Bulletin of the American Physical Society
APS March Meeting 2014
Volume 59, Number 1
Monday–Friday, March 3–7, 2014; Denver, Colorado
Session L29: Electronic and Valleytronic Properties of Graphene |
Hide Abstracts |
Chair: Michael Fogler, University of California, San Diego Room: 603 |
Wednesday, March 5, 2014 8:00AM - 8:12AM |
L29.00001: Magnetic field confinement in graphene with Gaussian deformations Martin Schneider, Daiara Faria, Silvia Viola Kusminskiy, Nancy Sandler It has been proposed that graphene holds the potential for novel transport properties under the combined effect of deformations and external fields. Strain produced by deformations results in a pseudomagnetic field that substantially modifies the real space density of states. Analogously, external magnetic fields provide a controllable mechanism to confine states in graphene. To investigate how strain and magnetic fields combine to produce peculiar electronic properties, we study a model for graphene in the presence of an out-of-plane deformation, in the continuum limit. In particular, we focus on a Gaussian height profile that produces an inhomogeneous pseudomagnetic field with trigonal symmetry. We address the question of confinement of electrons due to this deformation, using a scattering formalism based on the Dirac equation description of graphene. Our results reveal a space dependent enhancement of the local density of states as the deformation is introduced. In analogy with the Landau level formation and confined states produced by constant magnetic fields, we discuss the formation of local Landau levels and confined states by the deformation and how the two combined effects affect the transport properties of the system. [Preview Abstract] |
Wednesday, March 5, 2014 8:12AM - 8:24AM |
L29.00002: Trapping electrons in graphene in a rotating saddle Johan Nilsson We consider particle motion in rotating saddle-shaped potentials. It is known that such rotating potentials can generate bounded motion for particles with a parabolic dispersion law through the combination of potential, centrifugal and Coriolis forces in the rotating frame. When applied to massless Dirac particles, for example electrons in graphene, such a potential is shown to lead to eigenstates that are spatially localized near the center of the saddle at certain energies. Although other states also exist at these energies, they have non-overlapping support in the oscillator basis, which tend to give the localized states a substantial life-time also when imperfections are present. Reference: J. Nilsson, PRL 111, 100403 (2013). [Preview Abstract] |
Wednesday, March 5, 2014 8:24AM - 8:36AM |
L29.00003: Variation of electronic and magnetic properties of bilayer zigzag graphene nanoribbons by sliding and electric field Ramazan Tugrul Senger, Mehmet Yagmurcukardes Structural, electronic and magnetic properties of bilayer zigzag graphene nanoribbons (BZGNR) are studied using density functional theory methods. We find that ground state stacking geometry of the layers depends on the width of BZGNR. Energy bandgap , edge-localized magnetic moments and the magnetic ordering are all modified by mechanical sliding of the layers and/or by external applied electric fields. These effects can be utilized in design of electro-mechanical and magneto-mechanical nano devices. [Preview Abstract] |
Wednesday, March 5, 2014 8:36AM - 8:48AM |
L29.00004: Tunable Quantum Temperature Oscillations in Graphene and Carbon Nanoribbons Justin Bergfield, Mark Ratner, Charles Stafford, Massimiliano Di Ventra We investigate the local electron temperature distribution in carbon nanoribbon (CNR) and graphene junctions subject to an applied thermal gradient. Using a realistic model of a scanning thermal microscope, we predict quantum temperature oscillations whose wavelength is related to that of Friedel oscillations but is not directly related to the local density of states. Experimentally, this wavelength can be tuned over several orders of magnitude by gating/doping, bringing quantum temperature oscillations within reach of the spatial resolution of existing measurement techniques. [Preview Abstract] |
Wednesday, March 5, 2014 8:48AM - 9:00AM |
L29.00005: Electron-phonon vertex correction and Migdal's theorem in monolayer graphene Bitan Roy, Jaydeep Sau Corrections to the electron-phonon vertex in three dimensional Fermi liquid system scales as the ratio of the electric to the ionic mass, and are therefore negligible. This outcome is often referred as Migdal's theorem. In this talk we will briefly review the applicability of the Migdal's theorem for non-relativistic Fermi liquids in two and one spatial dimensions. In th later part of the talk we will concentrate on the electron-phonon vertex corrections for quasi-relativistic Dirac fermions in graphene. We here consider take into account only the acoustic phonon and its coupling with Dirac fermions. We will present the relevance of the electron-phonon vertex corrections, which otherwise depends only the ratio of the velocity of acoustic phonon and the Fermi velocity. If time permits, relevance of the electron-phonon vertex function in bilayer graphene will also be discussed. [Preview Abstract] |
