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The excitation of quasi-particles near the extrema of the electronic bandstructure is a gateway to electronic phase transitions in condensed matter. Ina many-body system, quasi-particle dynamics are strongly influenced by theelectronic single-particle structure and have been extensively studied in theweak optical excitation regime. Yet, under strong optical excitation, wherelight fields coherently drive carriers, the dynamics of many-body interactionsthat can lead to new quantum phases remain largely unresolved. Here, we inducesuch a highly non-equilibrium many-body state through strong optical excitationof charge carriers near the van Hove singularity in graphite. We investigatethe system's evolution into a strongly-driven photo-excited state withattosecond soft X-ray core-level spectroscopy. Surprisingly, we find anenhancement of the optical conductivity of nearly ten times the quantumconductivity and pinpoint it to carrier excitations in flat bands. Thisinteraction regime is robust against carrier-carrier interaction with coherentoptical phonons acting as an attractive force reminiscent of superconductivity.The strongly-driven non-equilibrium state is markedly different from thesingle-particle structure and macroscopic conductivity and is a consequence ofthe non-adiabatic many-body state.

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