Enhanced optical conductivity and many-body effects in strongly-driven photo-excited semi-metallic graphite

TL;DR

This paper explores how strong optical excitation in graphite induces a non-equilibrium many-body state, significantly enhancing optical conductivity and revealing complex interactions beyond single-particle effects, with potential implications for quantum phase control.

Contribution

It demonstrates the induction of a highly non-equilibrium many-body state in graphite through strong optical excitation, revealing enhanced conductivity and complex carrier-phonon interactions.

Findings

Optical conductivity increases nearly tenfold due to carrier excitations in flat bands.

The non-equilibrium state differs markedly from the single-particle structure.

Carrier-phonon interactions suggest a superconductivity-like attraction.

Abstract

The excitation of quasi-particles near the extrema of the electronic band structure is a gateway to electronic phase transitions in condensed matter. In a many-body system, quasi-particle dynamics are strongly influenced by the electronic single-particle structure and have been extensively studied in the weak optical excitation regime. Yet, under strong optical excitation, where light fields coherently drive carriers, the dynamics of many-body interactions that can lead to new quantum phases remain largely unresolved. Here, we induce such a highly non-equilibrium many-body state through strong optical excitation of charge carriers near the van Hove singularity in graphite. We investigate the system's evolution into a strongly-driven photo-excited state with attosecond soft X-ray core-level spectroscopy. Surprisingly, we find an enhancement of the optical conductivity of nearly ten times the quantum conductivity and pinpoint it to carrier excitations in flat bands. This interaction regime is robust against carrier-carrier interaction with coherent optical phonons acting as an attractive force reminiscent of superconductivity. The strongly-driven non-equilibrium state is markedly different from the single-particle structure and macroscopic conductivity and is a consequence of the non-adiabatic many-body state.