Effect of the tetragonal distortion on the electronic structure, phonons and superconductivity in the Mo3Sb7 superconductor

Effect of tetragonal distortion on the electronic structure, dynamical properties and superconductivity in Mo$_3$Sb$_7$ is analyzed using first principles electronic structure and phonon calculations. Rigid muffin tin approximation (RMTA) and McMillan formulas are used to calculate the electron-phonon coupling constant $\lambda$ and superconducting critical temperature. Our results show, that tetragonal distortion has small, but beneficial effect on superconductivity, slightly increasing $\lambda$, and the conclusion that the electron-phonon mechanism is responsible for the superconductivity in Mo$_3$Sb$_7$ is supported. The spin-polarized calculations for the ordered (ferromagnetic or antiferromagnetic), as well as disordered (disordered local moment) magnetic states yielded non-magnetic ground state. We point out that due to its experimentally observed magnetic properties the tetragonal Mo$_3$Sb$_7$ might be treated as noncentrosymmetric superconductor, which could have influence for the pairing symmetry. In this context the relativistic band structure is calculated and spin-orbit interaction effects are discussed

Superconductivity near the Mott-Ioffe-Regel limit in the high-entropy alloy superconductor (ScZrNb)1−x_{1-x}(RhPd)x_x with a CsCl-type lattice

Theoretical analysis of the electronic structure of the high-entropy-type superconductor (ScZrNb)$_{1-x}$(RhPd)$_x$, $x \in (0.35, 0.45)$ is presented. The studied material is a partially ordered CsCl-type structure, with two sublattices, randomly occupied by Sc, Zr, Nb (first sublattice) and Nb, Rh, and Pd (second sublattice). Calculations were done using the Korringa-Kohn-Rostoker method with the coherent potential approximation (KKR-CPA) and take into account the substitutional disorder. Our total energy calculations confirm the preference for the partially ordered structure over the fully random {\it bcc}-type one. Electronic densities of states $N(E)$, dispersion relations, and McMillan-Hopfield parameters $\eta$ (electronic contribution to electron-phonon coupling) are studied as a function of composition. The computed increasing trends in $N(E_F)$ and $\eta$ with $x$ are opposite to what we expected based on the experimental results, where the decrease in the critical temperature with increasing $x$ was found. Very strong electron scattering due to disorder is observed, as the electronic dispersion relations are strongly smeared. As a result, the computed electronic lifetimes $\tau$ are very short, leading to a small mean-free path of electrons of the order of interatomic distance, which puts (ScZrNb)$_{1-x}$(RhPd)$_x$ near the Mott-Ioffe-Regel limit. The trend in $\tau(x)$ is similar to the trend observed experimentally in $T_c(x)$, suggesting that disorder may be the factor that influences $T_c$ in this series of alloys.Comment: 13 pages, 12 figures, accepted in Physical Review

Local distortions of the crystal structure and their influence on the electronic structure and superconductivity of the high-entropy alloy (TaNb)0.67(HfZrTi)0.33

Local distortions of the crystal structure and their influence on the electronic structure, electron-phonon interaction, and superconductivity are theoretically studied in the superconducting high-entropy alloy (HEA) (TaNb)$_{0.67}$(HfZrTi)$_{0.33}$. Distortions of the crystal lattice are caused by the relaxation of atomic positions and are studied in the twelve models of supercells. The largest relative changes in the interatomic distances due to relaxation reach 8\%. On average, local distortions tend to lower the density of states at the Fermi level and significantly reduce the electron-phonon coupling parameter $\lambda$. As a result, the calculated superconducting critical temperature is reduced to about 50\% of the initial value, which shows the strong impact of structural disorder on superconductivity in this prototype high-entropy alloy. With the reduced value of $\lambda$, the theoretical $T_c$ is closer to the experiment for typical values of the Columomb pseudopotential parameter $\mu^*$. The experimental $T_c$ can be reproduced by taking a slightly enhanced $\mu^* = 0.176$, which leaves little room for the possibility of suppression of superconductivity by disorder.Comment: 12 pages, 7 figures + Supplemental Material. Accepted for publication in Physical Review