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.     

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.     

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.     

Effect of deposition temperature on properties of CVD prepared Nb3Ge superconductor

We have examined properties of the Nb₃Ge superconductor prepared by CVD at temperatures between 750 °C and 900 °C. The microstructure, critical temperature, lattice parameter, normal state resistivity, critical current density and the slope of critical current density in magnetic field were investigated as functions of the deposition temperature. Relations between the critical temperature, lattice parameter and the normal state resistivity, as well as between the micro-structure and the critical current density, are analysed. Possible mechanisms ofT _c andB _c2 increases are discussed.The authors would like to thank I. Holicky for providing SEM investigation.     

Superconductivity and resistivity of amorphous niobium-germanium alloys

The superconducting transition temperature, T_c, and the normal state resistivity, π, for amorphous niobium-germanium alloys were shown to vary smoothly with composition. Most notably, no anomaly was observed in the amorphous state at the composition Nb₃Ge.     

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.     

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.     

High T c,structural instabilities and electronic properties of Nb3Ge

Abstract

A number of Nb₃Ge samples with rather high T _c,on (T _c,on ? 22.5 K) were prepared by CVD and characterized by the electrical resistance, X-ray diffraction and scanning electron microscope patterns. It was found that the crystal growth patterns are roughly divided into two categories, (i) a particles-pattern in a nearly single phase of Nb₃Ge and (ii) a network pattern with a fairly large amount of the Nb₅Ge₃ phase but with rather a large resistance ratio and a small resistivity at T ? T _c,on. The latter type of samples shows generally higher T _c,on. It is deduced that the origin of high T _c,on comes mainly from a deformation of the atomic potential at the Nb-site into an anharmonic and rather unstable shape due to the coexistence of the Nb₅Ge₃ phase. The mechanism of the formation of different patterns is discussed on the basis of the theory of non-equilibrium thermodynamics of Prigogine.     

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：     

The CESR and spin wave studies of A-15 films in normal and superconducting states

The electron spin resonance studies have been reported for A-15 superconductors, namely Nb₃Ge, Nb₃Si and V₃Si possessing different T_c values and CESR, Platzmann-Wolff type spin waves, and spin waves of antiferromagnetic type are observed in all the samples. It is found that T_c of Nb₃Ge depends upon the presence and separation of spin wave absorptions from the CESR, and T_c is found to increase when the separation is reduced. It is concluded that the exchange interactions in the conduction band, as manifested by the behaviour of spin waves, are of antiferromagnetic type and they are responsible for superconductivity in A-15 materials studied.     

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.     

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.     

Deconfinement transition for nonzero baryon density in the field correlator method

Deconfinement phase transition due to the disappearance of confining colorelectric field correlators is described using a nonperturbative equation of state. The resulting transition temperature T _c (μ) at any chemical potential μ is expressed in terms of the change of the gluon condensate ΔG ₂ and absolute value of the Polyakov loop L _fund(T _c ), which is known from the lattice and analytic data, and is in good agreement with the lattice data for ΔG ₂ ≈ 0.0035 GeV⁴; e.g., T _c (0) = 0.27, 0.19, and, 0.17 GeV for n _f = 0, 2, and 3, respectively. The text was submitted by the authors in English.     

Degradation universality and fröhlich lattice instability of A-15 superconductors

Striking correlations of T_c and lattice constant have recently been observed in ion bombarded thin films and bulk samples of Nb₃Ge. Anomalies in inelastic neutron scattering intensities in b.c.c.Zr-Nb alloys have also been reported. Both experiments can be understood in terms of ordering of strain fields clustered on point defects; the ordering is a characteristic feature of dynamically unstable lattices.     

Superconductivity and normal electrical conductivity of amorphous lead films nucleated with molybdenum

Lead films vapor quenched onto nucleating monolayers of Mo or W exhibit strong lattice disorder and can be considered to be amorphous. The amorphous-to-crystalline transformation temperatureT _tr is indicated by a sharp drop of the electrical resistivity in the course of annealing.T _tr is found to be proportional tod ^–2 for Pb thicknessd smaller than 30 nm. The superconducting transition temperatureT _c is by 0.6 to 1 K smaller in the amorphous state than after crystallization. In both states,T _c is proportional tod ^–1. Prenucleation with about half a monolayer of Mo leads to quite the sameT _c depression as observed earlier by Strongin et al. on Pb films vapor quenched onto predeposited films of SiO, Ge or Al₂O₃. For comparison, experiments have been carried out with 2.5 nm Ge predeposits. As with Mo prenucleation, a well defined transformation temperatureT _tr of about the same value has been observed.T _c of bulk amorphous Pb can be extrapolated to be about 6.6 K.     

Round-robin study of Tc measurement of superconducting “Nb3Ge” thin films

To determine the reproductibility in the measurement of T_c a round-robin experiment was conducted. The samples consisted of five high T_c “Nb₃Ge” thin films. The results of this experiment show that only the midpoint of the T_c curve is reproducible to within ≈±0.2 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.     

FeTe合金结构分析及其薄膜制备

利用固态反应法制备了名义成分为FeTe的合金, 采用X射线粉末衍射技术和Rietveld全谱拟合分析方法测定了其相组成和晶体结构. 研究表明,主相为Fe_1.08Te,空间群为P4/nmm,点阵参数 a = 3.8214(3) Å, c = 6.2875(3) Å, Z = 2, Fe原子占据2a和2c晶位, Te原子占据2c晶位. 利用脉冲激光沉积技术制备的FeTe薄膜超导转变起始温度为13.2 K,零电阻温度为9.8 K. 关键词： FeTe Rietveld结构精修 超导薄膜     

Superconductivity with onset above 23° K in NbGe sputtered films

Sputtered films of nominal composition Nb₃Ge deposited onto hot substrates, of the type recently found by Gavaler to have superconductivity onsets at 22.3° K, have now been obtained with a maximum onset temperature of 23.2 ± 0.2° K and a transition width of ～ 1.2° K. The sputtering conditions necessary to achieve these results are discussed and suggest that the high T_c phase may be detrimentally affected by the presence of high energy particles present under normal sputtering conditions.     

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.