Theory of enhanced interlayer tunneling in optically driven high $T_c$ superconductors

TL;DR

This paper demonstrates that optical driving can enhance interlayer Josephson coupling in high-temperature superconductors by inducing a non-equilibrium state, explaining recent experimental observations of improved c-axis transport.

Contribution

It introduces a theoretical mechanism showing how optical pulses can enhance Josephson coupling in high-Tc superconductors through parametric driving, extending to bilayer systems.

Findings

Enhanced imaginary part of low-frequency conductivity under optical drive.

Josephson coupling increases when pump frequency exceeds plasma frequency.

Emergence of a genuine non-equilibrium superconducting state.

Abstract

Motivated by recent pump-probe experiments indicating enhanced coherent $c$-axis transport in underdoped YBCO, we study Josephson junctions periodically driven by optical pulses. We propose a mechanism for this observation by demonstrating that a parametrically driven Josephson junction shows an enhanced imaginary part of the low-frequency conductivity when the driving frequency is above the plasma frequency, implying an effectively enhanced Josephson coupling. We generalize this analysis to a bilayer system of Josephson junctions modeling YBCO. Again, the Josephson coupling is enhanced when the pump frequency is blue-detuned to either of the two plasma frequencies of the material. We show that the emergent driven state is a genuine, non-equilibrium superconducting state, in which equilibrium relations between the Josephson coupling, current fluctuations, and the critical current no longer hold.