Bulletin of the American Physical Society
APS March Meeting 2015
Volume 60, Number 1
Monday–Friday, March 2–6, 2015; San Antonio, Texas
Session G17: Graphene: Quantum Hall Effects |
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Sponsoring Units: DCMP Chair: Fan Zhang, University of Texas at Dallas Room: 102AB |
Tuesday, March 3, 2015 11:15AM - 11:27AM |
G17.00001: Collective Edge Modes near the onset of a graphene quantum spin Hall state Ganpathy Murthy, Efrat Shimshoni, Herbert Fertig Graphene subject to a strong, tilted magnetic field exhibits an insulator-metal transition tuneable by tilt-angle, which is attributed to the transition from a canted antiferromagnetic (CAF) to a ferromagnetic (FM) bulk state at filling factor $\nu=0$. We develop a theoretical description for the spin and valley edge textures in the two phases, and the implied evolution in the nature of edge modes through the transition. Based upon numerical Hartree-Fock calculations, we derive a simple description of the spin-valley domain wall for arbitrary Zeeman energy $E_z$, parameterized by $two$ canting angles. Low-energy charged excitations can be constructed by imposing a slowly varying spin rotation on this state. In the CAF, these involve binding a vortex (meron) of the bulk state to a spin twist at the edge, so that the $bulk$ spin stiffness controls the excitation energy. As the CAF-FM transition is approached ($E_z\rightarrow E_z^c$), the bulk stiffness vanishes linearly with $(E_z^c-E_z)$ and the vortex unbinds from the edge, yielding a gapless edge excitation characteristic of a quantum spin Hall state. Our model predicts the $E_z$-dependence of the activation gap in edge transport, and offers a qualitative picture of how this transport should evolve with filling factor. [Preview Abstract] |
Tuesday, March 3, 2015 11:27AM - 11:39AM |
G17.00002: Oscillatory magnetotransport between co-propagating quantum Hall edge channels in graphene p-n junctions Sei Morikawa, Satoru Masubuchi, Rai Moriya, Kenji Watanabe, Takashi Taniguchi, Tomoki Machida We conducted magnetotransport measurements in high-quality dual-gated graphene n-p-n junctions. As we used hexagonal boron nitride as a dielectric layer, Fabry-Perot interference patterns\footnote{A. F. Young \textsl{et al}., \textbf{Nat. Phys.} 5, 222.} can be observed clearly in zero magnetic fields, owing to the extremely high carrier mobility of our devices.\footnote{S. Masubuchi, S. Morikawa \textsl{et al}., \textbf{Jpn J. Appl. Phys.} 52, 110105.} Moreover, the two-terminal resistance $R$ exhibited oscillatory behavior as a function of the magnetic field $B$, whose oscillation period $\Delta B$ differed from both the conventional Shubnikov-de Haas effect ($\Delta B \propto B$) and the Aharonov-Bohm effect with magnetic flux penetrating through the gated region ($\Delta B = \mathrm{const}$). The oscillatory behavior of $R$ was well reproduced by our numerical calculation under the assumption that $R$ oscillated as a function of the magnetic flux penetrating through the insulating region between the co-propagating p and n quantum Hall edge channels.\footnote{S. Morikawa \textsl{et al}., submitted.} [Preview Abstract] |
Tuesday, March 3, 2015 11:39AM - 11:51AM |
G17.00003: Ising quantum Hall ferromagnetic states in bilayer graphene Ren\'e C\^ot\'e, Wenchen Luo, Alexandre B\'edard-Vall\'ee We present a study of the phase diagram of the chiral two-dimensional electron gas (C2DEG) in the higher Landau levels, $\left\vert N\right\vert\geq 1,$ of a chirally stacked bilayer graphene as a function of magnetic field $B$ and interlayer electrical bias $\Delta _{B}$. In the Hartree-Fock approximation, the ground states of the C2DEG are respectively valley-pseudospin or spin Ising quantum Hall ferromagnets at odd or even filling factors of the quartet of states in levels $\left\vert N\right\vert\geq 1$ [1]. Changing the magnetic field or the bias introduces first order phase transitions between the different Ising ground states that are characterized by a discontinuity in the transport gap $\Delta _{t}$. The C2DEG shows an hysteretic behavior with respect to the bias $\Delta _{B}$ with a marked difference between positive $N>0$ and negative $N<0$ Landau levels [2]. We discuss the relevance of our results with recent experimental measurements of broken-symmetry gaps in bilayer graphene [3].\\[4pt] [1] Wenchen Luo, R. C\^{o}t\'{e}, and Alexandre B\'{e}dard-Vall\'{e}e, Phys. Rev. B \textbf{90}, 075425 (2014). \\[0pt] [2] Wenchen Luo and R. C\^{o}t\'{e}, arXiv:1410.4232 (2014). \\[0pt] [3] Kayoung Lee et al., Science \textbf{345}, 58 (2014). [Preview Abstract] |
