Bulletin of the American Physical Society
APS March Meeting 2014
Volume 59, Number 1
Monday–Friday, March 3–7, 2014; Denver, Colorado
Session D29: Electrical Transport Properties of Bilayer Graphene |
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Sponsoring Units: DCMP Chair: Ki Wook Kim, North Carolina State University Room: 603 |
Monday, March 3, 2014 2:30PM - 2:42PM |
D29.00001: The dangerous negative energy sea and the N=0 fractional quantum Hall effect in bilayer graphene Rohit Hegde, Inti Sodemann, Fengcheng Wu, Allan H. MacDonald The $N=0$ Landau level of bilayer graphene is nearly eight-fold degenerate because it contains states with $n=0$ and $n=1$ cyclotron quantum numbers in addition to spin and valley indices. Because the self energy due to exchange with the negative energy sea is $n$-dependent, it plays an essential role in choosing between competing correlated states at both integer and fractional filling factors. We show that this interaction must be included in a systematic theory controlled by the ratio of Coulomb to cyclotron energies. We will discuss the implications of this vacuum exchange effect for integer and fractional quantum Hall ground states and low lying charged and neutral excitations, and its interplay with the valley symmetry breaking and Zeeman terms that are also important in single-layer graphene. [Preview Abstract] |
Monday, March 3, 2014 2:42PM - 2:54PM |
D29.00002: Bilayer graphene with parallel magnetic field and twisting: Phases and phase transitions in a highly tunable Dirac system Kun Yang, Bitan Roy The effective theory for bi-layer graphene, subject to parallel/in-plane magnetic fields is discussed. We show that with a sizable in-plane magnetic field the trigonal warping becomes irrelevant, and one ends up with two Dirac points in the vicinity of each valleys in the low-energy limit, similar to the twisted bi-layer graphene. Combining twisting and parallel field thus gives rise to a Dirac system with tunable Fermi velocity and ultra violet cutoff. If the interactions are sufficiently strong, several fully gapped states can be realized in these systems, in addition to the ones in pristine setup. Symmetry based classification of the order parameters will be discussed. We also present the quantum critical behavior of various phase transitions driven by the twisting and the magnetic field. Effects of an additional perpendicular fields, and possible ways to realize the some of the new massive phases will be highlighted. [Preview Abstract] |
Monday, March 3, 2014 2:54PM - 3:06PM |
D29.00003: Topological phases in the zeroth Landau level of bilayer graphene Zlatko Papic, Dmitry Abanin We study the phase diagram of the zeroth Landau level of bilayer graphene in the presence of strong mixing between two degenerate orbital sublevels, as well as the screening of the effective Coulomb interaction. Using large scale exact diagonalization calculations, we find stable quantum Hall states at filling factors $\nu=-1, -4/3, -5/3, -8/5, -1/2$. We discuss the nature of these ground states and their collective excitations in terms of the known states in GaAs semiconductors using a truncated interaction model. Furthermore, we present evidence that the $\nu=-1/2$ fraction, which was recently reported experimentally, is unlikely a two-component ``331" state, but instead is of non-Abelian nature and related to the Moore-Read Pfaffian wave function. [Preview Abstract] |
Monday, March 3, 2014 3:06PM - 3:18PM |
D29.00004: Superfluid-Insulator transition of quantum Hall domain walls in bilayer graphene H.A. Fertig, Victoria Mazo, Chia-Wei Huang, Efrat Shimshoni, Sam Carr We consider the zero-filled quantum-Hall ferromagnetic state of bilayer graphene subject to a kink-like perpendicular electric field, which generates domain walls in the electronic state and low-energy collective modes confined to move along them. In particular, it is shown that two pairs of collective helical modes are formed at opposite sides of the kink, each pair consisting of modes with identical helicities. We derive an effective field theoretical model of these modes in terms of two weakly coupled anisotropic quantum spin-ladders, with parameters tunable through control of the electric and magnetic fields. This yields a rich phase diagram, where due to the helical nature of the modes, distinct phases possess very different charge conduction properties. Most notably, this system can potentially exhibit a transition from a superfluid to an insulating phase. [Preview Abstract] |
