43 research outputs found
Andreev reflection at the interface with an oxide in the quantum Hall regime
Quantum Hall/superconductor junctions have been an attractive topic as the
two macroscopically quantum states join at the interface. Despite longstanding
efforts, however, experimental understanding of this system has not been
settled yet. One of the reasons is that most semiconductors hosting
high-mobility two-dimensional electron systems (2DES) usually form Schottky
barriers at the metal contacts, preventing efficient proximity between the
quantum Hall edge states and Cooper pairs. Only recently have relatively
transparent 2DES/superconductor junctions been investigated in graphene. In
this study, we propose another material system for investigating
2DES/superconductor junctions, that is ZnO-based heterostrcuture. Due to the
ionic nature of ZnO, a Schottky barrier is not effectively formed at the
contact with a superconductor MoGe, as evidenced by the appearance of Andreev
reflection at low temperatures. With applying magnetic field, while clear
quantum Hall effect is observed for ZnO 2DES, conductance across the junction
oscillates with the filling factor of the quantum Hall states. We find that
Andreev reflection is suppressed in the well developed quantum Hall regimes,
which we interpret as a result of equal probabilities of normal and Andreev
reflections as a result of multiple Andreev reflection at the
2DES/superconductor interface.Comment: 18 pages, 8 figure
Andreev Reflection at the Interface with an Oxide in the Quantum Hall Regime
Quantum Hall/superconductor junctions have been an attractive topic as the two macroscopically quantum states join at the interface. Despite longstanding efforts, however, experimental understanding of this system has not been settled yet. One of the reasons is that most semiconductors hosting high-mobility two-dimensional electron systems (2DES) usually form Schottky barriers at the metal contacts, preventing efficient proximity between the quantum Hall edge states and Cooper pairs. Only recently have relatively transparent 2DES/superconductor junctions been investigated in graphene. In this study, we propose another material system for investigating 2DES/superconductor junctions, that is ZnO-based heterostructure. Due to the ionic nature of ZnO, a Schottky barrier is not effectively formed at the contact with a superconductor MoGe, as evidenced by the appearance of Andreev reflection at low temperatures. With applying magnetic field, while clear quantum Hall effect is observed for ZnO 2DES, conductance across the junction oscillates with the filling factor of the quantum Hall states. We find that Andreev reflection is suppressed in the well developed quantum Hall regimes, which we interpret as a result of equal probabilities of normal and Andreev reflections as a result of multiple Andreev reflection at the 2DES/superconductor interface
MgZnO/ZnO heterostructures with electron mobility exceeding 1 × 10⁶ cm²/Vs
The inherently complex chemical and crystallographic nature of oxide materials has suppressed the purities achievable in laboratory environments, obscuring the rich physical degrees of freedom these systems host. In this manuscript we provide a systematic approach to defect identification and management in oxide molecular beam epitaxy grown MgZnO/ZnO heterostructures which host two-dimensional electron systems. We achieve samples displaying electron mobilities in excess of 1 × 10⁶ cm²/Vs. This data set for the MgZnO/ZnO system firmly establishes that the crystalline quality has become comparable to traditional semiconductor materials