Nuclear relaxation and impurity magnetism in superconducting La1−c Gd c Al2 and La1−c Ce c Al2

The¹³⁹La spin-lattice relaxation rate in the normal and superconducting states of La_1–c Gd _c Al₂ and La_1–c Ce _c Al₂ alloys has been studied as a probe of the effect of magnetic impurites on superconducting quasiparticle excitations in these systems. Comparison of the experimental data with the theory of pair breaking by magnetic impurities verifies the predicted broadening of the peak in the spectrum of quasiparticle excitations. The dependence of the relaxation rate on the strength of the coupling between conduction electrons and impurities is not fully resolved in the present experiments.Work supported in part by the U.S. National Science Foundation.Laboratoire associé au CNRS.     

Vortex motion in high-T c superconductors near the upper critical fieldH c2

The vortex dynamics in layered superconductors such as YBaCuO and BiSrCaCuO near the upper critical field H_c2 is considered. The magnetic field is parallel to the layers and a transport current flows along the layers perpendicular toH. There exists a finite pinning force for the vortex motion across the layers in a macroscopically homogeneous superconductor. The critical current vanishes exponentially in the limit of weakly layered structure, _c (T)s. The current-voltage characteristic is calculated in case _c (T)s. The initial part of the characteristic may have a negative slope depending on the magnetic fieldH.     

H c2 anisotropy in the system niobium-nitrogen

Systematic experiments on the impurity and temperature dependence of the H _c2anisotropy in niobium are reported. A combined evaluation of these two dependences within the framework of a recent nonlocal theory is shown to permit a separation of different sources contributing to the H _c2anisotropy. Whereas the l = 4 component of the anisotropy is mainly due to band-structure anisotropies of the normal state, the l = 6 component in pure niobium originates predominantly from anisotropies of the energy gap. The rms anisotropy of the energy gap in niobium is estimated to be 0.08. The anisotropy of the flux line lattice predicted on the basis of the microscopic anisotropy parameters as deduced from the H _c2measurements is in agreement with experimental observations.     

Reproducibility ofT c in a Bi2Sr2CaCu208 superconductor

The reproducibility ofT _c in superconductive Bi₂Sr₂CaCu₂O₈ is known to be very poor. In our study, the reproducibility was found to depend greatly on preparation conditions. DTA, TGA and powder X-ray diffraction methods were used to study the Bi₂Sr₂CaCu₂O₈ superconductors prepared by different routes. Resistivity and diamagnetic susceptibility were taken as the measure of superconductivity. It was found that those superconductors prepared from the oxides/carbonates (one-step) process resulted in scatteredT _c data which were less reproducible. The two-step synthesis from precalcined precursors of Bi-Sr-O and Sr-Ca-Cu-0 containing mixtures, resulted in a much improved reproducibility with the predominantT _c = 80 K superconducting phase (more than 95%). It was also found that the amount ofT _c = 115 K phase tends to decrease after repeated pulverization and sintering, leading to a single 80 K phase state. Compaction of the pulverized powder at a pressure > 4.5 ton cm^–2 was found to induce a preferred (00L) orientation during sintering.     

Measurements ofH c2(T) in Bi-Sr-Cu-O

H _c2(T) has been measured for thin BSCO films at temperatures down to 65 mK and pulsed fields up to 35 T.H _c2(T) diverged anomalously as the temperature decreased. At the lowest temperature, it was five times that expected for a conventional superconductor.     

Symmetry considerations forH c2 anisotropy experiments in cubic superconductors

We report on an evaluation ofH _c2 anisotropy effects in arbitrary planes and directions of cubic superconductors. Using Mercator projections, information on the three-dimensional variation ofH _c2 is obtained. The results are discussed for the three main crystallographic planes of the cubic lattice and evaluated in terms of their usefulness for an accurate experimental determination of the anisotropy coefficientsA ₁ . Although some planes show interesting features because of special symmetry conditions, we conclude that careful measurements in the (110) plane represent the optimum choice for an accurate determination of the anisotropy coefficients.This work was supported in part by Fonds zur Förderung der Wissenschaftlichen Forschung, Wien, under contract No. 3973.     

