arXiv subjects

Allan H. MacDonald

Publications and source records attributed to Allan H. MacDonald.

At least 199 records · Page 11Linked to original sources

arXiv2012

Transport Properties of Graphene Nanoroads in Boron-Nitride Sheets

We demonstrate that the one-dimensional (1D) transport channels that appear in the gap when graphene nanoroads are embedded in boron-nitride (BN) sheets are more robust when they are inserted at AB/BA grain boundaries. Our conclusions are based on ab-initio electronic structure calculations for a variety of different crystal orientations and bonding arrangements at the BN/C interfaces. This property is related to the valley-Hall conductivity present in the BN band structure and to the topologically protected kink states that appear in continuum Dirac models with position dependent masses.

cond-mat.mes-hall↗

arXiv2012

Pseudospin Transfer Torques in Semiconductor Electron Bilayers

We use self-consistent quantum transport theory to investigate the influence of electron-electron interactions on interlayer transport in semiconductor electron bilayers in the absence of an external magnetic field. We conclude that, even though spontaneous pseudospin order does not occur at zero field, interaction-enhanced quasiparticle tunneling amplitudes and pseudospin transfer torques do alter tunneling I-V characteristics, and can lead to time-dependent response to a dc bias voltage.

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arXiv2012

Hund's Rules for the N=0 Landau Levels of Trilayer Graphene

The N=0 Landau levels of ABC and ABA trilayer graphene both have approximate 12-fold degeneracies that are lifted by interactions to produce strong quantum Hall effects (QHE) at all integer filling factors between nu=-6 and nu=6. We discuss similarities and differences between the strong-magnetic-field weak-disorder physics of the two trilayer cases, and between trilayer and bilayer cases. These differences can be used to identify the stacking order of high-quality trilayer samples by studying their quantum Hall effects.

cond-mat.str-el↗

arXiv2012

Pseudospin Order in Monolayer, Bilayer, and Double-Layer Graphene

Graphene is a gapless semiconductor in which conduction and valence band wavefunctions differ only in the phase difference between their projections onto the two sublattices of the material's two-dimensional honeycomb crystal structure. We explain why this circumstance creates openings for broken symmetry states, including antiferromagnetic states in monolayer and bilayer graphene and exciton condensates in double-layer graphene, that are momentum space analogs of the real-space order common in systems with strong local interactions. We discuss some similarities among, and some differences between, these three broken symmetry states.

cond-mat.mes-hall↗

arXiv2011

Distinguishing Spontaneous Quantum Hall States in Graphene Bilayers

Chirally stacked N-layer graphene with N>=2 is susceptible to a variety of distinct broken symmetry states in which each spin-valley flavor spontaneously transfers charge between layers. In mean-field theory the neutral bilayer ground state is a layer antiferromagnet (LAF) state that has opposite spin-polarizations in opposite layers. In this Letter we analyze how the LAF and other competing states are influenced by Zeeman fields that couple to spin and by interlayer electric fields that couple to layer pseudospin, and comment on the possibility of using response and edge state signatures to identify the character of the bilayer ground state experimentally.

cond-mat.str-el↗

arXiv2011

Electronic Highways in Bilayer Graphene

Bilayer graphene with an interlayer potential difference has an energy gap and, when the potential difference varies spatially, topologically protected one-dimensional states localized along the difference's zero-lines. When disorder is absent, electronic travel directions along zero-line trajectories are fixed by valley Hall properties. Using the Landauer-Büttiker formula and the non-equilibrium Green's function technique we demonstrate numerically that collisions between electrons traveling in opposite directions, due to either disorder or changes in path direction, are strongly suppressed. We find that extremely long mean free paths of the order of hundreds of microns can be expected in relatively clean samples. This finding suggests the possibility of designing low power nanoscale electronic devices in which transport paths are controlled by gates which alter the inter-layer potential landscape.

