Proximity-Induced Superconductivity with Subgap Anomaly in Type II Weyl Semi-Metal WTe2

Proximity Effect-Induced Magnetoresistance Enhancement in a Fe3GeTe2/NbSe2/Fe3GeTe2 Magnetic Tunnel Junction

The coupling between van der Waals-layered magnetic and superconducting materials holds the possibility of revealing novel physical mechanisms and realizing spintronic devices with new functionalities. Here, we report on the realization and investigation of a maximum ∼17-fold magnetoresistance (MR) enhancement based on a vertical magnetic tunnel junction of Fe3GeTe2 (FGT)/NbSe2/FGT near the NbSe2 layer's superconducting critical temperature (TC) of 6.8 K. This enhancement is attributed to the band splitting in the atomically thin NbSe2 spacer layer induced by the magnetic proximity effect on the material interfaces. However, the band splitting is strongly suppressed by the interlayer coupling in the thick NbSe2 layer. Correspondingly, the device with a thick NbSe2 layer displays no MR increase near TC but a current dependent on transport properties at extremely low temperatures. This work carefully investigates the mechanism of MR enhancement, paving an efficient way for the modulation of spintronics' properties and the achievement of spin-based integrated circuits.

Anisotropic proximity-induced superconductivity and edge supercurrent in Kagome metal, K1-xV3Sb5

Materials with Kagome nets are of particular importance for their potential combination of strong correlation, exotic magnetism, and electronic topology. KV₃Sb₅ was discovered to be a layered topological metal with a Kagome net of vanadium. Here, we fabricated Josephson Junctions of K_1-xV₃Sb₅ and induced superconductivity over long junction lengths. Through magnetoresistance and current versus phase measurements, we observed a magnetic field sweeping direction-dependent magnetoresistance and an anisotropic interference pattern with a Fraunhofer pattern for in-plane magnetic field but a suppression of critical current for out-of-plane magnetic field. These results indicate an anisotropic internal magnetic field in K_1-xV₃Sb₅ that influences the superconducting coupling in the junction, possibly giving rise to spin-triplet superconductivity. In addition, the observation of long-lived fast oscillations shows evidence of spatially localized conducting channels arising from edge states. These observations pave the way for studying unconventional superconductivity and Josephson device based on Kagome metals with electron correlation and topology.

Structure modulation of two-dimensional transition metal chalcogenides: recent advances in methodology, mechanism and applications

Together with the development of two-dimensional (2D) materials, transition metal dichalcogenides (TMDs) have become one of the most popular series of model materials for fundamental sciences and practical applications. Due to the ever-growing requirements of customization and multi-function, dozens of modulated structures have been introduced in TMDs. In this review, we present a systematic and comprehensive overview of the structure modulation of TMDs, including point, linear and out-of-plane structures, following and updating the conventional classification for silicon and related bulk semiconductors. In particular, we focus on the structural characteristics of modulated TMD structures and analyse the corresponding root causes. We also summarize the recent progress in modulating methods, mechanisms, properties and applications based on modulated TMD structures. Finally, we demonstrate challenges and prospects in the structure modulation of TMDs and forecast potential directions about what and how breakthroughs can be achieved.

Tunable Photoresponse in a Two-Dimensional Superconducting Heterostructure

The photo-induced superconducting phase transition is widely used in probing the physical properties of correlated electronic systems and to realize broadband photodetection with extremely high responsivity. However, such photoresponse is usually insensitive to electrostatic doping due to the high carrier density of the superconductor, restricting its applications in tunable optoelectronic devices. In this work, we demonstrate the gate voltage modulation to the photoresponsivity in a two-dimensional NbSe₂-graphene heterojunction. The superconducting critical current of the NbSe₂ relies on the gate-dependent hot carrier generation in graphene via the Joule heating effect, leading to the observed shift of both the magnitude and peak position of the photoresponsivity spectra as the gate voltage changes. This heating effect is further confirmed by the temperature and laser-power-dependent characterization of the photoresponse. In addition, we investigate the spatially-resolved photocurrent, finding that the superconductivity is inhomogeneous across the junction area. Our results provide a new platform for designing tunable superconducting photodetector and indicate that the photoresponse could be a powerful tool in studying the local electronic properties and phase transitions in low-dimensional superconducting systems.