Tuesday, March 3, 2015 11:51AM - 12:03PM |
G17.00004: Quantum Hall ferromagnetism in gapped bilayer graphene with trigonal warping effects Xiao Li, Fan Zhang, Qian Niu, Allan MacDonald The interplay between nontrivial Fermi surface topology and electron-electron interactions often leads to interesting phenomena in the quantum Hall regime. Bilayer graphene provides a unique platform to explore such physics, because the combined effects of trigonal warping and interlayer bias give rise to a nontrivial bandstructure at low energies. In the presence of a small perpendicular magnetic field, the highest valence-band Landau level of gapped bilayer graphene becomes three-fold degenerate excluding the spin degrees of freedom, with the three Landau levels corresponding to semiclassical orbits centered on different points in momentum space. Such a Landau level structure has been observed in a recent experiment [Phys. Rev. Lett. 113, 116602 (2014)]. In this work we construct a theory to show how the electron-electron interactions break this three-fold orbital degeneracy, and give rise to a gapped quantum Hall state at all intermediate integer filling factors. We further demonstrate that the resulting ground state breaks rotational symmetry and discuss some experimental consequences. [Preview Abstract] |
Tuesday, March 3, 2015 12:03PM - 12:15PM |
G17.00005: Chiral symmetry breaking and the Quantum Hall Effect in monolayer graphene Malcolm Kennett, Bitan Roy Monolayer graphene in a strong magnetic field exhibits quantum Hall states at filling fractions $\nu = 0$ and $\nu = \pm 1$ that are not explained within a picture of non-interacting electrons. We propose that these states arise from interaction induced chiral symmetry breaking orders. We argue that when the chemical potential is at the Dirac point, weak onsite repulsion supports an easy-plane antiferromagnet state, which simultaneously gives rise to ferromagnetism oriented parallel to the magnetic field direction, whereas for $|\nu|=1$ easy-axis antiferromagnetism and charge-density-wave order coexist. We perform self-consistent calculations of the magnetic field dependence of the activation gap for the $\nu = 0$ and $|\nu| = 1$ states and obtain excellent agreement with recent experimental results. [Preview Abstract] |
Tuesday, March 3, 2015 12:15PM - 12:27PM |
G17.00006: Quantum Hall Effect (QHE) in ABA stacked trilayer graphene Petr Stepanov, Yafis Barlas, Nathaniel Gillgren, Takashi Taniguchi, Jeanie Lau Since its experimental discovery in 2004 graphene was under extensive research as a promising counterpart of silicon for the future electronics application as well as an excellent model of 2 dimensional electron gas. Here we investigate quantum Hall effect in ABA trilayer graphene -- hexagonal boron nitride heterostructures. Landau Levels (LL) crossings at low filling factors were observed and explored at different external electric fields. The formation of the QH states as an interaction of monlayer-like and bilayer-like branches will be discussed. We will present the most recent experimental results. [Preview Abstract] |
Tuesday, March 3, 2015 12:27PM - 12:39PM |
G17.00007: Optical study of nonuniform quantum-Hall ferromagnetic states in bilayer and trilayer graphene Manuel Barrette, Ren\'e C\^ot\'e The chiral two-dimensional electron gas in the $N=0$ Landau level of a Bernal-stacked bilayer graphene is host to a variety of broken-symmetry ground states that can be described as layer, spin, or orbital quantum Hall ferromagnets (QHFs). At filling factors $\nu =1,3,$ an externally applied electric field between the two layers can induce a transition from uniform to nonuniform orbital QHF states with an helical or skyrmionic texture of electric dipoles [1]. A similar skyrmionic texture can also arise in the $N=0$ Landau level of an ABC-stacked trilayer graphene. In this talk, we discuss the optical properties of these textured ground states. We compute their electromagnetic absorption as well as the Kerr and Faraday rotations induced by their collective excitations and show that each textured phase has a distinct optical signature. \\[4pt] [1] R. C\^{o}t\'{e}, J. P. Fouquet, and Wenchen Luo, Phys. Rev. B \textbf{84}, 235301 (2011). [Preview Abstract] |
