Bulletin of the American Physical Society
APS March Meeting 2014
Volume 59, Number 1
Monday–Friday, March 3–7, 2014; Denver, Colorado
Session G37: Focus Session: Quantum Hall Effect in Graphene |
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Sponsoring Units: DMP Chair: Zhigang Jiang, Georgia Institute of Technology Room: 705/707 |
Tuesday, March 4, 2014 11:15AM - 11:51AM |
G37.00001: Quantum Hall effect in epitaxial graphene - a metrological perspective Invited Speaker: Alexander Tzalenchuk Although the QHE has been used successfully for more than two decades to realise the resistance scale, graphene has potential to supersede conventional semiconductors as the material of choice for quantum electrical metrology. The physical mechanisms giving graphene, grown on SiC in particular, an edge over the conventional semiconductors include the pinning of the filling factor over an extraordinarily broad range of magnetic field, large inter-Landau level spacing and a very short energy relaxation time. Together they lead to a very robust quantum Hall state opening an opportunity to realise the quantum resistance standard of greatly reduced cost and complexity operating at high temperatures, low magnetic fields and high signal-to-noise ratio. I will review the progress achieved in graphene engineering, physical understanding and metrology from the first accurate QHE measurements performed on exfoliated samples (with precision of $15$ parts in $10^6$) and on graphene on SiC ($3$ parts in $10^9$) to a direct comparison between graphene on SiC and GaAs demonstrating equivalence of the quantised values of the Hall resistance with a relative uncertainty of $8.6$ parts in $10^{11}$. [Preview Abstract] |
Tuesday, March 4, 2014 11:51AM - 12:03PM |
G37.00002: Transport Properties of Topological 1D Zero-Line Mode in Graphene Zhenhua Qiao, Jeil Jung, Chungwei Lin, Allan MacDonald, Qian Niu When the inversion symmetry of graphene systems is broken, e.g. graphene subjected to a staggered sublattice potential or bilayer under an applied interlayer potential difference, a bulk band gap opens to support the quantum valley-Hall state. When the potential varies spatially, a topological one-dimensional conducting channel is formed along the zero-line of the potential. We find that such a state shows the property of zero bend resistance. And if two straight zero lines crosses, we show that the splitting of the zero line mode obeys a counterintuitive current partition law. We provide a theory to understand the physics behind these novel characteristics. [Preview Abstract] |
Tuesday, March 4, 2014 12:03PM - 12:15PM |
G37.00003: Hall effect in triangular antidot graphene under a weak magnetic field Yuan Zheng In triangular antidot graphene (TAG),a quasi-gap is formed by enhanced electron-electron interaction when the charge carrier density is very low, in conjunction with the appearance of a small effective mass for the TAG, owing to the altered dispersion relation. In the gap, a very long de-phasing length (10micron) has been observed at 2K. It means that there is an enlarged mesoscopic region in TAG. Within the quasigap, the inelastic scatterings are exponentially suppressed at low temperatures. Physics of electrons transport can therefore be treated with only elastic scatterings in the low temperature regime. Classical Hall effect is an effective way to probe the type of charge carriers and charge carrier density in semiconductor. When charge carrier density is low and the geometric size of the scattering features is also small, the Fermi wavelength of electrons can become comparable to the size of the scattering features. In the weak magnetic field regime, the giant Hall effect as well as the vanishing Vxy are both possible under certain conditions. In this talk we present both theoretical simulations and experimental results for the Hall effect in the antidot graphene. [Preview Abstract] |
Tuesday, March 4, 2014 12:15PM - 12:27PM |
G37.00004: Quantum Hall effect in tunable superlattice in graphene Sudipta Dubey, Mandar Deshmukh Superlattice in graphene is created by a bottom gate and an array of top gates pinned to the same potential. The difference in charge density between region with and without top gate creates the amplitude of superlattice potential and thus can be tuned by the gate voltage. The superlattice period is 150 nm. We study the effect of magnetic field in this array of p-n junctions when the magnetic length is smaller than the superlattice period. Depending on the gate voltage applied in the top and bottom gate, the edge states circulate in the same or opposite direction. As the filling fraction in the adjacent region can be controlled, we can tune the backscattering between alternate regions by changing the charge density in the region between them. When the gate voltages are so tuned that we have electrons and holes in the adjacent region, the edge states circulating in the opposite direction in the p and n region bring electrons and holes at the p-n interface. In this regime, we observe a large longitudinal resistance. [Preview Abstract] |
Tuesday, March 4, 2014 12:27PM - 12:39PM |
