arXiv subjects

Xingjun Wu

Publications and source records attributed to Xingjun Wu.

11 recordsLinked to original sources

arXiv2026

Microwave Response of the Superconducting Diode Effect in Proximitized Bilayer Graphene Interferometers

Microwave irradiation has emerged as a promising means to tune the superconducting diode effect (SDE) in Josephson junction devices. Previous experimental studies have mainly focused on the adiabatic-driving regime, in which the diode efficiency increases monotonically with microwave power and can approach the ideal value of unity. Beyond this regime, however, the microwave response of the SDE remains largely unexplored experimentally. In this work, we investigate the microwave response of the SDE in bilayer-graphene-based superconducting quantum interference devices (SQUIDs) under a broad range of driving frequencies. We show that increasing the driving frequency changes the response characteristics of the diode efficiency to microwave power--the dependence of the diode efficiency evolves from monotonic enhancement with increasing microwave power in the adiabatic regime to non-monotonic behavior beyond this regime, and ultimately to sign-reversal as well oscillatory characteristics at sufficiently high frequencies. We find that these experimentally observed frequency-dependent power response characteristics of the diode efficiency can be qualitatively captured by simulations based on the resistively shunted junction model using the device current-phase relations extracted from the experiments. These results establish SQUIDs made from bilayer graphene as a versatile platform for studying dynamic properties of superconducting junction devices.

cond-mat.mes-hall↗

arXiv2026

Spin splitting, Kondo correlation and singlet-doublet quantum phase transition in a superconductor-coupled InSb nanosheet quantum dot

We realize a superconductor-coupled quantum dot (QD) in an InSb nanosheet, a 2D platform promising for studies of topological superconductivity. The device consists of a superconductor-QD-superconductor junction, where a bottom bilayer gate defines the QD and allows tuning of its coupling to the superconducting leads. The QD exhibits large $g$-factors and strong spin-orbit coupling. Transport measurements reveal Coulomb diamond-shaped differential conductance features with even-odd alternating sizes and pronounced conductance lines associated with the superconducting gap, confirming a few-electron, superconductor-coupled regime. At an odd electron occupation, Kondo signatures emerge, including a zero-bias peak that splits with magnetic field and is logarithmically suppressed at elevated temperatures. We further observe a doublet-singlet quantum phase transition, manifested by a clear change of Andreev bound states from crossing to anticrossing as the coupling strength increases. These results underscore the rich physics of InSb nanosheet QDs and their promise for topological quantum devices.

cond-mat.supr-con↗

arXiv2025

Tunable superconducting diode effect in higher-harmonic InSb nanosheet interferometers

Superconducting diodes, characterized by the nonreciprocal supercurrent flow, have gained significant attention for their potential in dissipationless electronics. This study presents a superconducting quantum interference device (SQUID) composed of two Al-InSb nanosheet Josephson junctions. Utilizing prepatterned local backgates, we achieve a gate- and flux-tunable superconducting diode with controllable efficiency in both amplitude and sign. Numerical simulations attribute the diode effect to higher harmonics in the current-phase relation. Crucially, fractional Shapiro step experiments provide direct insights into the evolution of these higher harmonics with flux tuning, showcasing significant enhancements in the second-harmonic signatures of the SQUID near half-integer flux quanta. Furthermore, we investigate the microwave-assisted diode response and experimentally show that the polarity of the diode effect can be switched by the microwave power. These results demonstrate the potential of InSb nanosheet-based hybrid devices as highly tunable elements for use in dissipationless electronics.

cond-mat.supr-con↗

arXiv2025

Gate Tunable Josephson Diode Effect in Josephson Junctions made from InAs Nanosheets

We report the observation of Josephson diode effect (JDE) in hybrid devices made from semiconductor InAs nanosheets and superconductor Al contacts. By applying an in-plane magnetic field ($B_{\mathrm{xy}}$), we detect non-reciprocal superconducting switching current as well as non-reciprocal superconducting retrapping current. The strength of the JDE depends on the angle between the in-plane magnetic field and the bias current ($I_{\mathrm{b}}$), reaching its maximum when $B_{\mathrm{xy}} \perp I_{\mathrm{b}}$ and dropping to nearly zero when $B_{\mathrm{xy}}\parallel I_{\mathrm{b}}$. Additionally, the diode efficiency is tunable via an electrostatic gate with a complete suppression at certain gate voltages. Our findings indicate that the observed JDE in InAs nanosheet-based Josephson junctions most likely arises from the Rashba spin-orbit interaction (SOI) in the nanosheets. Such gate-tunable JDE in Josephson junctions made from semiconductor materials with SOI is useful not only for constructing advanced superconducting electronics but also for detecting novel superconducting states.

cond-mat.mes-hall↗

arXiv2024

Ubiquitous missing first Shapiro step in Al-InSb nanosheet Josephson junctions

The absence of odd-order Shapiro steps is a predicted signature of topological superconductors. Experimentally, the missing first-order Shapiro step has been reported in both putative topological superconducting systems and topologically trivial superconductor-semiconductor Josephson junctions. Here, we revisit this phenomenon in topologically trivial Al-InSb nanosheet Josephson junctions under microwave irradiation. The missing first Shapiro step coincides with a sharp voltage jump during superconducting switching, yet reappears when the jump is lowered and softened by increasing microwave power, temperature, or magnetic field. It also reappears at higher microwave frequencies, consistent with qualitative results from an RSJ model incorporating the sharp jump. These observations indicate that the absence of the first Shapiro step, associated with the sharp switching jump, simply results from their location within the measurement blind region. This work identifies a common but overlooked mechanism underlying the missing first Shapiro step, offering new insights into fractional Josephson effect experiments.

