An unusual continuous paramagnetic-limited superconducting phase transition in 2D NbSe 2

Reversible modulation of superconductivity in thin-film NbSe2 via plasmon coupling

In recent years, lightwave has stood out as an ultrafast, non-contact control knob for developing compact superconducting circuitry. However, the modulation efficiency is limited by the low photoresponse of superconductors. Plasmons, with the advantages of strong light-matter interaction, present a promising route to overcome the limitations. Here we achieve effective modulation of superconductivity in thin-film NbSe2 via near-field coupling to plasmons in gold nanoparticles. Upon resonant plasmon excitation, the superconductivity of NbSe2 is substantially suppressed. The modulation factor exceeds 40% at a photon flux of 9.36 × 1013 s-1mm-2, and the effect is significantly diminished for thicker NbSe2 samples. Our observations can be theoretically interpreted by invoking the non-equilibrium electron distribution in NbSe2 driven by the plasmon-associated evanescent field. Finally, a reversible plasmon-driven superconducting switch is realized in this system. These findings highlight plasmonic tailoring of quantum states as an innovative strategy for superconducting electronics.

Fundamental Limits of Few-Layer NbSe2 Microbolometers at Terahertz Frequencies

The rapid development of infrared spectroscopy, observational astronomy, and scanning near-field microscopy has been enabled by the emergence of sensitive mid- and far-infrared photodetectors. Superconducting hot-electron bolometers (HEBs), known for their exceptional signal-to-noise ratio and fast photoresponse, play a crucial role in these applications. While superconducting HEBs are traditionally crafted from sputtered thin films such as NbN, the potential of layered van der Waals (vdW) superconductors is untapped at THz frequencies. Here, we introduce superconducting HEBs made from few-layer NbSe2 microwires. By improving the interface between NbSe2 and metal leads, we overcome impedance mismatch with RF readout, enabling large responsivity THz detection (0.13 to 2.5 THz) with a minimal noise equivalent power of 7 pW/ H z and nanosecond-range response time. Our work highlights NbSe2 as a promising platform for HEB technology and presents a reliable vdW assembly protocol for custom bolometer production.

Anisotropic Superconducting Nb2 CTx MXene Processed by Atomic Exchange at the Wafer Scale

Superconductivty has recently been induced in MXenes through surface modification. However, the previous reports have mostly been based on powders or cold-pressed pellets, with no known reports on the intrinsic superconsucting properties of MXenes at the nanoale. Here, it is developed a high-temperature atomic exchange process in NH3 atmosphere which induces superconductivity in either singleflakes or thin films of Nb2 CTx MXene. The exchange process between nitrogen atoms and fluorine, carbon, and oxygen atoms in the MXene lattice and related structural adjustments are studied using both experiments and density functional theory. Using either single-flake or thin-film devices, an anisotropic magnetic response of the 2D superconducting transformation has been successfully revealed. The anisotropic superconductivity is further demonstrated using superconducting thin films uniformly deposited over a 4 in. wafers, which opens up the possibility of scalable MXene-based superconducting devices.

Topotactic fabrication of transition metal dichalcogenide superconducting nanocircuits

Superconducting nanocircuits, which are usually fabricated from superconductor films, are the core of superconducting electronic devices. While emerging transition-metal dichalcogenide superconductors (TMDSCs) with exotic properties show promise for exploiting new superconducting mechanisms and applications, their environmental instability leads to a substantial challenge for the nondestructive preparation of TMDSC nanocircuits. Here, we report a universal strategy to fabricate TMDSC nanopatterns via a topotactic conversion method using prepatterned metals as precursors. Typically, robust NbSe₂ meandering nanowires can be controllably manufactured on a wafer scale, by which a superconducting nanowire circuit is principally demonstrated toward potential single photon detection. Moreover, versatile superconducting nanocircuits, e.g., periodical circle/triangle hole arrays and spiral nanowires, can be prepared with selected TMD materials (NbS₂, TiSe₂, or MoTe₂). This work provides a generic approach for fabricating nondestructive TMDSC nanocircuits with precise control, which paves the way for the application of TMDSCs in future electronics.

