# Transiently enhanced interlayer tunneling in optically driven high $T_c$ superconductors

## TL;DR

This paper develops a theoretical model to explain the transient enhancement of interlayer tunneling in high-temperature superconductors induced by optical pulses, aligning with recent experimental observations and predicting larger effects under specific driving conditions.

## Contribution

The paper introduces a pump-probe formalism for Josephson junction stacks to explain transient tunneling enhancement, highlighting the role of parametric driving near plasma frequencies.

## Key findings

- Transient Josephson coupling can be enhanced with pulsed optical excitation.
- Maximum effect occurs when driven above the plasma frequency.
- Model reproduces experimental features below T_c.

## Abstract

Recent pump-probe experiments reported an enhancement of superconducting transport along the $c$-axis of underdoped YBa$_2$Cu$_3$O$_{6+\delta}$ (YBCO), induced by a mid-infrared optical pump pulse tuned to a specific lattice vibration. To understand this transient non-equilibrium state, we develop a pump-probe formalism for a stack of Josephson junctions, and we consider the tunneling strengths in presence of modulation with an ultrashort optical pulse. We demonstrate that a transient enhancement of the Josephson coupling can be obtained for pulsed excitation and that this can be even larger than in a continuously driven steady-state. Especially interesting is the conclusion that the effect is largest when the material is parametrically driven at a frequency immediately above the plasma frequency, in agreement with what is found experimentally. For bilayer Josephson junctions, an enhancement similar to that experimentally is predicted below the critical temperature $T_c$. This model reproduces the essential features of the enhancement measured below $T_c$. To reproduce the experimental results above $T_c$, we will explore extensions of this model, such as in-plane and amplitude fluctuations, elsewhere.