Tuesday, March 3, 2015 12:39PM - 12:51PM |
G17.00008: Broken Symmetry States in Twisted Bilayer Graphene Youngwook Kim, Jong Mok Ok, Jun Sung Kim, Jaesung Park, Suyong Jung, Dong Su Lee, Intek Song, Hee Cheul Choi, K. Watanabe, T. Taniguchi Graphene bilayer with multiple degeneracy provides an access to rich quantum Hall states (QHS) with broken symmetry, arising from electron-electron interactions and Zeeman splitting. Here, we present quantum Hall effect in high-quality twisted bilayer graphene. At high density regime, we found several QH plateaus are suppressed or emerged with magnetic fields, indicating transitions between different QH states. We ascribe this to imperfect screening of twisted bilayer, which results in different Landau levels formation on each layer and their mixings. As low density regime, odd integer QHS are observed, suggesting an important role of the interlayer charge transfer for stabilizing broken symmetry QHS [Preview Abstract] |
Tuesday, March 3, 2015 12:51PM - 1:03PM |
G17.00009: Collective spin excitations in magnetically ordered graphene Quantum Hall insulator Jose Lado, Joaquin Fernandez-Rossier At half filling, the application of a perpendicular magnetic field in graphene results in a very large density of states at the Fermi energy, due to the n=0 Landau levels. Interactions are known to lead to some sort of electronic order that leads to a band-gap opening. Experimental evidence [1] suggest that electronic order is antiferromagnetic, and can be tuned into ferromagnetic upon application of a large in-plane field. This behaviour is properly captured by mean field Hubbard model [2]. In this talk, by using tight binding models and calculating responses within the random phase approximation [3], we show the different collective modes of bulk and edges associated to the different electronically ordered phases. Furthermore, we discuss the coupling of these spin waves to quasiparticles transport at the edges and discuss how this can affect chiral spin transport. \\[4pt] [1] A. F. Young et al, Nature 505, 528-532 (2014)\\[0pt] [2] J. L. Lado and J. Fern\'andez-Rossier, Phys. Rev. B 90, 165429 (2014)\\[0pt] [3] J. L. Lado and J. Fern\'andez-Rossier, in preparation [Preview Abstract] |
Tuesday, March 3, 2015 1:03PM - 1:15PM |
G17.00010: Imaging the charge profile of graphene in quantum Hall states Yongtao Cui, Eric Ma, Georgi Diankov, Francois Amet, Vitto Han, Michael Kelly, David Goldhaber-Gordon, Cory Dean, Zhi-Xun Shen Under quantum Hall conditions, discrete energy levels (Landau levels) form in a two dimensional electron gas (2DEG) system. Spatial reconstructions of carriers due to electrostatics can occur for a non-uniform charge profile, giving rise to highly insulating incompressible regions. In this work we use microwave impedance microscope to image the quantum Hall states in graphene devices. First, 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 Landau levels. Furthermore, a finite voltage bias on the tip can induce a local charge perturbation, which leads to an extra incompressible ring that moves along with the tip during scanning. Such incompressible ring can be used to probe the variations of the local carrier profile. Our results indicate that the carrier density in graphene tuned by the back gate tends to increase toward the edge due to electrostatic screening. This is in contrast to the case of conventional semiconductor 2DEG systems, where the carrier density always decreases toward the edge due to charge depletion. We will discuss how this charge profile affects the formation of the incompressible stripes. [Preview Abstract] |
Tuesday, March 3, 2015 1:15PM - 1:27PM |
G17.00011: Measurement of energy gaps of integer and fractional quantum Hall states in suspended bilayer graphene devices Yanmeng Shi, Yongjin Lee, Shi Che, Ziqi Pi, Tim Espiritu, Kevin Myhro, Petr Stepanov, Nathanial Gillgreen, Dmitry Smirnov, Chun Ning Lau Single- and few-layer graphene have emerged as interesting 2D systems for the investigation of novel integer and quantum Hall states. Recently clear fractional quantum Hall states in bi-layer graphene have been observed, though studies of the magnitudes of the gaps and their dependence on electric field are very limited. Here, using dual-gated suspended bilayer graphene device, we measure the Landau level gaps for the nu$=$1 and nu$=$2/3 states, and explore their dependence on electric field. [Preview Abstract] |
