Bulletin of the American Physical Society
APS March Meeting 2015
Volume 60, Number 1
Monday–Friday, March 2–6, 2015; San Antonio, Texas
Session F52: Invited Session: Tunable Topological States in Monolayer and Bilayer Graphene |
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Sponsoring Units: DCMP Chair: Allan MacDonald, University of Texas at Austin Room: Grand Ballroom C2 |
Tuesday, March 3, 2015 8:00AM - 8:36AM |
F52.00001: Direct capacitive probe of valley order in a bilayer graphene quantum Hall ferromagnet Invited Speaker: Andrea Young Bilayer graphene is a highly tunable quantum Hall system: both the spin and valley splitting can be manipulated independently of the electron density. Recently, a number of studies have revealed a rich phase diagram of gapped ground states corresponding to diverse spin/valley isospin polarizations; however, all of these studies are indirect, relying on the dependence of features associated with gapped states on electric or magnetic field to infer the isospin ordering of these states. In this talk, I will describe a capacitive technique capable of directly detecting the valley component of the isospin order. By sensitively measuring the difference in capacitance between top and bottom gates of a dual-gated bilayer graphene device, we extract layer-charge response--the tendency of the layer polarization to change in response to a change in chemical potential of the entire bilayer. The asymmetric capacitance reflects the ground state layer polarization, and thus, in the zero Landau level (zLL), the valley polarization. In agreement with theoretical expectations, we find that the phase diagram of the octet zLL is, for the most part, characterized by the filling of pairs of degenerate sublevels with different orbital quantum numbers. Surprisingly, however, we also observe numerous additional states at intermediate electric fields, associated with an ``odd'' layer occupation. Finally, we find that phase transitions between states with different layer polarization are characterized by large anomalies in the asymmetric capacitance, which persist well into the compressible center of the Landau level. [Preview Abstract] |
Tuesday, March 3, 2015 8:36AM - 9:12AM |
F52.00002: Topological phases in the zeroth Landau level of bilayer graphene Invited Speaker: Zlatko Papic Over the past years, a remarkable variety of novel correlated states was discovered in graphene and its bilayer in a magnetic field. These states exhibit previously unseen richness due to an interplay of electron spin, valley and orbital degrees of freedom. They are furthermore distinguished by their high degree of tunability, e.g. via the in-plane magnetic or the perpendicular electric field, which allows one to probe their properties in a more flexible and direct way than in GaAs semiconductor systems. In this talk I will present a theoretical overview of the phase diagram of the partially-filled zeroth Landau level of bilayer graphene. Using realistic large-scale numerical calculations [1] that incorporate strong mixing between orbitally degenerate sublevels, as well as the screening of the Coulomb interaction, we identify several robust quantum Hall states with odd denominators such as $\nu=-4/3, -5/3, -8/5$. Although these states bear some relation to their more familiar analogs in GaAs, their collective excitations are expected to be different, as we illustrate on an example of the $\nu=-1$ state that acquires a neutral gap in bilayer graphene. Furthermore, we find evidence for the existence of an incompressible, even-denominator $\nu=-1/2$ state, and argue that this state is in the universality class of the non-Abelian Moore-Read state or its particle-hole conjugate, while other candidates such as the 331 state are unlikely to describe it. Finally, it will be shown that symmetry breaking, induced by an electric field applied perpendicular to the basal plane, is a useful experimental knob to tune the quantum phase transitions between integer or fractional states in bilayer graphene at a fixed filling factor [2]. These results illustrate the potential of bilayer graphene as a model platform to study the emergent topologically ordered phases and transitions between them via symmetry breaking.\\[4pt] [1] Z. Papic and D. Abanin, Phys. Rev. Lett. 112, 046602 (2014).\\[0pt] [2] Patrick Maher, Lei Wang, Yuanda Gao, Carlos Forsythe, Takashi Taniguchi, Kenji Watanabe, Dmitry Abanin, Zlatko Papic, Paul Cadden-Zimansky, James Hone, Philip Kim, and Cory R. Dean, Science 345, 61-64 (2014). [Preview Abstract] |
Tuesday, March 3, 2015 9:12AM - 9:48AM |
