Electrical transport properties of a quasi-one-dimensional Nb3Te4 single crystal

The electrical resistance of a linear chain metal Nb₃Te₄ were measured from 1.3 to 320 K. The residual resistance ratio $\text{R(300}\text{K}\text{)}\text{R(4.2}\text{K}\text{)}$ is about 3. Nb₃Te₄ shows an anomaly in the resistivity vs temperature at about 80 K, suggesting an occurrence of a charge-density-wave transition. The transverse and longitudinal magnetoresistance at 4.2 K are proportional to the magnetic field in the range of 2–58 kOe. In the superconducting region close to the transition temperature T_c, the critical magnetic field H_c2 is proportional to δT=T_c?T. The angular dependence of H_c2 fits well with the fluxoid model of the Ginzburg-Landau theory. The ratio of the critical fields parallel and perpendicular to the chain direction is 4.8.     

Superconductivity of powder-in-tube Sr0.6K0.4Fe2As2 wires

Nb-sheathed Sr_0.6K_0.4Fe₂As₂ superconducting wires have been fabricated using the powder-in-tube (PIT) method for the first time and the superconducting properties of the wires have been investigated. The transition temperature (T_c) of the Sr_0.6K_0.4Fe₂As₂ wires is confirmed to be as high as 35.3 K. Most importantly, Sr_0.6K_0.4Fe₂As₂ wires exhibit a very weak J_c-field dependence behavior even the temperature is very close to T_c. The upper critical field H_c2(0) value can exceed 140 T, surpassing those of MgB₂ and all the low temperature superconductors. Such high H_c2 and superior J_c-field performance make the 122 phase SrKFeAs wire conductors a powerful competitor potentially useful in very high field applications.     

Metal-nonmetal transition and superconductivity in frozen aluminium-hydrogen films

A study of the influence of the hydrogen concentration c_H on the superconducting transition temperature T_c and the d.c.-conductivity σ of Al-films has been performed. The samples were prepared by co-deposition of Al and H (H₂) onto a sapphire substrate held at a temperature of about 5 K. Within the experimental errors, there is no difference between the behaviour of Al-H and Al-H₂ respectively. σ shows a decrease with increasing c_H and a metal-insulator transition at c_H?0.68. The T_c values exhibit a maximum T_c ? 5.3 K for 0.22 ? c_H ? 0.5. Annealing experiments result in a decrease of T_c and an increase of σ. An analysis of the annealing experiments leads us to the conclusion that the as-prepared Al-H (Al-H₂) films have an amorphous structure, which transforms during annealing in a granular system of Al grains embedded in insulating AlH₃. No H-specific effect has been detected.     

Effect of temperature and magnetic field on the evolution of a vortex structure of the granular YBa2Cu3O7 − δ high-temperature superconductor

The temperature dependences of the electrical resistivity ρ(T) _{H = const} have been measured in external magnetic fields H _ext (0 ≤ H _ext ≤ 1420 Oe) at temperatures ranging from 70 to 273 K for samples of the granular YBa₂Cu₃O_{7 ? δ} high-temperature superconductor (HTSC). Cooling of the samples to the minimum temperature T _min (70 K) has been performed in external magnetic fields (FC mode) and in the absence of a magnetic field (ZFC mode). Moreover, the dependences ρ(T) _{H = 0} for samples cooled in the FC mode have been measured in a zero field. The curves ρ(T)H = const have been converted into isotherms of the magnetore-sistance ρ(H _ext) _{T = const}. A comparative analysis of the specific features in the behavior of the curves ρ(H) _{T = const} for samples with different “magnetic prehistories” has made it possible to elucidate the nature and mechanisms of the influence of the particular scenario of the magnetic treatment of granular HTSCs on the behavior of their galvanomagnetic properties. The temperature dependences of the critical magnetic fields of superconducting grains (H _c1g , H _c2g ) and Josephson weak links (H _c2J ) have been determined, and the H-T phase diagrams of granular YBa₂Cu₃O_{7 ? δ} HTSCs have been recovered.     

Observation of a transition from BCS to HTSC-like superconductivity in Ba1 − x KxBiO3 single crystals

The temperature dependences of the upper critical field B _c2(T) and surface impedance Z(T) = R(T) + iX(T) have been measured in Ba_{1 ? x} K_xBiO₃ single crystals with transition temperatures 6 ≤ T _c ≤ 32 K (0.6 > x > 0.4). A transition from the BCS to an unusual type of superconductivity has been revealed: B _c2(T) curves of the crystals with T _c > 20 K have positive curvature (as in some HTSCs), and those of the crystals with T _c < 15 K described by the usual Werthamer-Helfand-Hohenberg (WHH) formula. The R(T) and X(T) dependences of the crystals with T _c ≈ 32 K and T _c ≈ 11 K in the temperature range T ? T _c are linear (as in HTSCs) and exponential (BCS), respectively. The experimental results are discussed using the extended saddle point model by Abrikosov.     