Orbital Fulde-Ferrell Pairing State in Moiré Ising Superconductors

In this Letter, we study superconducting moiré homobilayer transition metal dichalcogenides where the Ising spin-orbit coupling (SOC) is much larger than the moiré bandwidth. We call such noncentrosymmetric superconductors, moiré Ising superconductors. Because of the large Ising SOC, the depairing effect caused by the Zeeman field is negligible and the in-plane upper critical field (B_{c2}) is determined by the orbital effects. This allows us to study the effect of large orbital fields. Interestingly, when the applied in-plane field is larger than the conventional orbital B_{c2}, a finite-momentum pairing phase would appear which we call the orbital Fulde-Ferrell (FF) state. In this state, the Cooper pairs acquire a net momentum of 2q_{B}, where 2q_{B}=eBd is the momentum shift caused by the magnetic field B and d denotes the layer separation. This orbital field-driven FF state is different from the conventional FF state driven by Zeeman effects in Rashba superconductors. Remarkably, we predict that the FF pairing would result in a giant superconducting diode effect under electric gating when layer asymmetry is induced. An upturn of the B_{c2} as the temperature is lowered, coupled with the giant superconducting diode effect, would allow the detection of the orbital FF state.

Tuning metal/superconductor to insulator/superconductor coupling via control of proximity enhancement between NbSe2monolayers

The interplay between charge transfer and electronic disorder in transition-metal dichalcogenide multilayers gives rise to superconductive coupling driven by proximity enhancement, tunneling and superconducting fluctuations, of a yet unwieldy variety. Artificial spacer layers introduced with atomic precision change the density of states by charge transfer. Here, we tune the superconductive coupling betweenNbSe2monolayers from proximity-enhanced to tunneling-dominated. We correlate normal and superconducting properties inSnSe1+δmNbSe21tailored multilayers with varying SnSe layer thickness (m=1-15). From high-field magnetotransport the critical fields yield Ginzburg-Landau coherence lengths with an increase of140%cross-plane (m=1-9), trending towards two-dimensional superconductivity form>9. We show cross-overs between three regimes: metallic with proximity-enhanced coupling (m=1-4), disordered-metallic with intermediate coupling (m=5-9) and insulating with Josephson tunneling (m>9). Our results demonstrate that stacking metal mono- and dichalcogenides allows to convert a metal/superconductor into an insulator/superconductor system, prospecting the control of two-dimensional superconductivity in embedded layers.

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.

Tunneling Spectroscopy of Two-Dimensional Materials Based on Via Contacts

We introduce a novel planar tunneling architecture for van der Waals heterostructures based on via contacts, namely, metallic contacts embedded into through-holes in hexagonal boron nitride (hBN). We use the via-based tunneling method to study the single-particle density of states of two different two-dimensional (2D) materials, NbSe₂ and graphene. In NbSe₂ devices, we characterize the barrier strength and interface disorder for barrier thicknesses of 0, 1, and 2 layers of hBN and study the dependence on the tunnel-contact area down to (44 ± 14)² nm². For 0-layer hBN devices, we demonstrate a crossover from diffusive to point contacts in the small-contact-area limit. In graphene, we show that reducing the tunnel barrier thickness and area can suppress effects due to phonon-assisted tunneling and defects in the hBN barrier. This via-based architecture overcomes limitations of other planar tunneling designs and produces high-quality, ultraclean tunneling structures from a variety of 2D materials.

Observation of Superconducting Collective Modes from Competing Pairing Instabilities in Single-Layer NbSe2

In certain unconventional superconductors with sizable electronic correlations, the availability of closely competing pairing channels leads to characteristic soft collective fluctuations of the order parameters, which leave fingerprints in many observables and allow the phase competition to be scrutinized. Superconducting layered materials, where electron-electron interactions are enhanced with decreasing thickness, are promising candidates to display these correlation effects. In this work, the existence of a soft collective mode in single-layer NbSe₂ , observed as a characteristic resonance excitation in high-resolution tunneling spectra is reported. This resonance is observed along with higher harmonics, its frequency Ω/2Δ is anticorrelated with the local superconducting gap Δ, and its amplitude gradually vanishes by increasing the temperature and upon applying a magnetic field up to the critical values (T_C and H_C2 ), which sets an unambiguous link to the superconducting state. Aided by a microscopic model that captures the main experimental observations, this resonance is interpreted as a collective Leggett mode that represents the fluctuation toward a proximate f-wave triplet state, due to subleading attraction in the triplet channel. These findings demonstrate the fundamental role of correlations in superconducting 2D transition metal dichalcogenides, opening a path toward unconventional superconductivity in simple, scalable, and transferable 2D superconductors.

van der Waals π Josephson Junctions

Proximity-induced superconductivity in a ferromagnet can induce Cooper pairs with a finite center-of-mass momentum and stabilize Josephson junctions (JJs) with π phase difference in superconductor-ferromagnet-superconductor heterostructures. The emergence of two-dimensional layered superconducting and magnetic materials promises a new platform for realizing π JJs with atomically sharp interfaces. Here we demonstrate a thickness-driven 0-π transition in JJs made of NbSe₂ (an Ising superconductor) and Cr₂Ge₂Te₆ (a ferromagnetic semiconductor). By systematically increasing the Cr₂Ge₂Te₆ weak link thickness, we observe a vanishing supercurrent at a critical thickness of ∼8 nm, followed by a re-entrant supercurrent. Near the critical thickness, we further observe unusual supercurrent interference patterns with vanishing critical current around zero in-plane magnetic field. They signify the formation of 0-π JJs (with both 0 and π regions), likely induced by the nanoscale magnetic domains in Cr₂Ge₂Te₆.

