Interfacial superconductivity in Cu/Cu2O and its effect on shielding ambient electric fields

Abstract

A model is presented for two-dimensional superconductivity at semiconductor-on-metal interfaces mediated by Coulomb interactions between electronically-active interface charges in the semiconductor and screening charges in the metal. The junction considered is native Cu₂O on Cu in which an interfacial double charge layer of areal density n, comprising superconducting holes in Cu₂O and mediating electrons in Cu, is induced in proportion to a sub-monolayer of adsorbed ⁴He atoms. Evidence for superconductivity on copper with prior air exposure is revealed in new analysis of previously published work function data. Based on a theory developed for layered superconductors, the intrinsic transition temperature T_C = β n^1/2/ζ is determined by n and transverse distance ζ ≃ 2.0 Å between the charge layers; β = 1.933(6) e²ƛ_C/k_B = 1247.4(3.7) K-Ų is a universal constant involving the reduced Compton wavelength of the electron ƛ_C. This model is applied to understanding the shielding of copper work-function patch and gravitational compression electric fields reported in the Witteborn-Fairbank gravitational electron free fall experiment. Interfacial superconductivity with n ≃ 1.6 × 10¹² cm⁻², T_C ≃ 7.9 K and Berezinskiĭ-Kosterlitz-Thouless temperature T_BKT ≃ 4.4 K accounts for the shielding observed at temperature T ≃ 4.2 K. Helium desorption and concomitant decreases in n and T_C replicate the temperature transition in ambient electric fields on falling electrons, as observed by Lockhart et al., and the vanishing of superconductivity above T ≃ 4.8 K.