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Register a new userOptically controllable magnetism in atomically thin semiconductors
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Electronic states in two-dimensional layered materials can exhibit a remarkable variety of correlated phases including Wigner-crystals, Mott insulators, charge density waves, and superconductivity. Recent experimental and theoretical research has indicated that ferromagnetic phases can exist in electronically-doped transition metal dichalcogenide (TMD) semiconductors, but a stable magnetic state at zero magnetic field has eluded detection. Here, we experimentally demonstrate that mesoscopic ferromagnetic order can be generated and controlled by local optical pumping in monolayer WSe2 at zero applied magnetic field. In a spatially resolved pump-probe experiment, we use polarization-resolved reflectivity from excitonic states as a probe of charge-carrier spin polarization. When the sample is electron-doped at density $n_e = 10^{12} cm^{-2}$, we observe that a local, circularly-polarized, microwatt-power optical pump breaks the symmetry between equivalent ferromagnetic spin configurations and creates magnetic order which extends over mesoscopic regions as large as 8 um x 5 um, bounded by sample edges and folds in the 2D semiconductor. The experimental signature of magnetic order is circular dichroism (CD) in reflectivity from the excitonic states, with magnitude exceeding 20% at resonant wavelengths. The helicity of the pump determines the orientation of the magnetic state, which can be aligned along the two principle out-of-plane axes. In contrast to previous studies in 2D materials that have required non-local, slowly varying magnetic fields to manipulate magnetic phases, the demonstrated capability to control long-range magnetism and corresponding strong CD with local and tunable optical pumps is highly versatile. This discovery will unlock new TMD-based spin and optical technologies and enable sophisticated control of correlated electron phases in two-dimensional electron gases (2DEGs).
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We show that a transition metal dichalcogenide monolayer with a radiatively broadened exciton resonance would exhibit perfect extinction of a transmitted field. This result holds for s- or p-polarized weak resonant light fields at any incidence angle, due to the conservation of in-plane momentum of excitons and photons in a flat defect-free two dimensional crystal. In contrast to extinction experiments with single quantum emitters, exciton-exciton interactions lead to an enhancement of reflection with increasing power for incident fields that are blue detuned with respect to the exciton resonance. We show that the interactions limit the maximum reflection that can be achieved by depleting the incoming coherent state into an outgoing two-mode squeezed state.
While conventional semiconductor technology relies on the manipulation of electrical charge for the implementation of computational logic, additional degrees of freedom such as spin and valley offer alternative avenues for the encoding of information. In transition metal dichalcogenide (TMD) monolayers, where spin-valley locking is present, strong retention of valley chirality has been reported for MoS$_2$, WSe$_2$ and WS$_2$ while MoSe$_2$ shows anomalously low valley polarisation retention. In this work, chiral selectivity of MoSe$_2$ cavity polaritons under helical excitation is reported with a polarisation degree that can be controlled by the exciton-cavity detuning. In contrast to the very low circular polarisation degrees seen in MoSe$_2$ exciton and trion resonances, we observe a significant enhancement of up to 7 times when in the polaritonic regime. Here, polaritons introduce a fast decay mechanism which inhibits full valley pseudospin relaxation and thus allows for increased retention of injected polarisation in the emitted light. A dynamical model applicable to cavity-polaritons in any TMD semiconductor, reproduces the detuning dependence through the incorporation of the cavity-modified exciton relaxation, allowing an estimate of the spin relaxation time in MoSe$_2$ which is an order of magnitude faster than those reported in other TMDs. The valley addressable exciton-polaritons reported here offer robust valley polarised states demonstrating the prospect of valleytronic devices based upon TMDs embedded in photonic structures, with significant potential for valley-dependent nonlinear polariton-polariton interactions.
Transition metal dichalcogenide monolayers are promising candidates for exploring new electronic and optical phenomena and for realizing atomically thin optoelectronic devices. They host tightly bound electron-hole pairs (excitons) that can be efficiently excited by resonant light fields. Here, we demonstrate that a single monolayer of molybdenum diselenide (MoSe2) can dramatically modify light transmission near the excitonic resonance, acting as an electrically switchable mirror that reflects up to 85% of incident light at cryogenic temperatures. This high reflectance is a direct consequence of the excellent coherence properties of excitons in this atomically thin semiconductor, encapsulated by hexagonal boron nitride. Furthermore, we show that the MoSe2 monolayer exhibits power- and wavelength-dependent nonlinearities that stem from exciton-based lattice heating in the case of continuous-wave excitation and exciton-exciton interactions when fast, pulsed laser excitation is used. These observations open up new possibilities for studying quantum nonlinear optical phenomena and topological photonics, and for miniaturizing optical devices.
Stacking order can significantly influence the physical properties of two-dimensional (2D) van der Waals materials. The recent isolation of atomically thin magnetic materials opens the door for control and design of magnetism via stacking order. Here we apply hydrostatic pressure up to 2 GPa to modify the stacking order in a prototype van der Waals magnetic insulator CrI3. We observe an irreversible interlayer antiferromagnetic (AF) to ferromagnetic (FM) transition in atomically thin CrI3 by magnetic circular dichroism and electron tunneling measurements. The effect is accompanied by a monoclinic to a rhombohedral stacking order change characterized by polarized Raman spectroscopy. Before the structural change, the interlayer AF coupling energy can be tuned up by nearly 100% by pressure. Our experiment reveals interlayer FM coupling, which is the established ground state in bulk CrI3, but never observed in native exfoliated thin films. The observed correlation between the magnetic ground state and the stacking order is in good agreement with first principles calculations and suggests a route towards nanoscale magnetic textures by moire engineering.
The realization of mixtures of excitons and charge carriers in van-der-Waals materials presents a new frontier for the study of the many-body physics of strongly interacting Bose-Fermi mixtures. In order to derive an effective low-energy model for such systems, we develop an exact diagonalization approach based on a discrete variable representation that predicts the scattering and bound state properties of three charges in two-dimensional transition metal dichalcogenides. From the solution of the quantum mechanical three-body problem we thus obtain the bound state energies of excitons and trions within an effective mass model which are in excellent agreement with Quantum Monte Carlo predictions. The diagonalization approach also gives access to excited states of the three-body system. This allows us to predict the scattering phase shifts of electrons and excitons that serve as input for a low-energy theory of interacting mixtures of excitons and charge carriers at finite density. To this end we derive an effective exciton-electron scattering potential that is directly applicable for Quantum Monte-Carlo or diagrammatic many-body techniques. As an example, we demonstrate the approach by studying the many-body physics of exciton Fermi polarons in transition-metal dichalcogenides, and we show that finite-range corrections have a substantial impact on the optical absorption spectrum. Our approach can be applied to a plethora of many-body phenomena realizable in atomically thin semiconductors ranging from exciton localization to induced superconductivity.
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