Photoinduced quantum spin and valley Hall effects, and orbital magnetization in monolayerMoS2

Abstract

We theoretically demonstrate that 100% valley-polarized transport in monolayers of MoS 2 and other group-VI dichalcogenides can be obtained using off-resonant circularly polarized light. By tuning the intensity of the off-resonant light the intrinsic band gap in one valley is reduced, while it is enhanced in the other valley, enabling single valley quantum transport. As a consequence, we predict (i) enhancement of the longitudinal electrical conductivity, accompanied by an increase in the spin polarization of the flowing electrons, (ii) enhancement of the intrinsic spin Hall effect, together with a reduction of the intrinsic valley Hall effect, and (iii) enhancement of the orbital magnetic moment and orbital magnetization. These mechanisms provide appealing opportunities to the design of nanoelectronics based on dichalcogenides. Monolayers of the transition metal dichalcogenides MX 2 (M = Mo, W; X = S, Se) are emerging as promising materials for a wide variety of applications in nanoelectronics, due to their exceptional band structures [1]. In particular, the exfoliation of MoS 2 monolayers has attracted significant interest since the realization of field-effect transistors with high on-off ratio [2]. Monolayers of MX 2 can be regarded as semiconductor analogs of graphene [3,4], resulting in similar phenomena such as spin and valley Hall effects [5,6]. Indeed, MoS 2 has a honeycomb lattice with an intrinsic direct band gap of 1 to 2 eV, which is in the visible range. The band edge is located at the energy degenerate valleys (corners of the hexagonal Brillouin zone) [7,8].Thanks to its direct band gap, MoS 2 is suitable for optical manipulations and opens access to many optoelectronic applications [7][8][9]. It has been predicted that both valley polarization and valley coherence can be achieved by optical pumping with circularly and linearly polarized light [5,6,10]. Experimental realizations have been reported for MoS 2 and WSe 2 [11][12][13][14], suggesting that monolayers of MX 2 could be used for integrated valleytronic devices. Experiments have shown 30% to 50% valley polarization with circularly polarized light in the resonance regime [10][11][12]. Recent works on the optoelectronic properties of MoS 2 indicate that the photoresponse of externally biased phototransistors is driven by conductivity alteration upon illumination [15][16][17]. A photovoltaic effect has been reported for MoS 2 devices in contact with metallic electrodes that generate large Schottky barriers [18,19]. In addition, ultrasensitive phototransistors with improved mobility have been demonstrated in Ref. [20]. These devices show a photoresponsitivity in the 400 to 680 nm range with a maximum of 880 A/W at a wavelength of 561 nm.In contrast with the on-resonant optical induction used till now, we propose in this paper a scheme to employ off-resonant light to influence the band structure and corresponding transport properties of MX 2 monolayers, enabling 100% valley polarization. An important motivation is the development of experimental ...

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