Controllable Synthesis Quadratic-Dependent Unsaturated Magnetoresistance of Two-Dimensional Nonlayered Fe7S8 with Robust Environmental Stability

Two-dimensional (2D) iron chalcogenides (FeX, X = S, Se, Te) are emerging as an appealing class of materials for a wide range of research topics, including electronics, spintronics, and catalysis. However, the controlled syntheses and intrinsic property explorations of such fascinating materials still remain daunting challenges, especially for 2D nonlayered Fe₇S₈ with mixed-valence states and high conductivity. Herein, we design a general and temperature-mediated chemical vapor deposition (CVD) approach to synthesize ultrathin and large-domain Fe₇S₈ nanosheets on mica substrates, with the thickness down to ∼4.4 nm (2 unit-cell). Significantly, we uncover a quadratic-dependent unsaturated magnetoresistance (MR) with out-of-plane anisotropy in 2D Fe₇S₈, thanks to its ultrahigh crystalline quality and high conductivity (∼2.7 × 10⁵ S m^-1 at room temperature and ∼1.7 × 10⁶ S m^-1 at 2 K). More interestingly, the CVD-synthesized 2D Fe₇S₈ nanosheets maintain robust environmental stability for more than 8 months. These results hereby lay solid foundations for synthesizing 2D nonlayered iron chalcogenides with mixed-valence states and exploring fascinating quantum phenomena.

Fermi surface chirality induced in a TaSe2 monosheet formed by a Ta/Bi2Se3 interface reaction

Spin-momentum locking in topological insulators and materials with Rashba-type interactions is an extremely attractive feature for novel spintronic devices and is therefore under intense investigation. Significant efforts are underway to identify new material systems with spin-momentum locking, but also to create heterostructures with new spintronic functionalities. In the present study we address both subjects and investigate a van der Waals-type heterostructure consisting of the topological insulator Bi₂Se₃ and a single Se-Ta-Se triple-layer (TL) of H-type TaSe₂ grown by a method which exploits an interface reaction between the adsorbed metal and selenium. We then show, using surface x-ray diffraction, that the symmetry of the TaSe₂-like TL is reduced from D_3h to C_3v resulting from a vertical atomic shift of the tantalum atom. Spin- and momentum-resolved photoemission indicates that, owing to the symmetry lowering, the states at the Fermi surface acquire an in-plane spin component forming a surface contour with a helical Rashba-like spin texture, which is coupled to the Dirac cone of the substrate. Our approach provides a route to realize chiral two-dimensional electron systems via interface engineering in van der Waals epitaxy that do not exist in the corresponding bulk materials.

Current limitations of spintronics devices based on bulk topological materials stimulate the search for new materials and structures with interesting spin properties. Here the authors report a chiral spin texture around the Fermi level related to structural symmetry breaking in a TaSe₂ layer grown on a Bi2Se3 surface.

Polytypic Phase Transition of Nb1-xVxSe2 via Colloidal Synthesis and Their Catalytic Activity toward Hydrogen Evolution Reaction

Polytypes of two-dimensional transition metal dichalcogenide can extend the architecture and application of nanostructures. Herein, Nb_1-xV_xSe₂ alloy nanosheets in the full composition range (x) were synthesized by a colloidal reaction. At x = 0.1-0.3, a phase transition occurred from various hexagonal (three 2H and one 4H types) phase NbSe₂ to an atomically homogeneous 1T phase VSe₂. Density functional theory calculations also revealed a polytypic phase transition at x = 0.3, which was shifted close to 0 in the presence of Se vacancies. Furthermore, the calculations validate favorable formation of Se vacancies at the phase transition. The sample at x = 0.3 exhibited enhanced electrocatalytic activity toward the hydrogen evolution reaction (HER) in 0.5 M H₂SO₄. The Gibbs free energy indicates that the catalytic HER performance is correlated with the active Se vacancy sites of polytypic structures.

