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.     

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.     

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.     

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：     

Superconducting critical current of sputtered Nb-Ge films

The temperature dependence of the superconducting critical current density J_c(T) in zero applied field from 4.2 K to T_c has been measured for 15 films of Nb-Ge with varying composition, deposition conditions, and radiation damage. The results show (i) the enhanced superconductivity observed in stoichiometric Nb₃Ge as well as nonstoichiometric films is “bulk like” rather than filamentary, (ii) a simple correlation of the form which is reasonably independent of how the T_c has been achieved, and (iii) some additional evidence that the enhanced T_c of the films is not due to their chemical composition alone.     

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.     

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.     

Critical field and specific heat of the anisotropic superconductor Nb3S4

We report on the first measurements of the critical field B_c2 and the specific heat of Nb₃S₄, which is a linear structured compound with a superconducting transition temperature of 3.65 K. The angular dependence of B_c2 is well described by the effective-mass model. The ratio of the critical fields parallel and perpendicular to the c-axis gives a value of 4.6. The small value of the specific heat jump at $\text{T}{}_{\mathrm{c}}\text{(}\text{Δc}\text{γT}{}_{\mathrm{c}}\text{= 0.95)}$ can be explained with an anisotropic gap function.     

Superconductivity in PdAlx hydrides

Hydrides of PdA$\text{l}$_x alloys prepared by electron beam co-evaporation and subsequent implantation of hydrogen ions are found to exhibit anomalous changes in the superconducting transition temperature with increasing A$\text{l}$ content. At large hydrogen concentrations, H/Pd ～ 1, the addition of aluminum increases T_c only slightly (from 8.1 to 8.6°K for 7 at. % A$\text{l}$), and then yields an abrupt decrease in T_c at 10% A$\text{l}$. This behavior is in sharp contrast to PdAg_xH_y alloys which exhibit an enhanced T_c ～ 16°K at relatively low silver content.     

Ferromagnetic ordering in the MMnF6 series (M = Ni,Cu, Zn,Pd)

Neutron diffraction experiments performed on MMnF₆ compounds (M = Ni, Cu, Zn, Pd) have shown that both NiMnF₆ and ZnMnF₆ crystallize in the rhombohedral R$\text{3}$ space group with a cationic ordering, whereas PdMnF₆ shows cationic disordering (R$\text{3}$c, a = 5.46 Å, α = 54.45°). CuMnF₆ has a monoclinic distortion derived from the MnF₃ structural type (a = 8.57 Å, b = 4.85 Å, c = 13.46 Å, β = 92.12°). The magnetic properties can be correlated with the structure: the R$\text{3}$ ordered compounds show a ferromagnetic behavior (T_C(NiMnF₆) = 39 K; T_C(ZnMnF₆) = 9.5 K; T_C(CdMnF₆) = 8 K), while the other two have an antiferromagnetic ordering.     

The effect of hydrogen on the superconducting and structural properties of b.c.c. Nb-Ru alloys

The superconducting transition temperature (T_c) has been measured before and after the introduction of hydrogen into Nb_(1?x)Ru_x$\text{(0.20?x?0.33)}$. In all cases, the presence of appreciable amounts of this interstitial component led to a sharp increase in the T_c. All the evidence suggests that conversion of the host metal lattice to f.c.c. is necessary for the appearance of the elevated T_c.     

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.     

Superconducting—normal phase boundaries of (La,Th)Ce systems

The dependence of the superconducting transition temperature T_c on Ce impurity concentration n is reported for various ($\text{La, Th}$)Ce systems. These results document for the first time the evolution of the superconducting— normal phase boundary (T_c vs n curve) as an impurity undergoes magnetic- nonmagnetic transition which for Ce impurities in ($\text{La, Th}$)Ce systems proceeds with increasing Th concentration. Observed for only the second time, re-entrant T_c vs n curves are reported for two La-rich La, Th host compositions.     

