11 research outputs found
Supercurrent, Multiple Andreev Reflections and Shapiro Steps in InAs Nanosheet Josephson Junctions
High-quality free-standing InAs nanosheets are emerging layered semiconductor
materials with potentials in designing planar Josephson junction devices for
novel physics studies due to their unique properties including strong
spin-orbit couplings, large Land\'e g-factors and the two dimensional nature.
Here, we report an experimental study of proximity induced superconductivity in
planar Josephson junction devices made from free-standing InAs nanosheets. The
nanosheets are grown by molecular beam epitaxy and the Josephson junction
devices are fabricated by directly contacting the nanosheets with
superconductor Al electrodes. The fabricated devices are explored by
low-temperature carrier transport measurements. The measurements show that the
devices exhibit a gate-tunable supercurrent, multiple Andreev reflections, and
a good quality superconductor-semiconductor interface. The superconducting
characteristics of the Josephson junctions are investigated at different
magnetic fields and temperatures, and are analyzed based on the
Bardeen-Cooper-Schrieffer (BCS) theory. The measurements of ac Josephson effect
are also conducted under microwave radiations with different radiation powers
and frequencies, and integer Shapiro steps are observed. Our work demonstrates
that InAs nanosheet based hybrid devices are desired systems for investigating
forefront physics, such as the two-dimensional topological superconductivity
Gate-tunable negative differential conductance in hybrid semiconductor-superconductor devices
Negative differential conductance (NDC) manifests as a significant
characteristic of various underlying physics and transport processes in hybrid
superconducting devices. In this work, we report the observation of
gate-tunable NDC outside the superconducting energy gap on two types of hybrid
semiconductor-superconductor devices, i.e., normal metal-superconducting
nanowire-normal metal and normal metal-superconducting nanowire-superconductor
devices. Specifically, we study the dependence of the NDCs on back-gate voltage
and magnetic field. When the back-gate voltage decreases, these NDCs weaken and
evolve into positive differential conductance dips; and meanwhile they move
away from the superconducting gap towards high bias voltage, and disappear
eventually. In addition, with the increase of magnetic field, the NDCs/dips
follow the evolution of the superconducting gap, and disappear when the gap
closes. We interpret these observations and reach a good agreement by combining
the Blonder-Tinkham-Klapwijk (BTK) model and the critical supercurrent effect
in the nanowire, which we call the BTK-supercurrent model. Our results provide
an in-depth understanding of the tunneling transport in hybrid
semiconductor-superconductor devices.Comment: 15+6 pages, 4+6 figure
A Versatile Method of Engineering the Electron Wavefunction of Hybrid Quantum Devices
With the development of quantum technology, hybrid devices that combine
superconductors (S) and semiconductors (Sm) have attracted great attention due
to the possibility of engineering structures that benefit from the integration
of the properties of both materials. However, until now, none of the
experiments have reported good control of band alignment at the interface,
which determines the strength of S-Sm coupling and the proximitized
superconducting gap. Here, we fabricate hybrid devices in a generic way with
argon milling to modify the interface while maintaining its high quality.
First, after the milling the atomically connected S-Sm interfaces appear,
resulting in a large induced gap, as well as the ballistic transport revealed
by the multiple Andreev reflections and quantized above-gap conductance
plateaus. Second, by comparing transport measurement with Schr\"odinger-Poisson
(SP) calculations, we demonstrate that argon milling is capable of varying the
band bending strength in the semiconducting wire as the electrons tend to
accumulate on the etched surface for longer milling time. Finally, we perform
nonlocal measurements on advanced devices to demonstrate the coexistence and
tunability of crossed Andreev reflection (CAR) and elastic co-tunneling (ECT)
-- key ingredients for building the prototype setup for realization of Kitaev
chain and quantum entanglement probing. Such a versatile method, compatible
with the standard fabrication process and accompanied by the well-controlled
modification of the interface, will definitely boost the creation of more
sophisticated hybrid devices for exploring physics in solid-state systems.Comment: 18 pages, 9 figure
Hole-type superconducting gatemon qubit based on Ge/Si core/shell nanowires
Abstract We demonstrate that superconducting gatemon qubits based on superconductor-semiconductor-superconductor Josephson junctions can be constructed on hole-type Ge/Si core/shell nanowires. The frequency of the qubit can be set firstly by controlling the diffusion of Al in the nanowire via thermal annealing, which yields a suitable critical supercurrent allowing the qubit frequency to be within the experimentally accessible range, and then by fine tuning of a gate voltage, by which an accurate adjustment of the frequency can be realized. On the resulted qubit, Rabi oscillation with an energy relaxation time T 1 ~ 180 ns was observed in the time domain, an average decoherence time T 2 * ~ 15 ns was obtained, and the gate voltage dependence of both T 1 and T 2 * was investigated. Such a hole-type superconducting gatemon qubit, based on materials with strong spin-orbit coupling and potentially the absence of hyperfine interaction after isotope purification, could be used for exploring the quantum coherence phenomena of hole-gas and even Majorana physics in Ge-based quantum devices