cond-mat.mes-hall↗

arXiv2011

Effective screening and the plasmaron bands in Graphene

Electron-plasmon coupling in graphene has recently been shown to give rise to a "plasmaron" quasiparticle excitation. The strength of this coupling has been predicted to depend on the effective screening, which in turn is expected to depend on the dielectric environment of the graphene sheet. Here we compare the strength of enviromental screening for graphene on four different substrates by evaluating the separation of the plasmaron bands from the hole bands using Angle Resolved PhotoEmission Spectroscopy. Comparison with G0W-RPA predictions are used to determine the effective dielectric constant of the underlying substrate layer. We also show that plasmaron and electronic properties of graphene can be independently manipulated, an important aspect of a possible use in "plasmaronic" devices.

cond-mat.str-el↗

arXiv2011

Valley-Hall Kink and Edge States in Multilayer Graphene

We report on a theoretical study of one-dimensional (1D) states localized at few-layer graphene system ribbon edges, and at interfaces between few-layer graphene systems with different valley Hall conductivities. These 1D states are topologically protected when valley mixing is neglected. We address the influence on their properties of stacking arrangement, interface structure, and external electric field perpendicular to the layers. We find that 1D states are generally absent at multilayer ribbon armchair direction edges, but present irrespective of crystallographic orientation at any internal valley-Hall interface of an ABC stacked multilayer.

cond-mat.mes-hall↗

arXiv2011

Enhancement of non-local exchange near isolated band-crossings in graphene

The physics of non-local exchange interactions in graphene sheets is studied within a $π$-orbital tight-binding model using a Hartree-Fock approximation and Coulomb interactions modified at short distances by lattice effects and at large distances by dielectric screening. We use this study to comment on the strong non-locality of exchange effects in systems with isolated band-crossings at energies close to the Fermi energy. We also discuss the role of lattice scale details of the effective Coulomb interaction in determining whether or not broken symmetry states appear at strong interaction strengths, and in determining the character of those states when they do appear.

cond-mat.mes-hall↗

arXiv2010

Lattice Theory of Pseudospin Ferromagnetism in Bilayer Graphene: Competing Orders and Interaction Induced Quantum Hall States

In mean-field-theory bilayer graphene's massive Dirac fermion model has a family of broken inversion symmetry ground states with charge gaps and flavor dependent spontaneous inter layer charge transfers. We use a lattice Hartree-Fock model to explore some of the physics which controls whether or not this type of broken symmetry state, which can be viewed as a pseudospin ferromagnet, occurs in nature. We find that inversion symmetry is still broken in the lattice model and estimate that transferred areal densities are $\sim 10^{-5}$ electrons per carbon atom, that the associated energy gaps are $\sim 10^{-2} {\rm eV}$, that the ordering condensation energies are $\sim 10^{-7} eV$ per carbon atom, and that the energy differences between competing orders at the neutrality point are $\sim 10^{-9} eV$ per carbon atom. We explore the quantum phase transitions induced by external magnetic fields and by externally controlled electric potential differences between the layers. We find, in particular, that in an external magnetic field coupling to spontaneous orbital moments favors broken time-reversal-symmetry states that have spontaneous quantized anomalous Hall effects. Our theory predicts a non monotonic behavior of the band gap at neutrality as a function of interlayer potential difference in qualitative agreement with recent experiments.

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arXiv2010

Spontaneous Quantum Hall States in Chirally-stacked Few-Layer Graphene Systems

Chirally stacked N-layer graphene systems with N >= 2 exhibit a variety of distinct broken symmetry states in which charge density contributions from different spins and valleys are spontaneously transferred between layers. We explain how these states are distinguished by their charge, spin, and valley Hall conductivities, by their orbital magnetizations, and by their edge state properties. We argue that valley Hall states have [N/2] edge channels per spin-valley.

cond-mat.str-el↗

arXiv2010

Band Structure of ABC-Stacked Graphene Trilayers

The ABC-stacked N-layer-graphene family of two-dimensional electron systems is described at low energies by two remarkably flat bands with Bloch states that have strongly momentum-dependent phase differences between carbon pi-orbital amplitudes on different layers, and large associated momentum space Berry phases. These properties are most easily understood using a simplified model with only nearest-neighbor inter-layer hopping which leads to gapless semiconductor electronic structure, with p^N dispersion in both conduction and valence bands. We report on a study of the electronic band structures of trilayers which uses ab initio density functional theory and k*p theory to fit the parameters of a pi-band tight-binding model. We find that when remote interlayer hopping is retained, the triple Dirac point of the simplified model is split into three single Dirac points located along the three KM directions. External potential differences between top and bottom layers are strongly screened by charge transfer within the trilayer, but still open an energy gap at overall neutrality.

