Bulletin of the American Physical Society
APS March Meeting 2011
Volume 56, Number 1
Monday–Friday, March 21–25, 2011; Dallas, Texas
Session W37: Focus Session: Graphene: Growth, Characterization, and Devices: Quantum Hall Effect |
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Sponsoring Units: DMP Chair: Aaron Bostwick, Lawrence Berkeley National Laboratory Room: C146 |
Thursday, March 24, 2011 11:15AM - 11:27AM |
W37.00001: A study of the quantum Hall effect in CVD graphene synthesized on Cu Tian Shen, Wei Wu, Helin Cao, Yong Chen, David Newell, Curt Richter, Qingkai Yu Graphene films grown by Chemical Vapor Deposition (CVD) have provided a viable way to large area, low cost graphene-based electronics. Graphene CVD grown on Cu was transferred to SiO2/Si substrates and Hall-bar devices with sizes as large as 150 $\mu $m were fabricated, exhibiting carrier mobility of $\sim $3000cm$^{2}$/Vs. At low temperatures, the half integer quantum-Hall effect (QHE) and Shubnikov-de Haas (SdH) oscillations confirmed the single layer quality of the transferred graphene films. Systematic measurements in the quantum Hall region such as the DC/AC current dependence, temperature dependence, and AC frequency dependence were carried out, and their impact on the breakdown of the QHE was investigated. From weak localization peak fitting, the phase coherence length of the CVD graphene is determined to be $\sim $3$\mu $m at 1.6K. [Preview Abstract] |
Thursday, March 24, 2011 11:27AM - 11:39AM |
W37.00002: Strain-induced pseudo-magnetic fields and charging effects on CVD-grown graphene R.T.-P. Wu, M.-L. Teague, N.-C. Yeh, S. Yeom, B.L. Standley, D.A. Boyd, M.W. Bockrath Atomically resolved imaging and spectroscopic characteristics of chemical vapor deposition (CVD) grown graphene on Cu are studied using scanning tunneling microscopy and spectroscopy. CVD-grown graphene remaining on Cu exhibits large ripples and appears strongly strained. Different regions show different lattice structures and electronic density of states (DOS). Ridges appear along the boundaries of different lattice structures, which reveal excess charging effects. The large, non-uniform strain induces pseudo-magnetic fields up to $\sim $50 Tesla, yielding integer and fractional quantum Hall effects (IQHE and FQHE) as quantized conductance peaks in the DOS. For CVD-grown graphene transferred from Cu to SiO$_{2}$, the average strain and the resulting charging effects and pseudo-magnetic fields are much reduced. Fourier transformation of the local DOS of strained samples as well as data on the effects of real magnetic fields versus pseudo-magnetic fields will be presented. This work was jointly supported by NSF and NRI. [Preview Abstract] |
Thursday, March 24, 2011 11:39AM - 11:51AM |
W37.00003: Transport phenomena of suspended graphene in the quantum Hall regime Hsin-Ying Chiu, Ching-Tzu Chen, David DiVincenzo, Phaedon Avouris Towards the goal of making qubits in graphene, it is important to develop graphene quantum point contact for quantum-gate operations. Klein tunneling effect implies that confining charge carriers in graphene using external potential is challenging. Hence, we resort to electric-field controlled band gap opening for charge confinement. We have successfully fabricated high quality suspended few-layer graphene devices with local electrostatic gates. In this talk, we present our observation of voltage-controlled band gap opening and anomalous quantum Hall features upon applying vertical electrical filed, revealing rich physics of symmetry breaking in the few-layer graphene system. [Preview Abstract] |
Thursday, March 24, 2011 11:51AM - 12:03PM |
W37.00004: Measurement of the 1/3 Fractional Quantum Hall Effect Energy Gap in Multi-terminal Suspended Graphene Devices Fereshte Ghahari Kermani, Yue Zhao, Paul Cadden-Zimansky, Kirill Bolotin, Philip Kim In recent investigations of transport properties in two-terminal high mobility suspended graphene devices, a quantized conductance corresponding to the 1/3 FQHE state has been observed. However, due to the inherent mixing between longitudinal and transverse resistivities in this two-terminal measurement, quantitative characterization of the observed FQHE states such as the FQHE energy gap is difficult. In this talk, we present the measurement of multi-terminal IQHE and FQHE states in ultraclean suspended graphene samples in low density regime. The energy gap of the 1/3 FQHE, measured by its temperature-dependent activation, is found to be much larger than the corresponding state found in the 2DEGs of high-quality GaAs heterostructures, indicating that stronger e-e interactions are present in graphene. [Preview Abstract] |
Thursday, March 24, 2011 12:03PM - 12:15PM |