A-B site-bond correlated percolation for antiferromagnetic Ising model: The Bethe cluster approximation

We study the site-bond percolation problem for clusters of holes and particles with antiferromagnetic order by means of the Bethe cluster approximation. We find that the droplets (i.e.P _B =1?e ^?|K|/2) diverge at the antiferromagnetic critical pointH=0,T=T _c; however forH≠0 they diverge along a percolation line which is different from the Antiferromagnetic Phase Boundary except atT=0.     

Superconductivity of (Sn1−z Pbz)1−x InxTe alloys

The temperature dependences of the electrical resistivity of (Sn_1?z Pb_z)_1?x In_xTe alloys with different concentrations of lead (z=0–0.60) and indium (x=0.03–0.20) were studied at temperatures T=0.4–4.2 K in magnetic fields from zero to H=15 kOe. A resistivity drop of no less than three-four orders of magnitude was observed in this range of alloy compositions. Application of a magnetic field above a critical level resulted in a recovery of the sample resistivity to the original value. The observed resistivity drop is identified with a superconducting transition. The critical parameters of the superconducting transition (T _c and H _c2) were determined at the drop to one half the normal resistivity level. Experimental dependences of the critical supercon-ducting-transition temperature T _c and of the second critical magnetic field H _c2 on the contents of lead (z) and indium (x) were measured. The data obtained confirm a strong localization of the In impurity states and are evidence of the extrinsic nature of superconductivity in the class of materials under study. It was established that as the Pb content in (Sn_1?z Pb_z)_1?x In_xTe increases, T _c and H _c2 decrease as the Fermi level E _F (fixed in the In impurity resonance band) leaves the Δ extremum and the superconductivity breaks down when E _F leaves the LΣ saddle point in the valence-band energy spectrum.     

Re-entrant superconductivity in a magnetically ordered superconductor: La1-xGdxRu2

We have measured the critical temperature (T_c) and the upper critical magnetic field (H_c2) of La_1-xGdxRu₂. At low concentrations of the magnetic impurity (Gd), the suppression of T_c follows the expected Abrikosov-Gorkov (A-G) pair breaking curve. However, for larger concentrations, strong deviations below A-G are observed. Samples in this region (4. ? × ? 5. at. %) exhibit two T_c's. La_1-xGd_xRu₂ is known to order magnetically, probably as a spin glass, and the magnetic ordering temperature (T_M) has been measured in the normal state. This T_M curve intersects the T_c curve in the concentration range where the T_c curve is re-entrant and we therefore attribute the re-entrant T_c behavior to the magnetic ordering of the Gd³⁺ ions.     

Anisotropy effects in cesium flouroxide tungsten bronze

The upper critical field H_c2(T) and the specific heat jump ΔC(T_c) are calculated for Cs_0.1WO_2.9F_0.1 using a two-band model. The model parameters are obtained by adjusting the theoretical H_c2(T) values to experimental results. The model calculation predicts an anomalous specific heat jump ΔC as a function of the inverse relation T_c(H).     

Surface superconductivity and twinning-plane superconductivity in aluminum

The critical supercooling field H _sc is measured in aluminum single crystals and twinned bicrystals in a temperature range slightly below T _c0 (T _c0 ? 0.055 K < T < T _c0), where T _c0 is the critical superconducting transition temperature. It is found that, even in this small temperature range, the H _sc(H _c) dependence, which is considered to be identical to the H _c3(H _c) dependence for single crystals, is substantially nonlinear. The H _sc(H _c) dependences of the twinned bicrystals and single crystals are shown to be significantly different. The qualitative features of the phase diagram of the twinned aluminum bicrystals coincide with those of the phase diagram of twinning-plane superconductivity obtained earlier for tin in [1]. These findings allow the conclusion that the phenomenon of twinning-plane superconductivity also exists in face-centered cubic crystal lattices.     

Influence of annealing on magnetic properties of Co-based metallic glasses

The influence of low-temperature annealing on the magnetic hysteresis loop parameters of magnetostrictive Co-Si-B and nonmagnetostrictive Co-Fe-Si-B glasses was studied. The dependence of Curie temperature (T_C), crystallization temperature (T_x), full-stress-relaxation temperature, saturation magnetostriction constant, saturation magnetization and coercive field H_c on metalloids contents for as quenched Co_100-x(Si_0.5B_0.5)_x glasses was determined.It was found that annealing enhances remanence magnetization for all investigated Co-Si-B glasses. The coercive field of these glasses is influenced by annealing owing to: stress relaxation (resulting in an H_c decrease) and domain structure stabilization (resulting in an H_c increase). Significant H_c reduction for both magnetostrictive and non-magnetostrictive glasses was observed, after annealing above T_C. For glasses with T_C<>T_x, it was necessary to apply an external magnetic field in ord er to decrease H_c.It was found that non-magnetostrictive metallic glasses with low Curie temperatures (T_C ? 450 K) exhibit the most stable magnetic hysteresis loop parameters.     