Two-Dimensional Van Der Waals Topological Materials: Preparation, Properties, and Device Applications

Over the past decade, 2D van der Waals (vdW) topological materials (TMs), including topological insulators and topological semimetals, which combine atomically flat 2D layers and topologically nontrivial band structures, have attracted increasing attention in condensed-matter physics and materials science. These easily cleavable and integrated TMs provide the ideal platform for exploring topological physics in the 2D limit, where new physical phenomena may emerge, and represent a potential to control and investigate exotic properties and device applications in nanoscale topological phases. However, multifaced efforts are still necessary, which is the prerequisite for the practical application of 2D vdW TMs. Herein, this review focuses on the preparation, properties, and device applications of 2D vdW TMs. First, three common preparation strategies for 2D vdW TMs are summarized, including single crystal exfoliation, chemical vapor deposition, and molecular beam epitaxy. Second, the origin and regulation of various properties of 2D vdW TMs are introduced, involving electronic properties, transport properties, optoelectronic properties, thermoelectricity, ferroelectricity, and magnetism. Third, some device applications of 2D vdW TMs are presented, including field-effect transistors, memories, spintronic devices, and photodetectors. Finally, some significant challenges and opportunities for the practical application of 2D vdW TMs in 2D topological electronics are briefly addressed.

Superconducting quantum interference effect in NbSe2/NbSe2van der Waals junctions

Layered materials with exotic properties, such as superconducting, ferromagnetic, and so on, have attracted broad interest. The advances in van der Waals (vdW) stacking technology have enabled the fabrication of numerous types of junction structures. The dangling-bond-free interface provides an ideal platform to generate and probe various physics phenomena. Typical progress is the realization of vdW Josephson junctions with high supercurrent transparency constructed of two NbSe₂layers. Here we report the observation of periodic oscillations of the voltage drop across a NbSe₂/NbSe₂vdW junctions under an in-plane magnetic field. The voltage-drop oscillations come from the interface and the magnitude of the oscillations has a non-monotonic temperature dependence which increases first with increasing temperature. These features make the oscillations different from the modulation of the critical current of a Josephson junction by the magnetic field and the Little-Parks effect. The oscillations are determined to be generated by the quantum interference effect between two superconducting junctions formed between the two NbSe₂layers. Our results thus provide a unique way to make an in-plane superconducting quantum interference device that can survive under a high magnetic field utilizing the Ising-paring nature of the NbSe₂.

Magnetoresistance studies of two-dimensional Fe3GeTe2nano-flake

The magneto-transport properties of two-dimensional (2D) Fe₃GeTe₂(FGT) nano-flakes are carefully investigated with the variation of the temperature and the direction of the applied magnetic field (B). Four magnetoresistance (MR) behavior are obtained at different temperatures withBparalleling the flake's surface, because of the competition between the merging of different domains, spin fluctuation, and the spin momentum flipping. Different from the reported negative MR of bulk FGT, 2D FGT shows a positive MR behavior with the increase ofBat a low temperature in a lowBrange, owning to the domination of the spin momentum flipping induced by the weakening of the coupling between different layers with the decrease of the thickness of the FGT flake. The angle-dependence of the FGT MR is also investigated and can be well explained by the competition mentioned above.

Two-Dimensional TeB Structures with Anisotropic Carrier Mobility and Tunable Bandgap

Two-dimensional (2D) semiconductors with desirable bandgaps and high carrier mobility have great potential in electronic and optoelectronic applications. In this work, we proposed α-TeB and β-TeB monolayers using density functional theory (DFT) combined with the particle swarm-intelligent global structure search method. The high dynamical and thermal stabilities of two TeB structures indicate high feasibility for experimental synthesis. The electronic structure calculations show that the two structures are indirect bandgap semiconductors with bandgaps of 2.3 and 2.1 eV, respectively. The hole mobility of the β-TeB sheet is up to 6.90 × 10² cm² V^-1 s^-1. By reconstructing the two structures, we identified two new horizontal and lateral heterostructures, and the lateral heterostructure presents a direct band gap, indicating more probable applications could be further explored for TeB sheets.

Atomically Thin Quantum Spin Hall Insulators

Atomically thin topological materials are attracting growing attention for their potential to radically transform classical and quantum electronic device concepts. Among them is the quantum spin Hall (QSH) insulator-a 2D state of matter that arises from interplay of topological band inversion and strong spin-orbit coupling, with large tunable bulk bandgaps up to 800 meV and gapless, 1D edge states. Reviewing recent advances in materials science and engineering alongside theoretical description, the QSH materials library is surveyed with focus on the prospects for QSH-based device applications. In particular, theoretical predictions of nontrivial superconducting pairing in the QSH state toward Majorana-based topological quantum computing are discussed, which are the next frontier in QSH materials research.

2D Polarized Materials: Ferromagnetic, Ferrovalley, Ferroelectric Materials, and Related Heterostructures

The emergence of 2D polarized materials, including ferromagnetic, ferrovalley, and ferroelectric materials, has demonstrated unique quantum behaviors at atomic scales. These polarization behaviors are tightly bonded to the new degrees of freedom (DOFs) for next generation information storage and processing, which have been dramatically developed in the past few years. Here, the basic 2D polarized materials system and related devices' application in spintronics, valleytronics, and electronics are reviewed. Specifically, the underlying physical mechanism accompanied with symmetry broken theory and the modulation process through heterostructure engineering are highlighted. These summarized works focusing on the 2D polarization would continue to enrich the cognition of 2D quantum system and promising practical applications.