Stoichiometry,electrical properties,and electronic structure of Cr1.70Nb2.30Se10

Both powder and single-crystal X-ray investigations show that Cr_{1 + x}Nb_3?xSe₁₀ which belongs to the FeNb₃Se structure type, exists only in a narrow range of stoichiometry close to x=0.70. In contrast to the Fe analog, Cr_1.70Nb_2.30Se₁₀ undergoes no disticnt metal-insulator transition; the resistivity and the thermoelectric power remain of the same order as the temperature is lowered from 300 K to 4.2 K. The thermoelectric powers S of Fe_{1 + x}Nb_3?xSe₁₀ (0.25<x<0.40) and FeVNb₂Se₁₀, however, follow such a temperature T dependence as $\text{S∝}\text{1}\text{T}$ even at room temperature. On the basis of light-binding band calculations, these observations are interpreted in terms of the contribution of Cr or Fe d-orbitals to electron conduction.     

Electrical conductivity in amorphous Si(O or H)-Au films

SiO films obtained by sputtering in an ArO mixture with an oxygen partial pressure less than 3% are similar to a-Si films: the resistivity is proportional to exp (T₀/T)^{$\text{1}\text{4}$} and T₀ increases with oxygen content and decreases with increasing Au concentration (? 3.7 at.%). On the other hand, above an oxygen partial pressure of 5% one obtains insulating amorphous SiO₂ films. Conductivity appears in such films for Au? 13 at.% (? the percolation threshold) and then the resistivity is proportional to exp (T₀/T)^{$\text{1}\text{2}$}. The same behavior is observed when oxygen is replaced by hydrogen.     

Low temperature magneto-resistance,logarithmic resistivity rise and anisotropic superconductivity in TaS2(pyridine)12

The superconducting intercalation complex TaS₂(pyridine)${}_{\mathrm{<mtext>1</mtext><mtext>2</mtext>}}$ has received considerable attention as a prototype of two dimensional superconductivity. In this material we have detected a substantial, anisotropic magneto-resistance 11 K° above the superconducting transition and an anisotropic logarithmic resistivity rise below 20°K. The resistively measured superconducting transition temperature is also anisotropic. T_c is the lowest when the current is perpendicular to the layers.     

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.     

Etude comparative des structures des alumines β stoechiometrique et non stoechiometrique: Transition de phase dans l'alumine β stoechiometrique (II)

The usual preparation methods of β alumina lead to a non stoichiometric compound $\text{(β“N.S.”)}$ of formula 11Al₂O₃?(1 + x) B₂O with x ≈-0.3; a metastable phase with a composition close to stoichiometry $\text{(β“S”}\text{and}\text{x ? 0)}$ can however also be obtained. X-Ray diffuse scattering studies of this stoichiometric form of silver β alumina reveals a sharp order—disorder phase transition at about 307 K. The low temperature ordered state of the silver ions is found to correspond to a 3D hexagonal superstructure with the lattice constants $\text{a}\text{3}\text{, a}\text{3}\text{, c}$. Above the transition temperature 3D short range order is observed up to about 315 K, where a cross over occurs towards a higher temperature 2D short range state, similar to that previously observed at low temperature in $\text{β}{}_{\mathrm{<mtext>Ag</mtext>}}\text{“N.S.”}$. Above 500 K the conducting silver ions are found to be in a 2D quasi liquid state. A similar type of order—disorder phase transition seems to occur in stoichiometric sodium β alumina at lower temperature. It is concluded that the very particular behaviour namely the absence of phase transition in the usual forms of β alumina is a direct consequence of non stoichiometry.     

Preparation of ternary lead molybdenum sulfide with high superconducting transition temperature

The high field superconducting compound PbMo₆S₈ has been prepared by an isothermal chemical vapour transport reaction at 1273 K. The reaction product was single phase PbMo₆S₈ with lattice parameters of $\text{a}{}_0\text{= 6.545}\text{A}\text{?}$, and α = 89° 12′ and has been found to transform to the superconducting state at a midpoint T_c of 14.7 K.