Upper critical field of the 122-type iron pnictide superconductors

The upper critical fields (H_c2) of the single crystals (Sr,Na)Fe₂As₂ and Ba_0.55K_0.45Fe₂As₂ were determined by means of measuring the electrical resistivity, ρ_xx(μ₀H), using the facilities of pulsed magnetic field at Los Alamos. In general, these compounds possess a very large upper critical field (H_c2(0)) with a weak anisotropic effect. The detailed curvature of H_c2(T_c) may depend on the magnetic field orientation and the sample compositions. We argue that such a difference mainly results from the multi-band effect, which might be modified via doping.     

Properties of disordered Mo3Ge thin films with A15 structure

Heavy ion irradiation of A15 Mo₃Ge with the low transition temperature T_c = 1.45 K raises T_c to ? 6 K. For the first time in A15 compounds a T_c degradation (≈ 3 K) after having passed through a maximum is observed until near 3.5 K the saturation value is reached. The effects on T_c are interpreted by variations of the deduced (H'_c2, ?) density of states at the Fermi level. This picture consistently explains the T_c degradation of the high-T_c A15 compounds.     

Superconducting properties of amorphous sputtered Zr70Mo30

Specific heat, resistivity and critical superconducting fields (H_c and H_c2) measurements show that amorphous Zr₇₀Mo₃₀ is an intermediate superconductor (T_c = 4.3 K; λ = 0.77). The coupling strength is mainly governed by the electronic properties. Like in the Zr?3d alloys previously investigated, a discrepancy between the measured and calculated upper critical field slope is reported. The effect of two-level systems (TLS) needs to be clarified.     

Neutron diffraction determination of the magnetic phases of CoCl2·2H2O

The metamagnetic behaviour of CoCl₂·2H₂O has been determined by means of neutron diffraction. At T = 4.2 K the reflections of the antiferromagnetic zero-field phase disappear, when a magnetic field of H_c1 = 31.8 kOe is applied along the b axis. The new ferrimagnetic phase vanishes at H_c2 = 46.1 kOe and the magnetically saturated configuration is reached. The three phases meet at a triple point at T_T = 8.9 K and H_T = 37.5 kOe. Two types of phase transition have been detected.     

Neutron scattering investigation of the tricritical point in DyPO4

Neutron scattering experiments on DyPO₄ were performed without and with external field at temperatures between 1.65 and 3.5 K in order to determine the phase diagram and critical properties. In contrast to previous results the M(H)-curve shows typical first order behaviour fot T ? 2 K and H ? 4.2. kG. The H_c(T)-curve agrees quite well with the previous results. Critical scattering was observed at 1.65 K around the (110) magnetic Gragg-peak. A structure refinement at room temperature was also done mainly in order to check for the importance of extinction.     

Superconducting and magnetic properties of La3−x In Cex

Measurements of the critical temperature, T_c(x), the critical field, H_c2(x,T) and the susceptibility, χ (T), as a function de la concentration, x, in the system La₃₋x Ce_xIn, are compared with the theories of Muller-Hartmann and Zittsrtz, and Abrikosov and Gor'kov. The H_c2(x, T = 0) and χ(T) measurements indicate the appearance of short range antiferromagnetic order around x = 0.04 which is probably responsible for the anomalous behavior of T_c(x).     

Effects of composition on the upper critical field of V-Ga

Measurements of the temperature dependence of the upper critical field, H_c2(T), for a series of V_100?xGa_x materials are presented for 20.5 ≤ × ≤ 29.6. Fits of the data to conventional theory for a paramagnetically limited, dirty, type II superconductor show: 1) a maximum in T_c and H_c2(0) for x ? 25; 2) a constant ( for x ≤ 25; 3) a slowly increasing value of λ_so with increasing x up to x ～ 25; and 4) good agreement with stoichiometric ordered and thermally disordered V₃Ga. Above $\text{x ? 25}$ broader transitions are observed. For x = 25, T_c = 15.3 K, (, λ_so = 0.3 and H_c2(0) = 23.4 tesla. The effects of inclusion of strong-coupling in the theory are discussed briefly.     

Upper critical fields of Nb3Ge thin film superconductors

The upper critical field H_c2(T) of the highest T_c(～23K) Nb₃Ge superconducting films has been found to be ≈370kG at 4.2K. Measurements on lower T_c films show very broad transitions reflecting nonuniformity. The H_c2(T) characteristics are consistent with other Nb₃X type II superconductors.