Probing and Tuning the Spin Textures of the K and Q Valleys in Few-Layer MoS2

The strong spin-orbit coupling along with broken inversion symmetry in transition metal dichalcogenides (TMDs) results in spin polarized valleys, which are the origins of many interesting properties such as Ising superconductivity, circular dichroism, valley Hall effect, etc. Herein, it is shown that encapsulating few-layer MoS2 between hexagonal boron nitride (h-BN) and gating the electrical contacts by ionic liquid pronounce Shubnikov-de Haas (SdH) oscillations in magnetoresistance. Notably, the SdH oscillations remain unchanged in tilted magnetic fields, demonstrating that the spins of the Q/Q ' valleys are firmly locked to the out-of-plane direction; therefore, Zeeman energy is insensitive to the in-plane magnetic field. Ionic liquid gating induces superconductivity on the surface of unencapsulated MoS2. The spins of Cooper pairs are strongly pinned to the out-of-plane direction by the effective Zeeman field, hence are protected from being realigned by an in-plane magnetic field, namely, Ising protection. As a result, superconductivity persists in an in-plane magnetic field up to 14 T, in which T-c only decreases by approximate to 0.3 K from T-c0 as approximate to 7 K. By applying back gate, the strength of Ising protection can be effectively tuned, where an increase in 70% is observed when back gate changes from +90 to -90 V

A Flip-Over Plasmonic Structure for Photoluminescence Enhancement of Encapsulated WS₂ Monolayers

Transition metal dichalcogenide (TMD) monolayers, with their direct band gaps, have attracted wide attention from the fields of photonics and optoelectronics. However, monolayer semiconducting TMDs generally suffer from low excitation absorption and emission efficiency, limiting their further applications. Here a flip-over plasmonic structure comprised of silver nano-disk arrays supporting a WS2 monolayer sandwiched by hexagonal boron nitride (h-BN) layers is demonstrated. The flip-over configuration optimizes the optical process with a free excitation/emission path from the top and a strong plasmonic interaction from the bottom. As a result, the photoluminescence from the TMD monolayers can be greatly enhanced more than tenfold by optimizing the metasurface, which can be further improved nearly tenfold by optimizing the thickness of bottom h-BN. This study shows the advantages of using the flip-over structure, where the plasmonic interaction between the metasurface and TMDs can be tuned by introducing optimized plasmonic arrays and h-BN layers with suitable thickness. This hybrid device configuration paves a reliable platform to study the light–matter interaction, achieving highly efficient plasmonic TMD devices

Correlated States in Strained Twisted Bilayer Graphenes Away from the Magic Angle

Graphene moiré superlattice formed by rotating two graphene sheets can host strongly correlated and topological states when flat bands form at so-called magic angles. Here, we report that, for a twisting angle far away from the magic angle, the heterostrain induced during stacking heterostructures can also create flat bands. Combining a direct visualization of strain effect in twisted bilayer graphene moiré superlattices and transport measurements, features of correlated states appear at "non-magic"angles in twisted bilayer graphene under the heterostrain. Observing correlated states in these "non-standard"conditions can enrich the understanding of the possible origins of the correlated states and widen the freedom in tuning the moiré heterostructures and the scope of exploring the correlated physics in moiré superlattices

Highly Conductive Metallic State and Strong Spin-Orbit Interaction in Annealed Germanane

Similar to carbon, germanium exists in various structures such as three-dimensional crystalline germanium and germanene, a two-dimensional germanium atomic layer. Regarding the electronic properties, they are either semiconductors or Dirac semimetals. Here, we report a highly conductive metallic state in thermally annealed germanane (hydrogen-terminated germanene, GeH), which shows a resistivity of similar to 10(-7) Omega.m that is orders of magnitude lower than any other allotrope of germanium. By comparing the resistivity, Raman spectra, and thickness change measured by AFM, we suggest the highly conductive metallic state is associated with the dehydrogenation during heating, which likely transforms germanane thin flakes to multilayer germanene. In addition, weak antilocalization is observed, serving as solid evidence for strong spin-orbit interaction (SOI) in germanane/germanene. Our study opens a possible new route to investigate the electrical transport properties of germanane/germanene, and the large SOI might provide the essential ingredients to access their topological states predicted theoretically

Quantum Phase Transitions in Clean Ising Superconductors

Superconductivity as a macroscopic quantum state has been intensively studied since its discovery in 1908. Its applications have been significantly broadened, from the high-field coil, and superconducting sensors to quantum information. Especially, the great demand for data processing capability in artificial intelligence has stimulated the vast development in superconductor-based quantum computing. Alongside the engineering application, the understanding of superconductivity has been extended. Exotic superconductivity has been proposed, such as spin-triplet pairing, pair density wave, quantum Griffiths state, and Fulde–Ferrell–Larkin–Ovchinnikov state, which will in turn stimulate the engineering application of superconductivity. This thesis focuses on the quantum phase transitions in two Ising superconductors, the ionic liquid-gated MoS2, and multilayer NbSe2. The Bose-metal-like states and quantum Griffiths states are studied in the ionic liquid-gated MoS2 under perpendicular fields. An unconventional Fulde–Ferrell–Larkin–Ovchinnikov state, induced by coupling of Ising SOC and orbital effect, is studied in multilayer NbSe2 under parallel magnetic fields

Orbital Fulde–Ferrell–Larkin–Ovchinnikov state in an Ising superconductor

The conventional Fulde-Ferrell-Larkin-Ovchinnikov (FFLO) state relies on the Zeeman effect of an external magnetic field to break time-reversal symmetry, forming a state of finite-momentum Cooper pairing. In superconductors with broken inversion symmetries, the Rashba or Ising-type spin-orbit coupling (SOC) can interact with either the Zeeman or the orbital effect of magnetic fields, extending the range of possible FFLO states, though evidence for these more exotic forms of FFLO pairing has been lacking. Here we report the discovery of an unconventional FFLO state induced by coupling the Ising SOC and the orbital effect in multilayer 2H-NbSe2. Transport measurements show that the translational and rotational symmetries are broken in the orbital FFLO state, providing the hallmark signatures of finite momentum cooper pairings. We establish the entire orbital FFLO phase diagram, consisting of normal metal, uniform Ising superconducting phase, and a six-fold orbital FFLO state. This study highlights an alternative route to finite-momentum superconductivity and provides a universal mechanism to prepare orbital FFLO states in similar materials with broken inversion symmetries