L. F. Schneemeyer

Critical currents and thermally activated flux motion in high-temperature superconductors

We have measured the resistance below Tc of single crystals of the high‐temperature superconductors Ba2YCu3O7 and Bi2.2Sr2Ca0.8Cu2O8+δ in magnetic fields up to 12 T. The resistive transition of both compounds is dominated by intrinsic dissipation which is thermally activated, resulting in an exponential temperature dependence of the resistivity well below Tc. The dissipation is significantly larger and of different character in the Bi‐Cu compound than in Ba2YCu3O7. The relation between the activated behavior and the depinning critical current is discussed.

Spin fluctuations and superconductivity in Ba2YCu

We report observation of spin-pair excitations in Ba/sub 2/YCu/sub 3/O/sub 6+//sub delta/, with delta in the range 0.0--0.9. The frequency of the excitations, and hence the value of the exchange constant, J = 950 cm/sup -1/, is similar to that reported recently for (nonsuperconducting) La/sub 2/CuO/sub 4/. The present observation of magnetic excitations in a superconducting material, and of their evolution as a function of delta, demonstrate a large influence of carrier concentration on the spin-fluctuation dynamics.

Critical current densities in single‐crystal Bi2.2Sr2Ca0.8Cu2O8+δ

We report transport and magnetic measurements of critical current densities in single crystals of high Tc superconductors in the Bi‐Sr‐Ca‐Cu‐O system. Transport measurements of Jc were made using a pulse method, while a vibrating sample magnetometer was used to measure M‐H loops of oriented crystals in fields up to 15 kOe. Values for transport Jc in zero applied field are higher than those of Ba2YCu3O7 single crystals at comparable temperatures. Also we observe a strong magnetic field dependence for Jc at temperatures above ∼20 K.We report transport and magnetic measurements of critical current densities in single crystals of high T/sub c/ superconductors in the Bi-Sr-Ca-Cu-O system. Transport measurements of J/sub c/ were made using a pulse method, while a vibrating sample magnetometer was used to measure M-H loops of oriented crystals in fields up to 15 kOe. Values for transport J/sub c/ in zero applied field are higher than those of Ba/sub 2/YCu/sub 3/O/sub 7/ single crystals at comparable temperatures. Also we observe a strong magnetic field dependence for J/sub c/ at temperatures above approx.20 K.

Superconductivity in rare earth cuprate perovskites

Abstract Superconducting cuprate perovskites isostructural with Ba2YCu3O7 incorporating a variety of rare earth ions are reported. Resistivity and magnetic susceptibility measurements show transition temperatures in excess of 90K for several materials, including Nd, Sm, Gd, Dy, Ho, Er and Lu-containing phases. Interestingly, Ce and Tb, with a readily accessible 4+ state, form phases other than the defect perovskite which are not superconductors. However, Pr, which also has an accessible 4+ state, does give a nonsuperconducting defect perovskite isostructural with Ba2YCu3O7. The observation of high temperature superconductivity in phases containing ions carrying magnetic moments is significant because it suggests that the magnetic moment on the rare earth ion interacts only weakly with the electrons making up the band crucial to superconductivity in these materials.

Low‐temperature deposition of zirconium and hafnium boride films by thermal decomposition of the metal borohydrides (M[BH4]4)

Conductive (150 μΩu2009cm), adherent films of zirconium and hafnium borides have been deposited on various substrates by the low‐temperature (100–270u2009°C) thermal decomposition of Zr[BH4]4 and Hf[BH4]4. Auger electron spectroscopy of these films shows that their composition is ZrB2 and HfB2. The film surfaces are oxidized and slightly carbon contaminated. However, the bulk contains less than 1 at.u2009% C or O. This synthesis is by far the lowest temperature preparation of these materials (plasma‐enhanced chemical vapor deposition of TiB2 requires 480–600u2009°C) and holds great promise for the use of these materials in electronics applications.

Superconducting and Normal State Parameters of Bi2.2Sr2Ca0.8Cu2O8+δ Single Crystals : A Comparison with Ba2YCu3O7

Abstract Magnetic and magnetotransport properties were studied on Bi 2.2 Sr 2 Ca 0.8 Cu 2 O 8+δ single crystals. The normal state resistivity in the a-b plane varies linearly with temperature and extrapolates to zero at T = 0. The slopes of the upper critical field are −4.5+1(−0.55) T/K for H perpendicular (parallel) to the c-axis. The coherence length is calculated to be ≈30A˚in the a-b plane but only ≈A˚perpendicular to it. ξ c is thus shorter than the 12A˚spacing between the Cu-O double layers. From H c1 and H c2 data, the Sommerfeld parameter γ is deduced.

Efficiency losses from carrier‐type inhomogeneity in tungsten diselenide photoelectrodes

The coexistence of macroscopic p‐ and n‐type domains in nominally smooth, single‐crystal tungsten diselenide photoelectrodes leads to substantial efficiency losses in these crystals. The presence of such regions is demonstrated by rotating‐disk electrode experiments, Seebeck coefficient measurements, and in situ topographic photogenerated carrier‐collection analysis with a scanning laser spot. These results indicate the importance of controlling growth parameters to yield homogeneously doped crystals to achieve maximum solar conversion efficiencies.

Magnetization and critical currents of Bi-Sr-Ca-Cu-O and Ba2YCu3O7 superconductors

We have measured the critical currrent density Jc as a function of temperature for a Bi‐Sr‐Ca‐Cu‐O sample containing a substantial fraction of a phase with Tc≊110 K and for a Ba2YCu3O7 sample (TTc≊92 K). The behavior of Jc was deduced from magnetization measurements. For both samples the magnetization at fixed temperature and field decayed logarithmically with time. The values of Jc so derived, allowing for the decay to occur, went to zero for temperatures above 35 K for the Bi‐Sr‐Ca‐Cu‐O ceramic and to zero above 70 K for the Ba2YCu3O7 sample. These values are substantially lower than those deduced from dynamic M‐H loops.

Infrared studies of high Tc superconductors

The infrared properties of the high temperature superconductors are dominated by the strong temperature dependent absorption of the free carriers at low frequency and a temperature independent midinfrared band. Dramatic changes in reflectance do occur as the material becomes superconducting but these changes cannot be used to determine the energy gap, since as a result of the development of the superconducting condensate, changes in the real part of the dielectric function dominate. Also, since the relaxation rate of the free carriers is of the order of 200 cm−1 at 100 K the materials are near the clean limit below this temperature and we do not expect to see structure at the energy of the superconducting gap. The reflectance edge observed in many experiments at 450 cm−1 is caused by the onset of the midinfrared absorption which has the character of a direct particle-hole excitation rather than the Holstein scattering of the free carriers.

Vibrational Raman Scattering from high T C single Crystals

Polarized Raman spectra have been measured on single crystals of the high T c materials A 2 BCu 3 O x with A=Ba and Sr, B=Y and La and 6 zz )on chemical composition is in agreement with its assignment to the O(1) axial stretching vibration. The intensity of the 600 cm 1 band, which has been ascribed to a defect induced Cu-O mode, strongly depends on the crystal preparation and resulting purity.