Results for "Allan H. MacDonald"
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Gapped broken symmetry states in ABC trilayer grapheneAug 01 2012We use a self-consistent Hartree-Fock approximation with realistic Coulomb interactions for $\pi$-band electrons to explore the possibility of broken symmetry states in weakly disordered ABC stacked trilayer graphene. The competition between gapped and ... More SU(3) and SU(4) singlet quantum Hall states at $ν=2/3$Jun 08 2015Sep 28 2015We report on an exact diagonalization study of fractional quantum Hall states at filling factor $\nu=2/3$ in a system with a four-fold degenerate $n$=0 Landau level and SU(4) symmetric Coulomb interactions. Our investigation reveals previously unidentified ... More Electronic Highways in Bilayer GrapheneJul 22 2011Bilayer 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 ... More Strong interface-induced spin-orbit coupling in graphene on WS2Aug 12 2015Interfacial interactions allow the electronic properties of graphene to be modified, as recently demonstrated by the appearance of satellite Dirac cones in the band structure of graphene on hexagonal boron nitride (hBN) substrates. Ongoing research strives ... More Quantum Wells in Polar-Nonpolar Oxide Heterojunction SystemsOct 05 2008May 19 2009We 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 ... More Trion Valley Coherence in Monolayer SemiconductorsNov 10 2016The emerging field of valleytronics aims to exploit the valley pseudospin of electrons residing near Bloch band extrema as an information carrier. Recent experiments demonstrating optical generation and manipulation of exciton valley coherence (the superposition ... More Effective screening and the plasmaron bands in GrapheneJul 21 2011Electron-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 ... More Coulomb Gaps in a Strong Magnetic Field, S.-RApr 08 1993We report on a study of interaction effects in the tunneling density-of-states of a disordered two-dimensional electron gas in the strong magnetic field limit where only the lowest Landau level is occupied. Interactions in the presence of disorder are ... More Transport Between Twisted Graphene LayersFeb 16 2010Commensurate-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 ... More Edge State Tunneling in a Split Hall Bar ModelJan 31 2005Feb 01 2007In this paper we introduce and study the correlation functions of a chiral one-dimensional electron model intended to qualitatively represent narrow Hall bars separated into left and right sections by a penetrable barrier. The model has two parameters ... More Theory of Native Orientational Pinning in Quantum Hall NematicsJul 21 2013The orientation of the electron-nematic states discovered in the quantum Hall regime of GaAs $[001]$ growth-direction quantum wells is pinned by a weak native source of anisotropy. In this Letter we explain that this property, which has remained mysterious ... More Exciton band structure of monolayer MoS2Jan 09 2015Feb 16 2015We address the properties of excitons in monolayer MoS$_2$ from a theoretical point of view, showing that low-energy excitonic states occur both at the Brillouin zone center and at the Brillouin-zone corners, that binding energies at the Brillouin-zone ... More ARPES, Neutrons, and the High-$T_c$ MechanismApr 02 2008Jul 26 2008Extensive ARPES and low-energy inelastic neutron scattering studies of cuprate superconductors can be successfully described using a weak-coupling theory in which quasiparticles on a square lattice interact via scalar and spin-dependent effective interactions. ... More Magnetic Oscillations of a Fractional Hall DotNov 30 1992We show that a quantum dot in the fractional Hall regime exhibits mesoscopic magnetic oscillations with a period which is a multiple of the period for free electrons. Our calculations are performed for parabolic quantum dots with hard-core electron-electron ... More Excitation of g modes in Wolf-Rayet stars by a deep opacity bumpJan 08 2006Feb 15 2006We examine the stability of l=1 and l=2 g modes in a pair of nitrogen-rich Wolf-Rayet stellar models characterized by differing hydrogen abundances. We find that modes with intermediate radial orders are destabilized by a kappa mechanism operating on ... More Anomalous Hall effect in ferromagnetic semiconductorsOct 22 2001We present a theory of the anomalous Hall effect in ferromagnetic (Mn,III)V semiconductors. Our theory relates the anomalous Hall conductance of a homogeneous ferromagnet to the Berry phase acquired by a quasiparticle wavefunction upon traversing closed ... More Hund's Rules for the N=0 Landau Levels of Trilayer GrapheneJan 26 2012The 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 ... More Persistent Current States in Bilayer GrapheneNov 07 2011Mar 02 2015We argue that at finite carrier density and large displacement fields, bilayer graphene is prone to $\ell =0$ and $\ell = 1$ Pomeranchuk Fermi surface instabilities. The broken symmetries are driven by non-local exchange interactions which favor momentum ... More Theory of spin waves in diluted-magnetic-semiconductor quantum wellsDec 14 2003Mar 19 2004We present a theory of collective spin excitations in diluted-magnetic-semiconductor quantum wells in which local magnetic moments are coupled via a quasi-two-dimensional gas of electrons or holes. In the case of a ferromagnetic state with partly spin-polarized ... More Tunneling current characteristics in bilayer quantum Hall systemsJul 01 2001Aug 11 2001Weakly disordered bilayer quantum Hall systems at filling factor $\nu=1$ show spontaneous interlayer phase coherence if the layers are sufficiently close together. We study the collective modes in the system, the current-voltage characteristics and their ... More Zero-bias conductance