Bulletin of the American Physical Society
APS March Meeting 2015
Volume 60, Number 1
Monday–Friday, March 2–6, 2015; San Antonio, Texas
Session S51: Invited Session: The Valley Hall Effect in van der Waals Materials |
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Sponsoring Units: DCMP Chair: Allan MacDonald, University of Texas at Austin Room: Grand Ballroom C1 |
Thursday, March 5, 2015 8:00AM - 8:36AM |
S51.00001: Detecting Topological Currents in Gapped Graphene Invited Speaker: Andre Geim If a gap is open in a Dirac spectrum, Hall-like currents can flow perpendicular to applied electric field even in the absence of a magnetic field. This is due to non-zero Berry curvature, intrinsic for electronic systems with massive Dirac fermions. In graphene, the Berry curvature forces charge carriers in the two valleys to move in opposite directions, which results in zero net electric current. Nonetheless, if intervalley scattering is weak, valley currents can be detected using the nonlocal geometry. We will overview our observations of strong valley currents in graphene-on-hBN superlattices and biased bilayer graphene. [Preview Abstract] |
Thursday, March 5, 2015 8:36AM - 9:12AM |
S51.00002: Persistent valley currents and topological transport in gapped graphene Invited Speaker: Leonid Levitov The anomalous Hall effect (AHE), arising due to Berry curvature in materials with broken inversion symmetry, results in topological currents flowing in system bulk transversely to the applied electric field. We will discuss recent work on AHE in materials with several valleys, such as e.g. graphene and transition metal dichalcogenide monolayers, where these currents have been observed [Mak et al., Science 344, 1489 (2014); Gorbachev et al., Science 346, 448 (2014)]. Interestingly, these materials do not fit the paradigm of topological materials with Chern bands and associated topologically protected edge modes dominating (quantized) Hall conductivity. Here, in contrast, gapless edge states may be absent since they are not enforced by topology or symmetry. Further, even when present, these states are not protected against backscattering due to roughness on the atomic scale. Naively, this would lead one to conclude that topological currents cease to exist. If true, this would imply that the key manifestations, such as the valley Hall conductivity and orbital magnetization, vanish in the gapped state. We will argue that the opposite is true: the absence of conducting edge modes does not present an obstacle since valley currents can be transmitted by the bulk states in the filled Fermi sea beneath the gap. This leads to an interesting behavior: rather than being vanishingly small, valley currents reach maximum value in the gapped state. Such undergap currents can also occur as persistent currents in the thermodynamic ground state and dominate orbital magnetization in valley-polarized gapped systems. We will conclude with discussing requirements for dissipationless valley transport and argue that they can be met under realistic conditions. [Preview Abstract] |
Thursday, March 5, 2015 9:12AM - 9:48AM |
S51.00003: Valley current generation by electrically induced Berry curvature in double gated bilayer graphene Invited Speaker: Seigo Tarucha Valley degree of freedom is defined for an electronic system having degenerate band structure in a certain crystal configuration and can be used to generate non-dissipative current with accompanying no net charge flow by breaking the spatial inversion symmetry. Graphene and transition metal dichalcogenide are two typical valley materials having K and K$'$ valleys due to the existence of two sub-lattices. The valley current has only recently been studied for monolayer graphene on h-BN where the spatial inversion symmetry is structurally broken by the superlattice potential. We use a double gated bilayer graphene device to electrically break the spatial inversion symmetry and control the Berry curvature. We use valley hall effect to generate a transverse pure valley current and inverse valley hall effect to detect the current. In this device the Fermi energy and the bandgap are independently varied and this allows to prove existence of valley hall effect in the insulating regime where the local resistivity increases with lowering temperature. The insulating regime is particularly interesting because the electric field to valley current conversion is less dissipative in contrast to the case for conventional spin or valley hall systems. [Preview Abstract] |
Thursday, March 5, 2015 9:48AM - 10:24AM |
S51.00004: Valley and spin currents in 2D transition metal dichalcogenides Invited Speaker: Wang Yao In two-dimensional (2D) transition metal dichalcogenides (TMDs), carriers are indexed by both the spin and the valley pseudospin (labelling the degenerate band extrema in momentum space). 2D TMDs is therefore an ideal laboratory for exploring these internal quantum degrees of freedom for new electronics, and controlling the flow of spin and pseudospin is at the heart of such applications. We will discuss two mechanisms for generating spin and valley currents of electrons in 2D group-VIB TMDs: (I) the valley and spin Hall effects arising from the Berry curvatures; and (II) the nonlinear valley and spin currents arising from Fermi pocket anisotropy. The two effects have distinct scaling with the electric field, and different dependence of the current direction on the field direction and crystalline axis. We will discuss the possibility to observe and distinguish the two effects as distinct patterns of polarized electroluminescence at p-n junction in 2D TMDs. The nonlinear current response also makes possible the generation of pure spin and valley flows without net charge current, either by an AC bias or by an inhomogeneous temperature distribution. We will also discuss the valley Hall effect of charged excitons in monolayer TMDs, which arises from the effective coupling of the excitonic valley pseudospin to its center of mass motion by the exchange interaction between the electron and hole constituents. [Preview Abstract] |
Thursday, March 5, 2015 10:24AM - 11:00AM |
S51.00005: Electron dynamics and valley relaxation in 2D semiconductors Invited Speaker: Kenan Gundogdu Single layer transition metal dichalcogenides are 2D semiconducting systems with unique electronic band structure. Two-valley energy bands along with strong spin-orbital coupling lead to valley dependent career spin polarization, which is the basis for recently proposed valleytronic applications. Since the durations of valley population provide the time window in which valley specific processes take place, it is an essential parameter for developing valleytronic devices. These systems also exhibit unusually strong many body affects, such as strong exciton and trion binding, due to reduced dielectric screening of Coulomb interactions. But there is not much known about the impact of strong many particle correlations on spin and valley polarization dynamics. Here we report direct measurements of ultrafast valley specific relaxation dynamics in single layer MoS$_{2}$ and WS$_{2}$. We found that excitonic many body interactions significantly contribute to the relaxation process. Biexciton formation reveals hole valley spin relaxation time. Our results also suggest initial fast intervalley electron scattering and electron spin relaxation leads to loss of electron valley polarization, which then facilitates hole valley relaxation via excitonic spin exchange interaction. [Preview Abstract] |
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