D.c. conductivity of amorphous CulnSe2 films

Journal of Materials Science Letters -     

Volume Dependence of H c1, H c2, and ΔP(H c1) in Magnetically Ordered bcc 3He

We have used constant-volume pressure measurements over the entire range of molar volumes of bcc solid ³He, 20>v>24 cm³/mole, to determine the zero-temperature critical field H _c1 and the pressure discontinuity ΔP(H _c1) between the two antiferromagnetically ordered phases. The upper critical field H _c2 has been determined for 20>v>21.75 cm³/mole. The ³He was cooled by direct (two-stage) adiabatic demagnetization from T≈100 µK and H=2.5 T where its entropy S≈0. We find the Gruneisen parameters for H _c2 and ΔP to be larger than in previous work, but still smaller than for H _c2 and other physical quantities such as the Neel temperature T _N and e ^1/2 ₂, where e ₂ is the coefficient of the second-order term in the free energy of the paramagnetic phase. These resugts are significant for testing the mugtiple-exchange model for solid ³He.     

Elementary pinning potential in type II superconductors nearH c2

The elementary pinning potential of a small defect in a type II superconductor is calculated near the upper critical magnetic field. The calculation applies at all temperatures and all background impurity contents of the metal. The pinning potential is found to decay according to the inverse impurity parameter when other impurities are added. A very simple formula for the pinning potential is obtained in the Ginzburg-Landau region. The boundary condition to be imposed on a Ginzburg-Landau theory at small defects is derived.     

Fluctuation-induced diamagnetism in type II superconductors above H c2

The fluctuation-induced diamagnetism in strong magnetic fields in type II superconductors is studied theoretically. It is shown that the fluctuation-induced diamagnetism at low temperatures (i.e., TT _c0) is described in terms of a universal function of [B \s- H _c2(T)]/H_s(T) for both dirty and pure superconductors, where H _s(T), the scaling field, is proportional to (T/T _c0)H_c2(0).Work supported by the National Science Foundation under Grant DMR 76-21032.     

A.c. conductivity for amorphous TeO2-P2O5 glass system

The a.c. conductivity for the TeO₂-P₂O₅ glassy system was measured in the temperature range 300–573 K and in the frequency range 100 Hz to 10 kHz. The a.c. conductivity () increased with frequency according to the relation ()^s. The frequency exponent s was found to decrease with increasing temperature. The composition dependence of the conductivity was also investigated. The density of states was also calculated using the Elliott model. The a.c. conductivity increased over the studied temperature range. The obtained experimental data have been analysed with reference to various theoretical models. The analysis shows that the correlated barrier hopping (CBH) model is the most appropriate mechanism for conduction in the TeO₂-P₂O₅ glass system.     

HighT c of liquid-quenched Bi1.6Pb0.4Sr2Ga2Cu3Ox

AlthoughT _c cannot be found for a liquid-quenched Bi_1.6Pb_0.4Sr₂Ca₂Cu₃O_x glassy sample, a highT _c is found after annealing for 24 h at 1100 K. The maximum offset temperature of the superconducting transition is 113.3 K at 2.2 × 10^–2mAmm^–2. The maximumT _{c off} is larger than that (the maximumT _{c off} is 103.4 K at 2.0 × 10^–2 mAmm^–2) of sintered specimens before liquid quenching.     

Hall effect of a dirty type II superconductor nearH c2

The theory of the Hall effect of dirty, type II superconductors nearH _c2 by Ebisawa based on the free-electron model is extended to the case of isotropic but arbitrary energy—momentum relation. It is also shown that the detailed form of the impurity potential, i.e., the momentum dependence of its Fourier transform, which is neglected in Ebisawa's work, also contributes to the effect and its contribution is calculated. The result shows that, besides the energy dependence of the density of states and electron velocity near the Fermi energy of the host metal, the detailed nature of the impurity also has a large influence on the Hall effect. The theory is compared with experiment.On leave of absence from Research Institute of Electrical Communication, Tohoku University, Sendai, Japan.     

Improved Transport J c in MgB2 Tapes by Graphene Doping

Graphene is a special form of carbon which can effectively enhance the critical current density J _c of MgB₂. In this work, a systematic investigation on the impact of sintering conditions and doping level was carried out for graphene-doped MgB₂ tapes. It is found that an appropriate addition level, i.e., 8 at% in this work, is very critical to obtain a high J _c in graphene-doped samples. The critical field and pinning force are improved obviously due to the graphene doping. The magnetic J _c of samples sintered at 800 °C with 8 at% graphene doping reached 1.78 × 10⁴ A/cm², at 5 T, 20 K. At the same time, the transport J _c was up to 2.38 × 10⁴ A/cm² at 10 T, 4.2 K. The lattice distortion caused by C substitution and residual C at the grain boundaries were thought to be the major factors affecting the J _c of graphene-doped MgB₂ samples.