arXiv:0704.3017v3 [cond-mat.mes-hall] 1 Aug 2007 Andreev reflection and strongly enhanced magnetoresistance oscillations in Ga x In 1-x As/InP heterostructures with superconducting contacts Igor E. Batov, 1, 2, 3, ∗ Thomas Sch¨ apers, 2 Nikolai M. Chtchelkatchev, 4 Hilde Hardtdegen, 2 and Alexey V. Ustinov 1 1 Physikalisches Institut III, Universit¨ at Erlangen-N¨ urnberg, Erwin-Rommel-Strasse 1, 91058 Erlangen, Germany 2 Institute of Bio- and Nanosystems (IBN-1) and cni-Center of Nanoelectronic Systems for Information Technology, Research Centre J¨ ulich GmbH, 52425 J¨ ulich, Germany 3 Institute of Solid State Physics, Russian Academy of Sciences, 142432 Chernogolovka, Russia 4 L. D. Landau Institute for Theoretical Physics, Russian Academy of Sciences, 117940 Moscow, Russia (Dated: November 3, 2018) We study the magnetotransport in small hybrid junctions formed by high-mobility GaxIn1-xAs/InP heterostructures coupled to superconducting (S) and normal metal (N) terminals. Highly transmissive superconducting contacts to a two-dimensional electron gas (2DEG) located in a GaxIn1-xAs/InP heterostructure are realized by using a Au/NbN layer system. The magnetore- sistance of the S/2DEG/N structures is studied as a function of dc bias current and temperature. At bias currents below a critical value, the resistance of the S/2DEG/N structures develops a strong oscillatory dependence on the magnetic field, with an amplitude of the oscillations considerably larger than that of the reference N/2DEG/N structures. The experimental results are qualitatively explained by taking Andreev reflection in high magnetic fields into account. I. INTRODUCTION Mesoscopic systems consisting of superconduc- tor/semiconductor hybrid structures have attracted considerable attention in recent years. 1,2,3 The carrier transport in superconductor/normal metal or super- conductor/semiconductor structures can be described in the framework of Andreev reflection. 4 During an Andreev reflection process an electron that travels from the semiconductor on a superconductor/semiconductor interface is retroreflected as a hole. Simultaneously, a Cooper pair is created on the superconductor side. A number of interesting phenomena based on Andreev reflection had been studied in the past, e.g. gate-control of a Josephson supercurrent, 5,6 superconducting quan- tum point contacts, 7 control of the supercurrent by hot carrier injection, 8,9 and supercurrent reversal in a quantum dot. 10 A two-dimensional electron gas (2DEG) in a semi- conductor offers the advantage of ballistic transport in the semiconductor part. A fascinating regime oc- curs in high magnetic fields as soon as the transport across the superconductor/2DEG is governed by the Lan- dau quantization in the 2DEG. 11,12,13,14,15,16 Microscopic calculations 13,14,15,16 revealed conductance oscillations in S/2DEG junctions as a function of magnetic field. It was theoretically shown by Hoppe et al. 14 that the current flow along the S/2DEG interface can be described in the framework of Andreev bound states formed by electron and hole edge state excitations. At lower magnetic fields one can view this process in a semiclassical picture, in which carrier propagation is maintained by skipping or- bits of electrons and holes along the interface. 17,18,19,20,21 In mesoscopic S/2DEG contacts where the phase co- herence is maintained during the quasiparticle propa- gation, the interference between electrons and Andreev- reflected holes can lead to the magnetoconductance os- cillations which are based on a Aharonov–Bohm type effect. 17,18,19,20 The semiclassical theory of the charge transport through the S-2DEG interface at large filling factors was developed in Refs. 19,21. Apart from the or- bital effects, a magnetic field can also be employed to induce Zeeman energy splitting in the 2DEG. This opens up the possibility to study spin-related effects in combi- nation with Andreev reflection. 16,22,23,24 From experimental point of view, it is challenging to fabricate highly transmissive superconducting con- tacts to a 2DEG using superconductors with high crit- ical magnetic fields. 25,26,27 Recently, Eroms et al. 28 found enhanced oscillations in the magnetoresistance of a Nb/InAs structure for magnetic fields below the critical field of Nb. In this work, we report on the magnetotransport across a NbN/Au/2DEG interface. The choice of the NbN/Au system was motivated by our previous studies, where an Au interlayer helped achieving a high S/2DEG interface transparency while maintaining a high critical field of the superconductor. 29 Complementary to the work of Eroms et al., 28 we observe a suppression of enhanced oscillations in the magnetoresistance when a dc bias current across the junction exceeds a critical value or the temperature is increased above a critical temperature. 29 We compare our measurements of the NbN/Au/2DEG structures to that of similar structures with normal metal electrodes connected to the 2DEG. Our interpretation of the ex- perimental findings is based on recent theoretical models describing Andreev reflection across a S/2DEG interface in the presence of a magnetic field.