Bulletin of the American Physical Society
APS March Meeting 2014
Volume 59, Number 1
Monday–Friday, March 3–7, 2014; Denver, Colorado
Session F29: Magnetotransport in Graphene |
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Sponsoring Units: DCMP Chair: Kathleen McCreary, U.S. Naval Research Laboratory Room: 603 |
Tuesday, March 4, 2014 8:00AM - 8:12AM |
F29.00001: Theoretical study of disorder induced magnetoresistance in graphene Shaffique Adam, Jinglei Ping, Indra Yudhistira, Navneeth Ramakrishnan, Sungjae Cho, Michael S. Fuhrer In this work we predict theoretically that carrier density inhomogeneity provides a new mechanism for classical magnetoresistance. For concreteness, we study the case of graphene where density inhomogeneity and carrier scattering is dominated by charged impurities, although the mechanism itself is quite general and applies to other systems in which there are large spatial fluctuations of the carrier density. Calculations using an effective medium approximation show that low-field magnetoresistance becomes a universal function of the ratio between the average carrier density and the fluctuations of the carrier density, and scales as a power-law when this ratio is large. Our finding is in excellent agreement with recent experimental results. This work is supported by the Singapore National Research Foundation NRF-NRFF2012-01. [Preview Abstract] |
Tuesday, March 4, 2014 8:12AM - 8:24AM |
F29.00002: Examining the Quantum Nature of Edge Magnetism in Graphene Nanoribbons Michael Golor, Cornelie Koop, Thomas C. Lang, Manuel J. Schmidt, Stefan Wessel Based on low-energy theories for interaction effects along graphene edges, which preserve the full quantum nature, we study edge magnetism in various types of graphene nanoribbons. We find that the relevant physics is well captured by an effective Heisenberg model with extended ferromagnetic interactions along and antiferromagnetic interactions across the ribbon edges. The basic principles of edge magnetism are then studied in short and narrow armchair ribbons, for which we predict magnetic response and STS signatures that could be probed in future experiments. For the case of macroscopically large chiral ribbons, we find the spin-spin-correlation length to grow exponentially with the ribbon width, demonstrating the importance of quantum fluctuations in these systems. [Preview Abstract] |
Tuesday, March 4, 2014 8:24AM - 8:36AM |
F29.00003: ABSTRACT WITHDRAWN |
Tuesday, March 4, 2014 8:36AM - 8:48AM |
F29.00004: Magnetoresistance induced by inhomogeneity in graphene Jinglei Ping, Indra Yudhistira, Navneeth Ramakrishnan, Sungjae Cho, Shaffique Adam, Michael Fuhrer We study the magnetoresistance of graphene samples with varying disorder as a function of carrier density. We observe a quadratic low-field classical magnetoresistance which is largest at low carrier density reflecting the inhomogeneous nature of transport in the electron-hole puddle dominated minimum conductivity region, as observed previously[\textit{Phys. Rev. B} \textbf{77}, 084102(R) (2008)]. However we observe the magnetoresistance persists to carrier densities well outside the electron-hole puddle region, where single band transport is expected. We find that the magnetoresistance for all samples follows a universal form which depends only on the ratio of the carrier density $n$ to the characteristic electron-hole puddle density $n*$. The results are in excellent quantitative agreement with a recent theory based on an effective medium approximation for disordered graphene. [Preview Abstract] |
Tuesday, March 4, 2014 8:48AM - 9:00AM |
F29.00005: Magnetotransport properties of graphene devices contacted by resist-free stencil lithography Ather Mahmood, Cheol-Soo Yang, Won Jin Choi, Jean-Fran\c{c}ois Dayen, Jeong-O Lee, Bernard Doudin We demonstrate large-scale fabrication of high-quality contaminant-free graphene devices, a prerequisite for chemical functionalization applications. We investigate CVD graphene transferred from Cu substrates to Si/SiO$_{2}$. Patterning of graphene and metal evaporation are performed through a multi-step mechanical stencils methodology. Microlithography through stencil masks is well known, but patterning graphene while keeping its outstanding electrical properties remains challenging. Magnetotransport measurements at low temperature show the existence of Shubnikov-de Hass oscillations and Quantum Hall plateaus. Weak (anti-) localization signatures of monolayer graphene validate the excellent intrinsic properties of our samples. Finally, we show this technique is extended to complex geometries and smaller device feature sizes. [Preview Abstract] |
Tuesday, March 4, 2014 9:00AM - 9:12AM |
