Bulletin of the American Physical Society
APS March Meeting 2010
Volume 55, Number 2
Monday–Friday, March 15–19, 2010; Portland, Oregon
Session L21: Focus Session: Graphene: Transport II |
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Sponsoring Units: DMP Chair: Eduardo Mucciolo, University of Central Florida Room: Portland Ballroom 251 |
Tuesday, March 16, 2010 2:30PM - 2:42PM |
L21.00001: Weak localization in high quality epitaxial graphene Wei Pan, Anthony Joseph Ross III, Taisuke Ohta, Steve Howell, Tom Friedmann In this talk, we will present our recent low temperature magneto- transport data obtained in a high quality epitaxial graphene grown on an n-type 6H-SiC(0001) substrate. A Hall bar (50$\mu$m $\times$ 6.25$\mu$m) device was fabricated using the conventional photolithographic technique. The as-grown two- dimensional electron system (2DES) density was $6 \times 10^{11} $ cm$^{-2}$ and mobility 14,000 cm$^2$/Vs. In this device, the so-called half-integer quantum Hall effect states were found at $\nu$ = 2, 6, and 10 in high magnetic fields. Close to $B=0$, a strong weak-localization (WL) peak was observed. We fitted this WL peak based on a well-developed theoretical model and obtained the values of the phase coherence ($L_{\phi}$), intravalley ($L_ {\star}$), and intervalley ($L_{iv}$) scattering lengths. $L_ {\phi}$ first increases with decreasing temperature ($T$), $L_ {\phi} \propto T^{-1/2}$, and then saturates below $T \sim 1$K. $L_{iv}$ and $L_{\star}$ are in the order of half nanometer and in general independent of $T$. [Preview Abstract] |
Tuesday, March 16, 2010 2:42PM - 2:54PM |
L21.00002: Efficient electrolyte gating of high mobility graphene X. Hong, K. Zou, J. Zhu We report the effect of highly efficient electrolyte top gating (PEO:LiClO$_4$) on the transport properties of graphene. Single layer graphene sheets are mechanical exfoliated on SiO$_2 $/doped Si substrates and fabricated into field effect transistor (FET) devices using standard e-beam lithography. Pristine devices exhibit field effect mobility ($\mu_{FE}$) of 6,000 to 13,000 cm$^2$/Vs. An electrolyte droplet is then applied to such devices and functions as a top gate. It exhibits gating efficiency of 1x10$^{13}$/cm$^2$V at low gate voltage ($V_t<$1), which is 140 times higher than the 300 nm SiO$_2$/Si back gate and corresponds to a Debye length of $<$3 nm. At larger $V_t$, the gating efficiency rises sharply. The top gate can induce high carrier densities up to 1x10$^{14} $/cm$^2$ with less than 2 V. At low carrier densities ($n$$<$1x10$^{13}$/cm$^2$), $\mu_{FE}$ of electrolyte gated graphene is comparable to that of the pristine device. At higher densities, the conductivity $\sigma$ shows very weak density dependence, leading to $\mu\sim1/n$. In this regime, Li$^+$ and ClO$_4^-$ ions in the close vicinity of graphene become the main scattering source. We explore the interplay between the high geometric capacitance of the electrolyte top gate and the quantum capacitance of the graphene 2D electron gas. [Preview Abstract] |
Tuesday, March 16, 2010 2:54PM - 3:06PM |
L21.00003: Measurements of the effect of a random vector potential in graphene Mark Lundeberg, Joshua Folk High-resolution images have consistently demonstrated the presence of ripples in graphene flakes. Strain associated with the ripples can, in principle, affect the electronic properties by inducing a random gauge field. We report measurements of the analogous effects of a random vector potential generated by applying an in-plane magnetic field to a graphene flake. Phase-coherent weak localization is suppressed, while quasi-random Lorentz forces lead to anisotropic magnetoresistance. Distinct signatures of these two effects enable an independent estimation of the ripple amplitude and correlation length. By comparing to the dephasing effects of the random vector potential, the intra-valley scattering rate associated with graphene's ripples can be calculated. [Preview Abstract] |
Tuesday, March 16, 2010 3:06PM - 3:42PM |
L21.00004: The transport properties of graphene: an overview Invited Speaker: Nuno Peres We present an introduction to the transport properties of graphene combining theoretical analysis with experimental results. In the theoretical description we use simple intuitive models to illustrate important points on the transport properties of graphene. [Preview Abstract] |
Tuesday, March 16, 2010 3:42PM - 3:54PM |
L21.00005: Minimal conductivity in graphene beyond linear response Eugene Mishchenko The independence of the ac conductivity of intrinsic graphene of frequency takes its origin in the compensation of the vanishing density of states by the diverging matrix element of the corresponding interband transition. The applicability of the linear response approach, however, breaks down when this matrix element becomes comparable with the inverse electron lifetime. We show that the physics of the ac conductivity in this regime is determined by Rabi oscillations and obtain it beyond the first order perturbation theory. Under strong applied electric fields the induced current eventually saturates at a value determined by the frequency and the lifetime. We also calculate the electromagnetic response of a graphene sheet and find that the optical transparency is increased by the non-linear effects and make experimental predictions. [Preview Abstract] |