G37.00005: Spectroscopy of snake states using a graphene Hall bar Slavisa Milovanovic, Massoud Ramezani Masir, Francois Peeters A novel approach to observe snake states in a graphene Hall bar containing a pn-junction is proposed. The magnetic field dependence of the bend resistance in a ballistic graphene Hall bar structure containing a tilted pn-junction oscillates as a function of applied magnetic field. We show that each oscillation is due to a specific snake state that moves along the pn-interface. Furthermore depending on the value of the magnetic field and applied potential we can control the lead in which the electrons will end up and hence control the response of the system. [Preview Abstract] |
Tuesday, March 4, 2014 12:39PM - 12:51PM |
G37.00006: Canted magnetism and edge transport in tunable quantum Hall phases in graphene Jose Lado, Joaquin Fernandez-Rossier Motivated by recent experimental results [1] we study theoretically the quantum Hall effect in graphene in the presence of strong in plane magnetic field considering short range electron electron interactions. The experiments show a variety of phase transitions that change the bulk spin order order between different states, including antiferromagnetic (AF), ferromagnetic (FM) and canted antiferromagnetic (CAF), resulting in dramatically different edge states that control the conductivity. Here we model the non-trivial phase diagram of this system using a Hubbard model for a wide ribbon in a non-collinear mean field approximation. Our theory is able to account for the main experimental findings and provides a comprehensive phase diagram with at least 4 different electronic phases: AF, FM, CAF and a ferrimagnetic phase. Specifically, our model describes the presence of counter-propagating spin-filtered edge states in the FM phase at half filling, as well as a fully polarized single edge channel when the FM phase is doped into a ferrimagnetic phase with an electron-hole gap. [1] A. F. Young, J. D. Sanchez-Yamagishi, B. Hunt, S. H. Choi, K. Watanabe, T. Taniguchi, R. C. Ashoori, P. Jarillo-Herrero, arXiv:1307.5104 [Preview Abstract] |
Tuesday, March 4, 2014 12:51PM - 1:03PM |
G37.00007: Broken SU(4) Symmetry and The Fractional Quantum Hall Effect in Graphene Inti Sodemann, Allan MacDonald We describe a simple variational approach to understand the spin-valley broken symmetry states in the fractional quantum Hall regime of graphene. Our approach allows to predict the incompressible ground states and their charge gaps and is able to explain the observed differences between filling factor ranges $|\nu|<1$ and $1<|\nu|<2$. We find that in the SU(4) invariant case the incompressible ground state at $|\nu|=1/3$ is a three-component incompressible state, not the Laughlin state, and discuss the competition between these two states in the presence of SU(4) spin-valley symmetry breaking terms. We find that the lowest energy fractionally charged quasi-particles involve spin/valley flips in several prominent fractions. We discuss the expected behavior of the gaps under tilting the magnetic field away from normal which allows to tune the relative strength of Zeeman and valley symmetry breaking interactions. [Preview Abstract] |
Tuesday, March 4, 2014 1:03PM - 1:15PM |
G37.00008: Quantum Hall effect in polycrystalline CVD graphene: grain boundaries impact Rebeca Ribeiro-Palau, Fabien Lafont, Felicien Schopfer, Wilfrid Poirier, Vincent Bouchiat, Zhen Han, Alessandro Cresti, Aron Cummings, Stephan Roche It was demonstrated by Janssen et al. (New J. Phys. 2011) that graphene could surpass GaAs for quantum Hall resistance standards with an accuracy better than $10^{-10}$. Graphene should render possible the realization of a standard operating at $T>4$ K and $B<4$ T, easing its dissemination towards industry. To materialize this goal scalable graphene with outstanding electronic transport properties is required. We present measurements performed in large area Hall bars made of polycrystalline CVD graphene on Si/SiO$_2$, with a carrier mobility of 0.6 T$^{-1}$. Even at 20.2 T and 300 mK, the Hall resistance plateaus are insufficiently quantized at $\nu=\pm2$ and $\pm6$. This is due to a high dissipation manifested by a longitudinal resistance which does not drop to zero. We pointed out unusual power-law temperature dependencies of $R_{xx}$ and an exponential magnetic field dependence. We do not observe the common thermally activated or VRH behaviors. This can be attributed to the grain boundaries in the sample that short-circuit the edge states, as supported by our numerical simulations. This reveals new and peculiar aspects of the quantum Hall effect in polycrystalline systems. Another unexpected feature is the observation of the $\nu=0$ and 1 states in such low mobility systems. [Preview Abstract] |
Tuesday, March 4, 2014 1:15PM - 1:27PM |
G37.00009: Proximity-Induced Anomalous Hall Effect in Graphene Zhiyong Wang, Chi Tang, Raymond Sachs, Yafis Barlas, Jing Shi Pre-patterned graphene devices are transferred from SiO$_{2}$/Si to atomically flat magnetic insulator thin films, yttrium iron garnet (YIG) deposited by a laser molecular beam epitaxial system on gadolinium gallium garnet (GGG) substrate. Room temperature Raman spectroscopy reveals both single-layer graphene and YIG characteristic peaks. In addition to the ordinary Hall effect, there is a clear non-linear Hall component correlated with the magnetization of the YIG films, which we attribute to the anomalous Hall effect (AHE). The magnitude of AHE in graphene/YIG devices decreases as temperature increases. With device-to-device variations, in some devices, AHE persists to room temperature, indicating a strong proximity-induced exchange interaction. By sweeping top gate voltages, one can tune the carrier density across the Dirac point. We also find that the carrier mobility is not significantly different in graphene/YIG. As the graphene is tuned from the electron- to hole-type, the ordinary Hall changes the sign as expected, but the sign of the AHE contribution remains the same. It suggests that AHE does not simply originate from the carrier density change which is responsible for the ordinary Hall effect, but is related to the spin-orbit interaction in the system. [Preview Abstract] |
