Lifshitz transition enhanced triplet pz -wave superconductivity in hydrogen-doped KCr3As3

Abstract

The recently synthesized air-insensitive hydrogen-doped ${\mathrm{KCr}}_{3}{\mathrm{As}}_{3}$ superconductor has aroused great research interest. This material has, for the first time in the research area of the quasi-one-dimensional (quasi-1D) Cr-based superconductivity (SC), realized a tunability through charge doping, which will potentially significantly push the development of this area. Here, based on the band structure from first-principles calculations, we construct a six-band tight-binding model equipped with multiorbital Hubbard interactions and adopt the random-phase-approximation approach to study the hydrogen-doping dependence of the pairing symmetry and superconducting ${T}_{c}$. Under the rigid-band approximation, our pairing phase diagram is occupied by the triplet ${p}_{z}$-wave pairing throughout the hydrogen-doping regime $x\ensuremath{\in}(0.4,1)$ in which SC has been experimentally detected. Remarkably, the $x$ dependence of ${T}_{c}$ shows a peak at the three-dimensional--quasi-1D Lifshitz transition point, although the total density of states exhibits a dip there. The corresponding doping level is near the experimental estimation of the optimal doping level. A thorough investigation of the band structure reveals type-II van Hove singularities (VHSs) in the $\ensuremath{\gamma}$ band, which favor the formation of the triplet SC. It turns out that the $\ensuremath{\gamma}$ Fermi surface (FS) comprises two flat quasi-1D FS sheets almost parallel to the ${k}_{z}=0$ plane and six almost perpendicular tubelike FS sheets, and the type-II VHS just lies in the boundary between these two FS parts. Furthermore, the $|{k}_{z}|$ of the van Hove planes reaches the maximum near the Lifshitz-transition point, which pushes the ${T}_{c}$ of the ${p}_{z}$-wave SC to the maximum. Our results appeal for more experimental access into this intriguing superconductor.