A—15型化合物的超导性经验规律

本文给出一张既能考虑A-15结构型变,又能描述A-15型化合物T_c(超导临界温度)变化规律的图。从而预言:所述A-15系列中大概不会存在T_c很高(>25K)的化合物;铌系列中Nb₃Ge的T_c可能是最高的;钒系列中T_c可望还能提高(>17.1K),V₃Ge可望提高到12K以上。检查Nb₃Si,得不到Geller那样的乐观估计,并认为:以新工艺得到的A-15相Nb₃Si,是以牺牲正当化学配比或改变正常原子体积为代价而获得的非理想结构。从T_c与半径比关系及理想A-15结构对半径比的要求来看,笔者认为文献上关于Nb₃Si在有较高T_c(>23K)的种种预言,都是缺乏充分依据而不足置信的。 关键词：     

Anomalous behavior of superconducting transition temperature of amorphous and crystalline Nb3Ge under pressure

The superconducting transition temperature (T_c) of amorphous Nb₃Ge was studied under both hydrostatic and quasihydrostatic pressure to 3.5 and 13 GPa, respectively. Whereas hydrostatic pressure causes T_c to initially decrease, T_c is found to increase under higher quasihydrostatic pressures. T_c(p) was also studied on an A-15 crystalline Nb₃Ge sample obtained from the amorphous sample by annealing.     

Study of the correlation between the structure and critical temperatureT c of sputtered Nb-Ge films

Nb-Ge layers with continuous change of chemical and phase composition were prepared by the d.c. sputtering method. The dependence of critical temperatureT _c on phase composition, Ge-content, lattice imperfections and composition irregularities were studied. Films with highT _c contain beside the A-15 Nb₃Ge phase also the hexagonal and tetragonal modification of the Nb₅Ge₃ phase. Correlation betweenT _c and Nb₃Ge phase composition determined from the lattice parameter was found. In samples with highestT _c the lattice parametera ₀=0·5135 nm corresponding to 22–23 at.% of germanium was determined.     

Specific heat of A-15 Nb3Si produced by explosive compression

The specific heat from 1.2 to 23 K has been measured on a new high T_c superconductor, A-15 Nb₃Si. The sample was prepared by explosive compression and has an onset of bulk superconductivity at 18.0 K, with a transition width of 0.7 K. The density of states for pure A-15 Nb₃Si implied from the specific heat data is 0.94 ± 0.20 states/eV-atom, ΔC/γ T_c is 2.0 ± 0.2.     

The superconducting transition temperature of bulk samples of the niobium-germanium A15-phase after implantation of Ge,Si, O and Ar ions

Bulk material of Nb₃ (Ge_0.8Nb_0.2) with A15 structure and a superconducting transition temperature T_c of 6.5 K has been implanted with Ge, Si, Ar and O ions and subsequently annealed at high temperatures. After annealing between 700 and 750°C the Ge implanted samples showed a strong increase in T_c up to 16.2 K. With Si ions only a T_c of 13 K was obtained, with Ar and O ions T_c remained below 9 K. From X-ray measurements carried out on high T_c Ge implanted samples it could be concluded that the implanted surface layer grows up to a high degree epitaxially on the single crystallites of the bulk material. The lattice constant $\text{a}$₀ of the implanted film was reduced by 0.02 Å with respect to the bulk material. This reduction in $\text{a}$₀ is stronger than expected from the transition temperature of the implanted surface layer.     

Superconductivity and phase compositions in annealed amorphous Nb-Si alloys under high pressure

We report the results of amorphous Nb₈₁Si₁₉ and Nb₇₇Si₂₃ alloys annealed at high pressure but at a pressure lower than that used to synthesize single-phase A15 Nb₃Si. The lower pressure gives rise to the decomposition of the alloys into a multi-phase state consisting of A15 Nb₃Si, Γ-Nb₅Si₃, L1₂-type Nb₃Si and Nb. The lattice constants are smaller and T_c higher than those obtained for the single-phase samples. We predict the T_c of stoichiometric A15 Nb₃Si to be 25 K.     

