Monolayer group-VI transition metal dichalcogenides have recently emerged as semiconducting alternatives to graphene in which the true two-dimensionality is expected to illuminate new semiconducting physics. Here we investigate excitons and trions (their singly charged counterparts), which have thus far been challenging to generate and control in the ultimate two-dimensional limit. Utilizing high-quality monolayer molybdenum diselenide, we report the unambiguous observation and electrostatic tunability of charging effects in positively charged (X þ ), neutral (X o ) and negatively charged (X À ) excitons in field-effect transistors via photoluminescence. The trion charging energy is large (30 meV), enhanced by strong confinement and heavy effective masses, whereas the linewidth is narrow (5 meV) at temperatures o55 K. This is greater spectral contrast than in any known quasitwo-dimensional system. We also find the charging energies for X þ and X À to be nearly identical implying the same effective mass for electrons and holes.

Van der Waals bound heterostructures constructed with two-dimensional materials, such as graphene, boron nitride and transition metal dichalcogenides, have sparked wide interest in device physics and technologies at the two-dimensional limit. One highly coveted heterostructure is that of differing monolayer transition metal dichalcogenides with type-II band alignment, with bound electrons and holes localized in individual monolayers, that is, interlayer excitons. Here, we report the observation of interlayer excitons in monolayer MoSe 2 -WSe 2 heterostructures by photoluminescence and photoluminescence excitation spectroscopy. We find that their energy and luminescence intensity are highly tunable by an applied vertical gate voltage. Moreover, we measure an interlayer exciton lifetime of B1.8 ns, an order of magnitude longer than intralayer excitons in monolayers. Our work demonstrates optical pumping of interlayer electric polarization, which may provoke further exploration of interlayer exciton condensation, as well as new applications in two-dimensional lasers, light-emitting diodes and photovoltaic devices.

As a consequence of degeneracies arising from crystal symmetries, it is possible for electron states at band-edges ('valleys') to have additional spin-like quantum numbers. An important question is whether coherent manipulation can be performed on such valley pseudospins, analogous to that implemented using true spin, in the quest for quantum technologies. Here, we show that valley coherence can be generated and detected. Because excitons in a single valley emit circularly polarized photons, linear polarization can only be generated through recombination of an exciton in a coherent superposition of the two valley states. Using monolayer semiconductor WSe2 devices, we first establish the circularly polarized optical selection rules for addressing individual valley excitons and trions. We then demonstrate coherence between valley excitons through the observation of linearly polarized luminescence, whose orientation coincides with that of the linearly polarized excitation, for any given polarization angle. In contrast, the corresponding photoluminescence from trions is not observed to be linearly polarized, consistent with the expectation that the emitted photon polarization is entangled with valley pseudospin. The ability to address coherence, in addition to valley polarization, is a step forward towards achieving quantum manipulation of the valley index necessary for coherent valleytronics.

