Phase Sensitive Shot Noise in an Andreev Interferometer

We investigate nonequilibrium noise in a diffusive Andreev interferometer, in which currents emerging from two normal metal/superconductor (N-S) interfaces can interfere. We observe a modulation of the shot noise when the phase difference between the two N-S interfaces is varied by a magnetic flux. This is the signature of phase-sensitive fluctuations in the normal metal. The effective charge inferred from the shot noise measurement is close to $q_{\mathrm{e}\mathrm{f}\mathrm{f}}=2e$ but shows phase-dependent deviations from $2e$ at finite energy, which we interpret as being due to pair correlations. Experimental data are in good agreement with predictions based on an extended Keldysh Green’s function approach.

Figure 1

(a) SEM picture of the device; (b) the device schematic; (c),(d) models of the device at integer and half-integer magnetic flux, respectively. N and S are, respectively, normal and superconducting reservoirs.

Figure 2

(a) Experimental data of the differential resistance vs bias voltage at different values of magnetic flux (mixing chamber temperature $T=43\mathrm{m}\mathrm{K}$). The inset shows magnetic field dependence of the zero-bias differential resistance. (b) Theoretical predictions for $E_C=30\mu \mathrm{e}\mathrm{V}$ and an electron temperature of $T=43\mathrm{m}\mathrm{K}$.

Figure 3

(a) Experimentally measured effective charge $q_{\mathrm{e}\mathrm{f}\mathrm{f}}$ for several values of magnetic flux. (b) Theoretical predictions for $E_C=30\mu \mathrm{e}\mathrm{V}$ and $T=43\mathrm{m}\mathrm{K}$. The inset shows the theory for $\Phi =0$ and $\Phi ={\Phi }_0/2$ at 43 mK and for $T=0$.