Extended x-ray absorption fine structure investigations of Nb3Ge0.26Al0.74

We have performed Extended X-ray Absorption Fine Structure (EXAFS) studies of the high-T_c ternary superconducting alloy Nb₃Ge_0.26Al_0.74 above the Ge K-edge. The EXAFS indicates that the local environment surrounding the Ge atoms is characterized by a high degree of structural order, similar to that found in previous investigations of high-T_c Nb₃Ge thin films. We also present direct “microscopic” structural evidence that this material is a solid solution of Nb₃Ge and Nb₃Al. No evidence of Nb₃Ge microcrystals is observed.     

Disorder in superconductors—a study on Cu2Mo6S8

Some of the recent work on disorder-induced changes inT _c is reviewed. Shock-pressures induce a disorder uncomplicated by antisite disorder typical of particle irradiation, and have generated interest because of the shock-synthesis of A-15 Nb₃Si. In this paper we present our results on laser-induced shock-damage, and compare it with the results on V₃Si and the results on particle irradiation of Chevrel phase superconductors.     

Formation of near stoichiometric A15Nb3Si from annealing an amorphous alloy under high pressure

The A15Nb₃Si with a composition closed to stoichiometric compound has been synthesized under high pressure from a starting material of Nb₇₇Si₂₃ amorphous alloys. High pressure annealing was carried out in Bridgman anvils apparatus. The amorphous alloy would decompose into A15Nb₃Si, bcc Nb solid solution and hexagonal phase when it was annealed under a pressure lower or a temperature higher than that for forming single phase A15Nb₃Si. The yielded A15Nb₃Si exhibited a superconducting transition temperatureT _c of 19.1 K, and has been indexed unambiguously with a lattice parameter ofa=0.5093 nm. Moreover, a nonlinear relationship betweenT _c anda has been constructed from our experimental data, and aT _c of 27 K for stoichiometric A15Nb₃Si can be expected.     

Synthesis of superconducting compounds by thermolysis of volatile hydrides and organometallic compounds on glowing wires

The synthesis of high temperature superconducting phases in the NbGe, NbSn, VSi, VGe, VSn, NbC and MoC systems is described by method consisting in the thermolysis of volatile hydrides or organometallic compounds on resistively heated wires. For face-centred cubic NbC a higher transition temperature than previously reported was obtained. The A15 phase boundary of NbGe is extended towards the stoichiometric 3:1 composition, affording samples of Nb₃ Ge with a T_c onset of 15.8°K.     

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.     

Epitaxial growth of Nb3Ge on Nb3Ir and Nb3Rh

This is a report on a cooperative research carried out in Stanford University to investigate the possibility of using epitaxy to prepare the high T_c superconductor Nb₃Ge in an A15 crystalline structure at the 3:1 stoichiometry.Nb₃Ir polycrystalline films with the A15 structure deposited on sapphire were used as substrates for the epitaxial growth of Nb₃Ge because of the favorable lattice parameter match. The experimental results clearly show that epitaxial growth indeed occurs and helps to extend the range of homogeneity of the A15 phase up to 26.3 at.% Ge as compared with the thermodynamic equilibrium boundary at 19 at.% Ge. We also used Nb₃Rh films as substrates and found them inferior to Nb₃Ir because of the multiphase nature of the films.In addition to extending the A15 phase boundary epitaxy results in a considerable rise in the superconducting transition temperature for Ge-rich samples together with a reduction in the transition width. The work suggests that polycrystalline epitaxy can be an important tool in the synthesis of thin-film intermetallic compounds.     

Specific heat of high Tc,A-15 Nb0.75Ga0.25

The specific heat from 1.4 to 27 K has been measured on a NbGa sample that was approximately 85% A-15 Nb_0.76Ga_0.24 with a T_c of 19.8 K and a transition width of 1 K. The data imply that the density of states for A-15 Nb_0.75Ga_0.25 is 1.7 ± 0.25 states/eV-atom, in good agreement with recent band structure calculations. The Debye temperature is found to increase approximately 1 K per degree from T_c down to 4 K, with θ_D(0) = 280 K.     