Monday, March 3, 2014 3:18PM - 3:30PM |
D29.00005: Electric Field Effects and Landau Level Crossing in Suspended Bilayer Graphene Kevin Myhro, Yongjin Lee, Michael Deo, David Tran, Jeanie Lau Bilayer graphene offers a versatile 2D platform for electron transport study due to its gate tunable band gap, large tensile strength and ultra-high electronic mobility. Here we report two-terminal differential conductance measurements of dual-gated suspended bilayer graphene devices as a function of applied back gate and top gate voltages in zero and finite magnetic fields. Multi-level electron beam lithography defines contactless top gates and the bilayer graphene flakes are suspended by wet-etching the oxide layer. Successful current annealing allows us to reach high mobility and an insulating state at low magnetic fields. We investigate the role of the applied perpendicular electric field from top-gated devices, compare results to single-gated measurements, and characterize Landau Level crossings as a function of electric field, charge carrier density and magnetic field. [Preview Abstract] |
Monday, March 3, 2014 3:30PM - 3:42PM |
D29.00006: Magneto and Hall drag in graphene double-layer Xiaomeng Liu, Yuanda Gao, Lei Wang, Patrick Maher, Kin Chung Fong, Kenji Watanabe, Takashi Taniguchi, James Hone, Philip Kim, Cory Dean Recent advancements in the assembly of 2D layered materials have enabled fabrication of large-area BN-encapsulated graphene that exhibits ballistic transport over length scales in excess of tens of micrometers. Exploiting these techniques to fabricate graphene double layers, consisting of two parallel graphene layers separated by few-layer BN dielectrics, we investigate Coulomb drag in the strongly interacting and ultra-clean limit. Magneto drag and Hall drag, under perpendicular magnetic fields up to 13T and temperature down to 5K, is reported. Using a dual gated structure to independently tune the charge carrier density and type in each layer, we correlate the finite-field drag response with Landau level filling, and compare with existing theories. [Preview Abstract] |
Monday, March 3, 2014 3:42PM - 3:54PM |
D29.00007: Mapping the chemical potential in bilayer graphene using double bilayer heterostructures Kayoung Lee, Babak Fallahazad, Jiamin Xue, Takashi Taniguchi, Kenji Watanabe, Emanuel Tutuc A key property of an electron system, the chemical potential (Fermi energy) captures the physics of both the energy-momentum band structure, as well as interaction-induced corrections to the single particle energy. In Bernal stacked bilayer graphene the band structure can also be reshaped by an applied transverse electric field. By performing transport measurement in double-bilayer graphene heterostructure, which consists of two bilayer graphene vertically separated by hexagonal boron nitride, 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. In a perpendicular magnetic field, the quantum Hall states stabilized by Landau levels (LL) spin and valley degeneracy lifting undergo transitions as a function of electric and magnetic fields, as a result of the interplay between LL spin and valley splitting. By directly measuring the LL energies we extract the LL spin and valley splitting, and their dependence on magnetic and electric field. [Preview Abstract] |
Monday, March 3, 2014 3:54PM - 4:06PM |
D29.00008: Nonlocal transport in dual-gated bilayer graphene Yuya Shimazaki, Michihisa Yamamoto, Kenji Watanabe, Takashi Taniguchi, Seigo Tarucha We report nonlocal transport measurement of biased bilayer graphene. Dual gated bilayer graphene Hall bars sandwiched between two h-BN insulating layers were prepared using the transfer technique with PMMA thin flims. We measured both local and non-local transport at temperatures between 1.5 K and 200 K. We found enhancement of the nonlocal resistance near the charge neutrality point when we increase the perpendicular electric field. Observed nonlocal resistance at 70K is much larger than what is expected as the Ohmic contribution from van der Pauw formula with measured local resistivity. This observation indicates additional contribution to the nonlocal transport in biased bilayer graphene. We present temperature and displacement field dependence of the nonlocal resistance and discuss its origin in terms of valley Hall effect and transport through disordered edge states. [Preview Abstract] |
