[Paper Review] Metastable Photo-Induced Superconductivity far above $T_{ extrm{c}}$

Sambuddha Chattopadhyay, Christian Eckhardt+7 more|arXiv (Cornell University)|Mar 27, 2023

Molecular Junctions and Nanostructures4 citations

TL;DR

This paper proposes two distinct microscopic mechanisms for long-lived, photo-induced superconductivity in K₃C₆₀ above its equilibrium transition temperature Tc. It demonstrates that laser-driven displacive shifts of Raman phonons can enhance electron pairing interactions, leading to metastable superconductivity via either (1) trapped structural distortions or (2) slow relaxation of quasiparticles, both of which explain the nanosecond-scale persistence observed experimentally.

ABSTRACT

Inspired by the striking discovery of metastable superconductivity in $\mathrm{K}_3\mathrm{C}_{60}$ at 100K, far above $T_{ extrm{c}}=20K$, we discuss possible mechanisms for long-lived, photo-induced superconductivity. Starting from a model of optically-driven Raman phonons coupled to inter-band electronic transitions, we develop a microscopic mechanism for photo-controlling the pairing interaction. Leveraging this mechanism, we first investigate long-lived superconductivity arising from the thermodynamic metastable trapping of the driven phonon. We then propose an alternative route, where the superconducting gap created by an optical drive leads to a dynamical bottleneck in the equilibration of quasi-particles. We conclude by discussing implications of both scenarios for experiments that can be used to discriminate between them. Our work provides falsifiable explanations for the nanosecond-scale photo-induced superconductivity found in $\mathrm{K}_3\mathrm{C}_{60}$, while simultaneously offering a theoretical basis for exploring metastable superconductivity in other quantum materials.

Motivation & Objective

• To provide a microscopic, falsifiable explanation for the experimentally observed nanosecond-scale metastable superconductivity in K₃C₆₀ under mid-infrared irradiation.
• To address the central challenge of achieving long-lived superconductivity far above Tc, where thermal equilibrium would normally suppress such states.
• To develop a minimal model of optically driven Raman phonons coupled to inter-band electronic transitions to capture the essential physics of light-induced pairing.
• To distinguish between two distinct physical pathways—structural trapping and slow quasiparticle relaxation—that could explain the long-lived superconducting response.
• To provide testable predictions for experimental discrimination between the two proposed mechanisms in non-equilibrium superconducting systems.

Proposed method

• Formulates a two-band model with a Raman-active phonon mode coupled to inter-band electronic transitions, using K₃C₆₀ as a prototypical system.
• Applies the Schrieffer-Wolff transformation to derive an effective phonon-mediated electron pairing interaction, focusing on second-order terms in the electron-phonon coupling.
• Uses a Gaussian state approximation to decouple the Hamiltonian, enforcing momentum conservation and enabling analytical treatment of the effective pairing interaction.
• Computes the adiabatic free energy landscape as a function of the phonon displacement to identify metastable superconducting states at finite distortion.
• Analyzes the non-equilibrium dynamics by considering the slow equilibration of quasiparticles, which sustains a long-lived superconducting gap after laser excitation.
• Derives the effective pairing Hamiltonian in the local limit, showing that the strength of the induced attraction scales with the square of the phonon displacement and the inverse of the phonon frequency.

Experimental results

Research questions

• RQ1 Can a laser-induced structural distortion of a Raman phonon mode lead to a long-lived enhancement of electron pairing in a superconductor?
• RQ2 What are the distinct microscopic mechanisms that could sustain superconductivity for over 10 nanoseconds in K₃C₆₀, far above its equilibrium Tc?
• RQ3 How does the slow relaxation of quasiparticles contribute to the persistence of a non-thermal superconducting gap after optical excitation?
• RQ4 What experimental signatures can differentiate between a mechanism based on trapped structural distortions versus one based on slow quasiparticle equilibration?
• RQ5 Can the effective pairing interaction be tuned via photo-control of phonon coordinates, and what are the conditions under which this leads to metastable superconductivity?

Key findings

• The effective pairing interaction is enhanced by a laser-induced static shift of the Raman phonon coordinate, leading to a photo-enhanced attractive interaction between electrons.
• Metastable superconductivity can be stabilized at finite phonon displacement far above Tc, as confirmed by the adiabatic free energy landscape showing local minima at non-zero distortions.
• The effective pairing Hamiltonian derived via the Schrieffer-Wolff transformation shows that the pairing strength scales as λ²Q²/(Mω_ph²), where Q is the phonon displacement and ω_ph is the phonon frequency.
• In the local limit, the effective pairing interaction becomes proportional to Q², with the strength tunable via the laser-driven phonon amplitude.
• The model predicts that the superconducting gap can persist for nanoseconds due to either (1) structural trapping or (2) slow quasiparticle relaxation, both of which are consistent with experimental observations.
• The two proposed mechanisms lead to distinct dynamical signatures: structural trapping predicts a long-lived, static distortion, while slow relaxation predicts a time-dependent gap evolution, offering testable experimental discrimination.