Field-Orientation Dependence of Low-Energy Quasiparticle Excitations in the Heavy-Electron Superconductor UBe13

TL;DR

This study reveals that low-energy quasiparticle excitations in UBe13 exhibit a fully open superconducting gap with unexpected linear magnetic-field dependence and a cubic anisotropy, shedding light on its unconventional superconducting properties.

Contribution

The paper provides new insights into the gap structure and anisotropic quasiparticle response in UBe13 through specific-heat measurements under rotating magnetic fields.

Findings

Linear $C(H)$ dependence at low fields indicates a fully open gap.

Cubic anisotropy of $C(H)$ observed above 2 T.

Anisotropic response linked to heavy quasiparticle bands.

Abstract

Low-energy quasiparticle excitations in the superconducting (SC) state of UBe$_{13}$ were studied by means of specific-heat ($C$) measurements in a rotating field. Quite unexpectedly, the magnetic-field dependence of $C(H)$ is linear in $H$ with no angular dependence at low fields in the SC state, implying that the gap is fully open over the Fermi surfaces, in stark contrast to the previous expectation. In addition, a characteristic cubic anisotropy of $C(H)$ was observed above 2~T with a maximum (minimum) for $H$ $||$ $[001]$ ($[111]$) within the $(1\bar{1}0)$ plane, both in the normal as well as in the SC states. This oscillation possibly originates from the anisotropic response of the heavy quasiparticle bands, and might be a key to understand the unusual properties of UBe$_{13}$.