Universal approach to light driven "superconductivity" via preformed pairs

TL;DR

This paper proposes a universal mechanism for light-induced superconductivity in strongly paired materials, where radiation redistributes fermions to enhance pairing correlations and produce a transient state with superconducting-like properties.

Contribution

It applies Eliashberg theory to a pseudogap phase, explaining light-induced superconductivity through fermion redistribution and fluctuation theory, providing a microscopic understanding of observed phenomena.

Findings

Photoinduced state shows enhanced fermion pairing.

Optical conductivity exhibits a $1/\omega$ dependence in $\sigma_2$.

Light-induced phase has short-range coherence without long-range order.

Abstract

While there are many different mechanisms which have been proposed to understand the physics behind light induced ``superconductivity", what seems to be common to the class of materials in which this is observed are strong pairing correlations, which are present in the normal state. Here we argue, that the original ideas of Eliashberg are applicable to such a pseudogap phase and that with exposure to radiation the fermions are redistributed to higher energies where they are less deleterious to pairing. What results then is a photo-induced state with dramatically enhanced number of nearly condensed fermion pairs. In this phase, because the a.c. conductivity, $\sigma(\omega) = \sigma_1(\omega) + i \sigma_2(\omega)$, is dominated by the bosonic contribution, it can be computed using conventional (Aslamazov Larkin) fluctuation theory. We, thereby, observe the expected fingerprint of this photoinduced ``superconducting" state which is a $1/\omega$ dependence in $\sigma_2$ with fits to the data of the same quality as found for the so-called photo-enhanced (Drude) conductivity scenario. Here, however, we have a microscopic understanding of the characteristic low energy scale which appears in transport and which is necessarily temperature dependent. This approach also provides insight into recent observations of concomitant diamagnetic fluctuations. Our calculations suggest that the observed light-induced phase in these strongly paired superconductors has only short range phase coherence without long range superconducting order.