Bulletin of the American Physical Society
APS March Meeting 2013
Volume 58, Number 1
Monday–Friday, March 18–22, 2013; Baltimore, Maryland
Session A8: Graphene: Quantum Hall Effect |
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Sponsoring Units: DCMP Chair: Herb Fertig, Indiana University Room: 307 |
Monday, March 18, 2013 8:00AM - 8:12AM |
A8.00001: Four-flux fractional quantum Hall states in suspended graphene Andrei Levin, Benjamin Feldman, Benjamin Krauss, Jurgen Smet, Amir Yacoby The interactions between charge carriers in ultra-clean graphene subject to a perpendicular magnetic field can drive the system to condense into one of a set of incompressible fractional quantum Hall (FQH) states. We use a scanning single-electron transistor to measure the local electronic compressibility of suspended graphene. In addition to observing incompressible behavior at fractional filling factors in the two-flux composite fermion sequence, we also observe FQH states arising from four-flux composite fermions, including states at filling factors $\nu =$ 1/5, 2/7, 2/9, 3/11, 5/7 and 6/5. We measure the energy gaps of these states as a function of magnetic field; most display approximately linear scaling. Interestingly, several four-flux FQH states are conspicuously absent near filling factors $\nu =$ 1 and 2, despite the robust appearance of their counterparts near $\nu =$ 0. [Preview Abstract] |
Monday, March 18, 2013 8:12AM - 8:24AM |
A8.00002: Measurements of Chiral Heat Current in Graphene in the Quantum Hall Regime Seung-Geol Nam, E.H. Hwang, Hu-Jong Lee Heat transport measurements can offer a new window to probe the low-energy physics in quantum-Hall systems, which cannot be provided by the electronic transport measurements. In this presentation, we report chiral heat transport measurements in monolayer graphene in the integer quantum Hall regime. We inject charge carriers at a higher temperature than the system bulk and measure the thermoelectric voltage corresponding to the local electron temperature at a distance from the injection point. We find that in graphene heat transport at the edge in the quantum Hall regime is chiral and its direction is dependent on both the carrier type and the magnetic field direction. Measured thermoelectric signals in unipolar regions can be understood by the Mott relation, but a severe deviation of the signals from the Mott relation is found at a p-n junction. Thermoelectric signal decays with distance from the heater and saturates with increasing heating power even though it increases linearly at low powers, which indicates that a part of heat is transferred out of the edge current. [Preview Abstract] |
Monday, March 18, 2013 8:24AM - 8:36AM |
A8.00003: Drude weight, cyclotron resonance, and the Dicke model of graphene cavity QED Marco Polini, Luca Chirolli, Vittorio Giovannetti, Allan MacDonald The unique optoelectronic properties of graphene make this two-dimensional (2D) material an ideal platform for fundamental studies of cavity quantum electrodynamics (QED) in the strong-coupling regime. The celebrated Dicke model of cavity QED can be approximately realized in this material when the cyclotron transition of its 2D massless Dirac fermion carriers is nearly resonant with a cavity photon mode. In this talk I will discuss the theory of strong matter-photon coupling in this circumstance, emphasizing the essential role of a dynamically generated matter energy term that is quadratic in the photon field and absent in graphene's low-energy Dirac model. [Preview Abstract] |
Monday, March 18, 2013 8:36AM - 8:48AM |
A8.00004: Theory of unconventional quantum Hall effect in strained graphene Zi-Xiang Hu, Bitan Roy, Kun Yang We show graphene discerns an unconventional sequence of quantized Hall conductivity, when subject to both magnetic fields (B) and strain through both theoretical arguments and numerical calculations. The strain produces time-reversal symmetric pseudo/axial magnetic fields (b). The single electron spectrum is composed of two inter-penetrating sets of Landau levels (LLs), located at $\pm\sqrt{2n|b \pm B|}$, n = 0, 1, 2,.... For $b > B$, these two sets of LLs have opposite chiralities, resulting in oscillating Hall conductivity between 0 and $\mp 2e^2/h$ in electron and hole doped system, respectively, as the chemical potential deviates from the neutrality point, but remains in its vicinity. The electron electron interactions stabilizes various correlated ground states, e.g., spin-polarized, quantum spin-Hall insulators at and near the neutrality point, and possibly anomalous Hall insulating phase at incommensurate filling. Such broken symmetry ground states have similarities as well as significant differences from there counterparts in the absence of strain. For realistic strength of magnetic fields and interactions, we present scaling of interaction induced gap for various Hall states within the zeroth Landau level. [Preview Abstract] |
Monday, March 18, 2013 8:48AM - 9:00AM |
