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Physical Review B

Supercurrent reversal in quantum dots induced by spin-pump effects

Hui Pan, Tsung-han Lin

Published May 4, 2007• 11 citations
Publisher

Abstract

The spin-pump effects caused by the rotating magnetic field on the supercurrent in a superconductor--quantum-dot--superconductor system are studied theoretically. By tuning the magnetic-field strength, the sign of the supercurrent can be changed from positive to negative, which results in the $\ensuremath{\pi}$-junction transition. The $\ensuremath{\pi}$-junction transition can also be tuned by the quantum dot energy level. The tunable $\ensuremath{\pi}$-junction transition is interpreted in the picture of the current carrying density of states, which clearly shows the enhancement, suppression, and reversion of the supercurrent.

DOI
10.1103/physrevb.75.195305

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The spin-pump effects caused by the rotating magnetic field on the supercurrent in a superconductor--quantum-dot--superconductor system are studied theoretically. By tuning the magnetic-field strength, the sign of the supercurrent can be changed from positive to negative, which results in the \ensuremathπ\ensuremath{\pi}\ensuremathπ-junction transition. The \ensuremathπ\ensuremath{\pi}\ensuremathπ-junction transition can also be tuned by the quantum dot energy level. The tunable \ensuremathπ\ensuremath{\pi}\ensuremathπ-junction transition is interpreted in the picture of the current carrying density of states, which clearly shows the enhancement, suppression, and reversion of the supercurrent.
Serial double quantum dots created in semiconductor nanostructures provide a versatile platform for investigating two-electron spin quantum states, which can be tuned by electrostatic gating and an external magnetic field. In this Rapid Communication, we directly measure the supercurrent reversal between adjacent charge states of an InAs nanowire double quantum dot with superconducting leads, in good agreement with theoretical models. In the even charge parity sector, we observe a supercurrent blockade with increasing magnetic field, corresponding to the spin singlet to triplet transition. Our results demonstrate a direct spin to supercurrent conversion, the superconducting equivalent of the Pauli spin blockade. This effect can be exploited in hybrid quantum architectures coupling the quantum states of spin systems and superconducting circuits.
The supercurrent through parallel-coupled double quantum dots connected to two superconductor leads is investigated theoretically. The possibility of controlling the supercurrent of the system is explored by tuning the interdot coupling, dot energy levels, total magnetic flux treading the ring connecting dots and leads, and flux difference between two subrings. Owing to the interdot coupling, one strongly and one weakly coupled states of the system can be formed. The magnetic flux can exchange the function of the two states, which leads to a swap effect. It is found that the oscillation period of the supercurrent with respect to the magnetic flux depends not only on the flux difference but also on the ratio of the two parts fluxes.
Dynamic 0−π0-π0−π transition induced by pumping mechanism
Using Nambu$\otimes $spin space Keldysh Green's function approach, we present a nonequilibrium charge and spin pumping theory of a quantum dot in the mico-cavity coupled to two superconducting leads. It is found that the charge currents include two parts: The dissipationless supercurrent standing for the transfer of coherent Cooper pairs and the pumped quasi-particle current. The supercurrent exhibits a dynamic 0−π0-π0−π transition induced by the frequency and strength of the σ−σ_{-}σ−​ polarized laser field. This dynamic transition is not affected by the strong Coulomb interaction. Especially, the spin current appears and is an even function of the phase difference between two superconductors when the frequency of the polarized laser field is larger than two times superconducting energy gap. Our theory serves as an extension to non-superconducting spintronics.
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We report the transport measurements on the InAs self-assembled quantum dots (SAQDs) which have a unique structural zero-dimensionality, coupled to a superconducting quantum interference device (SQUID). Owing to the SQUID geometry, we directly observe a π phase shift in the current phase relation and the negative supercurrent indicating π junction behavior by not only tuning the energy level of SAQD by back-gate but also controlling the coupling between SAQD and electrodes by side-gate. Our results inspire the future quantum information devices which can link optical, spin, and superconducting state.
Combining the Josephson effect with magnetism, or spin dependence in general, creates novel physical phenomena. The spin-asymmetric Josephson effect is a predicted phenomenon where a spin-dependent potential applied across a Josephson junction induces a spin-polarized Josephson current. Here, we propose an approach to observe the spin-asymmetric Josephson effect with spin-dependent superlattices, realizable, e.g., in ultracold atomic gases. We show that observing this effect is feasible by studying numerically the quantum dynamics of the system in one dimension. Furthermore, we show that the enhancement, or tunability, of the critical supercurrent in ferromagnetic Josephson junctions [F. S. Bergeret, A. F. Volkov, and K. B. Efetov, Phys. Rev. Lett. 86, 3140 (2001)] can be explained by the spin-asymmetric Josephson effect.