Andreev magnetotransport in low-dimensional proximity structures: Spin-dependent conductance enhancement

Abstract

We study the excess conductance due to the superconducting proximity effect in a ballistic two-dimensional electron system subject to an in-plane magnetic field. We show that under certain conditions the interplay of the Zeeman spin splitting and the effect of a screening supercurrent gives rise to a spin-selective Andreev enhancement of the conductance and anomalies in its voltage, temperature and magnetic field characteristics. The magnetic-field influence on Andreev reflection is discussed in the context of using superconducting hybrid junctions for spin detection. DOI: 10.1103/PhysRevB.71.094517 PACS number͑s͒: 74.45.ϩc, 73.23.Ad, 72.25.Dc Advances in nanotechnology of semiconductorsuperconductor junctions 1 have created a unique opportunity to investigate the interplay between superconducting phasecoherence and various electronic properties of lowdimensional semiconductors. Recent developments in this field include the realization of a long-range Josephson coupling mediated by Andreev reflection 2 of ballistic twodimensional electrons and controlled by the injection of hot carriers, 3,4 the observation of a giant proximity-induced enhancement of the conductance of two-dimensional electron systems ͑2DES͒, 5 theoretical 6 and experimental 7 studies of Andreev billiards, and classical 8 and quantum 9,10 Andreev edge states in high magnetic fields.In this paper we discuss a possibility of using the superconducting proximity effect for detecting the spin of transport carriers in low-dimensional systems, a problem closely related to the ongoing work on spin injection in semiconductors. 11 In nonmagnetic normal metalsuperconductor ͑NS͒ junctions spin resolving transport measurements were first reported in Ref. 12 where the magnetic field spin splitting of the quasiparticle density of states in thin superconducting films served as an electron filter. Applied to ferromagnet-superconductor systems, this idea has developed into a sensitive technique of analyzing the spin polarization of ferromagnetic metals. 13 We note that the findings of Ref. 12 are specific to low-transparency tunnel junctions where the superconducting proximity effect is negligible and hence the electron transport is predominantly a quasiparticle one.In structures with improved interfacial quality the penetration of the superconducting order parameter into the normal system is accompanied by the conversion of a quasiparticle current into a supercurrent via Andreev reflection which manifests itself as a low-bias conductance enhancement. [14][15][16][17][18][19][20][21][22] One of the most striking examples of the proximity effect occurs in ballistic semiconductor quantum wells ͑2DES͒ with a lateral superconducting contact ͓see Fig. 1͑a͔͒. In this case the conductance in the plane of the quantum well can nearly twice exceed the normal-state value 5,7,10 due to a proximity-induced mixing of particle and hole states characterized by a superconducting minigap E g in the excitation spectrum of the 2DES. 18 For such proximity structures we study the ...

Andreev reflection without Fermi surface alignment in high-Tcvan der Waals heterostructures

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Evidence for a new excitation at the interface between a high-Tcsuperconductor and a topological insulator

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Transport in Nb-InAs structures: from phase coherence to the edge state regime

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