Andreev reflection and bound state formation in a ballistic two-dimensional electron gas probed by a quantum point contact

摘要

We study coherent transport and bound-state formation of Bogoliubovquasiparticles in a high-mobility In$_{0.75}%$Ga$_{0.25}$As two-dimensionalelectron gas (2DEG) coupled to a superconducting Nb electrode by means of aquantum point contact (QPC) as a tunable single-mode probe. Below thesuperconducting critical temperature of Nb, the QPC shows a single-channelconductance greater than the conductance quantum $2e^{2}/h$ at zero bias, whichindicates the presence of Andreev-reflected quasiparticles, time-reversedstates of the injected electron, returning back through the QPC. The markedsensitivity of the conductance enhancement to voltage bias and perpendicularmagnetic field suggests a mechanism analogous to reflectionless tunneling--ahallmark of phase-coherent transport, with the boundary of the 2DEG cavityplaying the role of scatters. When the QPC transmission is reduced to thetunneling regime, the differential conductance vs bias voltage probes thesingle-particle density of states in the proximity area. Measured conductancespectra show a double peak within the superconducting gap of Nb, demonstratingthe formation of Andreev bound states in the 2DEG. Both of these results,obtained in the open and closed geometries, underpin the coherent nature ofquasiparticles, i.e., phase-coherent Andreev reflection at the InGaAs/Nbinterface and coherent propagation in the ballistic 2DEG.