92 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
Blue light-emitting diode based on ZnO
A near-band-edge bluish electroluminescence (EL) band centered at around 440
nm was observed from ZnO p-i-n homojunction diodes through a semi-transparent
electrode deposited on the p-type ZnO top layer. The EL peak energy coincided
with the photoluminescence peak energy of an equivalent p-type ZnO layer,
indicating that the electron injection from the n-type layer to the p-type
layer dominates the current, giving rise to the radiative recombination in the
p-type layer. The imbalance in charge injection is considered to originate from
the lower majority carrier concentration in the p-type layer, which is one or
two orders of magnitude lower than that in the n-type one. The current-voltage
characteristics showed the presence of series resistance of several hundreds
ohms, corresponding to the current spread resistance within the bottom n-type
ZnO. The employment of conducting ZnO substrates may solve the latter problem.Comment: 13 pages, 4 figures. Jpn. J. Appl. Phys. in pres
Stabilization of a honeycomb lattice of IrO octahedra in superlattices with ilmenite-type MnTiO
In the quest for quantum spin liquids, thin films are expected to open the
way for the control of intricate magnetic interactions in actual materials by
exploiting epitaxial strain and two-dimensionality. However, materials
compatible with conventional thin-film growth methods have largely remained
undeveloped. As a promising candidate towards the materialization of quantum
spin liquids in thin films, we here present a robust ilmenite-type oxide with a
honeycomb lattice of edge-sharing IrO octahedra artificially stabilized by
superlattice formation with an ilmenite-type antiferromagnetic oxide MnTiO.
The stabilized sub-unit-cell-thick Mn-Ir-O layer is isostructural to MnTiO,
having the atomic arrangement corresponding to ilmenite-type MnTiO not
discovered yet. By spin Hall magnetoresistance measurements, we found that
antiferromagnetic ordering in the ilmenite Mn sublattice is suppressed by
modified magnetic interactions in the MnO planes via the IrO planes.
These findings lay the foundation for the creation of two-dimensional Kitaev
candidate materials, accelerating the discovery of exotic physics and
applications specific to quantum spin liquids
Gate-electric-field and magnetic-field control of versatile topological phases in a semi-magnetic topological insulator
Surface states of a topological insulator demonstrate interesting quantum
phenomena, such as the quantum anomalous Hall (QAH) effect and the quantum
magnetoelectric effect. Fermi energy tuning plays a role in inducing phase
transitions and developing future device functions. Here, we report on
controlling the topological phases in a dual-gate field-effect transistor of a
semi-magnetic topological insulator heterostructure. The heterostructure
consists of magnetized one-surface and non-magnetic other-surface. By tuning
the Fermi energy to the energy gap of the magnetized surface, the Hall
conductivity becomes close to the half-integer quantized Hall
conductivity , exemplifying parity anomaly. The dual-gate control
enables the band structure alignment to the two quantum Hall states with
and 0 under a strong magnetic field. These states are
topologically equivalent to the QAH and axion insulator states, respectively.
Precise and independent control of the band alignment of the top and bottom
surfaces successively induces various topological phase transitions among the
QAH, axion insulator, and parity anomaly states in magnetic topological
insulators.Comment: 20 pages, 4 figure
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
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