Commensurate Stacking Phase Transitions in an Intercalated Transition Metal Dichalcogenide

Intercalation and stacking-order modulation are two active ways in manipulating the interlayer interaction of transition metal dichalcogenides (TMDCs), which lead to a variety of emergent phases and allow for engineering material properties. Herein, the growth of Pb-intercalated TMDCs-Pb(Ta_1+x Se₂ )₂ , the first 124-phase, is reported. Pb(Ta_1+x Se₂ )₂ exhibits a unique two-step first-order structural phase transition at around 230 K. The transitions are solely associated with the stacking degree of freedom, evolving from a high-temperature (high-T) phase with ABC stacking and R3m symmetry to an intermediate phase with AB stacking and P3m1, and finally to a low-temperature (low-T) phase again with R3msymmetry, but with ACB stacking. Each step involves a rigid slide of building blocks by a vector [1/3, 2/3, 0]. Intriguingly, gigantic lattice contractions occur at the transitions on warming. At low-T, bulk superconductivity with T_c  ≈ 1.8 K is observed. The underlying physics of the structural phase transitions are discussed from first-principle calculations. The symmetry analysis reveals topological nodal lines in the band structure. The results demonstrate the possibility of realizing higher-order metal-intercalated phases of TMDCs and advance the knowledge of polymorphic transitions, and may inspire stacking-order engineering in TMDCs and beyond.

Observation of Coexisting Weak Localization and Superconducting Fluctuations in Strained Sn1-xInxTe Thin Films

Topological superconductors have attracted tremendous excitement as they are predicted to host Majorana zero modes that can be utilized for topological quantum computing. Candidate topological superconductor Sn_1-xIn_xTe thin films (0 < x < 0.3) grown by molecular beam epitaxy and strained in the (111) plane are shown to host quantum interference effects in the conductivity coexisting with superconducting fluctuations above the critical temperature T_c. An analysis of the normal state magnetoresistance reveals these effects. A crossover from weak antilocalization to localization is consistently observed in superconducting samples, indicating that superconductivity originates dominantly from charge carriers occupying trivial states that may be strongly spin-orbit split. A large enhancement of the conductivity is observed above T_c, indicating the presence of superconducting fluctuations. Our results motivate a re-examination of the debated pairing symmetry of this material when subjected to quantum confinement and lattice strain.

Phase-Tunable Synthesis and Etching-Free Transfer of Two-Dimensional Magnetic FeTe

Two-dimensional (2D) Fe-chalcogenides (e.g., FeS, FeSe, and FeTe, etc.) have sparked extensive interest due to their rich phase diagrams including superconductivity, magnetism, and topological state, as well as versatile applications in electronic devices and energy related fields. However, the phase-tunable synthesis and green transfer of such fascinating materials still remain challenging. Herein, we develop a temperature-mediated chemical vapor deposition (CVD) approach to grow ultrathin nonlayered hexagonal and layered tetragonal FeTe nanosheets on mica substrates, with their thicknesses down to ∼2.3 and ∼4.0 nm, respectively. Interestingly, we have observed exciting ferromagnetism with the Curie temperature approaching ∼300 K and high conductivity (∼1.96 × 10⁵ S m^-1) in 2D hexagonal FeTe. More significantly, we have designed a swift, high-efficiency, and etching-free method for the transfer of 2D FeTe nanosheets onto arbitrary substrates, and such a transfer strategy enables the cyclic utilization of growth substrates. These results should propel the further development of phase-tunable synthesis and green transfer of 2D Fe-chalcogenides, as well as their potential applications in spintronic devices.

Molecular Approach to Engineer Two-Dimensional Devices for CMOS and beyond-CMOS Applications

Two-dimensional materials (2DMs) have attracted tremendous research interest over the last two decades. Their unique optical, electronic, thermal, and mechanical properties make 2DMs key building blocks for the fabrication of novel complementary metal-oxide-semiconductor (CMOS) and beyond-CMOS devices. Major advances in device functionality and performance have been made by the covalent or noncovalent functionalization of 2DMs with molecules: while the molecular coating of metal electrodes and dielectrics allows for more efficient charge injection and transport through the 2DMs, the combination of dynamic molecular systems, capable to respond to external stimuli, with 2DMs makes it possible to generate hybrid systems possessing new properties by realizing stimuli-responsive functional devices and thereby enabling functional diversification in More-than-Moore technologies. In this review, we first introduce emerging 2DMs, various classes of (macro)molecules, and molecular switches and discuss their relevant properties. We then turn to 2DM/molecule hybrid systems and the various physical and chemical strategies used to synthesize them. Next, we discuss the use of molecules and assemblies thereof to boost the performance of 2D transistors for CMOS applications and to impart diverse functionalities in beyond-CMOS devices. Finally, we present the challenges, opportunities, and long-term perspectives in this technologically promising field.

