Josephson quantum electron pump: theory and experiment

Pumping is a transport mechanism that exploits the time-dependence of some properties of a nano-scale conductor to produce a dc charge current in the absence of an applied voltage [1–3]. So far, nano-scale pumps have been realised only in systems exhibiting strong Coulombic effects, whereas evidence for pumping in the absence of Coulomb-blockade has been elusive. A pioneering experiment [4] evidenced the difficulty of modulating in time the properties of an open mesoscopic conductor at cryogenic temperatures without generating undesired bias voltages due to stray capacitances. One possible solution to this problem is to use the ac Josephson effect to induce periodically time-dependent Andreev-reflection amplitudes in a hybrid normal-superconducting system [5]. Here we report an experimental and theoretical investigation of charge flow in an unbiased InAs nanowire (NW) embedded in a superconducting quantum interference device (SQUID). In this system, pumping may occur via the cyclic modulation of the phase of the order parameter of different superconducting electrodes. The symmetry of the current with respect to the enclosed magnetic flux and bias SQUID current is a discriminating signature of pumping. Currents exceeding 20 pA are measured at 250 mK, and exhibit symmetries compatible with a pumping mechanism in this setup. We have extended the theoretical framework for pumping, based on the dynamical scattering approach, to take into account the fact that the normal and the superconducting parts of the circuit have no common ground. [1] D. J. Thouless, Phys. Rev. B 27, 6083 (1983). [2] M. Büttiker, H. Thomas, and A. Prêtre, Z. Phys. B 94,133 (1994). [3] P. W. Brouwer, Phys. Rev. B 58, R10135 (1998). [4] M. Switkes et al., Science 283, 1905 (1999). [5] S. Russo, et al., Phys. Rev. Lett. 99, 086601 (2010).

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