Superconductivity in A-15 compounds

Superconductivity in A-15 compounds is examined in terms of electron pairing induced by exchange of acoustic plasmons. The electronic band structure of Nb₃Sn, V₃Si, and similar materials favor transition temperatures of T_c 20 K.     

Metastable A-15 phase in liquid-quenched Nb-Si binary alloys

Metastable A-15 and amorphous phases have been prepared in liquid-quenched Nb-Si binary alloys. The results suggest that the presence of interstitial impurities has a stabilizing influence on the A-15 structure.     

The density of states of b.c.c. Nb3Au: A comparison with A-15 Nb3Au

When Nb₃Au is converted from the A-15 structure to the b.c.c. allotrope, a sharp decrease in the superconducting transition temperature, T_c, occurs (11.1 to 1.5 K). To study possible causes for this sharp decrease, specific heat measurements have been made on the b.c.c. form of Nb₃Au yielding values of γ=19.2 mJ/mole-K², β=0.274 mJ/mole-K⁴ and θ_D=305 K. These parameters compared with published values for the A-15 Nb₃Au indicate that a sharp drop in the density of states at the Fermi level, N(0), occurs when the A-15 structure is converted to the b.c.c. form.     

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.     

Superconductivity and structural transformation in HfRuAs

HfRuAs exists in two structural modifications. The high-temperature phase (ZrRuSi-type structure) exhibits superconductivity at 4.93-4.37 K, whereas the low-temperature phase (TiFeSi-type superstructure) remains normal at 1 K. The lattice distortion giving rise to the superstructure indicates zig-zag chains of metal atoms are crucial to superconductivity in this class of compounds.     

Electrical resistivity, Hall effect and superconductivity of some b.c.c. titanium-molybdenum alloys*

The electrical resistivities and the Hall coefficients of a series of body centered cubic Ti-Mo alloys have been measured between 1.1°K and room temperature, in magnetic fields up to 30 kG, and for concentrations of Mo between 7 and 24 atomic per cent. For the lower Mo concentration alloys the resistivities are high (≈ 140 microhm cm), and are slightly larger at 4.2°K than at 300°K. The resistivities decrease with increase of Mo over the entire range of alloy concentrations studied. The Hall coefficients are positive and are appreciably concentration and temperature dependent only below solute concentrations of about 12 atomic per cent Mo. The alloys are all superconducting at liquid helium temperatures, their resistive superconducting transition temperatures increasing with increase of Mo. Atomic ordering and structural transformation, concentration and temperature dependent electronic factors, and spin-disorder scattering are considered as possible explanations for the anomalous concentration and temperature dependence of the resistivity.     

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.     

Superconductivity and magnetism in dilute CdMn alloys. Strong depression in Tc

The superconducting transition temperature in dilute CdMn is depressed by 220K/at.% Mn. The Mn content was determined by Curie constant, resistance and atomic absorption measurements on the same sample. A small amount of the Mn is magnetically passive; the remaining Mn atoms have an effective magnetic moment of 4.80μ_B.     

Electronic properties of nonhomovalent alloys: A study of the b.c.c. Li−Mg solid solution

Summary The influence of the alloying on the electronic properties of the b.c.c. Li−Mg solid solution has been investigated within the KKR-CPA framework. Such an influence has been studied by computing the band structure, Fermi surface, soft X-ray spectra, residual resistivity and Auger spectra for chosen values of the composition. The theoretical results we obtained are in fair agreement with the surface existing experimental data and support the view that also in ?simple metals? alloys care has to be taken in studying such systems in terms of low-order perturbation approaches.

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Riassunto Sono state studiate varie proprietà elettroniche delle leghe di Li−Mg nell’intervallo di concentrazione in cui questi sistemi cristallizzano nel reticolo b.c.c. In particolare sono state calcolate la struttura a bande, le superfici di Fermi, la resistività residua, gli spettri X ed Auger e si sono confrontati i risultati con i dati sperimentali esistenti. Il buon accordo fra risultati teorici e dati sperimentali indica che, anche per leghe di metalli semplici, gli approcci perturbativi a basso ordine possono essere insufficienti a descrivere il comportamento di tali sistemi.