Ising pairing in atomically thin superconductors

Ising-type pairing in atomically thin superconducting materials has emerged as a novel means of generating devices with resilience to a magnetic field applied parallel to the two-dimensional (2D) plane. In this mini-review, we canvas the state of the field by giving a historical account of 2D superconductors with strongly enhanced in-plane upper critical fields, together with the type-I and type-II Ising pairing mechanisms. We highlight the vital role of spin-orbit coupling in these superconductors and discuss other effects such as symmetry breaking, atomic thicknesses, etc. Finally, we summarize the recent theoretical proposals and highlight the open questions, such as exploring topological superconductivity in these systems and looking for more materials with Ising pairing.

Unconventional supercurrent phase in Ising superconductor Josephson junction with atomically thin magnetic insulator

In two-dimensional (2D) NbSe₂ crystal, which lacks inversion symmetry, strong spin-orbit coupling aligns the spins of Cooper pairs to the orbital valleys, forming Ising Cooper pairs (ICPs). The unusual spin texture of ICPs can be further modulated by introducing magnetic exchange. Here, we report unconventional supercurrent phase in van der Waals heterostructure Josephson junctions (JJs) that couples NbSe₂ ICPs across an atomically thin magnetic insulator (MI) Cr₂Ge₂Te₆. By constructing a superconducting quantum interference device (SQUID), we measure the phase of the transferred Cooper pairs in the MI JJ. We demonstrate a doubly degenerate nontrivial JJ phase (ϕ), formed by momentum-conserving tunneling of ICPs across magnetic domains in the barrier. The doubly degenerate ground states in MI JJs provide a two-level quantum system that can be utilized as a new dissipationless component for superconducting quantum devices. Our work boosts the study of various superconducting states with spin-orbit coupling, opening up an avenue to designing new superconducting phase-controlled quantum electronic devices.

Van der Waals structures provide a new platform to explore novel physics of superconductor/ferromagnet interfaces. Here, NbSe₂ Josephson junction with Cr₂Ge₂Te₆ enables non-trivial Josephson phase by spin-dependent interaction, boosting the study of superconducting states with spin-orbit coupling and phase-controlled quantum electronic device.

Spin-textured Chern bands in AB-stacked transition metal dichalcogenide bilayers

While transition-metal dichalcogenide (TMD)-based moiré materials have been shown to host various correlated electronic phenomena, topological states have not been experimentally observed until now [T. Li et al., Quantum anomalous Hall effect from intertwined moiré bands. arXiv [Preprint] (2021). https://arxiv.org/abs/2107.01796 (Accessed 5 July 2021)]. In this work, using first-principle calculations and continuum modeling, we reveal the displacement field-induced topological moiré bands in AB-stacked TMD heterobilayer [Formula: see text]/[Formula: see text] Valley-contrasting Chern bands with nontrivial spin texture are formed from interlayer hybridization between [Formula: see text] and [Formula: see text] bands of nominally opposite spins. Our study establishes a recipe for creating topological bands in AB-stacked TMD bilayers in general, which provides a highly tunable platform for realizing quantum-spin Hall and interaction-induced quantum anomalous Hall effects.

Magnetic Field-Induced "Mirage" Gap in an Ising Superconductor

Superconductivity is commonly destroyed by a magnetic field due to orbital or Zeeman-induced pair breaking. Surprisingly, the spin-valley locking in a two-dimensional superconductor with spin-orbit interaction makes the superconducting state resilient to large magnetic fields. We investigate the spectral properties of such an Ising superconductor in a magnetic field taking into account disorder. The interplay of the in-plane magnetic field and the Ising spin-orbit coupling leads to noncollinear effective fields. We find that the emerging singlet and triplet pairing correlations manifest themselves in the occurrence of "mirage" gaps: at (high) energies of the order of the spin-orbit coupling strength, a gaplike structure in the spectrum emerges that mirrors the main superconducting gap. We show that these mirage gaps are signatures of the equal-spin triplet finite-energy pairing correlations and due to their odd parity are sensitive to intervalley scattering.

