Transient superconductivity without superconductivity

TL;DR

This paper offers an alternative explanation for transient superconductivity-like signals observed after THz pulses, suggesting they result from non-equilibrium states with negative conductivity and spontaneous polarization, not actual superconductivity.

Contribution

It introduces a phenomenological model where negative conductivity induces a non-equilibrium state with coupled oscillations, explaining experimental observations without requiring true superconductivity.

Findings

Negative conductivity leads to instability and spontaneous polarization.

Coupled oscillations of entropy and charge modify reflectivity.

Predicted dependencies on polarization and incidence angle.

Abstract

Recent experiments on K$_3$C$_{60}$ and layered copper-oxide materials have reported substantial changes in the optical response following application of an intense THz pulse. These data have been interpreted as the stimulation of a transient superconducting state even at temperatures well above the equilibrium transition temperature. We propose an alternative phenomenology based on the assumption that the pulse creates a non-superconducting, though non-equilibrium situation in which the linear response conductivity is negative. The negative conductivity implies that the spatially uniform pre-pulse state is unstable and evolves to a new state with a spontaneous electric polarization. This state exhibits coupled oscillations of entropy and electric charge whose coupling to incident probe radiation modifies the reflectivity, leading to an apparently superconducting-like response that resembles the data. Dependencies of the reflectivity on polarization and angle of incidence of the probe are predicted and other experimental consequences are discussed.