Antonio Santacesaria | Searcharxiv

arXiv2026

Two-phonon pairing and superconductivity in $SrTiO_3$

We explore the possibility that the two-phonon pairing mechanism proposed by Ngai can explain superconductivity in doped strontium titanate (STO). The two-phonon deformation potential is evaluated using two distinct theoretical estimates based on first-principles calculations and two empirical estimates based on experimental data, yielding very large values of the same order of magnitude. We derive the effective electron-electron interaction mediated by two-phonon exchange. Crucial to our computations is the strong dispersion of the involved soft phonons. Because the scale of the two-phonon interaction is much larger than the Fermi energy, the Migdal theorem does not apply. We obtain a one-loop Eliashberg equation that can be solved in the weak-coupling limit. We find that despite the significant electron-phonon matrix element, phase space considerations considerably reduce the effective coupling. A two-phonon mechanism can not be excluded based on the $T_c$ magnitude, but its value is dominated by the high-energy-frequency physics and is weakly affected by the soft-phonon behavior. The theory predicts a $T_c$ that monotonously increases with doping, which is not in accordance with the experiment. We identify the conditions for a two-phonon mechanism to be effective in other materials and discuss possible candidates.

cond-mat.supr-con↗

arXiv2025

Ternary nickel hydrides: a new platform for unconventional superconductivity and quantum magnetism

In this letter, we propose ternary nickel hydrides MNiH2 (M = Li, Na) as new materials that mimic cuprate physics but have important differences and interesting properties. Ni-H bands are wider than in oxides due to shorter bond lengths and covalency is larger than in Ni oxides which leads to a large scale of magnetic interactions. The charge transfer energy is smaller than in LaNiO2 which in cuprates translates to a larger Tc. We notice the formation of the electride band close to the Fermi surface which appears due to H vacancy along the c lattice vector. The considerable difference with cuprates arises from dz2 orbitals hybridization with interstitial orbitals allowing charge transfer to an apical vacancy state and self-doping the cuprate like Ni dx2-y2 H s antibonding bands which suggests that stoichiometric NaNiH2 may already be metallic and superconducting.

cond-mat.str-el↗