Flux periodicities in loops and junctions with d-wave superconductors

Abstract

The magnetic flux periodicity in superconducting loops is reviewed. Whereas quantization of the magnetic flux with hc/2e prevails in sufficiently thick loops with current free interior, the supercurrent in narrow loops is either hc/2e- or hc/e-periodic with the external magnetic flux. The periodicity depends on the properties of the condensate state, in particular on the Doppler shift of the energy spectrum. For an s-wave superconductor in a loop with diameter larger than the coherence length ξ0, the Doppler shift is small with respect to the energy gap, and the hc/2e-periodic behavior of its flux dependent thermodynamic properties is maintained. However, for smaller s-wave loops and, more prominently, narrow d-wave loops of any diameter R, the Doppler shift has a strong effect on the supercurrent carrying state; as a consequence, the fundamental flux periodicity is in fact hc/e. It is shown analytically and numerically that the hc/e-periodic component in the supercurrent decays only algebraically as 1/R for large d-wave loops. For nodal superconductors the discrete nature of the eigenergies close to the Fermi energy has to be respected in the evaluation of the Doppler shift. Furthermore, we investigate, whether the Doppler shift modifies the supercurrent through Josephson junctions with d-wave superconductors. For transparent junctions, the Josephson current behaves similar to the persistent supercurrent in a loop. These distinct physical phenomena can be compared, if the magnetic flux Φ = φ ⋅ hc/e is identified with the phase variation of the order parameter δϕ through 2πφ = δϕ/2. Correspondingly, the Josephson current can display a 4π-periodicity in δϕ, if the Doppler shift is sufficiently strong which is true for transparent junctions of d-wave superconductors. Moreover, a 4π-periodicity is also valid for the current-flux relation of field-threaded junctions. In the tunneling regime the microscopic theory reproduces the results of the Ginzburg-Landau description for sufficiently wide Josephson junctions.