cond-mat.mtrl-sci↗

arXiv2010

Transport Between Twisted Graphene Layers

Commensurate-incommensurate transitions are ubiquitous in physics and are often accompanied by intriguing phenomena. In few-layer graphene (FLG) systems, commensurability between honeycomb lattices on adjacent layers is regulated by their relative orientation angle theta, which is in turn dependent on sample preparation procedures. Because incommensurability suppresses inter-layer hybridization, it is often claimed that graphene layers can be electrically isolated by a relative twist, even though they are vertically separated by a fraction of a nanometer. We present a theory of interlayer transport in FLG systems which reveals a richer picture in which the specific conductance depends sensitively on theta, single-layer Bloch state lifetime, in-plane magnetic field, and bias voltage. We find that linear and differential conductances are generally large and negative near commensurate values of theta, and small and positive otherwise.

cond-mat.mes-hall↗

arXiv2010

Critical Tunneling Currents in Quantum Hall Superfluids: Pseudospin-Transfer Torque Theory

At total filling factor $ν=1$ quantum Hall bilayers can have an ordered ground state with spontaneous interlayer phase coherence. The ordered state is signaled experimentally by dramatically enhanced interlayer tunnel conductances at low bias voltages; at larger bias voltages inter-layer currents are similar to those of the disordered state. We associate this change in behavior with the existence of a critical current beyond which static inter-layer phase differences cannot be maintained, and examine the dependence of this critical current on sample geometry, phase stiffness, and the coherent tunneling energy density. Our analysis is based in part on analogies between coherent bilayer behavior and spin-transfer torque physics in metallic ferromagnets. Comparison with recent experiments suggests that disorder can dramatically suppress critical currents.

cond-mat.mes-hall↗

arXiv2009

Renormalized Landau Levels and Particle-Hole Symmetry in Graphene

In this proceedings paper we report on a calculation of graphene's Landau levels in a magnetic field. Our calculations are based on a self-consistent Hartree-Fock approximation for graphene's massless-Dirac continuum model. We find that because of graphene's chiral band structure interactions not only shift Landau-level energies, as in a non-relativistic electron gas, but also alter Landau level wavefunctions. We comment on the subtle continuum model regularization procedure necessary to correctly maintain the lattice-model's particle hole symmetry properties.

cond-mat.mes-hall↗

arXiv2009

Influence of a Transport Current on Magnetic Anisotropy in Gyrotropic Ferromagnets

Current-induced torques are commonly used to manipulate non-collinear magnetization configurations. In this article we discuss current-induced torques present in a certain class of collinear magnetic systems, relating them to current-induced changes in magnetic anisotropy energy. We present a quantitative estimate of their characteristics in uniform strained ferromagnetic (Ga,Mn)As.

cond-mat.mtrl-sci↗

arXiv2009

Quantum Wells in Polar-Nonpolar Oxide Heterojunction Systems

We address the electronic structure of quantum wells in polar-nonpolar oxide heterojunction systems focusing on the case of non-polar BaVO$_3$ wells surrounded by polar LaTiO$_3$ barriers. Our discussion is based on a density functional description using the local spin density approximation with local correlation corrections (LSDA+U). We conclude that a variety of quite different two-dimensional electron systems can occur at interfaces between insulating materials depending on band line-ups and on the geometrical arrangement of polarity discontinuities.

cond-mat.str-el↗

arXiv2008

Gilbert Damping in Conducting Ferromagnets I: Kohn-Sham Theory and Atomic-Scale Inhomogeneity

We derive an approximate expression for the Gilbert damping coefficient α_G of itinerant electron ferromagnets which is based on their description in terms of spin-density-functional-theory (SDFT) and Kohn-Sham quasiparticle orbitals. We argue for an expression in which the coupling of magnetization fluctuations to particle-hole transitions is weighted by the spin-dependent part of the theory's exchange-correlation potential, a quantity which has large spatial variations on an atomic length scale. Our SDFT result for α_G is closely related to the previously proposed spin-torque correlation-function expression.

cond-mat.mes-hall↗