F29.00006: Effect of spin-orbit interaction on the conductance fluctuation in disordered graphene Duk-Hyun Choe, K.J. Chang Recent findings of topological insulators have demonstrated the importance of spin-orbit interaction in low dimensional systems. In particular, the spin-orbit coupling gives rise to the formation of topological surface states that are protected by time-reversal symmetry. The universal conductance fluctuation (UCF) in spin-orbit coupled systems, however, has received comparatively little attention. It has been known that the universality characterized by the value of UCF only depends on the dimensionality and symmetry ($\beta = $ 1,2,4 according to the random matrix theory) of the system. Here, we investigate the effect of spin-orbit interaction on the UCF behavior in disordered graphene by considering Kane-Mele (KM) and Rashba type interactions. Following the random matrix theory, both KM and Rashba Hamiltonians belong to the circular symplectic ensemble ($\beta =$ 4), because in both cases time-reversal symmetry is maintained while spin-rotational symmetry is broken. Interestingly, conductance fluctuation in the KM Hamiltonian exhibits the same UCF value as that for the circular unitary ensemble ($\beta =$ 2). We reveal the origin of such inconsistency and furthermore find that there exist new types of universality class, different from the conventional ones. [Preview Abstract] |
Tuesday, March 4, 2014 9:12AM - 9:24AM |
F29.00007: Electronic transport in graphene sheets in a random magnetic field Caio Lewenkopf, Rhonald Burgos, Jesus Warnes, Leandro Lima We present a theoretical study of the effect of ripples and strain fields in the transport properties of diffusive deposited graphene flakes. Defects in the crystalline structure, adsorbed atomic impurities and charge inhomogeneities at the substrate are believed to be the dominant disorder sources for the electronic transport in graphene at low temperatures. We show that intrinsic ripples also effect the conductivity, in particular, its quantum corrections. To this end, we analyze recent experimental results on the conductivity of rippled monolayer graphene sheets subjected to a strong magnetic field parallel to the graphene-substrate interface, $B_\parallel$ [M. B. Lundeberg and J. A. Folk, Phys. Rev. Lett. 105, 146804 (2010)]. In this setting, $B_\parallel$ gives rise to a random magnetic field normal to graphene sheet, that depends on the local curvature of the smooth disordered ripples. The analysis of the weak localization corrections of the magnetoconductance allows to establish the dependence of electronic dephasing rate on the magnitude of the random magnetic field. We compare the results for $B_\parallel$ with the conductivity and weak localization corrections due to the pseudo-magnetic fields originated by intrinsic ripples and strain fields. [Preview Abstract] |
Tuesday, March 4, 2014 9:24AM - 9:36AM |
F29.00008: Density-matrix renormalization group studies on a magnetic impurity in graphene Tomonori Shirakawa, Seiji Yunoki Motivated by recent experiments on the emergence of magnetism in graphene induced by lattice defects or by magnetic adatoms, we have theoretically studied the ground state properties of single magnetic impurity in grapheme. First, we have developed a new numerical technique within the density-matrix renormalization group (DMRG) scheme for magnetic impurity model in general to study site-dependent quantities including local density of states even away from magnetic impurity site, Friedel density oscillations, and spin-spin correlation functions between the magnetic impurity and the surrounding conduction electrons. This new technique is applied to three different models: (i) a magnetic adatom on grapheme, (ii) a substitutional magnetic impurity in grapheme, and (iii) a model on defect in the graphene. Our systematic study of these models reveals that, in the presence of particle-hole symmetry, the ground state of model (i) exhibits the formation of local moments without Kondo-screening, whereas the others behave very similarly to the Kondo singlet states. We also discuss the real-space decay of spin-spin correlations between magnetic impurity and surrounding conduction electrons in these models. [Preview Abstract] |
Tuesday, March 4, 2014 9:36AM - 9:48AM |
