Thermodynamic parameters of atomically thin superconductors derived from the upper critical field

Abstract

The amplitude of the ground-state superconducting energy gap Δ(0) and relative jump in the electronic specific heat at the transition temperature are the primary fundamental parameters of any superconductor. Several well-established techniques are available for measuring these values in bulk samples. However, a limited number of techniques can be used to measure these parameters in atomically thin superconductors. Here, we propose a new approach for extracting Δ(0) and in atomically thin superconductors by utilizing the upper critical field data from perpendicular, B c2,Δ(T) (when a magnetic field is applied perpendicular to the film surface) and parallel, B c2,‖(T) (when a magnetic field is applied in the direction parallel to the film surface), external field directions. The deduced parameters for few-layer-thick Al, Sn, NbSe2, MoS2, magic angle twisted trilayer graphene, and WTe2 are well-matched values expected for strongly and moderately strongly coupled electron-phonon-mediated superconductors. In many reports, the enhancement of B c2,‖(0) above the Pauli–Clogston–Chandrasekhar limiting field (i.e. magnetic field required to break the Cooper pair) in atomically thin superconductors has been explained based on the assumption of exotic pairing mechanisms, for instance, Ising-type pairing. Here, we explain the observed B c2,‖(0) enhancement based on the geometrical enhancement factor that originates from the sample geometry. This approach does not assume the existence of novel exotic pairing mechanisms in atomically thin superconductors.