Twist-Induced Modification in the Electronic Structure of Bilayer WSe2.

The recent discovery of strongly correlated phases in twisted transition-metal dichalcogenides (TMDs) highlights the significant impact of twist-induced modifications on electronic structures. In this study, we employed angle-resolved photoemission spectroscopy with submicrometer spatial resolution (µ-ARPES) to investigate these modifications by comparing valence band structures of twisted (5.3°) and nontwisted (AB-stacked) bilayer regions within the same WSe2 device. Relative to the nontwisted region, the twisted area exhibits pronounced moiré bands and ∼90 meV renormalization at the Γ-valley, substantial momentum separation between different layers, and an absence of flat bands at the K-valley. We further simulated the effects of lattice relaxation, which can flatten the Γ-valley edge but not the K-valley edge. Our results provide a direct visualization of twist-induced modifications in the electronic structures of twisted TMDs and elucidate their valley-dependent responses to lattice relaxation.

Nodal and Nematic Superconducting Phases in NbSe_{2} Monolayers from Competing Superconducting Channels.

Transition metal dichalcogenides like 2H-NbSe_{2} in their two-dimensional (2D) form exhibit Ising superconductivity with the quasiparticle spins being firmly pinned in the direction perpendicular to the basal plane. This enables them to withstand exceptionally high magnetic fields beyond the Pauli limit for superconductivity. Using field-angle-resolved magnetoresistance experiments for fields rotated in the basal plane we investigate the field-angle dependence of the upper critical field (H_{c2}), which directly reflects the symmetry of the superconducting order parameter. We observe a sixfold nodal symmetry superposed on a twofold symmetry. This agrees with theoretical predictions of a nodal topological superconducting phase near H_{c2}, together with a nematic superconducting state. We demonstrate that in NbSe_{2} such unconventional superconducting states can arise from the presence of several competing superconducting channels.

Determining Interaction Enhanced Valley Susceptibility in Spin-Valley-Locked MoS2.

Two-dimensional transition metal dichalcogenides (TMDCs) are recently emerged electronic systems with various novel properties, such as spin-valley locking, circular dichroism, valley Hall effect, and superconductivity. The reduced dimensionality and large effective masses further produce unconventional many-body interaction effects. Here we reveal strong interaction effects in the conduction band of MoS2 by transport experiment. We study the massive Dirac electron Landau levels (LL) in high-quality MoS2 samples with field-effect mobilities of 24 000 cm2/(V·s) at 1.2 K. We identify the valley-resolved LLs and low-lying polarized LLs using the Lifshitz-Kosevitch formula. By further tracing the LL crossings in the Landau fan diagram, we unambiguously determine the density-dependent valley susceptibility and the interaction enhanced g-factor from 12.7 to 23.6. Near integer ratios of Zeeman-to-cyclotron energies, we discover LL anticrossings due to the formation of quantum Hall Ising ferromagnets, the valley polarizations of which appear to be reversible by tuning the density or an in-plane magnetic field. Our results provide evidence for many-body interaction effects in the conduction band of MoS2 and establish a fertile ground for exploring strongly correlated phenomena of massive Dirac electrons.

Odd-Integer Quantum Hall States and Giant Spin Susceptibility in p-Type Few-Layer WSe_{2}.

We fabricate high-mobility p-type few-layer WSe_{2} field-effect transistors and surprisingly observe a series of quantum Hall (QH) states following an unconventional sequence predominated by odd-integer states under a moderate strength magnetic field. By tilting the magnetic field, we discover Landau level crossing effects at ultralow coincident angles, revealing that the Zeeman energy is about 3 times as large as the cyclotron energy near the valence band top at the Γ valley. This result implies the significant roles played by the exchange interactions in p-type few-layer WSe_{2}, in which itinerant or QH ferromagnetism likely occurs. Evidently, the Γ valley of few-layer WSe_{2} offers a unique platform with unusually heavy hole carriers and a substantially enhanced g factor for exploring strongly correlated phenomena.

Van der Waals Encapsulation by Ultrathin Oxide for Air-Sensitive 2D Materials.

The ambient stability is one of the focal points for applications of 2D materials, especially for those well-known air-sensitive ones, such as black phosphorus (BP) and transitional metal telluride. Traditional methods of encapsulation, such as atomic layer deposition of oxides and heterogeneous integration of hexagonal boron nitride, can hardly avoid removal of encapsulation layer when the 2D materials are encapsulated for further device fabrication, which causes complexity and damage during the procedure. Here, a van der Waals encapsulation method that allows direct device fabrication without removal of encapsulation layer is introduced using Ga2O3 from liquid gallium. Taking advantage of the robust isolation ability against ambient environment of the dense native oxide of gallium, hundreds of times longer retention time of (opto)electronic properties of encapsulated BP and MoTe2 devices is realized than unencapsulated devices. Due to the ultrathin high-κ properties of Ga2O3, top-gated devices are directly fabricated with the encapsulation layer, simultaneously as a dielectric layer. This direct device fabrication is realized by selective etching of Ga2O3, leaving the encapsulated materials intact. Encapsulated 1T' MoTe2 exhibits high conductivity even after 150 days in ambient environment. This method is, therefore, highlighted as a promising and distinctive one compared with traditional passivation approaches.

Giant nonlinear Hall effect in twisted bilayer WSe2.