Monday, March 3, 2014 4:06PM - 4:18PM |
D29.00009: Interlayer spacing of bilayer graphene determined accurately by photon energy dependent photoelectron intensity oscillation Ku-Ding Tsuei, Cheng-Mau Cheng, Jun-Hao Deng, Meng-Shiung Chiang, Chia-Jen Hsu We have carried out an angle resolved photoemission spectroscopic study on high quality bilayer graphene grown epitaxially on a SiC(0001) surface over a wide range of photon energies from 30 to 130 eV. The band intensities are maximized along the K$\Gamma$ direction in the first Brillouin while vanishing along the opposite KM direction due to matrix element effect [1]. For bilayer graphene the two valence bands further oscillate in intensity with varying photon energies [2]. We analyze the data by expressing the intensity asymmetry (I$_{1}$-I$_{2}$)/(I$_{1}$+I$_{2}$) for each photon energy thus avoid the uncertainty by comparing band intensities at different photon energies. The intensity asymmetry can be well simulated by a tight-binding model which attributes the intensity oscillation to the interference of photoelectron amplitudes emitted from the two layers. The oscillation period thus provides an accurate measure of the interlayer spacing. The obtained interlayer spacing is very near that of graphite from literature. [1] E. L. Shirley et al., Phys. Rev. B 51, 13614 (1995). [2] T. Ohta et al., Phys. Rev. Lett. 98, 206802 (2007). [Preview Abstract] |
Monday, March 3, 2014 4:18PM - 4:30PM |
D29.00010: Direct observation of asymmetric band structure of bilayer graphene through quantum capacitance measurements Kaoru Kanayama, Kosuke Nagashio, Tomonori Nishimura, Akira Toriumi Although upper conduction and valence sub-bands in bilayer graphene are known to be asymmetric, a detailed analysis based on the electrical measurements is very limited due to the infirm quality of gate insulator. In this study, the electrical quality of the top-gate Y$_{\mathrm{2}}$O$_{\mathrm{3}}$ insulator is drastically improved by the high-pressure O$_{\mathrm{2}}$ post-deposition annealing at 100 atm and the carrier density of $\sim $8*10$^{\mathrm{13}}$ cm$^{\mathrm{-2}}$ is achieved. In quantum capacitance measurements, the drastic increase of the density of states is observed in addition to the van Hove singularity, suggesting that the Fermi energy reaches upper sub-band. At the same carrier density, the sudden reduction of the conductivity is observed, indicating that the inter-band scattering occurs. The estimated carrier density required to fill the upper sub-bands is different between electron and hole sides, i.e., asymmetric band structure between upper conduction and valence bands is revealed by the electrical measurements. [Preview Abstract] |
Monday, March 3, 2014 4:30PM - 4:42PM |
D29.00011: Interaction-driven capacitance in graphene electron-hole bilayer in the quantum Hall regime Bahman Roostaei, Yogesh Joglekar Fabrication of devices made by isolated Graphene or Graphene-like single layers ( such as h-BN) has opened up possibility of examining highly correlated states of electron systems in parts of their phase diagram that is impossible to access in their counterpart devices such as semiconductor heterostructures. An example of such states are Graphene (or Graphene like) double layer electron-hole systems under strong magnetic fields where the separation between layers can be of the order of one magnetic length with interlayer tunneling still suppressed. In those separations correlations between electrons and holes are of crucial importance and must be included in determination of observables. Here we report a thorough mean-field study of the coherent and crystalline ground states of the interacting balanced electron-hole Graphene systems in small and intermediate separations with each layer occupying up to four lowest lying Landau levels. We calculate the capacitance of such states as a function of layer separation and filling factor. Our calculations show significant enhancement of the capacitance compared to geometrical value due to quantum mechanical corrections. [Preview Abstract] |
Monday, March 3, 2014 4:42PM - 4:54PM |
