Bulletin of the American Physical Society
APS March Meeting 2020
Volume 65, Number 1
Monday–Friday, March 2–6, 2020; Denver, Colorado
Session F53: Transport Properties of 2D Materials |
Hide Abstracts |
Sponsoring Units: DCMP Chair: Alexey Lipatov Room: Mile High Ballroom 1F |
Tuesday, March 3, 2020 8:00AM - 8:12AM |
F53.00001: A hidden quantum phase in two-dimensional WTe2 Sanfeng Wu Correlations, spin-orbit coupling and topology are all important in the atomically thin WTe2 layers. In this talk I will introduce our recent quantum transport studies of two-dimensional WTe2 where we discover a hidden quantum phase. |
Tuesday, March 3, 2020 8:12AM - 8:24AM |
F53.00002: Anisotropic magnetotransport in few-layer Nb-doped WSe2 Hiroyuki Nakamura, Avaise Mohammed, Wilfried Sigle, Yi Wang, Peter Wochner, John Villanova, Jeb Stacy, Salvador Barraza-Lopez, Hugh Churchill, Peter van Aken, Hidenori Takagi Hole-doped epitaxial WSe2 films with thicknesses of three to six monolayers were grown by a hybrid pulsed-laser deposition, in which metallic alloy targets were laser-ablated under a controlled Se vapor. The Hall effect indicated active Nb-dopant concentrations of 10-50 at%. Scanning transmission electron microscopy revealed that Nb atoms occupy W sites, and 2H- as well as 3R-stacking coexist in the films. A striking anisotropy between magnetoconductance (MC) for in-plane (B // I) and out-of-plane (B ⊥ I) magnetic fields was found, showing a complete absence of weak antilocalization for B ⊥ I, while it restores for B // I at intermediate temperatures. The band structure of 2H-3R Nb:WSe2 obtained by first-principles calculations is shown, based on which a few possible origins of the MC will be discussed. |
Tuesday, March 3, 2020 8:24AM - 8:36AM |
F53.00003: Observation of monolayer-like quantum transport in few-layer MoS2 Tianyi Han, Xu Han, Jiangxiazi Lin, Benjamin A. Piot, Xiangbin Cai, Ding Pan, Junwei Liu, Ning Wang The conduction band minimum of MoS2 shifts from K valley to Q valley when its thickness increases from monolayer to bulk. Previously, K valley quantum transport has been extensively studied in monolayer MoS2, revealing its intriguing physical properties such as the spin-valley locking effect and the interaction enhanced Zeeman splitting. However, the fabrication of monolayer MoS2 device requires considerable efforts mainly due to the challenges of achieving good electrical contact. Here we report the observation of monolayer-like K valley quantum transport in high quality few-layer MoS2. Well-developed two-fold quantum oscillations are presented, with valley Zeeman splitting and spin polarized Landau levels at high magnetic field. Further, we observe multi K band conduction in the Landau fan diagram, from which the spin-orbit coupling splitting energy is estimated. Our observation offers the possibility of switching from Q valley to K valley transport in few-layer MoS2. |
Tuesday, March 3, 2020 8:36AM - 8:48AM |
F53.00004: Preferential out-of-plane conduction and quasi-one-dimensional electronic states in layered van der Waals material 1T-TaS2 Edoardo Martino, Konstantin Semeniuk, Alla Arakcheeva, Carsten Putzke, Philip Moll, helmuth berger, Ivo Batistic, Eduard Tutiš, Ana Akrap, Laszlo Forro Layered metallic transition-metal dichalcogenides are conventionally seen as two-dimensional conductors, despite a scarcity of systematic studies of the interlayer charge transport. Motivated by the ascending strategy of functionalizing 2D materials by vertical heterostructures we initiated an in-depth study of out-of-plane charge dynamics and emergent properties arising from interlayer coupling. |
Tuesday, March 3, 2020 8:48AM - 9:00AM |