The recently discovered nonlinear Hall effect (NHE) in a few non-interacting systems provides a novel mechanism for generating second-harmonic electrical Hall signals under time-reversal-symmetric conditions. Here, we introduce a new approach to engineering an NHE by using twisted moiré structures. We found that the twisted WSe2 bilayer exhibited an NHE when the Fermi level was tuned to the moiré flat bands. When the first moiré band was half-filled, the nonlinear Hall signal exhibited a sharp peak with a generation efficiency that was at least two orders of magnitude greater than those obtained in previous experiments. We discuss the possible origins of the diverging generation efficiency in twisted WSe2 based on resistivity measurements, such as moiré-interface-induced correlation effects and mass-diverging-type continuous Mott transition. This study demonstrates not only how interaction effects can combine with Berry curvature dipoles to produce novel quantum phenomena, but also the potential of NHE measurements as a new tool for studying quantum criticality.

Bridging the gap between atomically thin semiconductors and metal leads.

Electrically interfacing atomically thin transition metal dichalcogenide semiconductors (TMDSCs) with metal leads is challenging because of undesired interface barriers, which have drastically constrained the electrical performance of TMDSC devices for exploring their unconventional physical properties and realizing potential electronic applications. Here we demonstrate a strategy to achieve nearly barrier-free electrical contacts with few-layer TMDSCs by engineering interfacial bonding distortion. The carrier-injection efficiency of such electrical junction is substantially increased with robust ohmic behaviors from room to cryogenic temperatures. The performance enhancements of TMDSC field-effect transistors are well reflected by the low contact resistance (down to 90 Ωµm in MoS2, towards the quantum limit), the high field-effect mobility (up to 358,000 cm2V-1s-1 in WSe2), and the prominent transport characteristics at cryogenic temperatures. This method also offers possibilities of the local manipulation of atomic structures and electronic properties for TMDSC device design.

Lattice reconstruction induced multiple ultra-flat bands in twisted bilayer WSe2.

Moiré superlattices in van der Waals heterostructures provide a tunable platform to study emergent properties that are absent in the natural crystal form. Twisted bilayer transition metal dichalcogenides (TB-TMDs) can host moiré flat bands over a wide range of twist angles. For twist angle close to 60°, it was predicted that TB-TMDs undergo a lattice reconstruction which causes the formation of ultra-flat bands. Here, by using scanning tunneling microscopy and spectroscopy, we show the emergence of multiple ultra-flat bands in twisted bilayer WSe2 when the twist angle is within 3° of 60°. The ultra-flat bands are manifested as narrow tunneling conductance peaks with estimated bandwidth less than 10 meV, which is only a fraction of the estimated on-site Coulomb repulsion energy. The number of these ultra-flat bands and spatial distribution of the wavefunctions match well with the theoretical predictions, strongly evidencing that the observed ultra-flat bands are induced by lattice reconstruction. Our work provides a foundation for further study of the exotic correlated phases in TB-TMDs.

Layer-dependent interface reconstruction and strain modulation in twisted WSe2.

Twistronics has emerged as one of the most attractive playgrounds for manipulating the interfacial structures and electronic properties of two-dimensional materials. However, the layer-dependent lattice reconstruction and resulted strain distribution in marginally twisted transition metal dichalcogenides still remain elusive. Here we report a systematic study by both electron diffraction quantification and atomic-resolution imaging on the interface reconstruction of twisted WSe2, which shows a strong dependence on the constituent layer numbers and twist angles. The competition between the interlayer interaction, which varies with local atomic configurations, and the intralayer elastic deformation, related to the layer thickness, leads to rich superlattice motifs and strain modulation patterns, i.e. triangular for odd and kagome-like textures for even layer numbers, against the rigid stacking moiré model. The strain effects of small twist angles are further demonstrated by electrical transport measurements, manifesting intriguing conducting states at low temperatures beyond the flat band features of large twist angles. Our work not only provides a comprehensive understanding of layer-dependent twist structures, but also may shed light on the future design of twistronic devices.

Interaction effects and superconductivity signatures in twisted double-bilayer WSe2.

Twisted bilayer graphene provides a new two-dimensional platform for studying electron interaction phenomena and flat band properties such as correlated insulator transition, superconductivity and ferromagnetism at certain magic angles. Here, we present experimental characterization of interaction effects and superconductivity signatures in p-type twisted double-bilayer WSe2. Enhanced interlayer interactions are observed when the twist angle decreases to a few degrees as reflected by the high-order satellites in the electron diffraction patterns taken from the reconstructed domains from a conventional moiré superlattice. In contrast to twisted bilayer graphene, there is no specific magic angle for twisted WSe2. Flat band properties are observable at twist angles ranging from 1 to 4 degrees. Our work has facilitated future study in the area of flat band related properties in twisted transition metal dichalcogenide layered structures.

Intrinsic valley Hall transport in atomically thin MoS2.

Electrons hopping in two-dimensional honeycomb lattices possess a valley degree of freedom in addition to charge and spin. In the absence of inversion symmetry, these systems were predicted to exhibit opposite Hall effects for electrons from different valleys. Such valley Hall effects have been achieved only by extrinsic means, such as substrate coupling, dual gating, and light illuminating. Here we report the first observation of intrinsic valley Hall transport without any extrinsic symmetry breaking in the non-centrosymmetric monolayer and trilayer MoS2, evidenced by considerable nonlocal resistance that scales cubically with local resistance. Such a hallmark survives even at room temperature with a valley diffusion length at micron scale. By contrast, no valley Hall signal is observed in the centrosymmetric bilayer MoS2. Our work elucidates the topological origin of valley Hall effects and marks a significant step towards the purely electrical control of valley degree of freedom in topological valleytronics.