Light-emitting diodes are of importance for lighting, displays, optical interconnects, logic and sensors [1][2][3][4][5][6][7][8] . Hence the development of new systems that allow improvements in their efficiency, spectral properties, compactness and integrability could have significant ramifications. Monolayer transition metal dichalcogenides have recently emerged as interesting candidates for optoelectronic applications due to their unique optical properties [9][10][11][12][13][14][15][16] . Electroluminescence has already been observed from monolayer MoS2 devices 17,18 . However, the electroluminescence efficiency was low and the linewidth broad due both to the poor optical quality of MoS2 and to ineffective contacts. Here, we report electroluminescence from lateral p-n junctions in monolayer WSe2 induced electrostatically using a thin boron nitride support as a dielectric layer with multiple metal gates beneath.This structure allows effective injection of electrons and holes, and combined with the high optical quality of WSe2 it yields bright electroluminescence with 1000 times smaller injection current and 10 times smaller linewidth than in MoS2 17,18 . Furthermore, by increasing the injection bias we can tune the electroluminescence between regimes of impurity-bound, charged, and neutral excitons. This system has the required ingredients for new kinds of optoelectronic devices such as spin-and valley-polarized light-emitting diodes, on-chip lasers, and two-dimensional electro-optic modulators. Main TextFew-layer group-VIB transition metal dichalcogenides (TMDs) represent a class of semiconductors in the two-dimensional (2D) limit 9,10,19 . Due to their large carrier effective mass and the reduced screening in 2D, electron-hole interactions are much stronger than in conventional semiconductors. This leads to large binding energies for both charged and neutral excitons which as a result are spectrally sharp, robust, and amenable to electrical manipulation 16,20,21 . In addition, the large spin-orbit coupling 22 and the acentric structure of TMDs provides a connection between spin and valley degrees of freedom 14 , light polarization 11,13,15,16 , and magnetic and electric fields 23 that can be exploited for new kinds of device operation.Although in bulk TMDs the band gap is indirect, in the limit of a single monolayer it becomes direct 9,10 , fulfilling the most basic requirement for efficient light emission. Indeed, electroluminescence (EL) has already been reported from monolayer MoS2 field-effect transistors (FETs), occurring near the Schottky contact with a metal 17 or with highly doped silicon 18 .However, the efficiency and spectral quality was much lower than has been demonstrated for other nanoscale light emitters such as carbon nanotubes 7 , for two reasons. First, efficient EL requires effective injection of both electrons and holes into the active region, which should therefore be within a p-n junction. Second, MoS2 is known to have poorer optical quality than other group VIB TMDs, possi...

Topological states of matter such as quantum spin liquids (QSLs) are of great interest because of their remarkable predicted properties including protection of quantum information and the emergence of Majorana fermions. Such QSLs, however, have proven difficult to identify experimentally. The most promising approach is to study their exotic nature via the wavevector and intensity dependence of their dynamical response in neutron scattering. A major search has centered on iridate materials which are proposed to realize the celebrated Kitaev model on a honeycomb lattice -a prototypical topological QSL system in two dimensions (2D). The difficulties of iridium for neutron measurements have, however, impeded progress significantly. Here we provide experimental evidence that a material based on ruthenium, α-RuCl 3 realizes the same Kitaev physics but is highly amenable to neutron investigation. Our measurements confirm the requisite strong spin-orbit coupling, and a low temperature 2 magnetic order that matches the predicted phase proximate to the QSL. We also show that stacking faults, inherent to the highly 2D nature of the material, readily explain some puzzling results to date. Measurements of the dynamical response functions, especially at energies and temperatures above that where interlayer effects are manifest, are naturally accounted for in terms of deconfinement physics expected for QSLs. Via a comparison to the recently calculated dynamics from gauge flux excitations and Majorana fermions of the pure Kitaev model we propose α-RuCl 3 as the prime candidate for experimental realization of fractionalized Kitaev physics.Exotic physics associated with frustrated quantum magnets is an enduring theme in condensed matter research. The formation of quantum spin liquids (QSL) The Kitaev model consists of a set of spin-1/2 moments � ���⃗ � arrayed on a honeycomb lattice. The Kitaev couplings, of strength K in eqn.(1) are highly anisotropic with a different spin component interacting for each of the three bonds of the honeycomb lattice. In actual materials a Heisenberg interaction (J) is also generally expected to be present, giving rise to the Heisenberg-Kitaev (H-K) Hamiltonian given by 11 .where, for example, m is the component of the spin directed along the bond connecting spins (i,j). The QSL phase of the pure Kitaev model (J=0), for both ferro and antiferromagnetic K, is stable for relatively small Heisenberg perturbations.Remarkably the Hamiltonian (1) has been proposed to accurately describe octahedrallycoordinated magnetic systems, Fig. 1 21 -27 . Whilst these studies lend support to the material as a potential Kitaev material, conflicting results centering on the low temperature magnetic properties have hindered progress. To resolve this we undertake a comprehensive evaluation of the magnetic and spin orbit properties of α-RuCl 3 , and further measure the dynamical response establishing this as a material proximate to the widely searched for quantum spin liquid.We begin by investigating the crystal and m...

