Search for magnetic field expulsion in optically driven K$_3$C$_{60}$

TL;DR

This study searches for transient Meissner diamagnetism in photoexcited K$_3$C$_{60}$ to determine if it exhibits non-equilibrium superconductivity, but current measurements do not conclusively confirm its presence due to experimental limitations.

Contribution

The paper provides an upper limit on diamagnetic response in photoexcited K$_3$C$_{60}$ and discusses the challenges in detecting transient superconductivity in this material.

Findings

No definitive evidence of Meissner diamagnetism was observed.

The experimental setup achieved a magnetic field resolution of ~50 nT.

Photo-induced phase shows weaker diamagnetism than zero-temperature superconducting K$_3$C$_{60}$.

Abstract

Photoexcited K$_3$C$_{60}$ displays several properties reminiscent of equilibrium superconductivity, including transient optical spectra, pressure dependence, and I-V characteristics. However, these observations do not decisively establish non-equilibrium superconductivity, which would be conclusively evidenced by transient Meissner diamagnetism, as shown recently in driven YBa$_2$Cu$_3$O$_{6.48}$. Here, we search for transient magnetic field expulsion in K$_3$C$_{60}$ by measuring Faraday rotation in a magneto-optic material placed in its vicinity. Unlike in the case of homogeneous, insulating YBa$_2$Cu$_3$O$_{6.48}$, inhomogeneous, metallic K$_3$C$_{60}$ powders reduce the size of the effect. With the $\sim50$ nT magnetic field resolution achieved in our experiments, we provide an upper limit for the photo-induced diamagnetic volume susceptibility ($\chi_v>-0.1$). On this basis, we conclude that the photo-induced phase has weaker diamagnetism than superconducting K$_3$C$_{60}$ at zero temperature. Yet, from recent nonlinear transport measurements in this granular material, we expect a light-induced state similar to the equilibrium superconductor near 0.8 T$_c$, for which $\chi_v>-0.1$. A definitive conclusion on the presence or absence of Meissner diamagnetism cannot be made for K$_3$C$_{60}$ with the current resolution.