K. A. Müller

Susceptibility Measurements Support High-Tc Superconductivity in the Ba-La-Cu-O System

The magnetic susceptibility of ceramic samples in the metallic BaLaCuO system has been measured as a function of temperature. This system had earlier shown characteristic sharp drops in resistivity at low temperatures. It is found that, for small magnetic fields of less than 0.1 T, the samples become diamagnetic at somewhat lower temperatures than the resistivity drop. The highest-temperature diamagnetic shift occurs at (33 ± 2) K, and may be related to shielding currents at the onset of percolative superconductivity. The diamagnetic susceptibility can be suppressed with external fields of 1 to 5 T.

Microwave Magneto-Surface Impedance of High-Tc Superconductors

Magnetic-field-induced microwave absorption in sintered high-Tc copper oxide superconductors is found to be proportional to the surface area of samples and not to their volume, establishing that absorption takes place near the sample surface. The surface resistance and reactance increase with magnetic field as a result of energy loss through fluxons driven by superconducting currents. When the magnetic field is modulated, absorptive and dispersive signals arise from modulation of the complex surface impedance through processes induced by the changing magnetic fields.

Numerical simulations of a high-Tc-superconductive glass model

Numerical simulations for anXY-spin-glass model are presented. The connection to recent experiments on high-Tc superconductors is outlined. In particular, the glass behaviour in experiments is reproduced by a 2-D model. A phase diagram important for future experiments is also provided. A substantial gain inTc is predicted for enhanced couplings between planes.

High-Tc superconductivity 1996 : ten years after the discovery

Preface. Part I: Introduction. The Discovery of HTc Superconductivity and its Decisive Circumstances J.G. Bednorz. Part II: General Problems. `S and `d Wave Symmetry Components in High-Temperature Cuprate Superconductors K.A. Muller, H. Keller. Theory Aspects of High-Temperature Superconductivity R. Micnas, S. Robaskiewicz. Part III: New Materials and Structures. Novel Two-Dimensional Perovskites J.G. Bednorz, et al. Mechanisms Generating Layered Cuprates and Oxycarbonates: Classification B. Raveau. Mercurocuprates: The Highest Transition-Temperature Superconductors P.P. Edwards, et al. High Pressure Crystal Growth and Properties of HgBa2Can-1CunO2n+2+delta, Sn1-xCuO2, CaCuO2 and Y2Ba4Cu6+nO14+n J. Karpinski, et al. Noncuprate Layered Perovskite Superconductor Y. Maeno, S. Ikeda. Non-Toxic HTSC Materials, Substitution in the Charge Reservoir Layer: `Cupro/Carbonate Cuprates and Related Materials M.A. Alario-Franco, et al. Part IV: Phase Separation, Electronic Inhomogeneities, and Related Problems. 1. General. The Two-Component Paradigm for Superconductivity in the Cuprates D. Mihailovic, K.A. Muller. Two- Component High Tc Superconductivity: Charge Reservoirs, Phase Separation and Optical Excitations D. Mihailovic. 2. The La-Cuprate System. Structural Phase Transitions and Superconductivity in Lanthanum Copper Oxydes M.K. Crawford, et al. Magnetic Properties of Single-Layer Cuprates D.C. Johnston, et al. Structural Phase Transitions and Incommensurate Spin Fluctuations in the La2CuO4+y System B.O.Wells, et al. Phase Separation and Stripe-Like Modulation Observation in Cu-Doped La2CuO4+ System Z.X. Zhao, et al. 3. The Bi-Cuprate System. From a Homogeneous CuO2 Plane to a Superlattice of Quantum Stripes A. Bianconi, et al. Lattice Effect and D-Electron Delocalization Effect Around Tc in Bi-Based High-Tc Superconductors Y. Li, et al. 4. The Y-Cuprate System and Related Compounds. Lattice Instabilities, Structural Anomalies and Phase Transitions in Y1-yCayBa2Cu3Ox Phases E. Kaldis. Raman Scattering in the YBCO Compounds E. Liarokapis. Local Structure Studies of the Underdoped-Overdoped Transition in Yba2Cu3Ox. Measurement of the Yttrium X-Ray Absorption-Fine-Structure J. Roehler, et al. Neutron Spectroscopy of Rare Earths in Superconducting Materials W. Henggeler, et al. Tm NMR and Cu NQR Studies of Phase Separation in TmBaCuO Compounds M.A. Teplov, et al. Part V: Applications. Applications of High-Tc Superconductors H. Rietschel. Index.