Tuesday, March 16, 2010 3:54PM - 4:06PM |
L21.00006: Quantum Linear Magnetoresistance in Multilayer Epitaxial Graphene Adam L. Friedman, Joseph L. Tedesco, Paul M. Campbell, James Culbertson, Edward Aifer, Keith Perkins, Rachel L. Myers-Ward, Jennifer K. Hite, Charles R. Eddy, Jr., D. Kurt Gaskill A conductor in an applied magnetic field normally exhibits a quadratic magnetoresistance (MR) that saturates at low fields and displays a relatively small MR ratio. However, the introduction of impurities or inhomogeneities into the system can result in a phenomenon where the normal quadratic dependence of the resistance on the field gives way to a non-saturating linear magnetoresistance (LMR) and an MR in the giant or colossal range. Due to its unique band structure, graphene is the perfect platform for the observation and control of LMR. Here, multilayer epitaxial graphene grown on SiC is shown to display linear magnetoresistance. Raman spectroscopy studies show that the material is quite inhomogeneous with varying thickness and grain size. Observed LMR is quantum in nature even up to room temperature as it is shown to not match the classical models for linear magnetoresistance. This is expected when comparing the results to LMR models. The magnitude of the magnetoresistance reaches as high as 250{\%} above the zero field resistance at 12 T and 4.2K, which opens the door to future giant magnetoresistance graphene devices and sensors. [Preview Abstract] |
Tuesday, March 16, 2010 4:06PM - 4:18PM |
L21.00007: The effect of a velocity barrier on the ballistic transport of Dirac fermions Andres Concha, Zlatko Tesanovic We propose a novel way to manipulate the transport properties of massless Dirac fermions by using velocity barriers, a region in which the Fermi velocity, $v_{F}$, has a value that differs from the one in the surrounding background. We find that the transmission through a velocity barrier is highly anisotropic, and that perfect transmission always occurs at normal incidence. We also analyzed consequences of such a barrier on the conductivity and Fano factors in the wide ribbon limit, and show that its effects should be easily mesurable with current available technology. Remarkably, when $v_{F}$ in the barrier is larger that the velocity outside the barrier, we find that a critical transmission angle exists, a Brewster-like angle for massless Dirac electrons. The later being the basic element to construct electron waveguides and related devices. [Preview Abstract] |
Tuesday, March 16, 2010 4:18PM - 4:30PM |
L21.00008: Fluctuation of magnetoresistance at the minimum conductivity point of monolayer graphene due to potential fluctuation Ryuta Yagi, Seiya Fukada, Yumi Shintani We have studied magnetoresistance fluctuation which appeared at minimum conductivity point of monolayer graphene devices at low temperatures. The fluctuation was sample-dependent and was reproducible from sweep to sweep. It was not random but was found to be superposition of some series of Shubnikov-de Haas (S- dH) oscillations though peaks are highly deformed. Peaks of the fluctuation could be assigned to some series of oscillations periodic in 1/B. Cyclotron masses for major peaks of the fluctuation were estimated from the temperature dependence of the amplitude. The cyclotron masses for each series of the oscillation and carrier density calculated from the periodicity assuming the oscillations to be S-dH effect, agreed with the characteristic relation between carrier density and cyclotron mass in monolayer graphene. The major oscillations were due to a few of the largest puddles of electron and/or hole rather than the universal conductance fluctuation. [Preview Abstract] |
Tuesday, March 16, 2010 4:30PM - 4:42PM |
L21.00009: Electron-Hole Puddle Induced Scattering and 1/f Noise Behavior in Graphene Guangyu Xu, Carlos Torres Jr., Yuegang Zhang, Fei Liu, Minsheng Wang, Yi Zhou, Caifu Zeng, Kang Wang We report the observation of electron-hole puddle induced scattering in both monolayer and bilayer bulk graphene at the low carrier density regime (near the Dirac point) using four-probe low frequency noise measurements. For monolayer graphene, the non-uniformity of the localized density of states in the presence of puddles results in an abnormal noise reduction in low carrier densities in the order of 1012 cm-2. For bilayer graphene, a similar noise reduction near the Dirac point is observed, but with a different carrier density dependence of the noise behavior due to the bandgap-related screening modulation by the gate bias. The noise decreases near the Dirac point since the carriers transport along the puddles and interact with fewer impurity scattering centers near the graphene-SiO2 interface. [Preview Abstract] |
