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Abstract Excitons in two-dimensional (2D) semiconductors have offered an attractive platform for optoelectronic and valleytronic devices. Further realizations of correlated phases of excitons promise device concepts not possible in the single particle picture. Here we report tunable exciton “spin” orders in WSe₂/WS₂moiré superlattices. We find evidence of an in-plane (xy) order of exciton “spin”—here, valley pseudospin—around exciton fillingv_ex = 1, which strongly suppresses the out-of-plane “spin” polarization. Upon increasingv_exor applying a small magnetic field of ~10 mT, it transitions into an out-of-plane ferromagnetic (FM-z) spin order that spontaneously enhances the “spin” polarization, i.e., the circular helicity of emission light is higher than the excitation. The phase diagram is qualitatively captured by a spin-1/2 Bose–Hubbard model and is distinct from the fermion case. Our study paves the way for engineering exotic phases of matter from correlated spinor bosons, opening the door to a host of unconventional quantum devices. 

Abstract Previous work that studied hexagonal boron nitride (h‐BN) memristor DC resistive‐switching characteristics is extended to include an experimental understanding of their dynamic behavior upon programming or synaptic weight update. The focus is on the temporal resistive switching response to driving stimulus (programming voltage pulses) effecting conductance updates during training in neural network crossbar implementations. Test arrays are fabricated at the wafer level, enabled by the transfer of CVD‐grown few‐layer (8 layer) or multi‐layer (18 layer) h‐BN films. A comprehensive study of their temporal response under various conditions–voltage pulse amplitude, edge rate (pulse rise/fall times), and temperature–provides new insights into the resistive switching process toward optimized devices and improvements in their implementation of artificial neural networks. The h‐BN memristors can achieve multi‐state operation through ultrafast pulsed switching (< 25 ns) with high energy efficiency (≈10 pJ pulse⁻¹). 

Abstract 2D layered semiconductors have attracted considerable attention for beyond‐Si complementary metal‐oxide‐semiconductor (CMOS) technologies. They can be prepared into ultrathin channel materials toward ultrascaled device architectures, including double‐gate field‐effect‐transistors (DGFETs). This work presents an experimental analysis of DGFETs constructed from chemical vapor deposition (CVD)‐grown monolayer (1L) molybdenum disulfide (MoS₂) with atomic layer deposition (ALD) of hafnium oxide (HfO₂) high‐k gate dielectrics (top and bottom). This extends beyond previous studies of DGFETs based mostly on exfoliated (few‐nm thick) MoS₂flakes, and advances toward large‐area wafer‐scale processing. Here, significant improvements in performance are obtained with DGFETs (i.e., improvements in ON/OFF ratio, ON‐state current, sub‐threshold swing, etc.) compared to single top‐gate FETs. In addition to multi‐gate device architectures (e.g., DGFETs), the scaling of the equivalent oxide thickness (EOT) is crucial toward improved electrostatics required for next‐generation transistors. However, the impact of EOT scaling on the characteristics of CVD‐grown MoS₂DGFETs remains largely unexplored. Thus, this work studies the impact of EOT scaling on subthreshold swing (SS) and gate hysteresis using current–voltage (I–V) measurements with varying sweep rates. The experimental analysis and results elucidate the basic mechanisms responsible for improvements in CVD‐grown 1L‐MoS₂DGFETs compared to standard top‐gate FETs. 

Abstract Group IV‐VI van der Waals crystals (MX, where M = Ge, Sn, and X = S, Se) are receiving increasing attention as semiconducting thermoelectric materials with nontoxic, earth‐abundant composition. Among them, SnSe is considered the most promising as it exhibits a remarkably high thermoelectric figure of merit (ZT), initially attributed to its low lattice thermal conductivity. However, it has been shown that the electronic band structure plays an equally important role in thermoelectric performance. A certain band shape, described as a “pudding mold” and characteristic for all MXs, has been predicted to significantly improveZTby combining good electrical conductivity with high Seebeck coefficient. This subtle feature is explored experimentally for GeS, SnS, and SnSe by means of angle‐resolved photoemission spectroscopy. The technique also allows for the determination of the effective mass and Fermi level position of as‐grown undoped crystals. The findings are supported by ab initio calculations of the electronic band structure. The results greatly contribute to the general understanding of the valence band dispersion of MXs and reinforce their potential as high‐performance thermoelectric materials, additionally giving prospects for designing systems consisting of van der Waals heterostructures. 

Abstract Janus transition metal dichalcogenides are an emerging class of atomically thin materials with engineered broken mirror symmetry that gives rise to long‐lived dipolar excitons, Rashba splitting, and topologically protected solitons. They hold great promise as a versatile nonlinear optical platform due to their broadband harmonic generation tunability, ease of integration on photonic structures, and nonlinearities beyond the basal crystal plane. Here, second and third harmonic generation in MoSSe and WSSe Janus monolayers is studied. Polarization‐resolved spectroscopy is used to map the full second‐order susceptibility tensor of MoSSe, including its out‐of‐plane components. In addition, the effective third‐order susceptibility and the second‐order nonlinear dispersion close to exciton resonances for both MoSSe and WSSe are measured at room and cryogenic temperatures. This work sets a bedrock for understanding the nonlinear optical properties of Janus transition metal dichalcogenides and probing their use in the next‐generation on‐chip multifaceted photonic devices. 

Two-dimensional (2D) Janus Transition Metal Dichalcogenides (TMDs) have attracted much interest due to their exciting quantum properties arising from their unique two-faced structure, broken-mirror symmetry, and consequent colossal polarisation field within the monolayer. While efforts have been made to achieve high-quality Janus monolayers, the existing methods rely on highly energetic processes that introduce unwanted grain-boundary and point defects with still unexplored effects on the material's structural and excitonic properties Through High-resolution scanning transmission electron microscopy (HRSTEM), density functional theory (DFT), and optical spectroscopy measurements; this work introduces the most encountered and energetically stable point defects. It establishes their impact on the material's optical properties. HRSTEM studies show that the most energetically stable point defects are single (V_S and V_Se) and double chalcogen vacancy (V_S–V_Se), interstitial defects (Mi), and metal impurities (MW) and establish their structural characteristics. DFT further establishes their formation energies and related localized bands within the forbidden band. Cryogenic excitonic studies on h-BN-encapsulated Janus monolayers offer a clear correlation between these structural defects and observed emission features, which closely align with the results of the theory. The overall results introduce the defect genome of Janus TMDs as an essential guideline for assessing their structural quality and device properties.