F53.00005: Band filling and cross quantum capacitance in ion gated semiconducting transition metal dichalcogenide monolayers Haijing Zhang, Christophe Berthod, helmuth berger, Thierry Giamarchi, Alberto Morpurgo Ionic liquid gated field-effect transistors based on semiconducting transition metal dichalcogenides are attracting significant scientific interest, but important aspects of how charge carriers are accumulated in these systems remain elusive. Here we present a thorough analysis of charge accumulation in MoSe2 and WSe2 monolayers. We identify the conditions when the chemical potential enters different valleys in the monolayer band structure (the K and Q valleys in the conduction band and the two spin-split K-valleys in the valence band) and find that an independent electron picture describes the occupation of states well. Unexpectedly, however, the same analysis shows that the total device capacitance cannot be simply described in terms of the series connection of a geometrical capacitance and of a quantum capacitance given by CQ=e2/(dμ/dn), as commonly assumed. This unexpected behavior is attributed to the presence of a cross quantum capacitance, which originates physically from mutual screening of the electric field generated by charges on one plate from charges sitting on the other plate. Our findings therefore reveal an important contribution to the capacitance of physical systems that had been virtually neglected until now. |
Tuesday, March 3, 2020 9:00AM - 9:12AM |
F53.00006: 2e charge transport in a quantum Hall - superconductor junction revealed by shot noise MANAS SAHU, Arup Kumar Paul, Jagannath Sutradhar, Kenji Watanabe, Takashi Taniguchi, Subroto Mukerjee, Sumilan Banerjee, Anindya Das Two remarkable phenomena of condensed matter physics, the quantum Hall (QH) effect and superconductivity, when combined, can give rise to exotic topological excitations and hold great promises for future quantum computation. Andreev reflection (AR) is the underlying phenomena, which determines the quasiparticle dynamics at the QH–superconductor (SC) junction. Even decades after the discoveries of QH and SC, the AR at the QH-SC junction remains elusive. Here we have carried out conductance and shot noise measurements in a graphene QH coupled to a MoRe superconductor. The QH plateaus are observed at 4(N+1/2)e2/h, which are identical to the conductance plateaus with a normal contact. This observation is consistent with our theoretical calculation of electron-hole mixing due to repetitive AR at QH - SC junction. By measuring shot noise, we observe the Fano factor close to unity when the bias energy is less than the superconducting gap. This is direct evidence of 2e charge transport at QH - SC junction. Above the superconducting gap, the Fano factor almost vanishes, as expected due to regular charge e transport. This work will pave the way for exploration of more exotic systems, such as junction of a fractional QH and a superconductor, by shot noise measurements. |
Tuesday, March 3, 2020 9:12AM - 9:24AM |
F53.00007: Direct measurement of layer polarizability in a van der Waals ferroelectric semimetal Qingrui Cao, Sergio C De La Barrera, Vineetha Bheemarasetty, Jiaqiang Yan, Di Xiao, Yang Gao, Benjamin Hunt A ferroelectric semimetal has coexistence of spontaneous electric polarization and metallicity. A recent study [1] demonstrated bilayer and trilayer 1T'-WTe2 exhibit an out-of-plane switchable polarization using graphene as an electric-field sensor. However, a direct measurement of its layer polarizability is still lacking. Here, we detect and quantify the ferroelectricity by layer-resolved capacitance measurements [2] in dual-gated bilayer WTe2, allowing us to control applied electric field and charge density independently. Moreover, we examine the density-dependent polarizability and extract the critical densities above which the polarization becomes weaker. All of our observations, including hysteresis, "near-layer capacitance enhancement", density dependence of the polarization, are consistent with the theoretical predictions based upon an electrostatic model of capacitance derived from a "tilted-Dirac-cone" model of bilayer WTe2. |
Tuesday, March 3, 2020 9:24AM - 9:36AM |
F53.00008: Electron-phonon scatterings and their roles in vertical charge transport through van der Waals heterostructures Suyong Jung Electron–phonon scattering in solid-state systems is a pivotal process in determining many of key physical quantities such as charged carrier mobilities and lattice thermal conductivities. Here, we report electron tunneling measurements with a semiconducting transition metal dichalcogenide, WSe2, as the tunnel medium. In van der Waals (vdW)-coupled vertical heterostructures, inelastic quantum tunnel events activated by turbostratic electron–phonon scatterings constructively establish an adjunct transport channel, which drastically increases its loads in vertical charge flows with increasing tunnel medium thickness. Phonons interacting with the tunnel electrons are layer-number dependent, varying by monolayer vs. bi- and triple-layer WSe2, with an out-of-plane flexural ZA mode as the common excitation. In addition, we find out that second-order electron–phonon scatterings become conspicuous in the multilayered films, involving assorted phonons in the tunnel media and the vdW heterojunctions. |
