Singularities in V3Si point-contact spectra

The V₃Si point-contact has been studied. The low-frequency mode of electron-phonon interaction has been observed by eV ～ 16 meV unknown previously. The “dirty” point-contacts exhibited the effects of the negative contribution to the dynamic electric resistance which might be the manifestation of the electron-electron correlations.     

Densities of phonon and electron states in V3Si and Cr3Si crystals

The regularization method has been used to solve the non-correct problem of finding out the phonon density of states from the temperature dependence of phonon heat capacity for two single crystals with A15 structure (the superconductor V₃Si and the non-superconductor Cr₃Si. The solution found agrees with the results of neutrongraphical and tunnel study. The electron density of states obtained in a narrow energy range near the Fermi level as the regularized solution of the reverse problem (from the temperature of the electron spin susceptibility) has at the Fermi energy the sharp maximum for V₃Si, but the flat minimum for Cr₃Si.     

The electronic structure of hypothetical Nb3Si and V3In

Self-consistent LMTO (Linear Muffin Tin Orbital) band calculations have been performed for Nb₃Si and V₃In. Calculated pressures show agreement with the results from existing Nb₃X and V₃X compounds, when the lattice parameters are 5.125 and 5.025 Å (± 0.025 Å) respectively. At these lattice dimensions, the superconducting transition temperatures are predicted to be approximately 18 K for Nb₃Si and 31 K for V₃In.     

Third order elastic constants of V3Si and V3Ge

General expressions have been derived for the second order elastic constants and third order elastic constants of the A-15 structure compounds with the nearest neighbour central interaction. The second order elastic constants, the third order elastic constants and the pressure derivatives of the second order elastic constants of V₃Si and V₃Ge are reported and compared with the available experimental measurements.     

Experimental proof for a quasi-one-dimensional band structure of V3Si

A positron annihilation experiment on V₃Si in the [100] direction is presented. It shows that the Fermi surface (FS) contains planar sections, proving the q-1-D nature of the electronic band structure. One possible way to account for the positions of the planar sections, is by a band structure with a FS at $\text{～ 0.9}\text{π}\text{a}$, belonging to the σ-band (m_l = 0) and a δ₂-band (m_l = ±2) with a density of states peak just below the Fermi level, which yields planar sections of the FS at $\text{～ 0.2}\text{π}\text{a}$ and $\text{～ 0.6}\text{π}\text{a}$, in agreement with a LCAO calculation.     

Electronic charge density of V3Si

The electronic charge density of V₃Si has been calculated using a self-consistent version of the linear-augmented-plane-wave (LAPW) method. The results exhibit a substantially nonspherical charge distribution at and beyond the V 3d-shell maximum, directional-bonding along the V-atom chains, and a net charge transfer from Si to V. While the two latter features are in qualitative agreement with the empirically-derived charge densities of Staudenmann et al., significant differences exist between the theoretical and empirical results.     

Grüneisen parameter and thermal expansion of V3Si and V3Ge

The Grüneisen parameter and lattice thermal expansion of the A-15 compounds V₃Si and V₃Ge at room temperature are evaluated on the basis of the method due to Brugger and Fritz [1] from the third order elastic constants reported earlier [2]. The calculated values are compared with available experimental values and are found to fit satisfactorily.     

The paramagnetic susceptability of neutron irradiated V3Ge single crystals

The paramagnetic susceptibility χ of V₃Ge single crystals was measured in the temperature range from 10 K to 350 K after irradiation with neutron fluences of $\text{(0.3?20) × 10}{}^{18}\text{n}\text{cm}{}^2\text{(E>1}\text{MeV}\text{)}$. Unirradiated crystals show a flat maximum at T=65±6 K, which disappears at high neutron doses. The temperature dependence and the absolute values of χ are continuously reduced by increasing neutron doses. This behaviour can be understood by the models of high density of states (DOS) proposed for A15 materials confirming the assumption that a sharp peak in the electronic density of states exists which is lowered and broadened by irradiation induced defects.     