Tuesday, March 4, 2014 1:27PM - 1:39PM |
G37.00010: Real space imaging of quantum hall edge states in graphene Yongtao Cui, Georgi Diankov, Eric Yue Ma, Francois Amet, Yongliang Yang, Michael Kelly, David Goldhaber-Gordon, Cory Dean, Zhi-Xun Shen At integer quantum hall filling factors in a two-dimensional electron gas, electrons in the bulk are localized, while near the edge it remains conductive as energy bands bend and cross the Fermi level. These conductive channels, known as the ``edge states,'' are immune to back scattering, giving rise to quantized resistance values -- the hallmark of the quantum hall effect. Here we use microwave impedance microscope to study the quantum hall edge states in graphene devices. Scanning images clearly show dividing regions of insulating bulk and conductive edges. We study the evolution of the edge patterns as the carrier density is tuned through multiple filling factors. Correlation between real space images and transport measurement demonstrates the robustness of the quantum hall effect -- even though the real space patterns are strongly affected by disorder, the quantization of resistance is retained due to the topological nature of the edge states. [Preview Abstract] |
Tuesday, March 4, 2014 1:39PM - 1:51PM |
G37.00011: Observation of new fractional quantum Hall states in the first Landau level in graphene Francois Amet, Andrew Bestwick, James Williams, Kenji Watanabe, Takahashi Taniguchi, David Goldhaber-Gordon We report on transport studies of the fractional quantum Hall effect in the n=0 and n=1 Landau level of monolayer graphene. The quality of the devices studied here -with mobilities up to 400 000cm2/Vs, and magnetic fields up to 45T- allows us to observe a variety of fractional quantum hall states following the composite fermion sequence, with denominators up to 9. The presence of odd numerator fractions between nu=1 and nu=2 is attributed to the breaking of the valley symmetry and correlates with a zero field insulating state observed at charge neutrality. We discuss the in-plane field dependence of the gaps, which is not trivial and shows dramatic differences between the n=0 and the n=1 Landau levels. [Preview Abstract] |
Tuesday, March 4, 2014 1:51PM - 2:03PM |
G37.00012: Broken SU(4) symmetry in quantum hall states in graphene: an exact diagonalization study Fengcheng Wu, Inti Sodemann, Yasufumi Araki, Thierry Jolicoeur, Allan MacDonald Electrons in graphene have four flavors due to low-energy spin and valley degrees of freedom. Long-range Coulomb interactions are SU(4) symmetric in spin and valley space, providing an experimental realization of the SU(4) fractional quantum hall effect. However, weak short-range electron-electron and electron-phonon interactions break the valley symmetry, and act as a source of isospin anisotropy. Using an exact diagonalization method that takes all four flavors into account, we study the SU(4) fractional quantum Hall effect, identifying singlet and broken symmetry ground states and low lying excitations at integer and fractional filling factors within the N=0 Landau level. We also account for the presence of valley-isospin anisotropy and Zeeman fields. For the quantum Hall states at neutrality we assess the impact of quantum fluctuations that are beyond the mean-field theory of quantum Hall ferromagnets. For the fractional quantum Hall states, we compute the energies of novel multi-component states and evaluate their prospects for experimental realization. A systematic symmetry analysis based on the SU(4) multiplet structure of the many body spectrum will be presented. [Preview Abstract] |
Tuesday, March 4, 2014 2:03PM - 2:15PM |
G37.00013: Quantum anomalous Hall effect in Co or Rh doped graphene Jun Hu, Ruqian Wu The recent discovery of topological insulators (TIs)---that act as insulator in the bulk yet possess quantized conducting edge or surface states---has triggered extensive interests in the field of condensed matter physics and materials science. One of the most interesting phenomena related to TIs is the quantum anomalous Hall effect (QAHE). Although there are a lot of theoretical predictions about the existence of the QAHE in different materials, the QAHE has been observed only in Be2Se3 so far, in an extreme experimental condition bellow 0.1 K due to the tiny TI gap. On the contrary, it was found that huge TI gaps can be induced in graphene by transition metal adatoms. In the present work, we predict that deposition of sparse Co or Rh adatoms on graphene can produce a TI gap of 37 or 100 meV around the Fermi level. Furthermore, we demonstrate that the QAH state is very robust, due to the strong perpendicular magnetic anisotropies. [Preview Abstract] |
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