Pressure effects on the unconventional superconductivity of noncentrosymmetric LaNiC2

Abstract

The unconventional superconductivity in the noncentrosymmetric LaNiC2, and its evolution with pressure, is analyzed basing on the ab initio computations and the full Eliashberg formalism. First principles calculations of the electronic structure, phonons and the electron-phonon coupling are reported in the pressure range 0-15 GPa. The thermodynamic properties of the superconducting state were determined numerically solving the Eliashberg equations. We found that already at p = 0 GPa, the superconducting parameters deviate from the BCS-type, and a large value of the Coulomb pseudopotential µ ⋆ = 0.22 is required to get the critical temperature Tc = 2.8 K consistent with experiment. If such µ ⋆ is used, the Eliashberg formalism reproduces also the experimentally observed values of the superconducting order parameter, the electronic specific heat jump at the critical temperature, and the change of the London penetration depth with temperature. This shows, that deviation of the above-mentioned parameters from the BCS predictions do not prejudge on the triplet or multiple gap nature of the superconductivity in this compound. Under the external pressure, calculations predict continuous increase of the electron-phonon coupling constant in the whole pressure range 0-15 GPa, consistent with the experimentally observed increase in Tc for the pressure range 0-4 GPa, but inconsistent with the drop of Tc above 4 GPa and the disappearance of the superconductivity above 7 GPa, reported experimentally. The disappearance of superconductivity may be accounted for by increasing the µ ⋆ to 0.36 at 7 GPa, which supports the hypothesis of the formation of a new high-pressure electronic phase, which competes with the superconductivity.

Unconventional superconductivity and an ambient-pressure magnetic quantum critical point in single-crystal LaNiC2

“…On the other hand, there is the idea that in LaNiC 2 other contributions to the driving mechanism are in play or that another high-pressure phase that competes with superconductivity may possibly exist. 25,29 This high-pressure phase could very well be the magnetic one found in the present work.…”

“…Here, Λ = N (E F ) < α 2 > /M < ω 2 >,ω D is the Debye frequency, Λ is the electron-phonon coupling constant, µ is the Coulomb pseudopotential, α is the electron-phonon matrix element, M is the atomic mass, and ω is the phonon frequency. Recent first-principles band-structure calculations under increasing external pressure in LaNiC 2 yielded a decrease in the density of states but an increase in Λ and, therefore, in T c 29. The numerical data, however, markedly fail to reproduce the entire T − P phase diagram, calling into question the pure electron-phonon interaction as the driving mechanism of superconductivity in this material.On the other hand, the sharpening of the superconducting transition with pressure may discard sample inhomogeneities as the origin of the broad transition at ambient pressure.This pressure-induced effect then opens the question about the physical source of the broadening.…”

Physical properties of hexagonal BaPtAs with noncentrosymmetric SrPtSb-type and centrosymmetric YPtAs-type crystal structures: Effects of spin-orbit coupling

“…This theoretical work [26] indicates that the impact on the electronic band structure of spin-orbit coupling (SOC) plus the absence of inversion is outstanding, and is much larger in the Pt compound due its heavier mass than that of Pd. First-principles calculations of the electronic structure, phonons, and the electronphonon coupling for LaNiC 2 have been carried out, while the thermodynamic properties of the superconducting state have been obtained numerically by solving the Eliashberg equations [27]. The results of this theoretical study [27] suggest that this noncentrosymmetric compound displays non-BCS superconducting properties, but with the electron-phonon coupling being the most likely the pairing mechanism.…”

“…First-principles calculations of the electronic structure, phonons, and the electronphonon coupling for LaNiC 2 have been carried out, while the thermodynamic properties of the superconducting state have been obtained numerically by solving the Eliashberg equations [27]. The results of this theoretical study [27] suggest that this noncentrosymmetric compound displays non-BCS superconducting properties, but with the electron-phonon coupling being the most likely the pairing mechanism. The electronic properties of noncentrosymmetric superconductor Ru 7 B 3 have been investigated by using the full potential linear muffin-tin orbital [15].…”

Nonunitary triplet pairing in the noncentrosymmetric superconductor LaNiC2

“…The normal state electronic structure was already published several times (see Refs. [46][47][48][49][50]). Hence, here we just briefly review the important properties.…”