Anisotropic Full-Gap Superconductivity in 2M-WS2 Topological Metal with Intrinsic Proximity Effect

Layered 2M-WS₂ is recently observed to show Majorana bound states in vortices, but its superconducting pairing mechanism remains unknown, hindering the understanding of its topological superconducting nature. Using the ab initio Migdal-Eliashberg theory and electron-phonon Wannier interpolation, we demonstrate that both bulk and bilayer 2M-WS₂ have a single anisotropic full-gap superconducting order of s-wave symmetry. We successfully reproduce the experimental superconducting critical temperature for the bulk and predict the bilayer 2M-WS₂, a two-dimensional (2D) Z₂ topological metal with nontrivial edge states right at the Fermi energy, to superconduct at 7 K, much higher than that in most 2D transition metal dichalcogenides (TMDs). A distinct proximity-enhanced surface superconductivity is further revealed by simulating quasiparticle density of states. This work unveils a universal electron-phonon full-gap pairing in 2M group VI TMDs and suggests a strong intrinsic surface-bulk proximity effect for 2M-WS₂, paving the way to engineering topological superconductivity in TMD-based nanoscale devices.

Nodal Flexible-surface Semimetals: Case of Carbon Nanotube Networks

Nodal surface-based topological semimetals (TSMs) are drawing attention due to their unique excitation and plasmon behaviors. However, only nodal flat-surface and nodal sphere TSMs are theoretically proposed due to strict symmetry requirements. Here, we propose that a series of surface-based topological phases can be realized in a tight-binding (TB) model with sublattice symmetry. These topological phases, named as nodal flexible-surface semimetals, include not only nodal surface and nodal sphere TSMs but also novel phases, like nodal tube, nodal crossbar, and nodal hourglass-like surface TSMs. According to the TB model, a family of carbon nanotube networks are then identified as nodal flexible-surface TSMs by first-principles calculations, and the topological phase transitions between these TSMs can be induced by strains. Moreover, the nodal flexible-surface TSMs with intrinsic high density of states at the Fermi level and special drumhead surface states are promising for studying high-temperature superconductors and strong correlation effects.

Theory of universal differential conductance of magnetic Weyl type-II junctions

We study the transport properties of junctions of normal and superconducting Weyl semimetal with tilted dispersion, in the presence of magnetization induced by magnetic strips. The sub gap tunnelling conductance shows robust signatures in the presence of different orientation and strength of magnetization of the magnetic strips. We obtain the analytical results for the normal-magnetic-superconducting junction in the thin barrier limit and demonstrate that these results have no analogues to their conventional counterparts and junctions with Dirac electrons in two-dimensions. We discuss possible experimental setups to test our theoretical predictions.

First-Principles Study of the Effect of Native Defects on Spin Polarization and Exchange Coupling Interaction in Semimetal V3O4

We use first-principles calculations to investigate the mechanism of the effect of native defects on the spin polarization and exchange coupling interaction in the V₃O₄ semimetal material. Our results reveal that, in contrast to other neutral defects, V vacancy defects in V₃O₄ at A/B sites are in favor of higher spin polarization degrees and lower defect formation energies. Compared to ideal V₃O₄, the V vacancy defects at A/B sites cause slightly lower spin polarization degrees but much higher exchange coupling interactions. Our results suggest an effective route to mediate the spin polarization and exchange coupling by defect engineering, which promotes the applications of the V₃O₄ semimetal material in spintronics.

Direct Evidence for Charge Compensation-Induced Large Magnetoresistance in Thin WTe2

Since the discovery of extremely large nonsaturating magnetoresistance (MR) in WTe₂, much effort has been devoted to understanding the underlying mechanism, which is still under debate. Here, we explicitly identify the dominant physical origin of the large nonsaturating MR through in situ tuning of the magneto-transport properties in thin WTe₂ film. With an electrostatic doping approach, we observed a nonmonotonic gate dependence of the MR. The MR reaches a maximum (10600%) in thin WTe₂ film at certain gate voltage where electron and hole concentrations are balanced, indicating that the charge compensation is the dominant mechanism of the observed large MR. Besides, we show that the temperature-dependent magnetoresistance exhibits similar tendency with the carrier mobility when the charge compensation is retained, revealing that distinct scattering mechanisms may be at play for the temperature dependence of magneto-transport properties. Our work would be helpful for understanding mechanism of the large MR in other nonmagnetic materials and offers an avenue for achieving large MR in the nonmagnetic materials with electron-hole pockets.