W37.00005: Symmetry breaking of zero energy landau level in monolayer graphene Yue Zhao, Paul Cadden-Zimansky, Fereshte Ghahari, Philip Kim We experimentally study the nature of the symmetry breaking of the zero energy landau level (LL) in monolayer graphene using Corbino geometry and Hall bar geometry devices. At high magnetic fields, in the absence of the edge state channel connection in Corbino devices, we observe a gap opening in $\nu=0$ QH state whose gap is independent of in-plane magnetic field. In Hall-bar geometry devices where edge state connection is allowed, we observe similar QH Insulator behavior independent of the in-plane magnetic field, indicating that the observed insulating behavior at the charge neutrality point of monolayer graphene at high magnetic field is originated from the degeneracy lifting of the zero LL via the valley pseudospin polarization rather spin polarization. [Preview Abstract] |
Thursday, March 24, 2011 12:15PM - 12:27PM |
W37.00006: Fractional quantum Hall effect in graphene on boron nitride Cory Dean, Andrea Young, Paul Cadden-Zimansky, Lei Wang, Hechen Ren, Kenji Watanabe, Takashi Taniguchi, Philip Kim, James Hone, Ken Shepard Graphene is a remarkable 2D material exhibiting many unique and surprising many-body effects resulting from strong electron interactions. A continuing challenge remains the fabrication of ultra-high mobility devices that allow the intrinsic character of graphene to be fully explored. In my talk I will discuss our recent advancements in fabricating very-high quality graphene devices on boron nitride. Magnetoresistance measurements under very large applied fields will be presented including our recent observation of the fractional quantum Hall effect in multi-terminal devices over a broad range of carrier densities. [Preview Abstract] |
Thursday, March 24, 2011 12:27PM - 1:03PM |
W37.00007: Transport on gated C-face epitaxial graphene Invited Speaker: We present transport and electronic properties on single layer and multilayered epitaxial graphene layers grown on 4H-SiC-(000-1) (C-face) by the Confinement Controlled Sublimation method [1]. Single layers present all the characteristics of isolated graphene layers. In particular quantum Hall effect plateaus develop at half-integer values, concomitant with vanishing longitudinal resistivity. High mobility up to $\mu $=14,000 cm$^{2}$/V.s at 300 K is achieved despite contamination and substrate steps. Multilayered epitaxial graphene (MEG) on the C-face consists of non-graphitic rotationally stacked graphene layers, exhibiting the band structure of a single graphene layer [2]. Transport in MEG presents also graphene characteristics. In some cases transport anomalies are observed indicating a much richer picture. \\[4pt] [1] R. Ming et al. Materials Science and Engineering -- Reports (submitted) \\[0pt] [2] M. Sprinkle et al, Phys. Rev. Lett. 103, 226803 (2009). [Preview Abstract] |
Thursday, March 24, 2011 1:03PM - 1:15PM |
W37.00008: Quantum Hall Effect of Hybrid Monolayer-bilayer Graphene Structures: Observation of Broken electron-hole Symmetry Yong P. Chen, Jifa Tian, Isaac Childres, Helin Cao Quantum Hall Effect (QHE) in both monolayer (1L) and bilayer (2L) graphene has been well studied in the past few years. Little attention has been paid to the magneto-transport across the 1L $\sim $2L graphene interface. Here, we present the magnetotransport measurements of several exfoliated graphene quasi-Hall bar devices which consist of partly 1L and partly 2L graphene. We focused on the Hall resistance (R$_{xy})$ across the interface between 1L and 2L graphene when the carrier types and densities are changed using a back gate voltage. We observed that when the carrier type is p type (hole), R$_{xy}$ typically shows QHE of bilayer graphene with filling factor of 4N, N being integers. When the carrier type is changed into n type (electron), the corresponding R$_{xy}$ typically shows QHE of single layer graphene with filling factor of 4(N+1/2), N being integers. We discuss possible explanations for the observed broken electron-hole symmetry in such hybrid structures. [Preview Abstract] |
Thursday, March 24, 2011 1:15PM - 1:27PM |
W37.00009: Multi-terminal transport property on hybrid structure of monolayer and bilayer graphene Yongjin Jiang, Jiangping Hu We study the transport properties of Hybrid structures formed by monolayer and bilayer graphene. The energy spectrum and edge states of various junctions in strong magnetic field are analyzed. The transport properties are affected by the mixing effect of interfaces and the scattering between edge states, and display many novel features in multi-terminal measurements, Our results explains recent experimental results on such hybrid structure systems. [Preview Abstract] |
Thursday, March 24, 2011 1:27PM - 1:39PM |