Macroscopic Flux Quantization and Microwave Excitation in Single-Crystal YBa2Cu3O7-δ

Series of microwave-absorption lines spaced uniformly in the magnetic field have been observed in single crystals of YBa2Cu3O7-?. As the crystals are rotated, the regularly periodic lines move in field so that the component of the magnetic field along a [110] direction remains constant. Line series appear at successive threshold microwave powers and temperatures, and the lines widen dramatically as either the microwave power or the temperature is increased. The absorptions are believed to arise from the mixing of macroscopic flux states, stabilized by the microwave excitation.

Spin-glass phase microwave study in high-TC superconductors

Abstract Measurement of low-field microwave absorption in single-crystal YBa 2 Cu 3 O 7−δ shows a series of equally spaced, narrow, anisotropic absorption lines. With the external magnetic field parallel to the twin planes, a typical line spacing is about 200 mOe with a linewidth of about 2 mOe. These lines are associated with the motion of fluxons into the regular array of twin planes. The widely observed absorption in the ceramic is therefore a powder spectrum of many small crystallites with similar line spectra, which are unresolved.

Measurement of the London penetration depths in YBa2Cu3Ox by means of muon spin rotation (μSR) experiments

Abstract To get accurate values of the London penetration depths in YBa2Cu3O x , μSR measurements were performed on a high quality, sintered sample and a c-axis-oriented polycrystal. For the sintered sample the temperature dependence of the effective penetration depth λeff is well described by the two-fluid model, with λeff(0) = 155(10) nm. This behavior of λeff(T) is consistent with conventional s-wave pairing. The anisotropy ratio λ c λ ab ⋍ 5(1) was determined from measurements on the polycrystal. These results were used to calculate λab(0) = 130(10) nm and λ c (0) ⋍ 500 – 800 nm .

Glass behavior of high-Tc superconductors

The glass behavior of high-Tc superconductors is reproduced in numerical simulations of a 2-D XY-spin-glass model. Higher critical temperatures are predicted for stronger interlayer couplings between the planes. Higher critical currents for a ceramic are expected for enhanced alignment of the planes in different grains of the material.

Nuclear Spin-Lattice Relaxation of 139La in the Superconducting Phase of La1.8Sr0.2CuO4-δ

We report on zero-field nuclear spin-lattice relaxation measurements of 139La in La1.8Sr0.2CuO4-δ in the normal and the superconducting state. Below the superconducting transition temperature Tc the relaxation rate behaves like 1/T1 ~ exp [- Δ/kT] with activation energy Δ = 11.5 meV leading to a gap energy of 2Δ = 23 meV. With Tc = 38 K the ratio 2Δ/kTc = 7.1 is obtained, which is twice as large as the weak-coupling value of 3.5.

Internal magnetic fields in the high-temperature superconductor YBa2Cu3O7−δ from muon spin rotation experiments

Muon spin rotation (μSR) measurements of the internal magnetic fields in sintered YBa2Cu3O7−δ samples are reported. Pronounced differences between field-cooled (FC) and zero-field-cooled (ZFC) μSR signals, hysteresis effects and flux trapping are observed. The μSR results are compared with the results of macroscopic experiments (susceptibility, magnetization, resistivity) and with predictions obtained from models of weakly-coupled superconducting regions.