Hybridization and bonding in transition metal compound superconductors: X-ray photoemission from crystalline and amorphous Nb3Ge.

An XPS (or ESCA) study of the high T_c crystalline and the low T_c amorphous phases of Nb₃Ge indicates a change of the Nb-Ge bonding from covalent to metallic upon the amorphous to crystalline transition. The high T_c superconductivity of Nb₃Ge does not appear to stem only from an unusually high electronic density of states at the Fermi level but rather from another property such as a resonant enhancement of the electron-phonon coupling resulting from cation-anion hybridization at E_F.     

Influence of composition on the superconducting transition temperature of alloys with the A-15 structure

Superconducting transition temperatures are presented for a wide range of compositions within the A-15 phase of the NbAl system. The maximum T_c of 19.0 K clearly occurs for a composition greater than that of stoichiometric Nb₃Al. This observation, together with a modern calculation by Shen, suggests that the important controlling factor for the maximum T_c in the A-15 class of materials is the approach to the “$\text{λ ? 2}$ limit” rather than structural instabilities or the stoichiometric composition.     

X-ray diffraction studies on He- and Ar-irradiated Nb3Ge thin films

We have performed measurements of relative integrated X-ray intensities in order to study the influence of radiation damage on the A15 structure of superconducting Nb₃Ge. With increasing He- and Ar-ion fluence an increase of the “temperature factor” and the lattice parameter has been observed in the fluence region where the depression of the superconducing transition temperature T_c was found to occur. Both parameters do not uniquely depend on the decrease of T_c. Further a decrease of the total relative X-ray intensity is found for particle fluences in the saturation region of T_c. The results are interpreted in terms of static atom displacements statistically distributed in the irradiated volume.     

The effects of composition and atomic ordering on superconductivity in the systems Nb3Ga and V3Ga

The strongly different variation of the superconducting transition temperature, T_c, observed from the A15 type compounds V₃Ga and Nb₃Ga as a function of the heat treatment has been correlated with the precise shapes of their corresponding phase fields at the vicinity of the stoichiometric composition. The change in T_c for V₃Ga is related to LRO effects only, while combined compositional and LRO effects are present in the case of Nb₃Ga. These two contributions, which have been separated in the present work, determine the behavior of all high T_c superconductors of the A15 phase.     

Spin resonance of conduction electrons and EPR of localized moments in a low-dimensional organic conductor [Pd(dddt)2]Ag1.5Br3.5

The EPR spectra of a quasi-two-dimensional organic metal [Pd(dddt)₂]Ag_1.5Br_3.5 contain signals due to the spin resonance of conduction electrons (CESR) and signals due to the localized magnetic moments of Ag²⁺. The system of Ag²⁺ ions exhibits a strong indirect antiferromagnetic exchange interaction characterized by the Weiss constant Θ=−280(25) K, which leads to magnetic ordering at a temperature of T ₀=70(10) K. The same temperature T ₀ corresponds to a strong anomaly in the CESR linewidth. The observed anomaly in the CESR linewidth, as well as some features of the temperature dependence of conductivity in the system studied, are explained by the interaction between conduction electrons and Ag²⁺ ions localized in the anion layers (π-d interaction) and by antiferromagnetic ordering of the spins of Ag²⁺ magnetic ions.     

The sputtering of high Tc A15 Nb3Si and V3Ge

We report some initial results on the preparation of A15 Nb₃Si and V₃Ge using a getter sputtering technique. Under sufficiently clean conditions we observe an increase in the superconducting transition temperature. DC onsets in excess of 14 and 11 K have been observed for Nb₃Si and V₃Ge respectively. In each case a positive identification of the A15 phase has been made.     

Specific heat of single phase Nb3Ge

For the first time, the specific heat of single phase, stoichiometric, high transition temperature (21.8 K) A-15 Nb₃Ge has been measured. From the data between 4 and 29 K, the linear term coefficient, γ, of the specific heat is found to be 30.3±1. mJ/mole-K² and the Debye temperature, ?_D, is 302±2 K. The bulk energy gap parameter, 2Δ/kT_c, is found to be 4.2±0.2, in agreement with tunneling measurements.