Abstract
Optical excitation in the cuprates has been shown to induce transient superconducting correlations above the thermodynamic transition temperature , as evidenced by the terahertz-frequency optical properties in the nonequilibrium state. In , 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 , 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 photosusceptibility for this phenomenon. Here, we make use of a newly developed nonlinear optical device, which generates widely tunable high-intensity femtosecond pulses, to excite throughout the entire optical spectrum (3–750 THz). In the far-infrared region (3–24 THz), signatures of nonequilibrium superconductivity are induced only for excitation of the 16.4- and 19.2-THz vibrational modes that drive -axis apical oxygen atomic positions. For higher driving frequencies (25–750 THz), a second resonance is observed around the charge transfer band edge at approximately 350 THz. These findings highlight the importance of coupling to the electronic structure of the planes, mediated either by a phonon or by charge transfer.
- Received 19 May 2019
- Revised 13 January 2020
- Accepted 27 January 2020
DOI:https://doi.org/10.1103/PhysRevX.10.011053
Published by the American Physical Society under the terms of the Creative Commons Attribution 4.0 International license. Further distribution of this work must maintain attribution to the author(s) and the published article’s title, journal citation, and DOI. Funded by the Max Planck Society.
Published by the American Physical Society
Physics Subject Headings (PhySH)
Popular Summary
To control high-temperature superconductors on ultrafast timescales, researchers have been turning to optical stimulation with far-infrared laser pulses, enabling the observation of transient superconducting correlations above the thermodynamic transition temperature. In the layered copper oxide , such “incipient photoinduced superconductivity” has been associated with the large-amplitude excitation of certain optically active vibrational modes. However, similar effects in other compounds have been reported also for excitation at visible wavelengths, far from any vibrational resonance. To date, researchers have not been able to comprehensively compare the frequency-dependent photosusceptibility for this phenomenon because of a lack of a suitable optical source. Here, we present such a source and use it to probe the nature of photoinduced superconductivity in .
Our newly developed optical device generates widely tunable high-intensity femtosecond pulses. We use these pulses to excite across a broad spectrum of frequencies, from 3 to 750 THz. In the far-infrared region (3–25 THz), we find signatures of nonequilibrium superconductivity only with the excitation of two specific vibrational modes that involve motions of the apical oxygen atoms (those above or below the superconducting crystallographic planes). However, a second resonance is also reported here for excitation frequencies in the visible section of the electromagnetic spectrum, peaking at approximately 350 THz.
Our findings suggest that a coupling of the incident field to the electronic structure of the copper-oxygen planes—either mediated by a phonon vibration or by direct charge transfer—might be the determining factor to induce transient superconductivity in this compound.