Tuesday, March 3, 2020 9:36AM - 9:48AM |
F53.00009: Orientation-dependent transport and Goos-Hanchen effect in phorsphorene LI Ruigang, Kwok Sum Chan, Zijing Lin By using the continuum model, we systematically investigate the transport property in the phorsphorene NPN junction. Different devices with different angles between the ribbon and armchair directions are studied. With the increase of this angle from 0°, it is found that the transmission first exhibits the oscillating behavior. Particularly, with the further increase of the angle close to 90°, the transmission goes through a fast drop to zero. We further find that the fast drop arises from the transformation of the transporting carriers from the Dirac quasiparticles into Schrodinger ones, and the angel of this transformation is related to the anisotropy of the effective mass, providing the possibility to shed light on the electronic device design. Furthermore, study of the Goos-Hanchen effect in the phorsphorene NPN junction is also addressed. It is interesting to find that even for the slight change of the angle between the ribbon and armchair directions, remarkable change of the incoming wave pocket is obtained, providing the promising candidate as the wave-pocket-selected filter. Finally, all above properties in NN’N junctions of the bulk phorsphorene are also addressed. |
Tuesday, March 3, 2020 9:48AM - 10:00AM |
F53.00010: Anomalous Coulomb Drag between InAs Nanowire and Graphene Heterostructures RICHA MITRA, MANAS SAHU, Kenji Watanabe, Takashi Taniguchi, Hadas Shtrikman, A K Sood, Anindya Das Correlated charge inhomogeneity breaks the electron-hole symmetry in two-dimensional bilayer heterostructures, responsible for non-zero drag appearing at the charge neutrality point. Although several theories predict this phenomenon, there is no consensus over the actual mechanism responsible for it. Here we report, Coulomb drag in a novel drag system consisting of a two-dimensional (2D) graphene and a one dimensional (1D) InAs nanowire (NW) heterostructures. For monolayer graphene (MLG)-NW heterostructures we observe an unconventional drag resistance peak near the Dirac point due to the correlated inter-layer charge puddles, which decreases monotonically with temperature (∼T-2) but increases rapidly with the magnetic field (∼B2). These anomalous responses together with the mismatched thermal conductivities of graphene and NWs establish the energy drag as the responsible mechanism of Coulomb drag in MLG-NW devices. In contrast, for bilayer graphene (BLG)-NW devices the drag voltage reverses sign across the Dirac Point, consistent with momentum drag but remains almost constant with magnetic field and temperature. These Coulomb drag measurements in 2D-1D systems, hitherto not reported before, will pave the future realization of correlated condensate states in novel systems. |
Tuesday, March 3, 2020 10:00AM - 10:12AM |
F53.00011: Gate-defined Quantum Confinement in few-layer Black Phosphorus Transistor Jiawei Yang, Shi Che, Takashi Taniguchi, Kenji Watanabe, Seongphil Moon, Dmitry Smirnov, Marc W Bockrath, Ruoyu Chen, Chun Ning Lau Black phosphorus is a novel two-dimensional(2D) semiconductor which has attracted considerable research interest due to its tunable band gap and high electron mobility. Here we demonstrate quantum confinement defined by split gate in devices based on few-layer black phosphorus. The tunability of split gate can be illustrated by the fact that a device can be tuned off by split gate alone. Quantized conductance is observed when the width of the quantum point contact is varied. In quantum Hall regime, p-p'-p junction formed by tuning split gate are studied. The work opens the door for using black phosphorus as platform for electronic and optoelectronic applications. |
Tuesday, March 3, 2020 10:12AM - 10:24AM |