A8.00005: Valley-kink in Bilayer Graphene at $\nu =$0: A Charge Density Signature for Quantum Hall Ferromagnetism Chia-Wei Huang, Efrat Shimshoni, Herbert Fertig We investigate the interaction-induced valley textured domain walls in bilayer graphene at the $\nu =$0 quantum Hall state, subject to a kink-like perpendicular electric field. Such a state can be realized in a double-gated suspended sample, where the electric field changes sign across a line in the middle of the sample. Using the Hartree-Fock approximation, we find that the Coulomb interaction opens a gap between the two lowest-lying states near the Fermi level, and yields a smooth valley texture throughout the domain walls. Moreover, our results suggest possibilities to visualize the resulting texture via measuring the charge density difference between the two graphene layers, which is predicted to exhibit a charge density wave. The width of the smooth texture and the resulting pattern can be tuned by the interplay between the magnetic field and gate electric fields. [Preview Abstract] |
Monday, March 18, 2013 9:00AM - 9:12AM |
A8.00006: Cyclotron-resonance-induced photovoltaic effect in high-mobility graphene in the quantum Hall regime Satoru Masubuchi, Masahiro Onuki, Miho Arai, Kenji Watanabe, Takashi Taniguchi, Tomoki Machida We have investigated the infrared photoinduced voltage $\Delta V$ in high-mobility graphene on hexagonal boron nitride in the quantum Hall regime. We observed $\Delta V$ of up to several $\mu$V at $\nu=\pm 2$ quantum Hall states under the cyclotron resonance conditions. The dependence of $\Delta V$ on the bias current indicates that $\Delta V$ signals derive from the photovoltaic effect rather than the bolometric effect. The dependence of $\Delta V$ on magnetic field direction and measurement geometry suggest the edge channel transport as an origin of photovoltaic effect. $\Delta V$ signals were robust up to $T=180$ K, indicating that $\Delta V$ signals can be used for developing novel terahertz photodetectors operating at high temperatures. [Preview Abstract] |
Monday, March 18, 2013 9:12AM - 9:24AM |
A8.00007: Landau-level mixing in the fractional quantum Hall effect in graphene Michael Peterson, Chetan Nayak We study the effects of Landau level mixing on the fractional quantum Hall effect in graphene. Landau level mixing in graphene is especially important since the ratio of the Coulomb energy to the cyclotron energy is independent of magnetic field and of order one. In particular, we derive an effective Hamiltonian that fully incorporates Landau level mixing by renormalizing the two-body Coulomb potential (renormalizing the Haldane pseudopotentials) and inducing particle-hole symmetry breaking three-body terms, cf. Bishara and Nayak, Phys. Rev. B 80, 121302(R) (2009). As opposed to the FQHE in GaAs semiconductor devices, graphene has no finite-thickness corrections since the two-dimensional graphene sheet is atomically thin and the Dirac nature of the electrons in graphene forces the particle-hole symmetry breaking three-body terms to exactly vanish in the lowest Landau level. [Preview Abstract] |
Monday, March 18, 2013 9:24AM - 9:36AM |
A8.00008: Self-similar occurrence of massless Dirac particles in graphene under magnetic field Jun-Won Rhim, Kwon Park Intricate interplay between the periodicity of the lattice structure and that of the cyclotron motion gives rise to a well-known self-similar fractal structure of the Hofstadter butterfly for an electron moving in lattice under magnetic field. Evolving from the $n=0$ Landau level, the central band of the Hofstadter butterfly is especially interesting since it may hold a key to the mysteries of the fractional quantum Hall effect in graphene. In this paper, we develop an effective Hamiltonian method that can be used to provide an accurate analytic description of the central Hofstadter band in the weak-field regime. One of the most important discoveries obtained in this work is that massless Dirac particles always exist inside the central Hofstadter band no matter how small the magnetic flux may become. In other words, with its bandwidth broadened by the lattice effect, the $n=0$ Landau level contains massless Dirac particles within itself. In fact, by carefully analyzing the self-similar recursive pattern of the central Hofstadter band, we conclude that massless Dirac particles should occur under arbitrary magnetic field. As a corollary, the central Hofstadter band also contains a self-similar structure of recursive Landau levels associated with such massless Dirac particles. [Preview Abstract] |
Monday, March 18, 2013 9:36AM - 9:48AM |
