic Radii Studied by Cu NQR.- 1. Introduction.- 2. Tc versus x for different RE in REBa2Cu3Ox.- 3. Cu NQR lines in different configurations.- 4. Experimental details.- 5. Results.- 5.1 NQR of Tm123.- 5.2 NQR of Nd123.- 6. Discussion.- 7. Conclusion.- The Role of the Additional Oxygen Atoms on the Superconducting Properties of La2CuO4-Related Compounds.- 1. Introduction.- 2. Experimental and Results.- 2.1 Potentiostatic experiments on ceramics.- 2.2 Galvanostatic experiments on ceramics.- 2.3 Electrochemical oxidation of thin films.- 2.4 Electrochemical oxidation of crystals.- 2.5 Role of the additional oxygen atoms.- 2.6 Mobility of “O?” in oxide lattices.- 3. Conclusion.- Structural Disorder and Phase Separation in TmBaCuO as Seen from Tm NMR.- 1. Experimental.- 2. Mean length of CuO chain segments.- 3. Localized Cu2+ centers.- 4. Phase separation in underdoped TmBa2Cu3O6+x.- VI Electron-Phonon Interaction I.- The Coupling of a Wigner Polaronic Charge Density Wave with a Fermi Liquid Arising from the Instability of a Wigner Polaron Crystal: A Possible Pairing Mechanism in High Tc Superconductors.- 1. Introduction.- 2. The Polaron Size and its Temperature Dependence.- 3. The Polaron Pair Filling Factor and the Insulator to Metal Transition.- 4. Ordering of Polarons as Function of Doping.- 5. Partial Charge Density for the Polaron Gas and the Fermi Liquid.- 6. The Polaron Gas Condenses in a Generalized Wigner CDW at v = 1 and in a Generalized Wigner Crystal at v = 2.- 7. On the Pairing Mechanism.- Pairing Interaction and Phonon Renormalization Favored by Phase Separation.- 1. Introduction.- 2. Unitary transformations.- 3. The Cooper pair interaction.- 4. The phonon renormalization.- 5. Appendix.- Electronic Excitations in Undoped and Lightly Oxygen Doped La2CuO4+y.- 1. Introduction.- 2. Charge-Transfer Excitation.- 3. Crystal-Field Excitations.- 4. Impurity Absorption.- 5. Conclusion.- VII Electron-Phonon Interaction II.- Electronic Structure and High Tc Superconductivity in the Cuprates.- 1. Introduction.- 2. Band structure of the cuprates.- 3. The Labbé Bok formula.- 4. The Coulomb repulsion between electrons.- 5. Coherence length and anisotropy effects.- 6. Phase separation and conclusion.- Apex Oxygen Anharmonicity and Low Energy Electronic Transitions Observed in 1-2-3 by IR-Excited Raman Spectroscopy.- 1. Introduction.- 2. IR excited FT-Raman spectroscopy.- 3. YBa2Cu3O6+x (x ? 2): IR-Raman scattering at low oxygen concentration.- 4. ErBa2Cu3O6+x: 0.1 ? x ? 0.9.- 5. 18O Metallic YBa2Cu3O6+x Substituted Samples.- 6. Untwinned 1-2-3 Crystal: Electronic Background.- Photoexcited Electons and Holes in YBa2Cu3O7??: A Study by Picosecond Raman Scattering.- 1. Introduction.- 2. Photoinduced carriers.- 3. Photoinduced (super)conductivity and the effects of carriers on Raman spectra.- 4. Discussion.- 5. Conclusion.- Anharmonic Effects Related to the Apical Oxygen Vibration in YBa2Cu3O7 by LAPW Calculations.- 1. Introduction.- 2. Method.- 3. Results.- VIII Further Theoretical Concepts.- Thermodynamics of the Emery Model.- 1. Introduction.- 2. Phase Diagrams Including Separated Phases.- 3. A Strong Coupling Ansatz for the Emery Model.- 4. Spiral Magnetic Order.- 5. Self Energy and DOS in Weak Coupling.- 6. Conclusions.- The Effect of Coulomb Interactions on Phase Separation.- 1. Introduction.- 2. The Phase Diagram.- 3. The Structure of the Ground States.- 4. Conclusion.- Options for the Effective Electron-Electron-Interaction from Unitary Transformations.- 1. Introduction.- 2. Unitary Transformations in Physics.- 3. Fröhlich’s Transformation.- 4. Moving Base Transformations.- 4.1 Moving Base in Q.- 4.2 Moving Base in P.- 5. A Didactical Cu - O - Cu Model.