F53.00012: Ultrafast currents in the monolayer of transition metal dichalcogenides Seyyedeh Azar Oliaei Motlagh, Vadym Apalkov, Mark I Stockman We theoretically predict that in a monolayer of the transition metal dichalcogenide (TMDCs), an applied ultrafast intense optical pulse with the polarization perpendicular to the TMDC’s axis of symmetry induces two types of currents: a trivial current, which flows in the direction of the applied pulse and a nontrivial current, which flows in the direction perpendicular to the polarization of the pulse. The applied pulse has a linear polarization with a high amplitude of several Volt per Angstrom and the duration of a few femtoseconds. Our results show that the ultrafast field-driven currents with asymmetric profiles transfer the electrons asymmetrically, which causes the monolayer of TMDC to become electrically polarized. The induced nontrivial current and the net electrical polarization are due to Berry curvature, which is nonzero near the K and K’ valleys [1]. |
Tuesday, March 3, 2020 10:24AM - 10:36AM |
F53.00013: On the nature of carrier states around the fermi level at the LAO/STO interface liviu chioncel, Tobias Strobl, Patrick Sailer, Daniel Braak, Thilo Kopp We propose a minimal tight-binding one-band model for the interface layer of LAO/STO consisting of “effective” carriers (of Ti-dxy character) in random magnetic fields. These fields result from the hybridization of Ti-d(eg)-bands and oxygen vacancies at the LAO/STO interface and can be seen as local moments (i.e. “effective impurities”) located at the Ti-site. Both the effects of a nonmagnetic on-site potential and the exchange interactions (Zeeman like) between carrier and magnetic impurities are taken into account. We apply the coherent potential approximation in studying this system. |
Tuesday, March 3, 2020 10:36AM - 10:48AM |
F53.00014: Electronic states and transport in graphene-TMD heterostructures Nohora Hernández Cepeda, Lilia Meza-Montes, Sergio E Ulloa The advent of graphene and other monolayer 2D materials has opened the possibility to combine them in stacks that show interesting effects. We study here one interesting example, as the proximity of graphene and transition-metal dichalcogenides (TMD) heterostructures (G-TMD) results on unusual electronic states and transport properties. We employ an effective Hamiltionian that describes the broken inversion symmetry in graphene due to the proximity of the TMD but preserves temporal inversion [1]. The latter symmetry can be broken when an external magnetic field is applied, generating gaps in the band structure and consequent changes in electronic states that we study as function of field direction and magnitude for different TMD systems. We focus on the competition with spin-orbit effects. We also present results on transport properties, calculating the spin-dependent transmission and reflection coefficients of carriers propagating on a graphene sheet decorated with TMD ribbons on top. |
Tuesday, March 3, 2020 10:48AM - 11:00AM |
F53.00015: Experimental Quantum Transport Strain Engineering in Graphene Guoqing Wei, Andrew C McRae, Linxiang Huang, Alexandre Champagne We measure ballistic charge conductivity in strained suspended graphene and observe the theoretically predicted [1] strain-induced scalar and vector potentials. To do so, we built an experimental platform for quantum transport strain engineering in 2D materials. This instrumentation permits low temperature (0.3 K- 70 K) transport, 0T - 9T magnetic fields, and a tunable uniaxial strain (up to 3%) which is independent from the gate-tunable charge density. We show slippage-free clamping of high aspect-ratio graphene crystals where atomically ordered edges are unnecessary for quantitative straintronics. We study in detail transport in a graphene channel whose length is 90 nm and width is 600nm. we observe that both the channel and contacts are ballistic. By applying strain, we find the strain-induced scalar potential which shifts the low energy band structure downward by up to 30 meV. We also clearly observe the effect of a gauge vector potential which reversibly suppresses the conductance by up to 13.6%. We next aim to demonstrate total suppression of conductivity [1] and explore straintronics in other 2D materials. |
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