Wednesday, March 5, 2014 9:00AM - 9:12AM |
L29.00006: Tunneling through graphene and topological insulators in presence of pn junction: transport properties and device prospects Redwan Sajjad, K.M. Masum Habib, Frank Tseng, Avik Ghosh We emphasize the role of pn junction (PNJ) in graphene electrical transport. In the ballistic regime, the resistance depends upon two key factors -- length to width aspect ratio and the PNJ formed between the doped region of graphene under metal contact. In the diffusive limit, these remain the deciding factors for the minimum conductivity. The PNJ allows us to demonstrate Klein tunneling - by either creating a PNJ electrostatically within the device or through the nature of Fabry-Perot oscillation between the two contacts. We then discuss the details of electron transport - the nature of peak device resistance, minimum contact resistance achievable with commonly used metals, effects such as electron hole asymmetry and negative differential resistance -- all being affected by the multiple PNJs formed near the contacts. We then show that PNJ acts as a filter for pseudo-spins in graphene and how this can be manipulated for gate modulation of resistance. The existence of a Dirac cone on the surface of a topological insulator has the potential of similar filtering action but for real spins instead of pseudo-spins. We adopt Non-Equilibrium Green's Function (NEGF) formalism and compare results with recent transport measurements. [Preview Abstract] |
Wednesday, March 5, 2014 9:12AM - 9:24AM |
L29.00007: Valley Polarization and Transport in Dual Gated Bilayer Graphene Patrick Maher, Kenji Watanabe, Takashi Taniguchi, Philip Kim The low energy band structures of graphene and its bilayer contain a valley degeneracy due to the two inequivalent points in the Brillouin zone. In bilayer graphene, this degree of freedom can be experimentally controlled through the breaking of layer symmetry by a transverse electric field. Notably, this can open a band gap at charge neutrality. Additionally, breaking of layer symmetry can give rise to broken symmetry quantum Hall states, and there are predictions that it can be used to create topological kink states. We report on transport measurements of ultra high quality dual-gated bilayer graphene samples encapsulated in hexagonal boron nitride. Our fabrication method involves no direct exposure of the graphene to resist, resulting in exceptionally low-disorder. In a magnetic field, we observe tunable symmetry-broken quantum Hall states. In addition, through the use of aligned split top and bottom gates, we can study transport along the one dimensional boundary between electric fields of opposite polarity. [Preview Abstract] |
Wednesday, March 5, 2014 9:24AM - 9:36AM |
L29.00008: VOI-based valley filters and valley valves in bilayer graphene quantum wires Ning-Yuan Lue, George Yu-Shu Wu Electrons in graphene have an inherent valley degree of freedom called valley pseudospin. Based on the valley-orbit interaction(VOI) in gapped graphene[1], we propose a valley filter/valve in gapped bilayer graphene(BLG)-based quantum wire device, and it's consistent with planar processing and is operated with electric gates producing an in-plane electric field transverse to the wire. The device consists of a quantum wire patterned in BLG by electrical gates, with the vicinity of the quantum wire being oxidized (or implanted with a line of point defects parallel to the wire). The transverse electric field produces a Rashba-type splitting in the valley subbands, and the oxidation (or defects) opens a pseudogap at the point where the two subbands cross. Valley polarization is generated when placing the Fermi level inside the pseudogap. We discuss the pseudogap, the valley polarization, and their dependence on the strength of the electric field and the distance between the oxidized region and the quantum wire. When the electric field is reversed, opposite valley polarity is attained. Therefore, the proposed valley filter can also be put together to form a valley valve. [1] Wu et al., PRB 84, 195463(2011); PRB 88, 125422(2013); Lee et al., PRB 86, 165411(2012). [Preview Abstract] |
Wednesday, March 5, 2014 9:36AM - 9:48AM |