F29.00009: Magneto-optics of general pseudospin-$s$ two-dimensional Dirac-Weyl fermions John Malcolm, Elisabeth Nicol The popularity of graphene--a pseudospin-$\frac{1}{2}$ two-dimensional Dirac-Weyl material--has prompted the search for related materials and the characterization of their properties. The magneto-optical conductivity is calculated for systems that obey the general pseudospin-$s$ two-dimensional Dirac-Weyl Hamiltonian, with particular focus on $s=\left\{\frac{1}{2},1,\frac{3}{2},2\right\}$. This follows previous work on the optical response of these systems in zero field [1]. In the presence of a magnetic field, Landau levels condense out of the $2s+1$ energy bands [2]. As the chemical potential in a system is shifted, patterns arise in the appearance and disappearance of certain peaks within the optical spectra. These patterns are markedly different for each case considered, creating unique signatures for potential experimental observations. The general structure of each spectrum and how they compare is discussed. \\ \\ \noindent [1] B. D\'{o}ra, J. Kailasvuori, and R. Moessner, Phys. Rev. B {\bf 84}, 195422 (2011). \\ \noindent [2] X. Lan, N. Goldman, A. Bermudez, W. Lu, and P. \"{O}hberg, Phys. Rev. B {\bf 84}, 165115 (2011). [Preview Abstract] |
Tuesday, March 4, 2014 9:48AM - 10:00AM |
F29.00010: Study of Magneto-transport in Niobium Nitride (NbN) - Graphene Josephson weak links Piranavan Kumaravadivel, Xu Du Proximity induced superconductivity in graphene - superconductor (SC) hybrid devices has revived interest in the study of electronic transport of Andreev bound states in the Quantum Hall (QH) regime. This is mainly due to the ability to fabricate ballistic superconducting weak links with type II SC where the interplay of proximity effect and QH effect can be studied at low magnetic fields ($\sim$ 0.5T) and extended up to the upper critical field (Hc$_{2})$ [1]. In our work we use sputtered NbN which has an upper critical field of 16T. Below the SC transition temperature of NbN (approx. 12K) we observe Andreev reflection and super-current which are indicative of transparent superconductor-normal interface. We study magneto- transport measurements at different fields applied perpendicular to the graphene channel. Our results suggest that the diamagnetic current and Abrikosov vortices which are formed in the NbN leads influence the SC-proximity effect in graphene. \\[4pt] [1] Mizuno, N. et al. Ballistic-like supercurrent in suspended graphene Josephson weak links. Nat. Commun. 4:2716 doi: 10.1038/ncomms3716 (2013) [Preview Abstract] |
Tuesday, March 4, 2014 10:00AM - 10:12AM |
F29.00011: Ballistic transport in CVD graphene V.E. Calado, S.E. Zhu, S. Goswami, Q. Xu, K. Watenabe, T. Taniguchi, G.C.A.M. Janssen, L.M.K. Vandersypen Chemical vapor deposition (CVD) synthesis of graphene is a scalable and controllable method for the production of single layer CVD graphene (CVDg). Up to now its electronic and structural quality is considered to be inferior to exfoliated graphene, and in particular no ballistic phenomena have been observed in CVDg. Here we synthesize and measure CVDg that shows ballistic transport on a micron length-scale at 4 K. With a dry transfer method we transferred 100 $\mu$m size single crystals of CVDg onto hexagonal boron nitride. Using non-local measurements we show that electrons can be ballistically directed by a magnetic field (transverse magnetic focussing) over length scales of about 1 micron. These findings suggest that CVD graphene is suitable for electron optics experiments. [Preview Abstract] |
Tuesday, March 4, 2014 10:12AM - 10:24AM |
F29.00012: Pseudogap opening and localization in disordered graphene: frustration effects at the Fermi energy due to the underlying triangular symmetry Eduardo Barrios-Vargas, Gerardo G. Naumis An intuitive explanation of the increase in localization observed near the Dirac point in doped graphene is presented. To do this, we renormalize the tight binding Hamiltonian in such a way that the honeycomb lattice maps into a triangular one [1]. Then, we investigate the frustration effects that emerge in this Hamiltonian. In this doped triangular lattice, the eigenstates have a bonding and antibonding contribution near the Dirac point, and thus there is a kind of Lifshitz tail [2]. The increase in frustration is related to an increase in localization, since the number of frustrated bonds decreases with disorder, while the frustration contribution raises. Then we show that states have a multifractal nature, with a fractal spectrum that approaches freezing as disorder increases [2]. We compute exacty the first spectral moments of the DOS using statistical averages and counting paths. Finally, the number of states at the Dirac point is obtained using a configurational counting [3]. \\[4pt] [1] Barrios-Vargas, Naumis, J. Phys.: Condens. Matter 23, 375501 (2011)\\[0pt] [2] Barrios-Vargas, Naumis, J. Phys.: Condens. Matter 24 (2012)\\[0pt] [3] Barrios-Vargas, Naumis, Solid State Communications 162, 23-27 (2013) [Preview Abstract] |
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