A8.00009: Observation of the Hofstadter butterfly in graphene on boron nitride Patrick Maher, Cory Dean, Carlos Forsythe, Lei Wang, Fereshte Ghahari, Pilkyung Moon, Mikito Koshino, Kenji Watanabe, Takashi Taniguchi, Ken Shepard, James Hone, Philip Kim In 1976, Douglas Hofstadter considered the general problem of 2D electrons subject to both a magnetic field and a periodic potential. His solution predicted a remarkably complex energy spectrum exhibiting self-similar fractal structure, termed the Hofstadter Butterfly. Experimental exploration of this problem has been limited by the difficulty of fabricating a system with a lattice constant on the order of the magnetic length. It has recently been shown that single layer graphene on hexagonal-BN develops a Moir\'{e} pattern with a length of up to 15 nm when the rotational angle between the two lattices approaches zero. We present data demonstrating that for bilayer graphene on hexagonal boron nitride, the effect of the modulation potential associated with the Moir\'{e} pattern is large enough to be observable by standard transport. Under large magnetic fields, additional gaps appear within the usual bilayer quantum Hall spectrum, consistent with calculations of the Hofstadter spectrum. We present the first direct experimental evidence of the longstanding theoretical prediction that the gaps arising from the superlattice are characterized by two integer quantum numbers. [Preview Abstract] |
Monday, March 18, 2013 9:48AM - 10:00AM |
A8.00010: Symmetry Breaking in Hofstadter's Butterfly in graphene Carlos Forsythe, Cory Dean, Lei Wang, Patrick Maher, Fereshte Ghahari, Pilkyung Moon, Mikito Koshino, Takashi Taniguchi, Kenji Watanabe, Ken Shepard, Jim Hone, Philip Kim We will present magnetotransport measurements in hBN encapsulated bilayer graphene devices where one of hBN substrates provides a weak modulation of lattice potential. Under a strong magnetic field, interplay between periodic electric potential and quantizing magnetic field lead to a fractal energy spectrum known as Hofstadter's butterfly. In graphene, while spin and layer symmetry breakings are expected in dual gated devices under large magnetic fields, valley symmetry breaking in the Hofstadter regime is not so easily understood. We will present the observance of these measured gaps along with a discussion of symmetry breaking in our BLG-hBN devices. Further quantitative analysis of these breakings will be presented through the temperature dependence of quantized conductance at these gaps. Through careful modulation of temperature and electron density, we have extracted a range of activation energies associated with symmetry breakings. [Preview Abstract] |
Monday, March 18, 2013 10:00AM - 10:12AM |
A8.00011: Andreev Reflections and Superconducting Proximity Effect in lateral BN/Graphene/NbSe$_{2}$ Heterostructures in the Integer Quantum Hall Regime Dmitri K. Efetov, Clevin Handschin, Cory Dean, Lei Wang, Philip Kim Inducing Superconductivity (SC) via proximity effect into the topological edge states of a 2D conductor in the Quantum Hall Regime (QHE) has been a long standing proposition which has recently reinvigorated attention. Here the combination of SC and QHE has a wide range of predictions such as the appearance of additional edge-states in the integer QHE. With the recent development of high mobility graphene on h-BN with an extremely low onset of the QHE (0.5T) and its high compatibility with various superconductors the road to test these predictions is now open. In this study we present lateral magneto-transport and electronic spectroscopy measurements of BN/graphene/NbSe2 heterostructures. We find that the NbSe2/graphene superconductor-normal metal interface (SN) has a very high transparency with extremely low electrical resistances of R$\sim$100Ohm and gives rise to Andreev reflections and a strong SC proximity effect in graphene below the critical SC transition temperature Tc $\sim$ 7.2K. The high mobility of the graphene on h-BN and the relatively high SC upper critical magnetic field of NbSe2 Hc2 $\sim$ 5T allow for a wide magnetic field range of 1-5T in which the SC and the QHE coexist. [Preview Abstract] |
Monday, March 18, 2013 10:12AM - 10:24AM |
A8.00012: Edge magnetoplasmons in graphene: determination of carrier drift velocity in Quantum Hall regime Ivana Petkovic, F.I.B. Williams, Keyan Bennaceur, Fabien Portier, Patrice Roche, D.C. Glattli Edge Magneto-Plasmons (EMP) are gapless quasi 1D elementary excitations which are split off from the bulk magneto-plasmon modes by the sample boundary, and are a tool of choice to investigate the structure of the edge of a 2D electron gas. We give a first experimental demonstration of their presence in graphene in the quantum Hall regime and use our results to evaluate the carrier drift velocity along the edge [1]. The group velocity of these modes is a sum of the Hall conductivity contribution and the carrier drift velocity at the edge. In graphene, due to its particular dynamics and an abrupt edge, the drift velocity is expected to be of the order of the Fermi velocity, thus becoming experimentally accessible. We show EMP to exist by timing the travel of narrow wave-packets on picosecond time scales around exfoliated samples. They show chiral propagation with low attenuation at a velocity which is quantized on Hall plateaus. We extract the carrier drift contribution and find it to be slightly less than the Fermi velocity, as expected for an abrupt edge. We also extract the spatial spread of edge accumulated charge and find it to be narrower than for soft edge systems.\\[4pt] [1] I. Petkovic, F.I.B. Williams, K. Bennaceur, F. Portier, P. Roche and D.C. Glattli, Phys. Rev. Lett.(2012). [Preview Abstract] |