D29.00012: Interlayer capacitance in graphene bilayers Andrea Young, Wang Lei, Cory Dean, Jim Hone, Raymond Ashoori Capacitance measurements of dual-gated bilayer electron systems provide a way to experimentally access both the total density of states as well as the interlayer polarizability. I will discuss capacitance measurements of clean, hexagonal boron nitride encapsulated graphene bilayers, in which we use this technique to probe layer polarization in response to an applied electric field at both zero magnetic field and in the quantum Hall regime. [Preview Abstract] |
Monday, March 3, 2014 4:54PM - 5:06PM |
D29.00013: Electron-hole asymmetric fractional quantum Hall effect in bilayer graphene Ben Feldman, Angela Kou, Andrei Levin, Bertrand Halperin, Kenji Watanabe, Takashi Taniguchi, Amir Yacoby At zero magnetic field, the electronic spectrum of bilayer graphene is electron-hole symmetric to first order. In a magnetic field, the lowest two orbital states occur at zero energy, and they combine with the spin and valley degrees of freedom to yield an eightfold degenerate lowest Landau level. Both external fields and electron-electron interactions can break these symmetries, leading to a uniquely rich and tunable phase diagram of many-body states. In this talk, I will present local electronic compressibility measurements of high quality bilayer graphene performed using a scanning single-electron transistor. We observe clear fractional quantum Hall states at filling factors $\nu$ = -10/3, -4/3, 2/3 and 8/3, with additional states appearing at $\nu$ = -17/5, -7/5, 3/5 and 13/5. Remarkably, this sequence breaks electron-hole symmetry and instead follows an even-odd pattern between integer quantum Hall states. Our results highlight the key role played by the orbital degree of freedom in the many-body physics of bilayer graphene. [Preview Abstract] |
Monday, March 3, 2014 5:06PM - 5:18PM |
D29.00014: Spontaneous layer polarization and conducting domain walls in bilayer graphene Erez Berg, Kusum Dhochak Bilayer graphene subjected to perpendicular magnetic and electric fields displays a subtle competition between different quantum Hall ferromagnetic phases, resulting from an interplay from the internal spin and valley degrees of freedom. The transition between different phases is often identified by the closing of the gap and an enhanced conductance. Here, we formulate a criterion for the existence of conducting edge states at domain walls between different phases. For example, for a spontaneously layer polarized state at filling factor $\nu=2$, domains walls between of regions of opposite polarization carry conducting edge modes. A microscopic analysis shows that lattice-scale interactions can favour such a layer polarized state in an intermediate range of magnetic field. We analyze the experiments of Weitz et al. (Science, 2010) in light of these results. [Preview Abstract] |
Monday, March 3, 2014 5:18PM - 5:30PM |
D29.00015: Local compressibility of bilayer graphene in the quantum hall regime Andrei Levin, Angela Kou, Benjamin Feldman, Bertrand Halperin, Kenji Watanabe, Takashi Taniguchi, Amir Yacoby In the presence of a strong magnetic field, the charge carriers in bilayer graphene (BLG) condense into a set of flat energy bands called Landau levels (LLs). Electronic compressibility measurements have historically been a powerful tool in studying the physics of partially filled LLs in two-dimensional electronic systems. In particular, electron-electron correlations arising from Coulomb interactions can introduce a negative component to the compressibility. Here we present measurements of electronic compressibility in BLG, performed locally using a scanning single electron transistor. We find that while the inverse compressibility is close to zero for $4 < |\nu| < 8$, it is markedly more negative in the lowest LL, $|\nu| < 4$. Moreover, within the lowest LL, the background inverse compressibility between integer filling also exhibits a stark even-odd asymmetry. It is more negative when starting to fill from an even filling factor than when starting to fill from an odd filling factor, exhibiting a $\nu \rightarrow \nu + 2$ symmetry and indicating the important role of the orbital degeneracy uniquely present in bilayer graphene. [Preview Abstract] |
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