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Резюме В рамках KKR-CPA исследуется влияние процесса сплавления на электронные свойства объемноцетририованного кубического твердого раствора Li−Mg. Проводятся вычисления зонной структуры, поверхности Ферми, спектров мягкого рентгеновского излучения, остаточного сопротивления и Ож-ye-спектров для выбранных величин состава. Полученные теоретические величины хорошо согласуются с существуюими экспериментальными данными.

     

Of heat capacity at homologous temperatures for metals showing h.c.p. ⇄ b.c.c. transformation

Some interesting aspects of the temperature dependence of the Planck’s functionφ and heat capacities of metals exhibiting the h.c.p. ⇄ b.c.c. transformation have been brought to light by the use of reduced temperature (T*) and Planck’s function (-φ T*). It has been shown that tangents drawn to the -φ T* vsT* plots of these metals at any chosen value ofT* intersect at a point whose coordinates are defined by the slope and intercept ofφ vs entropy plots at any homologous temperature and the selectedT* value. A generalized expression obtained for the temperature dependence ofφ has been used to demonstrate that the heat capacity of these metals may be visualized to have structural and material components.     

Superconductivity and magnetic susceptibility of TiU alloys

The superconducting transition temperatures of $\text{TiU}$ alloys have been measured. They peak at a temperature of 3.8 K at 20 a/o U. Similar behavior is seen for $\text{Ti}$Cr, $\text{Ti}$Mo, and $\text{Ti}$W indicating that U in Ti behaves like a hexavalent transition metal and is not magnetic. Magnetic susceptibility measurements support this viewpoint.     

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.     

Superconductivity and electron-phonon interaction in TaV alloys

The electron-phonon coupling in TaV alloys is estimated from resistive measurements. It decreases rapidly with increasing V in Ta in qualitative agreement with the observed transition temperature, T_c. A calculation of T_c from the strong-coupling equation gives good agreement with observed values for a constant Coulomb pseudo-potential and one adjustable parameter.     

Interatomic forces in the b.c.c. transition metals

Using information obtained from X-ray scattering data, a model for the charge density in the transition metals is constructed. In this model the charge density is given by the sum of the charges situated at the ionic and bond centre sites. Within this framework, expressions for the second order elastic constants and the phonon frequencies in the cubic transition metals are obtained. These quantities have been calculated with and without the bond charge contributions for iron. The comparison with the experimental results shows that the introduction of bond charges, and hence angularity in the force-fields, in iron leads to distinctly better agreement with experiment. From a study of the elastic constants of the other b.c.c. transition metals some observations as to the nature of the angular force fields are made.     

Superconductivity of Hf-Ta and Ta-W alloys under pressure

Measurements on ten binary alloys of the body-centered cubic part of the 5d-alloy series Hf-Ta and Ta-W yield a linear relationship between the derivatives of the quantityg both with respect to pressure and to valence electron concentration, whereg is defined byT _c ∝ θ exp (?1/g). This result agrees with observations on alloys of the 4d-series, Zr-Nb and Nb-Mo. It is concluded that for these alloys composed from the central part of the 4d and 5d transition series, the interaction responsible for superconductivity is less subject to complications than for other transition metals and is probably governed by the electronic density of states. Data on the variation of the transition temperature with concentration in Hf-Ta alloys are included.     

Superconductivity of vapour quenched beryllium and beryllium-based alloys

The properties of Be films, quench-condensed upon a³He cooled substrate, have been investigated by resistance and tunneling measurements. The superconducting transition temperature,T _c , of Be films increased with thickness and a thick film limit of 9.95 K could be estimated. Alloying with Al or Pb decreasedT _c. The ratios between energy gaps andT _c 's indicated that Be is a weak coupling superconductor, and no phonon induced structure could be traced in tunneling curves neither in pure Be nor in the Be based alloys. Resistance change during annealing as well as superconducting data indicated that the vapour quenched Be films were amorphous as deposited.     

Superconductivity in singlet La-Pr and in nonmagnetic La-rare earth alloys

We show that concentration dependence of the superconducting transition temperature of d.h.c.p. $\text{La}$Pr alloys is almost identical to that of $\text{La}$Y and $\text{La}$Lu alloys. This suggests that Pr is in a singlet ground state and that lifetime broadening caused by the 4? electrons of Pr have little effect on the superconducting process in $\text{La}$Pr. The depression of the transition temperature arises largely from pair weakening (or possibly crystal field effects) rather than pair breaking effects.