Monday, March 18, 2013 10:24AM - 10:36AM |
A8.00013: Quantum spin Hall effect in the graphene zero energy Landau level - Part I Andrea F. Young, Javier D. Sanchez-Yamagishi, Ben Hunt, Pablo Jarillo-Herrero, Ray C. Ashoori, Takashi Taniguchi, Kenji Watanabe Shortly after the experimental discovery of graphene, it was predicted that Zeeman splitting of the graphene zero energy Landau level results in a quantum spin Hall phase, characterized by counterpropagating spin-filtered edge states. However, experimental realization of this state has been obscured by the existence of competing Coulomb interaction-driven insulating phases. We address this problem by fabricating monolayer graphene devices in which the Coulomb interaction is heavily screened by a proximal graphite gate. Despite the reduction in the strength of intralayer interactions, the resulting high mobility samples show all the usual signatures of Coulomb-driven symmetry breaking in high magnetic fields, with a strong insulating state developing at charge neutrality at fields of $\sim$1 Tesla. Unlike in conventional samples, however, we observe a continuous transition from this insulating state to a conducting state of order e$^2$/h as a function of in-plane field. Simultaneous high-sensitivity capacitance measurements reveal that the sample bulk remains gapped throughout the transition. The observation of finite conduction in the presence of a bulk insulator strongly implies that transport occurs via the edge states characteristic of the quantum spin Hall state. [Preview Abstract] |
Monday, March 18, 2013 10:36AM - 10:48AM |
A8.00014: Quantum spin Hall effect in the graphene zero energy Landau level - Part II Javier D. Sanchez-Yamagishi, Andrea F. Young, Benjamin Hunt, Kenji Watanabe, Takashi Taniguchi, Ray C. Ashoori, Pablo Jarillo-Herrero Zeeman splitting of graphene's zeroth Landau level has been predicted to lead to a quantum spin Hall effect, but a competing interaction-driven insulating state has hampered previous attempts to drive the graphene into this regime. By using a proximal graphite gate to screen Coulomb interactions in the graphene, we are able to reduce the strength of this competing insulating state and observe a continuous transition to a conductive state as a function of in-plane field. We study this transition simultaneously in capacitance and transport, and find that despite conduction increasing by many orders of magnitude with in-plane field the bulk remains gapped throughout the transition. These observations indicate the continuous closing of a transport gap along the edge of the sample, with resulting counter-propagating edge states that are characteristic of the quantum spin Hall effect. We discuss the behavior of this transition across multiple samples with various levels of Coulomb screening, and present nonlocal multiterminal transport measurements designed to probe the nature of backscattering within the edge states. We also comment on the implications of our work for the rest of the graphene phase diagram at high magnetic fields. [Preview Abstract] |
Monday, March 18, 2013 10:48AM - 11:00AM |
A8.00015: Role of the charge inhomogeneity on the breakdown of the quantum Hall effect in narrow single layer graphene devices Cenk Yanik, Ismet Kaya The breakdown of the quantum Hall effect, which is observed as an abrupt escalation in the longitudinal resistance with an associated loss in the quantization of Hall voltage is the major obstacle against improving the resistance standard which is currently based on this effect. Graphene is inherently a 2D material and has an unusual band structure that allows the quantization of the Hall resistance even at room temperature. These unique properties of graphene make it a good candidate as a high precision metrological characterization tool for the quantum Hall resistance. The uncertainty in the quantum Hall resistance in graphene has been rapidly improving recently and graphene samples have already been shown to reach the precision of the current best 2DEG samples. In this talk, experimental results on the breakdown of the quantum Hall effect in graphene on SiOx is presented. In narrow graphene samples of 1 micrometer width, the charge inhomogeneity is quite prominent and strongly affects the nondissipative transport in the quantum Hall regime. It is observed that in such samples the quantization of the Hall resistance can retain at high current densities in the excess of 1 A/m even in the presence of dissipative potential along the longitudinal probes. [Preview Abstract] |
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