Moiré superlattices provide a powerful tool to engineer novel quantum phenomena in twodimensional (2D) heterostructures, where the interactions between the atomically thin layers qualitatively change the electronic band structure of the superlattice. For example, mini-Dirac points, tunable Mott insulator states, and the Hofstadter butterfly can emerge in different types of graphene/boron nitride moiré superlattices, while correlated insulating states and superconductivity have been reported in twisted bilayer graphene moiré superlattices 1-12 . In addition to their dramatic effects on the single particle states, moiré superlattices were recently predicted to host novel excited states, such as moiré exciton bands [13][14][15] . Here we report the first observation of moiré superlattice exciton states in nearly aligned WSe 2 /WS 2 heterostructures.These moiré exciton states manifest as multiple emergent peaks around the original WSe 2 A exciton resonance in the absorption spectra, and they exhibit gate dependences that are distinctly different from that of the A exciton in WSe 2 monolayers and in large-twist-angle WSe 2 /WS 2 heterostructures. The observed phenomena can be described by a theoretical model where the periodic moiré potential is much stronger than the exciton kinetic energy and creates multiple flat exciton minibands. The moiré exciton bands provide an attractive platform to explore and control novel excited state of matter, such as topological excitons and a correlated exciton Hubbard model, in transition metal dichalcogenides.

Moiré superlattices are emerging as a new route for engineering strongly correlated electronic states in two-dimensional van der Waals heterostructures, as recently demonstrated in the correlated insulating and superconducting states in magic-angle twisted bilayer graphene and ABC trilayer graphene/boron nitride moiré superlattices 1-4 . Transition metal dichalcogenide (TMDC) moiré heterostructures provide another exciting model system to explore correlated quantum phenomena 5 , with the addition of strong light-matter interactions and large spin-orbital coupling. Here we report the optical detection of strongly correlated phases in semiconducting WSe2/WS2 moiré superlattices. Our sensitive optical detection technique reveals a Mott insulator state at one hole per superlattice site (ν = 1), and surprising insulating phases at fractional filling factors ν = 1/3 and 2/3, which we assign to generalized Wigner crystallization on an underlying lattice 6-9 . Furthermore, the unique spin-valley optical selection rules 10-12 of TMDC heterostructures allow us to optically create and investigate low-energy spin excited states in the Mott insulator. We reveal an especially slow spin relaxation lifetime of many microseconds in the Mott insulating state, orders-of-magnitude longer than that of charge excitations. Our studies highlight novel correlated physics that can emerge in moiré superlattices beyond graphene.

Van der Waals heterostructures are synthetic quantum materials composed of stacks of atomically thin two-dimensional (2D) layers. Because the electrons in the atomically thin 2D layers are exposed to layer-layer coupling, the properties of van der Waals heterostructures are defined not only by the constituent monolayers, but also by the interactions between the layers. Many fascinating electrical, optical, and magnetic properties have recently been reported in different types of van der Waals heterostructures. In this review we focus on unique excited-state dynamics in transition metal dichalcogenide (TMDC) heterostructures. TMDC monolayers are the most widely studied 2D semiconductors, featuring prominent exciton states and accessibility to the valley degree of freedom. Many TMDC heterostructures are characterized by a staggered band alignment. This band alignment has profound effects on the evolution of the excited states in heterostructures, including ultrafast charge transfer between the layers, the formation of interlayer excitons, and the existence of longlived spin and valley polarization in resident carriers. Here we review recent experimental and theoretical efforts to elucidate electron dynamics in TMDC heterostructures, extending from time scales of femtoseconds to microseconds, and comment on the relevance of these effects for potential applications in optoelectronic and valleytronic/spintronic devices. Main text Advances in the isolation and manipulation of atomically-thin sheets of two-dimensional (2D) crystals, starting with the investigations of graphene a decade ago, have ushered in a new era of basic scientific research and technological innovation. 2D layers with diverse properties can now be prepared separately and then stacked together to form new types of quantum materials, known as van der Waals (vdW) heterostructures. The ability to combine materials with monolayer precision enables the design and creation of functional 2D materials that do not exist in nature. Today we have at our disposal a wide variety of atomically thin 2D layers, ranging from semiconducting MoS 2 and insulating hexagonal boron-nitride (h-BN) to magnetic CrI 3 and superconducting NbSe 2 , that can be stacked one upon the other. Since the electrons in atomically thin layers are exposed, different quantum states found in the individual layers can interact and couple to one another in ways that are not possible in other systems. VdW heterostructures constitute a vast family of new quantum materials, since they are defined not only by the combination of constituent monolayer materials, but also by the stacking sequence and relative crystallographic alignment of the layers. Further control of physical properties in 2D vdW heterostructures can be achieved through the application of electrostatic gating and fields, as well as substrate and strain engineering. Many fascinating physical phenomena have been reported in different vdW heterostructures, as exemplified by transport measurements revealing Hofstadter butterfly states,...

Black phosphorus (b-P) and more recently black phosphorus-arsenic alloys (b-PAs) are candidate 2D materials for the detection of mid-wave and potentially long-wave infrared radiation. However, studies to date have utilized laser-based measurements to extract device performance and the responsivity of these detectors. As such, their performance under thermal radiation and spectral response has not been fully characterized. Here, we perform a systematic investigation of gated-photoconductors based on b-PAs alloys as a function of thickness over the composition range of 0-91% As. Infrared transmission and reflection measurements are performed to determine the bandgap of the various compositions. The spectrally resolved photoresponse for various compositions in this material system is investigated to confirm absorption measurements, and we find that the cutoff wavelength can be tuned from 3.9 to 4.6 μm over the studied compositional range. In addition, we investigated the temperature-dependent photoresponse and performed calibrated responsivity measurements using blackbody flood illumination. Notably, we find that the specific detectivity (D*) can be optimized by adjusting the thickness of the b-P/b-PAs layer to maximize absorption and minimize dark current. We obtain a peak D* of 6 × 10 cm Hz W and 2.4 × 10 cm Hz W for pure b-P and b-PAs (91% As), respectively, at room temperature, which is an order of magnitude higher than commercially available mid-wave infrared detectors operating at room temperature.