Theory of anomalous proximity effects in phase-coherent structures.

Abstract

We present theoretical results for the change \ensuremath{\delta}G in the electrical conductance G of a mesoscopic sample due to the switching on of superconductivity. Due to competition between normal and Andreev scattering, the sign of \ensuremath{\delta}G depends in detail on the impurity configuration within a device. In contrast with universal conductance fluctuations, we demonstrate that \ensuremath{\delta}G can scale with the system size and therefore, as well as being negative, can have a magnitude much greater than 2${\mathit{e}}^{2}$/h. For clean systems, this anomalous behavior arises from low-angle quasiparticle scattering at normal-superconducting interfaces. For dirty systems it arises from the presence of normal-state conductance resonances. We also examine the magnetic-field dependence of \ensuremath{\delta}G and show that fields on the scale of a flux quantum through a sample can change the sign of \ensuremath{\delta}G and suppress its magnitude. For a superconducting order parameter of magnitude ${\mathrm{\ensuremath{\Delta}}}_{0}$, we present results for the \ensuremath{\Delta} susceptibility ${\mathrm{\ensuremath{\chi}}}_{\mathrm{\ensuremath{\Delta}}}$=${\mathrm{lim}}_{\mathrm{\ensuremath{\Delta}}0}$\ensuremath{\rightarrow}0\ensuremath{\partial}G(${\mathrm{\ensuremath{\Delta}}}_{0}$)/\ensuremath{\partial}${\mathrm{\ensuremath{\Delta}}}_{0}^{2}$. For clean systems, where the normal-state conductance is quantized in units of 2${\mathit{e}}^{2}$/h, we predict that ${\mathrm{\ensuremath{\chi}}}_{\mathrm{\ensuremath{\Delta}}}$ diverges at normal-state conductance steps. For dirty systems, it is shown that ${\mathrm{\ensuremath{\chi}}}_{\mathrm{\ensuremath{\Delta}}}$ is sensitive to the local environment of single impurity atoms.