Pump frequency resonances for light-induced incipient superconductivity in YBa$_2$Cu$_3$O$_{6.5}$

TL;DR

This study systematically explores how different pump frequencies induce transient superconductivity in YBa$_2$Cu$_3$O$_{6.5}$, revealing resonances linked to specific lattice vibrations and electronic transitions using a novel tunable femtosecond laser source.

Contribution

It introduces a widely tunable high-intensity femtosecond laser system to systematically study frequency-dependent photo-induced superconductivity in cuprates.

Findings

Resonances at 16.4 and 19.2 THz excite specific lattice modes linked to superconductivity.

A second resonance at ~350 THz relates to the charge transfer band edge.

Coupling to electronic structure and lattice vibrations is crucial for light-induced superconductivity.

Abstract

Optical excitation in the cuprates has been shown to induce transient superconducting correlations above the thermodynamic transition temperature, $T_C$, as evidenced by the terahertz frequency optical properties in the non-equilibrium state. In YBa$_2$Cu$_3$O$_{6+x}$ this phenomenon has so far been associated with the nonlinear excitation of certain lattice modes and the creation of new crystal structures. In other compounds, like La$_{2-x}$Ba$_x$CuO$_4$, similar effects were reported also for excitation at near infrared frequencies, and were interpreted as a signature of the melting of competing orders. However, to date it has not been possible to systematically tune the pump frequency widely in any one compound, to comprehensively compare the frequency dependent photo-susceptibility for this phenomenon. Here, we make use of a newly developed optical parametric amplifier, which generates widely tunable high intensity femtosecond pulses, to excite YBa$_2$Cu$_3$O$_{6.5}$ throughout the entire optical spectrum (3 - 750 THz). In the far-infrared region (3 - 25 THz), signatures of non-equilibrium superconductivity are induced only for excitation of the 16.4 THz and 19.2 THz vibrational modes that drive $c$-axis apical oxygen atomic positions. For higher driving frequencies (25 - 750 THz), a second resonance is observed around the charge transfer band edge at ~350 THz. These observations highlight the importance of coupling to the electronic structure of the CuO$_2$ planes, either mediated by a phonon or by charge transfer.