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Switching of Andreev Current with Spin-Hall Effect

Katsumoto Group

Two-dimensional electron gas (2DEG) in InAs quantum well has been often adopted as a material for middle layers in Superconductor-Normal conductor-Superconductor (SNS) sandwiches due to its low Schottky barrier to metals. At the same time it is well-known that the spin-orbit interaction (SOI) is generally strong in such materialswith narrow band gaps and InAs 2DEG actually shows spin-interference phenomena due to strongRashba-type SOI. The spin Hall effect originates from the SOI draws apart a spin up-down pair of electrons to form a Cooper pair, thus is supposed to work against the Andreev reflection, and to affect the SNS transport or the Andreev bound states (ABSs) formed in the normal layer.

Fig. 1. (a) Differential conductance of Nb-InAs-Nb SNS junction at 0.5K as a function of the source-drain bias voltage. The parameter is the external magnetic field perpendicular to the sample. The inset shows schematic view of the sample. (b) The same measurements as those in (a) under zero magnetic field with transverse currents through the InAs strip from zero to 1.2μA.

Here we report suppression of electric current enhanced by Andreev reflection with transverse current flow, which breaks the time-reversal symmetry in two-dimensional system (2DES) of InAs. The inset of Fig.1 schematically shows the sample structure, in which a 200nm wide InAs 2DES stripe is sandwitched by two Nb electrodes. The sample was cooled down to 0.5K. During the electric measurement, we kept the crossing point of Nb and InAs 2DES at the ground level. In order for that, we formed circuits for sweeping the potentials of the four terminals assuming Ohmic contact resistances.

At zero magnetic field and with no transverse current, the differential conductance oscillates with the source-drain voltage Vsd making a large peak structure at the origin. The characteristic lineshape can be interpreted with assuming the formation of ABSs as follows. The zero-bias peak reflects the resonance between the two Fermi levels of superconducting electrodes via the virtual tunneling through ABSs while the two side peaks are direct resonances between one of the superconductors and an ABS. Because the mean free path of InAs 2DES exceeds the width of the 2DES strip, the transport between the two Nb electrodes should be ballistic, that is, ABSs should be formed.

In Fig.1(a) we show the response to the magnetic field, in which the structure is squeezed to the origin and disappears at 0.75T. This behavior manifests that the structure is superconductivity origin and we can explain it with considering some kind of interference effect. Because the additional phase due to the voltage accumulates for Andreev type (electron-hole) shuttling, the Aharonov-Bohm (AB) phase should shift the positions of ABSs. Figure 1(b) shows the differential conductance (G) again as a function of Vsd with the transverse currents through the InAs 2DES strip from 108 nA to 1.2 μA. The transverse current also strongly diminishes the G-Vsd structure. The difference between the response to the magnetic field and that to the transverse current is apparent. This suggests that the latter comes not from the orbital effect and that the spin-Hall effect induced by the transverse current reduces the formation of ABSs.


References
  • [1] Y. Takahashi, Y. Hashimoto, Y. Iye, and S. Katsumoto, J. Cryst. Growth, published online: http://www.sciencedirect.com/science/article/pii/S0022024813000705.
Authors
  • S. Katsumoto, T. Nakamura, and Y. Hashimoto