Phonon density of states of V3Si

The phonon density of states of V₃Si at 300°K has been calculated from a microscopic model. It exhibits three pronounced peaks, in qualitative agreement with experimental results of Schweiss obtained from inelastic neutron scattering. By reducing the effective charges of both ions the agreement can be improved, but at the expense of agreement for the mode softening.     

Radiation damage in the non-metal sublattices of V3Ge and V3Si single crystals

Channeling measurements on V₃Ge and V₃Si single crystals, which have been irradiated with He-ions, revealed a considerable narrowing of the angular yield curves for the B-atom sublattices. Analysing the data with a computer simulation, the best agreement with the experiment was achieved by adding a static contribution to the thermal vibration amplitudes. After irradiation with He-fluences, which cause a saturation in the degradation of the superconducting transition temperature in the V₃Ge and V₃Si films, it was concluded that the average static displacement amplitude was 0.006 nm for the Ge-atoms and 0.01 nm for the Si-atoms.     

First order phase transition in V3Si

Thermal expansion measurements on a single crystal of V₃Si for the first time give evidence that the low temperature structural transition is of first order. The associated relative volume change is of the order of 10^-5.     

Photoelectron spectra of V3Si,a high Tc superconductor with a 15 structure

We present a high resolution (0.55 eV FWHM) overview of the valence band of V₃Si by X-ray induced photoelectronspectroscopy. A comparison of the spectrum with band structure calculations allows the determination of band energies at points of high symmetry. A new correlation between the core line asymmetry and the superconducting transition temperature has been established for Ti, V and V₃Si.     

Specific heat of a transforming V3Si crystal

The specific heat of a transforming V₃Si crystal in the normal, mixed, and superconducting states has been measured from 6 to 30K in zero and 52.3 kG magnetic fields. An analysis has been carried out in a self-consistent way based on the second-order phase transition from the normal to the superconducting state in zero magnetic field. Various physical parameters characterizing the superconducting and normal states are derived from the thermodynamics and the BCS weak coupling theory. The most important parameter obtained in this analysis is 2Δ(0)/kT_c = 3.46, which indicates a weak coupling in our V₃Si sample.     

Pressure dependence of positron lifetimes in V3Si

We have measured positron lifetimes in V₃Si at hydrostatic pressures of 0 to 20 kb, at room temperature. The lifetimes were independent of pressure, within the experimental uncertainty of ± 2 ps; the results of eight measurements all were between 129 and 133 ps.     

Heat capacity measurements and phase transitions in V3Si

The superconducting transition temperatures of both cubic and tetragonal phases of V₃Si have been observed unambigously for the first time, by heat capacity measurements based on steady state calorimetry technique. They are separated by about 1°K. In pressed powder compacts of V₃Si, another superconducting phase due to residual stress is found to occur with a transition temperature near 8°K.     

Comment on specific heat of a transforming V3Si crystal

At variance with results published recently in this journal, it is shown that a careful analysis of the specific heat of transforming samples leads to the following conclusions: (1) γ tetragonal < γ cubic; (2) the gap ratio is characteristic of moderately strong coupling.     

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.     

The electron distribution in superconducting alloys: I. Analysis of V3Si at room temperature

Accurate X-ray diffraction data are used to obtain the charge distribution in the superconducting alloy V₃Si. The main interaction is found to be strong covalent bonding between adjacent vanadium atoms within the infinite chains. Integration of the charge around the Si and V atoms leads to a best estimate of 1.8–2.4 electrons for the charge transfer from the silicon to the vanadium atoms.     

X-ray photoemission spectra of valence electrons in V3X and Nb3X compounds

The X-ray photoemission spectra (XPS) of the A15 type compounds V₃Au, Nb₃Os, Nb₃Ir, Nb₃Pt and Nb₃Au have been studied. The inner level binding energies of the different components and the valence electron distribution were measured. The Nb4d and the X5d energy bands of the Nb₃X compounds appear to be more and more separate with increasing atomic number of the X component. The comparison between the results from X-ray emission spectroscopy (XES) and X-ray photoelectron spectroscopy (XPS) of corresponding V₃X and Nb₃X compounds points out the similarity of their electronic structures.