Recent Advances in 2D Superconductors

The emergence of superconductivity in 2D materials has attracted much attention and there has been rapid development in recent years because of their fruitful physical properties, such as high transition temperature (T_c ), continuous phase transition, and enhanced parallel critical magnetic field (B_c ). Tremendous efforts have been devoted to exploring different physical parameters to figure out the mechanisms behind the unexpected superconductivity phenomena, including adjusting the thickness of samples, fabricating various heterostructures, tuning the carrier density by electric field and chemical doping, and so on. Here, different types of 2D superconductivity with their unique characteristics are introduced, including the conventional Bardeen-Cooper-Schrieffer superconductivity in ultrathin films, high-T_c superconductivity in Fe-based and Cu-based 2D superconductors, unconventional superconductivity in newly discovered twist-angle bilayer graphene, superconductivity with enhanced B_c , and topological superconductivity. A perspective toward this field is then proposed based on academic knowledge from the recently reported literature. The aim is to provide researchers with a clear and comprehensive understanding about the newly developed 2D superconductivity and promote the development of this field much further.

Concurrent Vacancy and Adatom Defects of Mo1-xNbxSe2 Alloy Nanosheets Enhance Electrochemical Performance of Hydrogen Evolution Reaction

Earth-abundant transition metal dichalcogenide nanosheets have emerged as an excellent catalyst for electrochemical water splitting to generate H₂. Alloying the nanosheets with heteroatoms is a promising strategy to enhance their catalytic performance. Herein, we synthesized hexagonal (2H) phase Mo_1-xNb_xSe₂ nanosheets over the whole composition range using a solvothermal reaction. Alloying results in a variety of atomic-scale crystal defects such as Se vacancies, metal vacancies, and adatoms. The defect content is maximized when x approaches 0.5. Detailed structure analysis revealed that the NbSe₂ bonding structures in the alloy phase are more disordered than the MoSe₂ ones. Compared to MoSe₂ and NbSe₂, Mo_0.5Nb_0.5Se₂ exhibits much higher electrocatalytic performance for hydrogen evolution reaction. First-principles calculation was performed for the formation energy in the models for vacancies and adatoms, supporting that the alloy phase has more defects than either NbSe₂ or MoSe₂. The calculation predicted that the separated NbSe₂ domain at x = 0.5 favors the concurrent formation of Nb/Se vacancies and adatoms in a highly cooperative way. Moreover, the Gibbs free energy along the reaction path suggests that the enhanced HER performance of alloy nanosheets originates from the higher concentration of defects that favor H atom adsorption.

Nonreciprocal superconducting NbSe2 antenna

The rise of two-dimensional (2D) crystalline superconductors has opened a new frontier of investigating unconventional quantum phenomena in low dimensions. However, despite the enormous advances achieved towards understanding the underlying physics, practical device applications like sensors and detectors using 2D superconductors are still lacking. Here, we demonstrate nonreciprocal antenna devices based on atomically thin NbSe₂. Reversible nonreciprocal charge transport is unveiled in 2D NbSe₂ through multi-reversal antisymmetric second harmonic magnetoresistance isotherms. Based on this nonreciprocity, our NbSe₂ antenna devices exhibit a reversible nonreciprocal sensitivity to externally alternating current (AC) electromagnetic waves, which is attributed to the vortex flow in asymmetric pinning potentials driven by the AC driving force. More importantly, a successful control of the nonreciprocal sensitivity of the antenna devices has been achieved by applying electromagnetic waves with different frequencies and amplitudes. The device’s response increases with increasing electromagnetic wave amplitude and exhibits prominent broadband sensing from 5 to 900 MHz.

Here, the authors observe reversible nonreciprocal charge transport in two-dimensional NbSe₂, and demonstrate antenna devices exhibiting strong sensitivity to driving AC electromagnetic waves in the superconducting regime.

Pseudogap and Weak Multifractality in 2D Disordered Mott Charge-Density-Wave Insulator

We investigate electronic states of Se-substituted 1T-TaS₂ by scanning tunneling microscopy/spectroscopy (STM/STS), where superconductivity emerges from the unique Mott-charge-density-wave (Mott-CDW) state. Spatially resolved STS measurements reveal that a pseudogap replaces the Mott gap with the CDW gaps intact. The pseudogap has little correlation with the unit-cell-to-unit-cell variation in the local Se concentration but appears globally. The correlation length of the local density of states (LDOS) is substantially enhanced at the Fermi energy and decays rapidly at high energies. Furthermore, the statistical analysis of LDOS indicates the weak multifractal behavior of the wave functions. These findings suggest a correlated metallic state induced by disorder and provide a new insight into the emerging superconductivity in two-dimensional materials.

