Superconductivity and hybrid soft modes in TiSe$_2$

TL;DR

This study investigates the mechanisms behind superconductivity in TiSe₂, revealing phonon-mediated pairing in intercalated samples and a hybrid phonon-exciton mechanism under pressure, with implications for excitonic insulator states.

Contribution

It provides detailed experimental and theoretical analysis showing different pairing mechanisms in TiSe₂ depending on intercalation or pressure conditions.

Findings

Superconductivity in intercalated TiSe₂ is phonon-mediated.

Pressure-induced superconductivity involves hybrid phonon-exciton modes.

TiSe₂ under pressure is close to an excitonic insulator state.

Abstract

The competition between superconductivity and other ground states of solids is one of the challenging topics in condensed matter physics. Apart from high-temperature superconductors [1,2] this interplay also plays a central role in the layered transition-metal dichalcogenides, where superconductivity is stabilized by suppressing charge-density-wave order to zero temperature by intercalation [3] or applied pressure [4-7]. 1T-TiSe$_2$ forms a prime example, featuring superconducting domes on intercalation as well as under applied pressure. Here, we present high energy-resolution inelastic x-ray scattering measurements of the CDW soft phonon mode in intercalated Cu$_x$TiSe$_2$ and pressurized 1T-TiSe$_2$ along with detailed ab-initio calculations for the lattice dynamical properties and phonon-mediated superconductivity. We find that the intercalation-induced superconductivity can be explained by a solely phonon-mediated pairing mechanism, while this is not possible for the superconducting phase under pressure. We argue that a hybridization of phonon and exciton modes in the pairing mechanism is necessary to explain the full observed temperature-pressure-intercalation phase diagram. These results indicate that 1T-TiSe$_2$ under pressure is close to the elusive state of the excitonic insulator.