Pressure-driven evolution of upper critical field and Fermi surface reconstruction in the strong-coupling superconductor Ti4Ir2O

Abstract

We report pressure-driven evolution of the superconducting transition temperature (${T}_{\mathrm{c}}$) and upper critical field ${B}_{\mathrm{c}2}$ $(0)$ of strong-coupling superconductor ${\mathrm{Ti}}_{4}{\mathrm{Ir}}_{2}\mathrm{O}$ that possesses an unusually large ${B}_{\mathrm{c}2} (0)$ at ambient pressure (AP). Our results reveal an extremely low-pressure coefficients of ${T}_{\mathrm{c}}$, i.e., $d{T}_{c}/dP\ensuremath{\approx}\ensuremath{-}0.047$ K/GPa for $P<15$ GPa and \ensuremath{-}0.017 K/GPa for $15<P<50$ GPa, presumably associated with an inherent large bulk modulus of $\ensuremath{\sim}252$ GPa. Interestingly, we find that its ${B}_{\mathrm{c}2}$ $(0)$ undergoes a smooth crossover at 35.6 GPa from well beyond to less than the Pauli paramagnetic limit ${B}_{\mathrm{p}}^{\mathrm{BCS}}(0)=1.84\phantom{\rule{0.16em}{0ex}}{T}_{c}$; i.e., the ${B}_{\mathrm{c}2}(0)\ensuremath{\approx}18.2 \mathrm{T}=1.7\phantom{\rule{0.16em}{0ex}}{B}_{\mathrm{p}}^{\mathrm{BCS}} (0)$ at AP decreases to $\ensuremath{\sim}5.8 \mathrm{T}=0.88\phantom{\rule{0.16em}{0ex}}{B}_{\mathrm{p}}^{\mathrm{BCS}} (0)$ at 50 GPa. The density-functional calculations predict the possible occurrence of pressure-induced Fermi surface reconstruction for the energy bands near the $K$ point with strong spin-orbit coupling in 31--41 GPa. The present work sheds more light on this intriguing superconductor capable of resisting large external compression and strong magnetic fields.