Correlated states in twisted double bilayer graphene

Electron-electron interactions play an important role in graphene and related systems and can induce exotic quantum states, especially in a stacked bilayer with a small twist angle. For bilayer graphene where the two layers are twisted by a "magic angle", flat band and strong many-body effects lead to correlated insulating states and superconductivity. In contrast to monolayer graphene, the band structure of untwisted bilayer graphene can be further tuned by a displacement field, providing an extra degree of freedom to control the flat band that should appear when two bilayers are stacked on top of each other. Here, we report the discovery and characterization of such displacement-field tunable electronic phases in twisted double bilayer graphene. We observe insulating states at a half-filled conduction band in an intermediate range of displacement fields. Furthermore, the resistance gap in the correlated insulator increases with respect to the in-plane magnetic fields and we find that the g factor according to spin Zeeman effect is ~2, indicating spin polarization at half filling. These results establish the twisted double bilayer graphene as an easily tunable platform for exploring quantum many-body states

Layer-by-Layer Epitaxy of Multilayer MoS2 Wafers

Two-dimensional (2D) semiconductor of MoS2 has great potential for advanced electronics technologies beyond silicon1-9. So far, high-quality monolayer MoS2 wafers10-12 are already available and various demonstrations from individual transistors to integrated circuits have also been shown13-15. In addition to the monolayer, multilayers have narrower band gaps but improved carrier mobilities and current capacities over the monolayer5,16-18. However, achieving high-quality multilayer MoS2 wafers remains a challenge. Here we report the growth of high quality multilayer MoS2 4-inch wafers via the layer-by-layer epitaxy process. The epitaxy leads to well-defined stacking orders between adjacent epitaxial layers and offers a delicate control of layer numbers up to 6. Systematic evaluations on the atomic structures and electronic properties were carried out for achieved wafers with different layer numbers. Significant improvements on device performances were found in thicker-layer field effect transistors (FETs), as expected. For example, the average field-effect mobility ({\mu}FE) at room temperature (RT) can increase from ~80 cm2V-1s-1 for monolayer to ~110/145 cm2V-1s-1 for bilayer/trilayer devices. The highest RT {\mu}FE=234.7 cm2V-1s-1 and a record-high on-current densities of 1.704 mA{\mu}m-1 at Vds=2 V were also achieved in trilayer MoS2 FETs with a high on/off ratio exceeding 107. Our work hence moves a step closer to practical applications of 2D MoS2 in electronics.Comment: 13 pages,4 Figure

Thermally induced band hybridization in bilayer-bilayer MoS2/WS2heterostructure

Publisher Copyright: © 2021 Chinese Physical Society and IOP Publishing Ltd.Transition metal dichalcogenides (TMDs), being valley selectively, are an ideal system hosting excitons. Stacking TMDs together to form heterostructure offers an exciting platform to engineer new optical and electronic properties in solid-state systems. However, due to the limited accuracy and repetitiveness of sample preparation, the effects of interlayer coupling on the electronic and excitonic properties have not been systematically investigated. In this report, we study the photoluminescence spectra of bilayer-bilayer MoS2/WS2 heterostructure with a type II band alignment. We demonstrate that thermal annealing can increase interlayer coupling in the van der Waals heterostructures, and after thermally induced band hybridization such heterostructure behaves more like an artificial new solid, rather than just the combination of two individual TMD components. We also carry out experimental and theoretical studies of the electric controllable direct and indirect infrared interlayer excitons in such system. Ourstudy reveals the impact of interlayer coupling on interlayer excitons and will shed light on the understanding and engineering of layer-controlled spin-valley configuration in twisted van der Waals heterostructures.Peer reviewe

Hot-Pressed Two-Dimensional Amorphous Metals and Their Electronic Properties

As an emerging research field, two-dimensional (2D) metals have been the subject of increasing research efforts in recent years due to their potential applications. However, unlike typical 2D layered materials, such as graphene, which can be exfoliated from their bulk parent compounds, it is hardly possible to produce 2D metals through exfoliation techniques due to the absence of Van der Waals gaps. Indeed, the lack of effective material preparation methods severely limits the development of this research field. Here, we report a PDMS-assisted hot-pressing method in glovebox to obtain ultraflat nanometer-thick 2D metals/metal oxide amorphous films of various low-melting-point metals and alloys, e.g., gallium (Ga), indium (In), tin (Sn), and Ga0.87Ag0.13 alloy. The valence states extracted from X-ray photoelectron spectroscopy (XPS) indicate that the ratios of oxidation to metal in our 2D films vary among metals. The temperature-dependent electronic measurements show that the transport behavior of 2D metal/metal oxide films conform with the 2D Mott’s variable range hopping (VRH) model. Our experiments provide a feasible and effective approach to obtain various 2D metals

Giant valley coherence at room temperature in 3R WS2 with broken inversion symmetry

| openaire: EC/H2020/820423/EU//S2QUIPBreaking the space-time symmetries in materials can markedly influence their electronic and optical properties. In 3R-stacked transition metal dichalcogenides, the explicitly broken inversion symmetry enables valley-contrasting Berry curvature and quantization of electronic angular momentum, providing an unprecedented platform for valleytronics. Here, we study the valley coherence of 3R WS2 large single-crystal with thicknesses ranging from monolayer to octalayer at room temperature. Our measurements demonstrate that both A and B excitons possess robust and thickness-independent valley coherence. The valley coherence of direct A (B) excitons can reach 0.742 (0.653) with excitation conditions on resonance with it. Such giant and thickness-independent valley coherence of large single-crystal 3R WS2 at room temperature would provide a firm foundation for quantum manipulation of the valley degree of freedom and practical application of valleytronics.Peer reviewe

Observation of logarithmic Kohn anomaly in monolayer graphene

| openaire: EC/H2020/820423/EU//S2QUIP | openaire: EC/H2020/834742/EU//ATOPElectron-phonon coupling in monolayer graphene breaks the adiabatic Born-Oppenheimer approximation and could lead to exotic logarithmic Kohn anomaly, manifested as logarithmic singularity in optical-phonon energy. However, unraveling unambiguously the fascinating logarithmic Kohn anomaly in monolayer graphene remains challenging due to the large carrier inhomogeneity originating from the unique massless Dirac-like band dispersion and the underneath substrate doping effect. Here we demonstrate a clear signature of intriguing logarithmic Kohn anomaly in monolayer graphene with ultralow carrier inhomogeneity via h-BN encapsulation. Significantly, the magnitude of anomalous phonon softening at 25 K shows an enhancement factor of 2 as compared to that previously observed in bilayer graphene at 12 K, even though bilayer graphene with nearly parabolic band dispersion is more immune to charged impurities. The uncovered unusual logarithmic Kohn anomaly in monolayer graphene can provide a firm basis for the understanding of various peculiar physics and may shed light on the nature of superconductivity in magic-angle graphene superlattices.Peer reviewe