Bulletin of the American Physical Society
APS March Meeting 2017
Volume 62, Number 4
Monday–Friday, March 13–17, 2017; New Orleans, Louisiana
Session Y22: Experimental Progress of Valley Transport in 2D MaterialsInvited
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Sponsoring Units: GMAG Chair: Fan Zhang, University of Texas, Dallas Room: New Orleans Theater A |
Friday, March 17, 2017 11:15AM - 11:51AM |
Y22.00001: Transport anomalies of high-mobility Q-valley electrons in few-layer WS2 and MoS2 Invited Speaker: Ning Wang Atomically thin transition metal dichalcogenides (TMDCs) have opened new avenues for exploring physical property anomalies due to their large band gaps, strong spin-orbit couplings, and rich valley degrees of freedom. Although novel optical phenomena such as valley selective circular dichroism, opto-valley Hall effect, and valley Zeeman effect have been extensively studied in TMDCs, investigation of quantum transport properties has encountered a number of obstacles primarily due to the low carrier mobility and strong impurity scattering. Recently, we successfully fabricated ultrahigh-mobility few-layer TMDC field-effect transistors based on the boron nitride encapsulation method and observed a number of interesting transport properties, such as even-odd layer-dependent magnetotransport of Q-valley electrons in WS2 and MoS2 and unconventional quantum Hall transport of $\Gamma $-valley hole carriers in WSe2. In few-layer samples of these TMDCs, the conduction bands along the $\Gamma $K directions shift downward energetically in the presence of interlayer interactions, forming six Q-valleys related by three-fold rotational symmetry and time reversal symmetry. In even-layers the extra inversion symmetry requires all states to be Kramers degenerate, whereas in odd-layers the intrinsic inversion asymmetry dictates the Q-valleys to be spin-valley coupled. In this talk, I'll demonstrate the prominent Shubnikov-de Hass (SdH) oscillations and the observation of the onset of quantum Hall plateaus for the Q-valley electrons. Universally in the SdH oscillations, we observe a valley Zeeman effect in all odd-layer TMDC devices and a spin Zeeman effect in all even-layer TMDC devices. In addition, we observe a series of quantum Hall states following an unconventional sequence predominated by odd-integer states under a moderate strength magnetic field in p-type few-layer TMDCs, indicating a large Zeeman energy associated with the carriers in the valence band at the $\Gamma $-valley. [Preview Abstract] |
Friday, March 17, 2017 11:51AM - 12:27PM |
Y22.00002: Valley and spin dependent physics in two-dimensional materials Invited Speaker: Kin Fai Mak Electrons in two-dimensional (2D) van der Waals' materials with a honeycomb lattice structure possess both the valley pseudospin and the spin degree of freedom (DOF). The valley DOF is associated with the degenerate conduction/valence band extrema at the K and the K' point of the Brillouin zone. When inversion symmetry is broken, interesting valley and spin dependent phenomena, such as spin-valley locking and the valley Hall effect (VHE), emerge. These unique properties are not only fundamentally important, but may also find applications in valley/spin based electronics and optoelectronics. In this talk, I will discuss our recent experiments on probing the valley and spin dependent properties in 2D transition metal dichalcogenides (TMDs). In particular, I will present results on the observation and the control of the VHE in TMD semiconductors. I will also discuss the influence of robust spin-valley locking on the physical properties of hole-doped TMD metals. [Preview Abstract] |
Friday, March 17, 2017 12:27PM - 1:03PM |
Y22.00003: Tunable valley symmetries of quantum Hall states in few-layer graphene Invited Speaker: Chun Ning Lau |
Friday, March 17, 2017 1:03PM - 1:39PM |
Y22.00004: Topological Valley Transport at Bilayer Graphene Domain Walls Invited Speaker: Long Ju Electron valley, a degree of freedom that is analogous to spin, can lead to novel topological phases in bilayer graphene. A tunable bandgap can be induced in bilayer graphene by an external electric field, and such gapped bilayer graphene is predicted to be a topological insulating phase protected by no-valley mixing symmetry, featuring quantum valley Hall effects and chiral edge states. Observation of such chiral edge states, however, is challenging because inter-valley scattering is induced by atomic-scale defects at real bilayer graphene edges. Recent theoretical work~has shown that domain walls between AB- and BA-stacked bilayer graphene can support protected chiral edge states of quantum valley Hall insulators. In this talk, I will discuss about our experimental effort on such stacking domain walls. We employed infrared near field nanoscopy to locate the domain walls in bilayer graphene and measure their electrical resistance in a dual-gated device configuration. Unlike single-domain bilayer graphene, which shows gapped insulating behaviour under a vertical electrical field, bilayer graphene domain walls feature one-dimensional valley-polarized conducting channels with a ballistic length of about 400 nanometres at 4 kelvin. Such topologically protected one-dimensional chiral states at bilayer graphene domain walls open up opportunities for exploring unique topological phases and valley physics in graphene. [Preview Abstract] |
Friday, March 17, 2017 1:39PM - 2:15PM |
Y22.00005: Magnetotransport of High Mobility Holes in Monolayer and Bilayer WSe$_{\mathrm{2}}$ Invited Speaker: Emanuel Tutuc Transition metal dichalcogenides have attracted significant interest because of their two-dimensional crystal structure, large band-gap, and strong spin-orbit interaction which leads to spin-valley locking. Recent advances in sample fabrication [1] have allowed the experimental study of low temperature magneto-transport of high mobility holes in WSe$_{\mathrm{2}}$ [2]. We review here the main results of these studies which reveal clear quantum Hall states in mono- and bilayer WSe$_{\mathrm{2}}$. The data allows the extraction of an effective hole mass of $m^{\ast }=$0.45$m_{e}$ ($m_{e}$ is the bare electron mass) in both mono and bilayer WSe$_{\mathrm{2}}$. A systematic study of the carrier distribution in bilayer WSe$_{\mathrm{2}}$ determined from a Fourier analysis of the Shubnikov-de Haas oscillations indicates that the two layers are weakly coupled. The individual layer density dependence on gate bias shows negative compressibility, a signature of strong electron-electron interaction in these materials associated with the large effective mass. We discuss the interplay between cyclotron and Zeeman splitting using the dependence of the quantum Hall state sequence on carrier density, and the angle between the magnetic field and the WSe$_{\mathrm{2}}$ plane. Work done in collaboration with B. Fallahazad, H. C. P. Movva, K. Kim, S. K. Banerjee, T. Taniguchi, and K. Watanabe. [1] H. C. P. Movva \textit{et al.}, \textit{ACS Nano} \textbf{9}, 10402 (2015). [2] B. Fallahazad \textit{et al.}, \textit{Phys. Rev. Lett.} \textbf{116}, 086601 (2016). [Preview Abstract] |
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