Conventional s-wave superconductivity and hidden peak effect in single crystals of Mo$_8$Ga$_41$ superconductor

TL;DR

This study investigates the superconducting properties of Mo$_8$Ga$_{41}$ crystals, revealing conventional s-wave behavior and a novel hidden peak effect in vortex pinning under field-cooling conditions.

Contribution

It provides the first detailed measurement of London and Campbell penetration depths in Mo$_8$Ga$_{41}$ and uncovers a previously unobserved hidden peak effect in vortex dynamics.

Findings

Superfluid density matches BCS s-wave theory without second gap.

Hysteresis in Campbell length indicates anharmonic vortex pinning.

Hidden peak effect observed in field-cooling protocol.

Abstract

London and Campbell penetration depths were measured in single crystals of the endohedral gallide cluster superconductor, Mo$_{8}$Ga$_{41}$. The full temperature range superfluid density is consistent with the clean isotropic $s-$wave weak-coupling BCS theory without any signs of the second gap or strong coupling. The temperature dependence of the Campbell length is hysteretic between zero-field cooling (ZFC) and field-cooling (FC) protocols, indicating an anharmonic vortex pinning potential. The field dependence of the effective critical current density, $j_{c}\left(H\right)$, reveals an unusual result. While in the ZFC protocol, $j_{c}\left(H\right)$ is monotonically suppressed by the magnetic field, it exhibits a profound ``hidden'' peak effect in the FC protocol, that is, without a vortex density gradient. We suggest a possible novel mechanism for the formation of the peak effect, which involves both static and dynamic aspects.