W37.00010: Electric field induced transition between spin to valley polarized $\nu $=0 quantum Hall state in dual-gated graphene bilayers Kayoung Lee, Seyoung Kim, Babak Fallahazad, Emanuel Tutuc Graphene bilayers in Bernal stacking exhibit a transverse electric field dependent energy gap, thanks to the on-site electron energy asymmetry between the two layers. In a perpendicular magnetic field, the applied transverse electric field ($E)$ will induce a quantum Hall state (QHS) at the charge neutrality point (filling factor $\nu $=0) marked by a insulating behavior of the longitudinal resistance ($\rho _{xx})$, and a plateau in the Hall conductivity. Using dual-gated graphene bilayers, we investigate here the $E$-field dependence of the $\nu $=0 QHS in high perpendicular magnetic fields ($B)$, up to 30T. The temperature dependence of $\rho _{xx}$ measured at $\nu $=0 shows an insulating behavior, which is strongest in the vicinity of $E$=0 as well as at large $E$-fields. At a fixed $B$-field, as a function of the applied $E$-field the $\nu $=0 QHS undergoes a transition, marked by a $\rho _{xx }$minimum, as well as a temperature independent $\rho _{xx}$ at a finite $E$-field value. This observation can be explained by a transition from a spin polarized $\nu $=0 QHS at small $E$-fields, to a valley (layer) polarized $\nu $=0 QHS at large $E$-fields. The $E$-field value at which the transition occurs follows a linear dependence on the applied perpendicular magnetic field, with a slope of $\sim $18 mV/nm$\cdot $T. [Preview Abstract] |
Thursday, March 24, 2011 1:39PM - 1:51PM |
W37.00011: Giant Spin-Hall Effect and Nonlocal Transport in Graphene Dmitry Abanin, K.S. Novoselov, A.K. Geim, L.S. Levitov Graphene provides a unique opportunity to explore quantum-relativistic phenomena in a condensed matter laboratory. Interesting phenomena associated with the parity anomaly, including quantum Hall effect in the absence of magnetic field and quantum spin-Hall effect in quantizing magnetic fields, have been theoretically proposed, but could not be observed so far largely due to disorder and density inhonogeneity. We show that weak magnetic field induces large bulk non-quantized spin-Hall effect in graphene. The effect occurs due to Zeeman spin splitting which generates the imbalance of the Hall resistivities of the two spin species. The spin-Hall effect is robust in the presence of disorder and interactions. It will manifest itself in large nonlocal transport mediated by long-lived spin currents, as well as in spin injection and spin accumulation experiments. The effect peaks at the Dirac point, and can serve as a hallmark of the relativistic character of carriers in graphene and other Dirac materials. [Preview Abstract] |
Thursday, March 24, 2011 1:51PM - 2:03PM |
W37.00012: Quantum Hall effect and Landau level crossing in trilayer graphene Thiti Taychatanapat, Kenji Watanabe, Takashi Taniguchi, Pablo Jarillo-Herrero We report the experimental observation of quantum Hall effect in Bernal stacked trilayer graphene (TLG) on hexagonal boron nitride substrate. The mobility of our TLG reaches 110,000~cm$^2 $/V$\cdot$s allowing us to observe the Shubnikov-de Haas oscillation at a magnetic field as low as $300$~mT and broken- symmetry states at high magnetic field. In addition, the unique band structure of Bernal stacked TLG which consists of monolayer-like and bilayer-like subbands at low energy allows us to observe the Landau level crossing between these two subbands. The positions of these crossings in magnetic field and filling factors enable us to estimate relevant Slonczewski-Weiss-McClure parameters. [Preview Abstract] |
Thursday, March 24, 2011 2:03PM - 2:15PM |
W37.00013: Suspended graphene electromechanics in quantum Hall regime Vibhor Singh, Ganesh Subramanian, Bushra Irfan, Hari Solanki, Mandar Deshmukh There has been a keen interest in the NEMS community in probing the coupling between charge and mechanical degrees in NEMS resonators. These experiments show that electron transport and mechanical motion of the resonator influence each other. Motivated by this, we have probed the electron transport in the ultra clean graphene devices in quantum Hall regime at low temperature while it is mechanically perturbed. There can be several mechanisms that can lead to the resistance change due to mechanical vibrations like, by strain due to deformation of the flake, redistribution of the carrier density etc, electron scattering within the flake due to changes in the energy landscape. In our study we find that upon mechanical vibrations the resistance of the device changes. We try to understand these changes caused by the non-linear dependence of resistance on carrier density. [Preview Abstract] |
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