Effective charges and statistical signatures in the noise of normal metal–superconductor junctions at arbitrary bias

Abstract

Shot noise is studied in a single normal metal--superconductor (NS) junction at finite frequency, and for branched NS junctions at zero frequency. The noise spectral density displays a singularity at the Josephson frequency $(\ensuremath{\omega}=2eV/\ensuremath{\Elzxh})$ when the applied bias is smaller than the gap of the superconductor. Yet, in the limit $\mathrm{eV}\ensuremath{\gg}\ensuremath{\Delta},$ quasiparticle contributions yield a singularity at $\ensuremath{\omega}=eV/\ensuremath{\Elzxh}$ analogous to that of a normal metal. The crossover between these two regimes shows new structures in the noise characteristic, pointing out the failure of the effective charge model. As an alternative to a finite-frequency measurement, if a sinusoidal external field is superposed to the constant bias (nonstationary Aharonov-Bohm effect), the second derivative of the zero-frequency noise with respect to the voltage exhibits peaks when the frequency of the perturbation is commensurate with the Josephson frequency. Finally, the statistical aspects of noise are studied with an analog of the Hanbury-Brown and Twiss experiment for fermions: a superconductor connected to two normal leads. Noise correlations are found to be either negative (fermionic) or positive (bosonic), due to the presence of evanescent Cooper pairs in the normal side of the junction, in the latter case.