First-principles calculations of pressure effects on the superconducting CaAlSi and SrAlSi

Pressure effects on the superconducting transition innH-CaAlSi

“…At P = 0, our phonon frequencies are all real, and even at higher pressures only a few become imaginary, while in Ref. 16 the whole out-of-plane branch becomes unstable. The most likely reason for this discrepancy is that we optimized the structural parameters, while the previous work assumed an isotropic compressibility.…”

“…13,14 The interplay of these two ingredients gives rise to intriguing effects both on the normal and the superconducting properties. The presence of an ultrasoft phonon mode at ∼7 meV was initially evidenced by first-principles calculations 11,12,15,16,17 and recently confirmed by neutron-scattering experiments. 17 The ultrasoft phonon modes are believed to induce strong eph coupling and to lead to an enhanced specific heat anomaly at T c as well as to a positive pressure dependence of T c in CaAlSi.…”

“…However, we will demonstrate here that the pressure dependence of T c of 1H-CaAlSi single crystals is markedly different from that of polycrystalline samples. Secondly, in these calculations 16 , the soft phonon branch in CaAlSi is unstable in some part of the brillouin zone (BZ), already at ambient pressure. This instability was not confirmed by recent ab-initio calculations and neutron scattering experiments.…”

“…16 for 1H−CaAlSi. At P = 0, our phonon frequencies are all real, and even at higher pressures only a few become imaginary, while in Ref.…”

Enhancing the Superconducting Transition Temperature ofBaSi2by Structural Tuning

“…Besides, a buckling of the boron honeycomb structure, as observed in ReB 2 [5], was proved to decrease the superconducting transition temperature T c . Nevertheless, recent studies on graphite intercalated superconductors, namely (Yb,Ca)C 6 [6], and the ternary silicide CaAlSi [7,8], characterized by large T c s, point to the importance in these systems of the electron-phonon (EP) coupling between the interlayer electrons and the outof-plane vibrational modes of the atoms composing the layer. In that case, the buckling phonon modes corresponding to the antiphase motion along the c-axis of the atoms in the sheets can lead to an enhancement of the EP coupling.…”