The creation of moiré patterns in crystalline solids is a powerful approach to manipulate their electronic properties, which are fundamentally influenced by periodic potential landscapes. In two-dimensional (2D) materials, a moiré pattern with a superlattice potential can form by vertically stacking two layered materials with a twist and/or finite lattice constant difference. This unique approach has led to emergent electronic phenomena, including the fractal quantum Hall effect 1-3 , tunable Mott insulators 4,5 , and unconventional superconductivity 6 . Furthermore, theory predicts intriguing effects on optical excitations by a moiré potential in 2D valley semiconductors 7-9 , but these signatures have yet to be experimentally detected. Here, we report experimental evidence of interlayer valley excitons trapped in a moiré potential in MoSe2/WSe2 heterobilayers. At low temperatures, we observe photoluminescence near the free interlayer exciton energy but with over 100 times narrower linewidths (~100 μeV). The emitter g-factors are homogeneous across the same sample and only take two values, -15.9 and 6.7, in samples with twisting angles near 60° and 0°, respectively. The g-factors match those of the free interlayer exciton, which is determined by one of two possible valley pairing configurations. At a twist angle near 20°, the emitters become two orders of magnitude dimmer, but remarkably, they possess the same g-factor as the heterobilayer near 60°. This is consistent with the Umklapp recombination of interlayer excitons near the commensurate 21.8° twist angle 7 . The emitters exhibit strong circular polarization, which implies the preservation of three-fold rotation symmetry by the trapping potential. Together with the power and excitation energy dependence, all evidence unambiguously points to their origin as interlayer excitons trapped in a smooth moiré potential with inherited valley-contrasting physics. Our results open opportunities for 2D moiré optics with twist angle as a unique control knob.

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Main TextIn monolayer transition metal dichalcogenides (TMDs), there is a valley pseudospin 1/2 which describes the two inequivalent but energy degenerate band edges (the ±K valleys) at the corners of the hexagonal Brillouin zone 1 . With broken inversion symmetry, electrons in the two valleys can have finite orbital contributions to their magnetic moments which are equal in magnitude but opposite in sign by time reversal symmetry. This orbital magnetic moment is thus linked to the valley pseudospin in the same way that the bare magnetic moment ( S) is linked to the real spin S, where is the Bohr magneton and is the Lande -factor. The orbital magnetic moment in turn has two parts: a contribution from the parent atomic orbitals, and a "valley magnetic moment" contribution from the lattice structure 1 (Fig. 1a, [18][19][20][21][22][23] , are subject to a momentum-dependent gauge field arising from electron-hole exchange, or valley-orbit coupling, which at zero magnetic field is predicted to result in massless and massive dispersion respectively within the light cone 24 . This implies the possibility of controlling excitonic valley pseudospin via the Zeeman effect in an external magnetic field.Our measurements of polarization-resolved photoluminescence (PL) in a perpendicular magnetic field are performed on mechanically exfoliated WSe2 monolayers. We have obtained consistent results from many samples. The data presented here are all taken from one sample at a temperature of 30 K. In order to resolve the splitting between the +K and -K valley excitons, which is significantly smaller than the exciton linewidth (~10 meV), we both excite and detect with a single helicity of light. In this way we address one valley at a time, and the splitting can be determined by comparing the peak positions for different polarizations. The splitting in the applied magnetic field breaks the valley degeneracy, enabling control of the valley polarization. To investigate this we measure the degree of PL polarization for both helicities of incident circular polarization. Figure 2a shows PL for σ -excitation with σ -(red) and σ + (orange) detection at a field of -7 T. The suppression of the σ + signal relative to the co-polarized σ -peak is a signature of optically pumped valley polarization 7-10 . The degree of exciton valley polarization is clearly larger for σ + excitation than for − (Fig. 2b). On the other hand, when the magnetic field is reversed to +7 T (Figs. 2c and d) the polarization becomes larger for − . This observation implies that, while the sign of the valley polarization is determined by the helicity of the excitation laser, its magnitude depends on the relationship between the helicity and the magnetic field direction. Figure 2e shows the degree of PL polarization for both σ + (blue) and σ -(red) excitation as a function of B between -7 T and +7 T for the neutral exciton peak. It is linear in B with a negative (positive) slope. This "X" pattern implies that the valley Zeeman splitting induces an asymmetry in the intervalle...