Tuesday, March 16, 2010 4:42PM - 4:54PM |
L21.00010: Effects of Electron-Phonon Interaction in Graphene: The First Principle Calculation J.T. Mullen, K.M. Borysenko, E.A. Barry, Y.G. Semenov, M. Buongiorno Nardelli, J.M. Zavada, K.W. Kim We investigate the electron-phonon interaction in intrinsic monolayer graphene using density functional perturbation theory. These results clearly show that the coupling energy is of the same order for all four in-plane phonon branches. Electron scattering rates calculated based on these matrix elements show the substantial contribution of scattering with optical phonons, as well as intervalley scattering with acoustic phonons at T $>$ 200 K. Based on obtained ``effective" deformation potential constants we suggest that the influence of a substrate in graphene is presently underestimated, which could explain the discrepancies in estimates of deformation potential constant based on available experimental data. The low-field mobilities calculated with the full-band Monte Carlo simulation based on the obtained scattering rates are in agreement with recent experiments. [Preview Abstract] |
Tuesday, March 16, 2010 4:54PM - 5:06PM |
L21.00011: Dirac Spectrum in One-Dimensional Potentials. L. Brey, D.P. Arovas, H.A. Fertig, Eun-Ah Kim, K. Ziegler We investigate the effect of one-dimensional potentials on the electronic properties of graphene using the continuum Dirac equation appropriate at low energies. In the case of a periodic potential of the form $V(x)=V_0 \cos (G_0 x)$, new zero energy Dirac points emerge whenever the condition $J_0 (\frac {2 V_0}{\hbar v_F G_0})$ is satisfied. In the case of piecewise constant potentials new Dirac points are present throughout the band structure, and in the special case of a particle-hole symmetric potential they occur, as in the cosine potential case, at zero energy. The analysis of the conductance parallel to the periodic potential demonstrates that resonances accompany the emergence of the induced Dirac points. We also consider the cases of a single trench and a $p$-$n$ junction embedded in neutral graphene. In both cases, we obtain that these structures support confined states that lead to interference effects in the conductance across these structures. [Preview Abstract] |
Tuesday, March 16, 2010 5:06PM - 5:18PM |
L21.00012: Edge states and transmission coefficients of monolayer-bilayer graphene zigzag interfaces Wenxin Ding, Zi-Xiang Hu, Oskar Vafek, Kun Yang We study the properties of a monolayer-bilayer graphene hybrid structure with a zigzag interface that preserves translational invariance in one direction. We first study the dispersion relations and edge states of the two different configurations of monolayer-bilayer graphene zigzag interfaces -that terminates at low energy sites (dangling sites) and at high energy sites (dimer sites), using both tight-binding model and continuum limit Hamiltonian. Then we calculate the reflection/transmission coefficients in the continuum limit. For both the dispersion and the reflection/transmission coefficients, prominent differences are observed between these two types of interfaces. For the high energy sites interface noticeable enhancement of reflection for states with energy below the interlayer coupling energy is found. [Preview Abstract] |
Tuesday, March 16, 2010 5:18PM - 5:30PM |
L21.00013: Time Reversal of a Pseudospin: General Properties and Application to Graphene R. Winkler, U. Z\"ulicke We show that pseudo-spin 1/2 degrees of freedom can be categorized in two types according to their behavior under time reversal. One type exhibits the behavior of ordinary fermionic spin-1/2 whose three Cartesian components are all odd under time reversal. For the second type, which is bosonic in nature, only one of the components is odd while the other two are even. This latter type is realized, e.g., by the pseudo-spin representation of the 2D isotropic harmonic oscillator (Schwinger model of spin-1/2). We show [1] that the sublattice-related pseudospin of charge carriers in graphene is likewise of the second type. Our results imply that, in the absence of true spin-orbit coupling, the quantum correction to the resistance of single-layer graphene will be positive (weak localization). This provides a natural explanation for the hitherto puzzling absence of the proposed [2] weak-antilocalization behaviour of graphene, as observed in recent experiments. [1] R. Winkler and U. Z\"ulicke, arXiv:0909.2169. [2] E. McCann et al, Phys. Rev. Lett. 97, 146805 (2006). [Preview Abstract] |
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