cond-mat.supr-con↗

arXiv2024

Microwave-assisted unidirectional superconductivity in Al-InAs nanowire-Al junctions under magnetic fields

Under certain symmetry-breaking conditions, a superconducting system exhibits asymmetric critical currents, dubbed the ``superconducting diode effect". Recently, systems with the ideal superconducting diode efficiency or unidirectional superconductivity have received considerable interest. In this work, we report the study of Al-InAs nanowire-Al Josephson junctions under microwave irradiation and magnetic fields. We observe an enhancement of superconducting diode effect under microwave driving, featured by a horizontal offset of the zero-voltage step in the voltage-current characteristic that increases with microwave power. Devices reach the unidirectional superconductivity regime at sufficiently high driving amplitudes. The offset changes sign with the reversal of the magnetic field direction. Meanwhile, the offset magnitude exhibits a roughly linear response to the microwave power in dBm when both the power and the magnetic field are large. The signatures observed are reminiscent of a recent theoretical proposal using the resistively shunted junction (RSJ) model. However, the experimental results are not fully explained by the RSJ model, indicating a new mechanism for unidirectional superconductivity that is possibly related to non-equilibrium dynamics or dissipation in periodically driven superconducting systems.

cond-mat.mes-hall↗

arXiv2023

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.

cond-mat.mes-hall↗

arXiv2023

Realization of independent contacts in barrier-separated InAs/GaSb quantum wells

InAs/GaSb double quantum wells (QWs) separated by a 100 \AA\ AlSb middle barrier are grown by molecular beam epitaxy. We report a nanofabrication technique that utilizes the surface Fermi level pinning position in InAs $[E_f^s(\rm InAs)]$ for realizing independent electric contacts to each well. In particular, separate ohmic contacts to the upper InAs quantum well are achieved by selectively etching down to the InAs, while contacts to the lower GaSb quantum well are obtained by the depletion method. For the latter, the upper InAs quantum well is locally pinched off by top etched trenches capped with a remaining 2-3 nm InAs layer. As a result of a relatively low $E_f^s(\rm InAs)$, applying a negative bias gate potential will create a conducting hole channel in GaSb, and hence a separate ohmic contact to the lower quantum well. This method is demonstrated with experiment and the support of a self-consistent band bending calculation. A number of experiments on separately probing Coulomb and tunnel-coupled InAs/GaSb systems now become accessible.

cond-mat.mes-hall↗

arXiv2022

Tunneling spectroscopic signatures of charge doping and associated Mott transition in ${\alpha}$-RuCl${_3}$ in proximity to graphite

The layered Mott insulator ${\alpha}$-RuCl${_3}$ has been extensively studied as a potential Kitaev quantum spin liquid candidate. Here, by constructing heterostructures with graphite, we employed electron tunneling measurements on few-layer ${\alpha}$-RuCl${_3}$ using a scanning tunneling microscopy/spectroscopy. Characteristic tunneling spectra were detected on ${\alpha}$-RuCl${_3}$ layers in proximity to graphite. In the single-layer ${\alpha}$-RuCl${_3}$ in direct contact with graphite, distinct states in the Mott-gap regime were observed. The in-gap states are demonstrated to be closely related to the electron orbitals in ${\alpha}$-RuCl${_3}$ and graphite, and to be sensitive to interfacial coupling, where a hybridization at the heterointerface is hypothesized. The in-gap states are also thought of as a charge reservoir for weakly doping the ${\alpha}$-RuCl${_3}$ upper-layers. It demonstrated that the weak doping effect causes a considerable decrease in the Mott-gap within the upper-layers, suggesting that an unconventional Mott-transition is occurring in these layers. The results show that the heterostructure comprised of ${\alpha}$-RuCl${_3}$ and graphite is a good platform for investigating the doping physics in ${\alpha}$-RuCl${_3}$. Therefore, tunneling into such a doped system is a useful probe for studying otherwise insulating spin-liquid candidates.

cond-mat.str-el↗

arXiv2019

Resistive signature of excitonic coupling in an electron-hole double layer with a middle barrier

We study the interlayer scattering mediated by long-range Coulomb interaction between electrons (density n) and holes (p) in a double-layer system. The gated device is made of InAs (e) and InGaSb (h) quantum wells separated by a AlSb middle barrier such that the interlayer tunneling is negligibly small. By using independent-layer contacts we measure the transport tensor \r{ho}_xx and \r{ho}_xy that are solely from the InAs layer, while sweeping p in the InGaSb layer. We found a strongly enhanced resistive scattering signal as the carrier densities approach a total charge neutrality, n = p, which cannot be described by the Fermi-liquid theory. Results of data analysis for density, temperature, and magnetic field dependences are consistent with the emergence of excitonic coupling between the two layers, stressing the dominance of Coulomb interaction even in the presence of disorder.

cond-mat.str-el↗

arXiv2017

Tuning Edge States in Strained-Layer InAs/GaInSb Quantum Spin Hall Insulators

We report on a class of quantum spin Hall insulators (QSHIs) in strained-layer InAs/GaInSb quantum wells, in which the bulk gaps are enhanced by up to five folds as compared to the binary InAs/GaSb QSHI. Remarkably, with consequently increasing edge velocity, the edge conductance at zero and applied magnetic fields manifests time reversal symmetry (TRS) -protected properties consistent with Z2 topological insulator. The InAs/GaInSb bilayers offer a much sought-after platform for future studies and applications of the QSHI.

cond-mat.mes-hall↗