Tuesday, March 3, 2015 1:27PM - 1:39PM |
G17.00012: Wess-Zumino-Witten terms in graphene Landau levels Junhyun Lee, Subir Sachdev We consider the interplay between the antiferromagnetic and Kekul\'e valence bond solid orderings in the zero energy Landau levels of neutral monolayer and bilayer graphene. We establish the presence of Wess-Zumino-Witten terms between these orders: this implies that their quantum fluctuations are described by the deconfined critical theories of quantum spin systems. We present implications for experiments, including the possible presence of excitonic superfluidity in bilayer graphene. [Preview Abstract] |
Tuesday, March 3, 2015 1:39PM - 1:51PM |
G17.00013: Coulomb drag in graphene quantum Hall bilayer systems Xiaomeng Liu, Lei Wang, Kin Chung Fong, Yuanda Gao, Patrick Maher, Kenji Watanabe, Takashi Taniguchi, James Hone, Cory Dean, Philip Kim Coulomb drag between electrons in closely spaced two-dimensional electron systems has provided an exciting avenue for research on quantum Hall bilayer systems. Employing dual-gated, encapsulated graphene double layers separated by a thin hBN dielectric, we investigate density tunable magneto and Hall drag in quantum Hall bilayer systems. Large variations of magneto-drag and Hall-drag are observed, which can be related to the Landau level (LL) filling status of both driving and drag layers. The measured drag resistivity tensor can be associated with the tensor product of the differential magneto-resistivity tensors of the drive and drag layers [1]. The temperature and field dependence of magneto-drag can be described in terms of the phase space for Coulomb scattering between LLs in the drag and drive layers. In the strong interaction regime and ultra-low temperature, we observe the effect of symmetry broken integer quantum Hall States in magneto and Hall drag signals. [1] F. von Oppen, S. Simon, and A. Stern, Phys. Rev. Lett. 87, 106803 (2001). [Preview Abstract] |
Tuesday, March 3, 2015 1:51PM - 2:03PM |
G17.00014: Fractional Quantum Hall Effect in the Second Landau Level of bilayer graphene Georgi Diankov, Francois Amet, Menyoung Lee, Andrew Bestwick, Kevin Tharratt, Chi-Te Liang, David Goldhaber-Gordon Bilayer graphene exhibits rich Quantum Hall physics due to valley, spin and orbital degrees of freedom that lead to a variety of polarization states. We study the Fractional Quantum Hall Effect (FQHE) in ultra-clean multiterminal bilayer graphene devices on boron nitride with a local graphite gate at magnetic fields of up to 45 T. We measure mobility of up to 1 million cm$^{\mathrm{2}}$/V.s and very low disorder. In addition to the broken-symmetry integer states, we unambiguously resolve a variety of fractions and focus on a series of fractions in the Second Landau Level, which do not follow particle-hole asymmetry. From the magnetic field dependence of the fractions, we find that some of these fractions have spin-polarized ground states while others are unpolarized, and we present a possible explanation for this difference. This work provides insights into how the symmetry-breaking electron-electron interactions and Zeeman splitting interact to produce a rich landscape of composite fermions in the Second Landau Level of bilayer graphene. [Preview Abstract] |
Tuesday, March 3, 2015 2:03PM - 2:15PM |
G17.00015: Effect of tunable superlattice on quantum Hall effect in graphene Sudipta Dubey, Mandar Deshmukh We have studied quantum Hall effect in tunable superlattice in graphene created using combination of back gate and an array of top-gates pinned to the same potential. In our device we are in the regime when superlattice period is larger than the magnetic length and superlattice amplitude can be tuned to be larger than Landau level spacing. We observe robust plateaus when charge carrier in adjacent region is of the same polarity. However when we have a series of p-n junction, the high superlattice amplitude leads to large local electric field in p-n junction causing collapse of Landau level and hence incomplete equilibration. We have also studied charge transport at low magnetic field where we have higher number of edge states circulating within a strip of back-gated or top-gated region. [Preview Abstract] |
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