Electrical tuning of valley magnetic moment through symmetry control in bilayer MoS2

Abstract

. We find that in bilayer MoS 2 the circularly polarized photoluminescence can be continuously tuned from −15% to 15% as a function of gate voltage, whereas in structurally non-centrosymmetric monolayer MoS 2 the photoluminescence polarization is gate independent. The observations are well explained as resulting from the continuous variation of orbital magnetic moments between positive and negative values through symmetry control.The Dirac-valley degree of freedom has recently been considered for new modes of electronic and photonic device operation 4,5,[9][10][11][12][13][14][15][16][17] following the arrival of atomically thin two-dimensional (2D) electronic systems 6,7,18,19 (Fig. 1a). In this context, phenomena such as valley polarization and anomalous valley-and spin-Hall effects have been discussed for the +K and −K Dirac valleys at opposite corners of the Brillouin zone in hexagonal systems [9][10][11][12]15 . The realization of these effects hinges on achieving control of valley contrast, that is, of properties that differ between the two valleys, in particular the magnetic moment (m) and Berry curvature ( ). Time-reversal symmetry dictates that each pseudovector, m as well as , has the same magnitude but opposite sign in the two valleys, whereas inversion symmetry requires them to have the same sign. Therefore, a necessary condition for valley-contrasting m and is inversion symmetry breaking 4 . Monolayer MoS 2 lacks structural inversion symmetry (Fig. 1a), and thus m and are non-zero, having equal magnitude but opposite signs in the two ±K valleys owing to timereversal symmetry. One direct consequence of non-zero m is valley-contrasting optical dichroism 5,8,9 , whereby charge carriers in the two valleys can be selectively excited by circularly polarized optical fields [9][10][11] . This effect permits optical generation of valley polarization, as recently demonstrated using polarized

Magnetoelectric effects and valley-controlled spin quantum gates in transition metal dichalcogenide bilayers

“…The strong PL peak associated with this transition suggests high efficiency of the injection 21,22,24,25 and remain open questions. Notably, for holes near K points, spin flip…”

“…Valley refers to the degenerate extrema of energy bands, which in many hexagonal 2D crystals are located at the corners of the hexagonal Brillouin zone (K points). It was predicted that in the absence of inversion symmetry valley degree of freedom can be associated with magnetic moment and optical circular dichroism 18,20 , which has made possible the first observations of optical pumping of valley polarization in TMDC monolayers [21][22][23] and biased bilayers 24 . As the spin and valley have magnetic moment that can be controlled by magnetic and optical means, while the layer pseudospin corresponds to an electrical polarization subject to electrical manipulation, a system allowing interplay of these different degrees of freedom may offer unprecedented possibilities to exploit their quantum control for new device concepts.…”

Gate-tunable topological valley transport in bilayer graphene

“…When the inversion symmetry is broken, however, time-reversal symmetry requires the Hall currents to have opposite signs and equal magnitudes in the two valleys (i.e., a valley The nonlocal transport persists up to room temperature and over long distances (up to 10 m). Our results represent major progress in the quest for a robust, tunable valley pseudospin system among various alternatives [3][4][5]11,12 , and indicate the possibility of using the nonlocal topological transport in practical applications under ambient conditions.The structure of our dual-gate graphene field-effect transistors (FETs) is shown in Fig. 1a and 1b.…”

“…Our results represent major progress in the quest for a robust, tunable valley pseudospin system among various alternatives [3][4][5]11,12 , and indicate the possibility of using the nonlocal topological transport in practical applications under ambient conditions.…”

Intervalley biexcitons and many-body effects in monolayerMoS2

“…These include valley-selective photoexcitation [2][3][4] , valley Hall effect 5 , valley-tunable magnetic moment 6 , and valley-selective optical Stark effect 7,8 . Unlike traditional semiconductors, the Coulomb interaction in monolayer TMDs is unusually strong because screening is greatly suppressed and spatial overlap of the interaction is much larger.…”