# Cooling quasiparticles in A$_3$C$_{60}$ fullerides by excitonic mid-infrared absorption

## TL;DR

This paper proposes a new mechanism involving excitonic mid-infrared absorption in A$_3$C$_{60}$ fullerides, explaining how IR-induced excitons can cool quasiparticles and enable transient superconductivity at higher temperatures.

## Contribution

It introduces a novel excitonic mechanism involving a super-exciton in A$_3$C$_{60}$, providing a new explanation for IR-induced transient superconductivity.

## Key findings

- Identification of a broad IR absorption peak as a super-exciton involving $t_{1u}$ to $t_{1g}$ electronic transition.
- Proposal that IR-induced excitons act as a cooling mechanism, extending superconductivity to higher temperatures.
- Explanation of the broad absorption feature's origin, challenging previous phonon-based interpretations.

## Abstract

Long after its discovery superconductivity in alkali fullerides A$_3$C$_{60}$ still challenges conventional wisdom. The freshest inroad in such ever-surprising physics is the behaviour under intense infrared (IR) excitation. Signatures attributable to a transient superconducting state extending up to temperatures ten times higher than the equilibrium $T_c\sim$ 20 K have been discovered in K$_3$C$_{60}$ after ultra-short pulsed IR irradiation -- an effect which still appears as remarkable as mysterious. Motivated by the observation that the phenomenon is observed in a broad pumping frequency range that coincides with the mid-infrared electronic absorption peak still of unclear origin, rather than to TO phonons as has been proposed, we advance here a radically new mechanism. First, we argue that this broad absorption peak represents a "super-exciton" involving the promotion of one electron from the $t_{1u}$ half-filled state to a higher-energy empty $t_{1g}$ state, dramatically lowered in energy by the large dipole-dipole interaction acting in conjunction with Jahn Teller effect within the enormously degenerate manifold of $\big(t_{1u}\big)^2\big(t_{1g}\big)^1$ states. Both long-lived and entropy-rich because they are triplets, the IR-induced excitons act as a sort of cooling mechanism that permits transient superconductive signals to persist up to much larger temperatures.