Optically induced lattice deformations, electronic structure changes, and enhanced superconductivity in YBa2Cu3O6.48

TL;DR

This study combines experimental and theoretical approaches to show that optical excitation induces structural and electronic changes in underdoped YBa2Cu3O6.48, leading to transient superconducting-like properties and enhanced interlayer coupling.

Contribution

It provides a theoretical prediction of electronic rearrangements caused by optically induced lattice deformations, supported by experimental x-ray absorption measurements.

Findings

Transient superconducting-like optical properties observed.

Predicted increase in hole-doping and interlayer Josephson coupling.

Experimental x-ray data consistent with theoretical predictions.

Abstract

Resonant optical excitation of apical oxygen vibrational modes in the normal state of underdoped YBa2Cu3O6+x induces a transient state with optical properties similar to those of the equilibrium superconducting state. Amongst these, a divergent imaginary conductivity and a plasma edge are transiently observed in the photo-stimulated state. Femtosecond hard x-ray diffraction experiments have been used in the past to identify the transient crystal structure in this non-equilibrium state. Here, we start from these crystallographic features and theoretically predict the corresponding electronic rearrangements that accompany these structural deformations. Using density functional theory, we predict enhanced hole-doping of the CuO2 planes. The empty chain Cu dy2-z2 orbital is calculated to strongly reduce in energy, which would increase c-axis transport and potentially enhance the interlayer Josephson coupling as observed in the THz-frequency response. From these calculations, we predict changes in the soft x-ray absorption spectra at the Cu L-edge. Femtosecond x-ray pulses from a free electron laser are used to probe these changes in absorption at two photon energies along this spectrum, and provide data consistent with these predictions.