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Register a new userValley Contrasting Magnetoluminescence in Monolayer MoS$_{2}$ Quantum Hall Systems
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The valley dependent optical selection rules in recently discovered monolayer group-VI transition metal dichalcogenides (TMDs) make possible optical control of valley polarization, a crucial step towards valleytronic applications. However, in presence of Landaul level(LL) quantization such selection rules are taken over by selection rules between the LLs, which are not necessarily valley contrasting. Using MoS$_{2}$ as an example we show that the spatial inversion-symmetry breaking results in unusual valley dependent inter-LL selection rules, which directly locks polarization to valley. We find a systematic valley splitting for all Landau levels (LLs) in the quantum Hall regime, whose magnitude is linearly proportional to the magnetic field and in comparable with the LL spacing. Consequently, unique plateau structures are found in the optical Hall conductivity, which can be measured by the magneto-optical Faraday rotations.
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Valleytronics targets the exploitation of the additional degrees of freedom in materials where the energy of the carriers may assume several equal minimum values (valleys) at non-equivalent points of the reciprocal space. In single layers of transition metal dichalcogenides (TMDs) the lack of inversion symmetry, combined with a large spin-orbit interaction, leads to a conduction (valence) band with different spin-polarized minima (maxima) having equal energies. This offers the opportunity to manipulate information at the level of the charge (electrons or holes), spin (up or down) and crystal momentum (valley). Any implementation of these concepts, however, needs to consider the robustness of such degrees of freedom, which are deeply intertwined. Here we address the spin and valley relaxation dynamics of both electrons and holes with a combination of ultrafast optical spectroscopy techniques, and determine the individual characteristic relaxation times of charge, spin and valley in a MoS$_{2}$ monolayer. These results lay the foundations for understanding the mechanisms of spin and valley polarization loss in two-dimensional TMDs: spin/valley polarizations survive almost two-orders of magnitude longer for holes, where spin and valley dynamics are interlocked, than for electrons, where these degrees of freedom are decoupled. This may lead to novel approaches for the integration of materials with large spin-orbit in robust spintronic/valleytronic platforms.
The Berry curvature dipole is a physical quantity that is expected to allow various quantum geometrical phenomena in a range of solid-state systems. Monolayer transition metal dichalcogenides provide an exceptional platform to modulate and investigate the Berry curvature dipole through strain. Here we theoretically demonstrate and experimentally verify for monolayer MoS$_rm{2}$ the generation of valley orbital magnetization as a response to an in-plane electric field due to the Berry curvature dipole. The measured valley orbital magnetization shows excellent agreement with the calculated Berry curvature dipole which can be controlled by the magnitude and direction of strain. Our results show that the Berry curvature dipole acts as an effective magnetic field in current-carrying systems, providing a novel route to generate magnetization.
The topological properties of a materials electronic structure are encoded in its Berry curvature, a quantity which is intimately related to the transverse electrical conductivity. In transition metal dichalcogenides with broken inversion symmetry, the nonzero Berry curvature results in a valley Hall effect. In this paper we identify a previously unrecognized consequence of Berry curvature in these materials: an electric field-induced change in the electrons charge density orientation. We use first principles calculations to show that measurements of the electric field-induced change in the charge density or local density of states in MoS$_2$ can be used to measure its energy-dependent valley and orbital Hall conductivity.
We study both the intrinsic and extrinsic spin Hall effect in spin-valley coupled monolayers of transition metal dichalcogenides. We find that whereas the skew-scattering contribution is suppressed by the large band gap, the side-jump contribution is comparable to the intrinsic one with opposite sign in the presence of scalar and magnetic scattering. Intervalley scattering tends to suppress the side-jump contribution due to the loss of coherence. By tuning the ratio of intra- to intervalley scattering, the spin Hall conductivity shows a sign change in hole-doped samples. Multiband effect in other doping regime is considered, and it is found that the sign change exists in the heavily hole-doped regime, but not in the electron-doped regime.
We study the electronic structures and topological properties of $(M+N)$-layer twisted graphene systems. We consider the generic situation that $N$-layer graphene is placed on top of the other $M$-layer graphene, and is twisted with respect to each other by an angle $theta$. In such twisted multilayer graphene (TMG) systems, we find that there exists two low-energy flat bands for each valley emerging from the interface between the $M$ layers and the $N$ layers. These two low-energy bands in the TMG system possess valley Chern numbers that are dependent on both the number of layers and the stacking chiralities. In particular, when the stacking chiralities of the $M$ layers and $N$ layers are opposite, the total Chern number of the two low-energy bands for each valley equals to $pm(M+N-2)$ (per spin). If the stacking chiralities of the $M$ layers and the $N$ layers are the same, then the total Chern number of the two low-energy bands for each valley is $pm(M-N)$ (per spin). The valley Chern numbers of the low-energy bands are associated with large, valley-contrasting orbital magnetizations, suggesting the possible existence of orbital ferromagnetism and anomalous Hall effect once the valley degeneracy is lifted either externally by a weak magnetic field or internally by Coulomb interaction through spontaneous symmetry breaking.
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