Spin-Orbit-Parity-Coupled Superconductivity in Topological Monolayer WTe_{2}

Recent experiments reported gate-induced superconductivity in the monolayer 1T^{'}-WTe_{2} which is a two-dimensional topological insulator in its normal state. The in-plane upper critical field B_{c2} is found to exceed the conventional Pauli paramagnetic limit B_{p} by one to three times. The enhancement cannot be explained by conventional spin-orbit coupling which vanishes due to inversion symmetry. In this Letter, we unveil some distinctive superconducting properties of centrosymmetric 1T^{'}-WTe_{2} which arise from the coupling of spin, momentum and band parity degrees of freedom. As a result of this spin-orbit-parity coupling (SOPC): (i) there is a first-order superconductor-metal transition at B_{c2} that is much higher than the Pauli paramagnetic limit B_{p}, (ii) spin-susceptibility is anisotropic with respect to in-plane directions and can result in possible anisotropic B_{c2}, and (iii) the B_{c2} exhibits a strong gate dependence as the spin-orbit-parity coupling is significant only near the topological band crossing points. The importance of SOPC on the topologically nontrivial inter-orbital pairing phase is also discussed. Our theory generally applies to centrosymmetric materials with topological band inversions.

Two-Dimensional Metallic NiTe2 with Ultrahigh Environmental Stability, Conductivity, and Electrocatalytic Activity

Two-dimensional (2D) metallic transition metal dichalcogenides (MTMDCs) supply a versatile platform for investigating newfangled physical issues and developing potential applications in electronics/spintronics/electrocatalysis. Among these, NiTe₂ (a type-II Dirac semimetal) possesses a Dirac point near its Fermi level. However, as-prepared 2D MTMDCs are mostly environmentally unstable, and little attention has been paid to synthesizing such materials. Herein, a general chemical vapor deposition (CVD) approach has been designed to prepare thickness-tunable and large-domain (∼1.5 mm) 1T-NiTe₂ on an atomically flat mica substrate. Significantly, ultrahigh conductivity (∼1.15 × 10⁶ S m^-1) of CVD-synthesized 1T-NiTe₂ and high catalytic activity in pH-universal hydrogen evolution reaction have been uncovered. More interestingly, the 2D 1T-NiTe₂ maintains robust environmental stability for more than one year and even after a variety of harsh treatments. These results hereby fill an existing research gap in synthesizing environmentally stable 2D MTMDCs, making fundamental progress in developing 2D MTMDC-based devices/catalysts.

Enhancement of critical current density in a superconducting NbSe2 step junction

We investigate the transport properties of a NbSe2 nanodevice consisting of a thin region, a thick region and a step junction. The superconducting critical current density of each region of the nanodevice has been studied as a function of temperature and magnetic field. We find that the critical current density has similar values for both the thin and thick regions away from the junction, while the critical current density of the thin region of the junction increases to approximately 1.8 times as compared with the values obtained for the other regions. We attribute such an enhancement of critical current density to the vortex pinning at the surface step. Our study verifies the enhancement of the critical current density by the geometrical-type pinning and sheds light on the application of 2D superconductors.

A new criterion for the prediction of solid-state phase transition in TMDs

The classical thermodynamic criterion for phase transition predicts whether the phase transition will occur according to whether the nth derivative of the state parameter is discontinuous, and the continuity verification of multi-order derivatives increases the difficulty and complexity of judgment for phase transition to a certain extent. Based on the reverse shifts of the DOS curves near the Fermi level, we propose a new criterion for solid-state phase transition named Conch Criterion, which has been verified in the TMD system. The new criterion can observe the occurrence of phase transition from another perspective besides the thermodynamic properties while mutually confirming the thermodynamic criterion.

Lateral heterostructures and one-dimensional interfaces in 2D transition metal dichalcogenides

The growth and exfoliation of two-dimensional (2D) materials have led to the creation of edges and novel interfacial states at the juncture between crystals with different composition or phases. These hybrid heterostructures (HSs) can be built as vertical van der Waals stacks, resulting in a 2D interface, or as stitched adjacent monolayer crystals, resulting in one-dimensional (1D) interfaces. Although most attention has been focused on vertical HSs, increasing theoretical and experimental interest in 1D interfaces is evident. In-plane interfacial states between different 2D materials inherit properties from both crystals, giving rise to robust states with unique 1D non-parabolic dispersion and strong spin-orbit effects. With such unique characteristics, these states provide an exciting platform for realizing 1D physics. Here, we review and discuss advances in 1D heterojunctions, with emphasis on theoretical approaches for describing those between semiconducting transition metal dichalcogenides MX ₂ (with M  =  Mo, W and X  =  S, Se, Te), and how the interfacial states can be characterized and utilized. We also address how the interfaces depend on edge geometries (such as zigzag and armchair) or strain, as lattice parameters differ across the interface, and how these features affect excitonic/optical response. This review is intended to serve as a resource for promoting theoretical and experimental studies in this rapidly evolving field.

