Anomalous Josephson Current in Junctions with Spin Polarizing Quantum Point Contacts

Abstract

We consider a ballistic Josephson junction with a quantum point contact in a two-dimensional electron gas with Rashba spin-orbit coupling. The point contact acts as a spin filter when embedded in a circuit with normal electrodes. We show that with an in-plane external magnetic field an anomalous supercurrent appears even for zero phase difference between the superconducting electrodes. In addition, the external field induces large critical current asymmetries between the two flow directions, leading to supercurrent rectifying effects. PACS numbers: 74.45.+c, 71.70.Ej, 72.25.Dc, 74.50.+r Josephson junctions (JJ) are the basic building blocks for superconducting electronics with applications that range from SQUID magnetometers to possible quantum computing devices. In superconductor-normal metal-superconductor (S-N-S) junctions the supercurrent flow is due to the Andreev states-a coherent superposition of electron and holes states. These states depend on the electronic structure of the normal material and on the properties of the S-N interface [1,2,3,4]. Modern technologies based on two dimensional electron gases (2DEGs) [5,6] or nanowires [7] allow for a precise control of such electronic properties, and thus of the JJ characteristics. Moreover, spin-orbit (SO) effects offer new alternatives to control the spin and charge transport [8,9].Superconducting rectifiers are among the new devices proposed and studied during the last few years. Most of these proposals are based on the dynamics of vortices [10,11]. Here we show that in systems with SO-coupling rectifying properties can be obtained by controlling the spin of the Andreev states. To this end we consider a ballistic JJ with a quantum point contact (QPC) in a 2DEG with SO interaction. The QPC can be tuned to control the number of transmitting channels and thus the critical current of the junction [3,4,5,12,13]. On the other hand, the QPC with SO coupling may act as a spin filter producing spin-polarized currents when embedded in a circuit with normal leads [14,15,16]. The normal current also generates an in-plane magnetization-perpendicular to the current -as well as out-of-plane spin-Hall textures [17]. Both effects are maximized at the core of the QPC [18]. As the SO-coupling preserves time-reversal symmetry (TRS), we expect that these peculiarities of the transmitting channels do not harm the Josephson effect when the leads become superconducting. However, the Josephson current itself breaks the TRS and, as we show below, it reveals striking effects of the SO-coupling. For example, the supercurrent generates spin polarization in the 2DEG [19] and the QPC in a similar way normal current does [17,18]. This is due to the distinctive spin texture of each Andreev state that contributes to the local magnetization in a supercurrent-carrying state.More striking effects take place if an external in-plane magnetic field is applied. Its effect on the supercurrent characteristics depends on the nature of the junction. In the absence of SO-coupling, the Zeeman field ...

Josephson junction through a disordered topological insulator with helical magnetization

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