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Register a new userUltrafast valley polarization in bilayer graphene
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We study theoretically interaction of a bilayer graphene with a circularly polarized ultrafast optical pulse of a single oscillation at an oblique incidence. The normal component of the pulse breaks the inversion symmetry of the system and opens up a dynamical band-gap, due to which a valley-selective population of the conduction band becomes sensitive to the angle of incident of the pulse. We show that the magnitude of the valley polarization can be controlled by the angle of incidence, the amplitude, and the angle of in-plane polarization of the chiral optical pulse. Subsequently, a sequence of a circularly polarized pulse followed by a linearly polarized femtosecond-long pulse can be used to control the valley polarization created by the preceding pulse. Generally, the linearly polarized pulse depolarizes the system. The magnitude of such a depolarization depends on the amplitude, and the in-plane polarization angle of the linearly polarized pulse. Our protocol provides a favorable platform for applications in valleytronics.
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Twisted bilayer graphene (TBG) aligned with hexagonal boron nitride (h-BN) substrate can exhibit an anomalous Hall effect at 3/4 filling due to the spontaneous valley polarization in valley resolved moire bands with opposite Chern number [Science 367, 900 (2020), Science 365, 605 (2019)]. It was observed that a small DC current is able to switch the valley polarization and reverse the sign of the Hall conductance [Science 367, 900 (2020), Science 365, 605 (2019)]. Here, we discuss the mechanism of the current switching of valley polarization near the transition temperature, where bulk dissipative transport dominates. We show that for a sample with rotational symmetry breaking, a DC current may generate an electron density difference between the two valleys (valley density difference). The current induced valley density difference in turn induces a first order transition in the valley polarization. We emphasize that the inter-valley scattering plays a central role since it is the channel for exchanging electrons between the two valleys. We further estimate the valley density difference in the TBG/h-BN system with a microscopic model, and find a significant enhancement of the effect in the magic angle regime.
Considering the difference of energy bands in graphene and silicene, we put forward a new model of the graphene-silicene-graphene (GSG) heterojunction. In the GSG, we study the valley polarization properties in a zigzag nanoribbon in the presence of an external electric field. We find the energy range associated with the bulk gap of silicene has a valley polarization more than 95%. Under the protection of the topological edge states of the silicene, the valley polarization remains even the small non-magnetic disorder is introduced. These results have certain practical significance in applications for future valley valve.
Coherence is a crucial requirement to realize quantum manipulation through light-matter interactions. Here we report the observation of anomalously robust valley polarization and valley coherence in bilayer WS2. The polarization of the photoluminescence from bilayer WS2 inherits that of the excitation source with both circularly and linearly polarized and retains even at room temperature. The near unity circular polarization of the luminescence reveals the coupling of spin, layer and valley degree of freedom in bilayer system, while the linear polarized photoluminescence manifests quantum coherence between the two inequivalent band extrema in momentum space, namely, the valley quantum coherence in atomically thin bilayer WS2. This observation opens new perspectives for quantum manipulation in atomically thin semiconductors.
We study transport in twisted bilayer graphene and show that electrostatic barriers can act as valley splitters, where electrons from the $K$ ($K$) valley are transmitted only to e.g. the top (bottom) layer, leading to valley-layer locked currents. We show that such a valley splitter is obtained when the barrier varies slowly on the moire scale and induces a Lifshitz transition across the junction, i.e. a change in the Fermi surface topology. Furthermore, we show that for a given valley the reflected and transmitted current are transversely deflected, as time-reversal symmetry is effectively broken in each valley separately, resulting in valley-selective transverse focusing at zero magnetic field.
The electron transport of different conical valleys is investigated in graphene with extended line-defects. Intriguingly, the electron with a definite incident angle can be completely modulated into one conical valley by a resonator which consists of several paralleling line-defects. The related incident angle can be controlled easily by tuning the parameters of the resonator. Therefore, a controllable 100% valley polarization, as well as the detection of the valley polarization, can be realized conveniently by tuning the number of line-defects and the distance between two nearest neighbouring line-defects. This fascinating finding opens a way to realize the valley polarization by line-defects. With the advancement of experimental technologies, this resonator is promising to be realized and thus plays a key role in graphene valleytronics.
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