Transport evidence of asymmetric spin-orbit coupling in few-layer superconducting 1Td-MoTe2

Two-dimensional transition metal dichalcogenides MX₂ (M = W, Mo, Nb, and X = Te, Se, S) with strong spin-orbit coupling possess plenty of novel physics including superconductivity. Due to the Ising spin-orbit coupling, monolayer NbSe₂ and gated MoS₂ of 2H structure can realize the Ising superconductivity, which manifests itself with in-plane upper critical field far exceeding Pauli paramagnetic limit. Surprisingly, we find that a few-layer 1T_d structure MoTe₂ also exhibits an in-plane upper critical field which goes beyond the Pauli paramagnetic limit. Importantly, the in-plane upper critical field shows an emergent two-fold symmetry which is different from the isotropic in-plane upper critical field in 2H transition metal dichalcogenides. We show that this is a result of an asymmetric spin-orbit coupling in 1T_d transition metal dichalcogenides. Our work provides transport evidence of a new type of asymmetric spin-orbit coupling in transition metal dichalcogenides which may give rise to novel superconducting and spin transport properties.

Ultrathin Magnetic 2D Single-Crystal CrSe

2D magnetic materials have generated an enormous amount of attention due to their unique 2D-limited magnetism and their potential applications in spintronic devices. Recently, most of this research has focused on 2D van der Waals layered magnetic materials exfoliated from the bulk with random size and thicknesses. Controllable growth of these materials is still a great challenge. In contrast, 2D nonlayered magnetic materials have rarely been investigated, not especially regarding their preparation. Cr_n X (X = S, Se and Te; 0 < n < 1), a class of nonlayered transition metal dichalcogenides, has rapidly attracted extensive attention due to its abundance of structural compounds and unique magnetic properties. Herein, the controlled synthesis of ultrathin CrSe crystals, with grain size reaching the sub-millimeter scale, on mica substrates via an ambient pressure chemical vapor deposition (CVD) method is demonstrated. A continuous CrSe film can also be achieved via precise control of the key growth parameters. Importantly, the CVD-grown 2D CrSe crystals possess obvious ferromagnetic properties at temperatures below 280 K, which has not been observed experimentally before. This work broadens the scope of the CVD growth of 2D magnetic materials and highlights their significant application possibilities in spintronics.

Location-selective growth of two-dimensional metallic/semiconducting transition metal dichalcogenide heterostructures

An electrical contact between metallic electrodes and semiconductors is critical for the performance of electronic and optoelectronic devices. Two-dimensional (2D) transition metal dichalcogenides (TMDs) contain semiconducting, metallic and insulating material members, which enables the fabrication of highly integrated electronic devices fully based on 2D TMDs. However, location-selective synthesis of metallic/semiconducting heterostructures by a chemical vapor deposition (CVD) method has rarely been reported. In this study, a two-step CVD method was applied to fabricate 2D metallic/semiconducting heterostructures. Semiconducting WS2 was first synthesized and served as the template for the following CVD growth of metallic NbS2. In the growth process, NbS2 flakes selectively nucleate at the edges of WS2 monolayers, thus resulting in the formation of NbS2 islands circling around the WS2 monolayers. The as-grown NbS2/WS2 heterostructure was further systematically characterized by Raman spectroscopy, atomic force microscopy (AFM) and scanning transition electron microscopy (STEM). The NbS2 layers epitaxially grown on the WS2 monolayers exhibit a 3R phase and there was no discernible lattice strain in the NbS2/WS2 van der Waals (vdW) heterostructure. The growth of the metallic/semiconducting 2D heterostructures could benefit the nanoelectronic device fabrication and provide a platform for the 2D contact resistance study.

Comprehensive optical characterization of atomically thin NbSe2

Transition-metal dichalcogenides (TMDCs) have offered experimental access to quantum confinement in one dimension. In recent years, metallic TMDCs like NbSe₂ have taken center stage with many of them exhibiting interesting temperature-dependent properties such as charge density waves and superconductivity. In this paper, we perform a comprehensive optical analysis of NbSe₂ by utilizing Raman spectroscopy, differential reflectance contrast, and spectroscopic ellipsometry. These analyses, when coupled with Kramers-Kronig analysis, allow us to extract the dielectric functions of bulk and atomically thin NbSe₂ and relate them to the resonant behavior of the Raman spectra.