L29.00009: The effect of Landau level mixing on the fractional quantum Hall effect in spin and valley polarized graphene Michael Peterson, Chetan Nayak The fractional quantum Hall effect (FQHE) in graphene presents many theoretical and experimental challenges and is not yet fully understood even in the lowest Landau level (LL). Besides spin and valley degrees of freedom being important, LL mixing is also important since it does not depend on the strength of the magnetic field (in contrast to the FQHE in semiconductors, i.e., parabolic bands) but instead depends on the dielectric of the substrate or lack thereof. Recently, we have produced an effective Hamiltonian for the FQHE in graphene that incorporates the effects of LL mixing. As a first step, we numerically study the FQHE in spin and valley polarized graphene in both the lowest and first excited LL while fully incorporating LL mixing. We find the interesting results that the lowest LL of graphene is nearly identical to that of semiconductors, even in the presence of LL mixing, and the anti-Pfaffian is stabilized in the half-filled first LL under moderate LL mixing. [Preview Abstract] |
Wednesday, March 5, 2014 9:48AM - 10:00AM |
L29.00010: Spin and valley skyrmions in Landau levels $\left\vert N\right\vert \geq 1$ of graphene and bilayer graphene Rene Cote, Wenchen Luo In a two-dimensional electron gas (2DEG), skyrmions are the lowest-energy charged excitations at filling factor $\nu =1$ while, for the chiral 2DEG in graphene, valley and spin skyrmions can exist up to Landau level $\left\vert N\right\vert =3$ $[1]$. In this talk, we discuss the excitation energy of spin and valley skyrmions in Landau level $\left\vert N\right\vert \geq 1$ in both graphene and bilayer graphene. In graphene, we consider a finite Zeeman term in order to compute the range of Zeeman coupling for which skyrmions are the lowest-energy charged excitations. We also show how the excitation energy is modified when screening is considered $[2]$. In bilayer graphene, we first derive the phase diagram of the chiral 2DEG at integer filling of the quartet of states in Landau levels $\left\vert N\right\vert \geq 1$ and show how a finite potential difference applied between the two layers can control the spin and pseudospin polarizations. We then compute the excitation energy of valley and spin skyrmions by using an anisotropic $\sigma $ model derived from the Hartree-Fock Hamiltonian and adding screening corrections.\\[4pt] [1] Kun Yang, S. Das Sarma, and A. H. MacDonald, Phys. Rev. B 74,075423 (2006).\\[0pt] [2] Wenchen Luo and R. Cote, Phys. Rev. B 88, 115417(2013). [Preview Abstract] |
Wednesday, March 5, 2014 10:00AM - 10:12AM |
L29.00011: Reading valley-hybridization and universal symmetry of graphene with mesoscopic conductance fluctuations Vidya Kochat, Atindra Nath Pal, Arindam Ghosh In graphene, the K and K' valleys act as spin-like entities, and can form the basis of valley-based electronics, having applications ranging from valley-based quantum computation, to valley filters or polarizers. The valleys hybridize to form new quantum states, such as the valley singlet and triplets, that lead to anti-localized quantum transport, non-locality and flavour Hall effect. Here we demonstrate a direct route for reading and manipulating the valley coherent states of disordered graphene by measuring the mesoscopic conductance fluctuations. We observe that the conductance fluctuations in graphene at low temperatures are reduced by a factor of four at high carrier densities, due to the gapping out of valley triplet states by short-range disorder. We also show that this results in a gate-tunable universal symmetry class, which is yet another unique and fundamental feature of the 2D honeycomb lattice of graphene. [Preview Abstract] |
Wednesday, March 5, 2014 10:12AM - 10:24AM |
L29.00012: Valley-dependent resonant inelastic transmission through a time-modulated region in graphene C.S. Chu, L. Chang, T.L. Liu Valley-dependent transmission is one of the key physical characteristics for valleytronics. In this work, we focus upon the valley-dependent nature of the quantum transport through a time-modulated-potential region in graphene, when the incident flow is collimated, with a given group velocity direction. Of particular interest is the interplay between the resonant sideband process and the trigonal-warping. The former causes transmission dip-structures which condition of occurrence is determined by sideband processes to a relevant band edge. The latter causes the relevant band-edge energy to become valley-dependent. The relevant band is a fixed-$k_{y}$ projection of the graphene energy band, where $k_{y}$ (along the time-modulate region interface) is conserved in the transmission. The valley polarization$ P$ in the transmission, for valley-unpolarized incident collimated beam, is calculated. Based on our understanding on the above valley-dependent nature, ways to optimize $P$ will be discussed. [Preview Abstract] |