F52.00003: Even-denominator fractional quantum Hall effect in multi-terminal suspended bilayer graphene Invited Speaker: Alberto Morpurgo I will discuss magneto-transport experiments through multi-terminal suspended bilayer graphene devices of very high quality (mean-free path larger than the device size; density of charge inhomogeneity 10$^{9}$ cm$^{-2})$. The multi-terminal geometry enables independent measurements of the longitudinal and transverse magneto resistance, which are essential to properly measure quantum Hall states. At high magnetic field, different fractional states emerge on the hole side, including states at $\nu = $ -4/3 and $\nu = $ -1/2 that are fully developed (plateau in R$_{\mathrm{xy}}$ quantized with an accuracy better than 0.5{\%}, and a concomitant minimum in R$_{\mathrm{xx}})$ and other states (e.g., at -5/2, -2/3, -8/5), which manifest themselves through a clear minimum in R$_{\mathrm{xx}}$ occurring at a fixed value of filling factor. The more pronounced states are consistent with predictions of a recent theory by Papic and Abanin, that describes the mixing of the degenerate, zero-energy N$=$0 and N$=$1 Landau levels of graphene bilayers due to e-e interactions, and which indicates that the even denominator $\nu = $-1/2 state is of the Moore-Read type. If time allows, I will also discuss our recent experiments of suspended multi-terminal 4-layer graphene, on which we made different interesting observations. One is an integer quantum Hall effect consistent with an even larger degeneracy of the E$=$0 Landau levels, for which it may be interesting to start exploring theoretically possible new physics in the fractional regime. The second is the occurrence of an unexpected gapped insulating state at zero magnetic field. Together with previous experiments on suspended mono, bi, and trilayers, this observation points to an even-odd effect of e-e interaction (at zero magnetic field) in graphene multilayers: even layers are gapped by e-e interactions while odd layers stay conducting, due to the presence of a Dirac-like band in their electronic structure. A comparison of the gapped state in bilayers and four-layers show that the magnitude of the effect of e-e interaction is not becoming smaller with increasing layer thickness, suggesting that interactions remain important in even thicker layers. \\[4pt] I am grateful to my collaborators, D.K. Ki, A. Grushina, D. Abanin, V. Falko, M. Koshino, E. McCann, M. Potemski, C. Fagueras, A. Nicolet. [Preview Abstract] |
Tuesday, March 3, 2015 9:48AM - 10:24AM |
F52.00004: Electron-hole asymmetry in the integer and fractional quantum Hall effect in bilayer graphene Invited Speaker: Angela Kou The nature of fractional quantum Hall (FQH) states is determined by the interplay between the Coulomb interaction and the symmetries of the system. The unique combination of spin, valley, and orbital degeneracies in bilayer graphene is predicted to produce an unusual and tunable sequence of FQH states. In this talk, I will present local electronic compressibility measurements of the lowest Landau level in bilayer graphene performed using a scanning single-electron transistor. In the integer quantum Hall regime, we find that the background compressibility between filling factors breaks particle-hole symmetry and instead obeys a $\nu \to \nu +$ 2 symmetry. We also find the above-mentioned $\nu \to \nu +$ 2 symmetry in the FQH regime; we observe incompressible FQH states at filling factors $\nu =$ 2$p +$ 2$/$3 with hints of additional states appearing at $\nu =$ 2$p +$ 3$/$5, where $p =$ -2, -1, 0 and 1. These observations highlight the importance of the orbital degeneracy for many-body states in bilayer graphene. [Preview Abstract] |
Tuesday, March 3, 2015 10:24AM - 11:00AM |
F52.00005: Chemical potential and tunneling in bilayer graphene using double bilayer graphene heterostructures Invited Speaker: Emanuel Tutuc Vertical heterostructures consisting of atomic layers separated by insulators can open a window to explore the role of electron interaction in these materials, otherwise not accessible in single layer devices. We describe here one such heterostructure, consisting of two bilayer graphene flakes separated by a hexagonal boron-nitride dielectric. Using the top layer as a resistively detected Kelvin probe we map the chemical potential of the bottom bilayer graphene as a function of electron density, perpendicular magnetic field, and transverse electric field. At zero magnetic field the chemical potential reveals a strongly non-linear dependence on density, with an electric field induced energy gap at charge neutrality. The data allow a direct measurement of the electric field-induced bandgap at zero magnetic field, the orbital Landau level energies, and the broken symmetry quantum Hall state gaps in high magnetic fields [1]. In samples where the two layers are rotationally aligned the interlayer tunneling current measured as a function of interlayer bias reveals a gate-tunable negative differential resistance thanks to momentum conserving tunneling [2]. Remarkably, the resonance width has a weak temperature dependence in the range 1.5 K to 300 K. \\[4pt] Work done in collaboration with K. Lee, B. Fallahazad, S. Kang, J. Xue, D. C. Dillen, K. Kim, L. F. Register, S. K. Banerjee, T. Taniguchi, and K. Watanabe. \\[4pt] [1] K. Lee \textit{et al.,} \textit{Science} \textbf{345}, 58 (2014).\\[0pt] [2] B. Fallahazad \textit{et al}., \textit{Nano Letters} ASAP, DOI: 10.1021/nl503756y (2014). [Preview Abstract] |
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