T. Lægreid

Wide-temperature instability in high-Tc superconductors, La1.85Ba0.15CuO4

Ultrasonic attenuation and velocity have been measured in the high-Tc superconductor La1.85Ba0.15CuO4 at 24 MHz. On cooling, a pronounced softening of the lattice is observed, beginning above room temperature. Below 200 K, the lattice remains soft and highly attenuating, suggestive of a near-instability state over the entire range 4 K to 200 K, partial stiffening occurs, but the attenuation remains higher than at room temperature, a feature suggestive of a possible glassy state.

Elastic and thermal behaviour of ceramic high-Tc superconductors studied by ultrasound, vibrating reed, and specific heat measurements

Abstract We report measurements of the elastic properties and specific heat performed on ceramic high-Tc superconductors. Characteristic features are observed in a wide range of temperatures. In YBCO we emphasize the dynamic behaviour observed between 220K and 270K, and suggest that a glass-like ordering phenomenon may lead to the observations. Hysteretic behaviour in the sound velocity, together with a broad lossy region, is found below 200K. Near Tc possible evidence of a structural instability is found. A specific heat anomaly is observed at about 220K. The elastic response of LABCO shows features which in many respects are similar to those observed in YBCO.

Restricted reversible region and strongly enhanced pinning in MPMG YBa2Cu3O7 with Y2BaCuO5 inclusions

Abstract Recent work has shown that the melt-powder-melt-grown (MPMG) high temperature superconductor YBa2Cu3O7 with Y2BaCuO5 inclusions sustains unusually high critical current in the bulk. In the present paper the magnetic irreversibility line of this material is studied by AC permeability measurements. The irreversibility line is much steeper and is moved to higher temperatures compared to single crystal YBa2Cu3O7 and the reversible region between the irreversibility line and the Hc2 line is drastically shrunk. From standard flux creep analysis the pinning energy is found to be increased by one order of magnitude as compared to YBa2Cu3O7. The measurements further show that the irreversibility temperature increases in a simple logarithmic fashion with increasing frequency. The magnetic AC field response is found to be highly nonlinear even when B ac B dc ∼10 -5 .

On the structural phase transition temperature in La2−x(Sr,Ba)xCuO4−y: Its dependence on pressure, doping concentration, and oxygen deficiency

Abstract The pressure dependence of the structural phase transition (SPT) temperature in La 2−x (Sr,Ba) x CuO 4−y compounds is phenomenologically ascribed to the coupling between strain field and structural order parameter. The influence of partial substitution of La by Sr or Ba and of oxygen deficiency on the SPT temperature is suggested to be physically equivalent to applying a strain field to the pure La 2 CuO 4 . Quantitative agreement with experimental observations is obtained by using a Landau free energy which includes the coupling energy between strain field and order parameter.

A Landau theoretical study of La2−x (Sr,Ba) xCuO4−y: Structural phase transitions and related elastic properties

Abstract A Landau free energy suitable for describing the structural phase transitions in La 2−x (Sr,Ba) x CuO 4−y is derived by means of group theory. It is demonstrated that the coupling energy due to the interaction between stress field and structural order parameter must include a term bilinear in two order parameter components. The structural phase transitions (SPTs) in La 2−x (Sr,Ba) x CuO 4−y are discussed. The calculations suggest that several SPTs may occur in these materials. The elastic moduli of lower-symmetry phases are determined and a phase diagram is given. The changes in the elastic moduli C 11 ,C 12 , C 13 , and C 33 caused by the transition from the 14/mmm phase to the Cmca phase in La 2−x Sr x CuO 4 are evaluated and three coupling constants are determined quantitatively on the basis of the available experimental results.

Temperature and field dependence of the weak-link behavior in superconducting YBa2Cu3O7-x

Abstract By measurement of the ac permeability of ceramic YBCO we determine a line in the B-T plabe separating regions of flux expulsion from those of large penetration. The line intersects the temperature axis below T c and does not scale with the sample radius, implying that simple critical state models are not appropriate to describe the nature of the magnetic hysteresis. A model where the flux penetration is caused by the onset of strong thermally activated creep across grain boundaries is proposed. The magnetic hysteresis are mainly governed by the time scale of the measurement and depends only weakly on the sample radius.

On the structural and elastic aspects of La2−x(Sr,Ba)xCuO4−y, superconductors: Landau theory

Using group theory and examining the symmetry of the La 2−x (Sr,Ba) x CuO 4−y superconductors, we construct a Landau free energy for these materials. It is demonstrated that the coupling energy due to the interaction between stress field and structural order parameter must include a term bilinear in two order parameter components. The structural phase transitions(SPTs) in La 2−x (Sr,Ba) x CuO 4−y superconductors are discussed. Calculation suggests that several SPTs may occur in these materials. The elastic moduli of the ordered phases are determined and a phase diagram is given. The changes in the elastic moduli C 11 , C 12 , C 13 , and C 33 caused by the transition from the I4/mmm phase to the Cmca phase in La 2−x Sr x CuO 4−y and three coupling constants involved in the SPT are evaluated quantitatively on the basis of the available experimental results.

Structural phase transition in single crystal La2−xSrxCuO4−y superconductors studied by ultrasound

Abstract We report the first elastic measurements on a single crystal La 2−x Sr x CuO 4−y high-T c superconductor. Ultrasonic measurements of longitudinal sound velocity and attenuation were performed at 13MHz. The strong anomalies in the elastic properties are ascribed to a tetragonal to orthorhombic phase transition at T 0 ⋍255K.

Contribution to specific heat and electrical conductivity of YBa2Cu3O7-† from fluctuations

Abstract Specific heat, C p , and electrical conductivity, σ, have been carefully studied in ceramic and single crystal YBa 2 Cu 3 O 7-† close to the superconducting transition at 92K. Fluctuation contributions to both C p and σ were observed above the superconducting transition. In the fluctuation-enhanced conductivity we find exponents of 1 2 and 2 3 caused by d=3 Gaussian fluctuations and critical fluctuations, respectiveley.

Specific heat, ac susceptibility, and dc conductivity in the Bi0.7Pb0.3Ca1Sr1Cu1.8Ox superconductor

Abstract A comparative study of dc conductivity, ac susceptibility, and especially specific heat on Bi 0.7 Pb 0.3 Ca 1 Sr 1 Cu 1.8 O x samples have been performed. The data indicate that a well developed high-T c phase was obtained by the Pb-substitution of Bi.