Bulletin of the American Physical Society
2009 APS March Meeting
Volume 54, Number 1
Monday–Friday, March 16–20, 2009; Pittsburgh, Pennsylvania
Session B1: New Experimental Developments in Graphene |
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Sponsoring Units: DCMP Chair: Eva Andrei, Rutgers University Room: Spirit of Pittsburgh Ballroom A |
Monday, March 16, 2009 11:15AM - 11:51AM |
B1.00001: Graphene and its chemical derivatives Invited Speaker: Graphene is a first two-dimensional atomic crystal. In my talk I'll overview our latest results on the electronic properties of graphene, and discuss a possibility of band structure engineering by chemical modification of this material. [Preview Abstract] |
Monday, March 16, 2009 11:51AM - 12:27PM |
B1.00002: The high-field state at the Dirac Point in graphene Invited Speaker: The discovery of the quantum Hall Effect in graphene has generated considerable interest in the state at the Dirac Point in a magnetic field $H$. In intense $H$, the 4-fold degeneracy of the $n=0$ Landau Level (LL) is lifted by the enhanced exchange energy. Among the broken symmetry states proposed are the quantum Hall ferromagnet, the quantum Hall insulator state, excitonic condensation, and charge-density-wave formation. A subset of these theories propose counter-propagating edge states that remain conducting in large $H$. We have performed measurements of the resistance $R_{xx}$ and Hall resistance $R_{xy}$ to fields of 33 T at temperatures $T$ from 0.3 to 50 K in $\sim$6 graphene samples. We find that, as $T$ decreases below 10 K, $R_0$ (= $R_{xx}$ at the Dirac Point) undergoes a steep increase with a divergence consistent with a field-driven transition to an insulating high-field state. The divergence in $R_0$ fits well to the Kosterlitz-Thouless (KT) form $\exp(b/\sqrt{h-1})$ with $h=H/H_c$ and $b\sim 1.4$. The critical field $H_c$ is sample dependent (12 T to 33 T ), and correlates with the disorder as measured by the offset gate voltage $V_0$ and the zero-$H$ mobility. The divergence in $R_0$ is strictly confined to the $n=0$ LL (bracketed by the sublevels $\nu = \pm 1$). The peaks with $n=\pm 1$ remain near the values $h/e^2$. Using an ultralow-power (3 fW), voltage-regulated technique, we show that the KT-fit to $R_0$ is valid over 3 decades (40 k$\Omega$ to 40 M$\Omega$). The steepness of the $R_0$ vs. $T$ curves implies a bulk gap $\Delta$ of magnitude 15-20 K that decreases when $H$ falls below $H_c$. We compare our findings with the various proposed models. We will also report thermopower and Nernst measurements taken to fields of 14 T.\\[4pt] Refs. J. G. Checkelsky, L. Li and N. P. Ong, prl {\bf100}, 206801 (2008); ibid. cond-mat arXiv:0808.0906v1. [Preview Abstract] |
Monday, March 16, 2009 12:27PM - 1:03PM |
B1.00003: Quantum Carrier Collimation in Locally Gated Graphene Heterojunction Devices Invited Speaker: While electron optics such as lensing and focusing have been demonstrated experimentally, building a collimated electron interferometer in two unconfined dimensions has remained a challenge due to the difficulty of creating electrostatic barriers that are sharp on the order of the electron wavelength. Owing to the suppression of backscattering experienced by the chiral quasiparticles, graphene provides an ideal medium to realize the quantum engineering of electron wave functions. In this presentation, we show our progresses in wave function engineering in graphene devices by demonstrating the conductance oscillations in extremely narrow graphene heterostructures where a resonant cavity is formed between two electrostatically created bipolar junctions. Analysis of the oscillations confirms that bipolar heterojunctions have a collimating effect on ballistically transmitted carriers. The robustness of the oscillatory conductance to scattering provides a novel probe of the ballistic physics of graphene at the Dirac point and makes graphene heterojunctions a promising tool for the coherent manipulation of carriers in mesoscopic systems. [Preview Abstract] |
Monday, March 16, 2009 1:03PM - 1:39PM |
B1.00004: Robustness of quantum Hall effect in locally gated graphene devices Invited Speaker: Two-terminal conductance of locally gated graphene {\it p-n-p} heterojunctions in the quantum Hall regime is quantized at integer and fractional values, owing to the edge states equilibration. We study the sensitivity of this quantization to finite longitudinal conductivity in the locally gated inner region of the junction. Taking a bulk conductivity approach, we solve spatially non-uniform conduction problem exactly by a conformal mapping method. We find that the robustness of the conductance quantization strongly depends on the geometry of the locally gated region, as well as on the densities in the inner and outer regions. We present a detailed comparison of our predictions with recent experimental data, finding good agreement. [Preview Abstract] |
Monday, March 16, 2009 1:39PM - 2:15PM |
B1.00005: Octet Quantum Hall Effect in Graphene Bilayers Invited Speaker: Interaction driven integer quantum Hall effects are anticipated [1]in graphene bilayers because of the near-degeneracy of eight Landau levels which appear near the neutral system Fermi level at filling factors between $\nu $=-4 and $\nu $=4. The bilayer graphene octet exhibtits a wide variety of broken symmetry states, with Ising, XY and Heisenberg character which can be controlled by an external field which creates an electric potential difference between the two layers. Because of the peculiarities of the bilayer graphene electronic structure states with n=0 and n=1 orbital character are degenerate. I will explain predictions that an intra-Landau-level cyclotron resonance signal will appear at some odd-integer filling factors, accompanied by collective modes which are nearly gapless and have approximate k$^{3/2}$ dispersion. This talk will be based on work performed in collaboration with Yafis Barlas, Rene Cote, Kentaro Nomura, and Jules Lambert. \\[4pt] [1] Y. Barlas \textit{et al. }, Phys. Rev. Lett. \textbf{101}, 097601(2008). [Preview Abstract] |
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