Wednesday, March 5, 2014 10:24AM - 10:36AM |
L29.00013: The Rashba Type Splitting of Valley Pseudospins in Gapped Graphene Quantum Wires Yen-Chun Chen, Ning-Yuan Lue, Miao-Lin Lin, George, Yu-Shu Wu A semiconductor with a strong spin-orbit interaction (SOI) exhibits pronounced Rashba spin splitting when subject to an electric field. Although gapped graphene is a semiconductor with an extremely weak SOI, a spin-like electron degree of freedom called valley pseudospin, in association with the doubly degenerate energy band valleys at Dirac points (K and K'), exists in graphene [1] and interacts with the orbital degree of freedom via the so-called valley-orbit interaction (VOI) [2]. In the presence of an in-plane electric field, the VOI induces the pseudospin splitting similar to the Rashba spin splitting. Here, we report our recent numerical study of Rashba type splitting of valley pseudopsins in gapped (monolayer and bilayer) graphene quantum wires subject to in-plane transverse electric fields. \\[4pt] [1] K. S. Novoselov et al. Science \textbf{306},666(2004); A. H. Casto Neto et al. Rev. Mod. Phys. \textbf{81},109(2009); A. Rycerz et al. Nature Phys.\textbf{3},172-175(2007); \\[0pt] [2] G. Y. Wu et al. PRB \textbf{84},195463(2011); M. K. Lee PRB \textbf{86},165411(2011); G. Y. Wu et al. PRB\textbf{ 88},125422(2013) [Preview Abstract] |
Wednesday, March 5, 2014 10:36AM - 10:48AM |
L29.00014: Valley-Spin Polarization in the Magneto-Optical Conductivity of Silicene and Other Buckled Honeycomb Lattices Calvin Tabert, Elisabeth Nicol The successful isolation of graphene made the field of two-dimensional (2D) crystals a reality. Recently, increasing attention has begun to focus on other 2D honeycomb systems such as those that map onto a low-energy Kane-Mele type Hamiltonian. These systems have an intrinsic band gap due to spin-orbit coupling. One such material is silicene, the silicon equivalent of graphene. Here, the silicon atoms form a buckled honeycomb lattice. This vertical buckling creates the possibility for a tunable band gap when an electric field is applied. As the electric field is varied, the system is predicted to transition between a topological insulator and a band insulator[1,2]. We show[3,4] that when this system is subjected to a magnetic field, we retain a Landau level (LL) spectrum similar to that of gapped graphene; however, the application of an electric field spin-splits the LLs at a given valley. By varying the electric field strength, one can elucidate signatures of the two insulating regimes. It is also possible to optically excite charge carriers of definite valley-spin polarization. [1]N.D. Drummond et al., PRB 85, 075423 (2012) [2]M. Ezawa, NJP 14, 033003 (2012) [3]C.J. Tabert and E.J. Nicol, PRL 110, 197402 (2013) [4]C.J. Tabert and E.J. Nicol PRB 88, 085434 (2013) [Preview Abstract] |
Wednesday, March 5, 2014 10:48AM - 11:00AM |
L29.00015: Disordered Dirac Fermions in Topological Superconductors and Artificial Graphene: Level Statistics, Chalker Scaling, and Quantum Hall Metal Yang-Zhi Chou, Matthew Foster We study a single Dirac fermion in 2D, subject to static vector potential disorder. This model describes the minimal surface state of a topological superconductor with time reversal and spin U(1) symmetries. Most of the zero-energy attributes are already known; in particular, the model is critically delocalized with multifractal wavefunctions. We numerically investigate various properties, especially those related to the finite energy states. The energy levels in the vicinity of zero energy (chiral point) show universal statistics while the multifractal spectra of the zero-energy wavefunction and the dynamical critical exponent reveal non-analyticity at the freezing transition. The two-wavefunction correlations in the chiral region show quantum critical scaling, even in the case of strong disorder. Moreover, we confirm that the finite energy states are delocalized, and their multifractal spectra are consistent with the integer quantum Hall plateau transition. Our model can also be possibly realized in the artificial materials like molecular graphene. [Preview Abstract] |
Follow Us |
Engage
Become an APS Member |
My APS
Renew Membership |
Information for |
About APSThe American Physical Society (APS) is a non-profit membership organization working to advance the knowledge of physics. |
© 2024 American Physical Society
| All rights reserved | Terms of Use
| Contact Us
Headquarters
1 Physics Ellipse, College Park, MD 20740-3844
(301) 209-3200
Editorial Office
100 Motor Pkwy, Suite 110, Hauppauge, NY 11788
(631) 591-4000
Office of Public Affairs
529 14th St NW, Suite 1050, Washington, D.C. 20045-2001
(202) 662-8700