anomaly in bilayer quantum Hall systemsFeb 03 2002Mar 25 2002Bilayer quantum Hall system at total filling factor $\nu=1$ shows a rich variety of broken symmetry ground states because of the competition between the interlayer and intralayer Coulomb interactions. When the layers are sufficiently close, a bilayer ... More Pseudospin Order in Monolayer, Bilayer, and Double-Layer GrapheneSep 01 2011Jan 18 2012Graphene 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 ... More Quantum Hall Superfluids in Topological Insulator Thin FilmsJul 14 2011Three-dimensional topological insulators have protected Dirac-cone surface states. In this paper we propose magnetic field induced topological insulator thin film ordered states in which coherence is established spontaneously between top and bottom surfaces. ... More Fractional-quantum-Hall edges at filling factor 1-1/mMay 20 1999We consider the edge of a two-dimensional electron system that is in the quantum-Hall-effect regime at filling factor 1-1/m with m being an odd integer, where microscopic theory explaining the occurrence of the quantum Hall effect in the bulk predicts ... More Addition Spectra of Quantum Dots in Strong Magnetic FieldsJun 03 1993We consider the magnetic field dependence of the chemical potential for parabolically confined quantum dots in a strong magnetic field. Approximate expressions based on the notion that the size of a dot is determined by a competition between confinement ... More Quantum-Hall Quantum-BitsOct 08 2001Aug 02 2002Bilayer quantum Hall systems can form collective states in which electrons exhibit spontaneous interlayer phase coherence. We discuss the possibility of using bilayer quantum dot many-electron states with this property to create two-level systems that ... More Von Neumann Algebras and Extensions of Inverse SemigroupsSep 04 2014Nov 26 2014In the 1970s, Feldman and Moore classified separably acting von Neumann algebras containing Cartan MASAs using measured equivalence relations and 2-cocycles on such equivalence relations. In this paper, we give a new classification in terms of extensions ... More Hartree-Fock Theory of Hole Stripe StatesMay 23 2000Jan 30 2001We report on Hartree-Fock theory results for stripe states of two-dimensional hole systems in quantum wells grown on GaAs (311)A substrates. We find that the stripe orientation energy has a rich dependence on hole density, and on in-plane field magnitude ... More Ferromagnetism in (III,Mn)V SemiconductorsNov 16 2001In this review article we briefly summarize the main experimental properties of (III,Mn)V ferromagnets and outline several different but related approaches that have been explored in an effort to gain insight into these materials. The main body of the ... More Valley-Hall Kink and Edge States in Multilayer GrapheneMay 18 2011We 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 ... More Half-metallic magnetism and the search for better spin valvesJul 20 2014We use a previously proposed theory for the temperature dependence of tunneling magnetoresistance to shed light on ongoing efforts to optimize spin valves. First we show that a mechanism in which spin valve performance at finite temperatures is limited ... More THz conductivity of graphene on boron nitrideJul 07 2015The conductivity of graphene on a boron nitride substrate exhibits features in the terahertz (THz) and infrared (IR) frequency regimes that are associated with the periodic moir\'e pattern formed by the weakly coupled two-dimensional materials. The THz ... More Dissipationless Spin Transport in Thin Film FerromagnetsNov 29 2000Oct 11 2001Metallic thin film ferromagnets generically possess spiral states that carry dissipationless spin currents. We relate the critical values of these supercurrents to micromagnetic material parameters, identify the circumstances under which the supercurrents ... More Transition-metal dimers and physical limits on magnetic anisotropyMar 23 2007Oct 24 2007Recent advances in nanoscience have raised interest in the minimum bit size required for classical information storage, i.e. for bistability with suppressed quantum tunnelling and energy barriers that exceed ambient temperatures. In the case of magnetic ... More Non-Local Transport Mediated by Spin-SupercurrentsAug 05 2014In thin film ferromagnets with perfect easy-plane anisotropy, the component of total spin perpendicular to the easy plane is a good quantum number and the corresponding spin supercurrent can flow without dissipation. In this Letter we explain how spin ... More Pseudospin Transfer Torques in Semiconductor Electron BilayersJan 26 2012Jan 27 2012We 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 ... More Band Structure of ABC-Stacked Graphene TrilayersApr 09 2010The 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 ... More Theory of Ferromagnetism in Diluted Magnetic SemiconductorsOct 29 2000Carrier-induced ferromagnetism has been observed in several (III,Mn)V semiconductors. We review the theoretical picture of these ferromagnetic semiconductors that emerges from a model with kinetic-exchange coupling between localized Mn spins and valence-band ... More Theory of Diluted Magnetic Semiconductor FerromagnetismJan 21 2000May 31 2000We present a theory of carrier-induced ferromagnetism in diluted magnetic semiconductors (III_{1-x} Mn_x V) which allows for arbitrary itinerant-carrier spin polarization and dynamic correlations. Both ingredients are essential in identifying the system's ... More Orbital and spin order in oxide two-dimensional electron gasesAug 11 2016We describe a variational theory of multi-band two-dimensional electron gases that captures the interplay between electrostatic confining potentials, orbital-dependent interlayer electronic hopping and electron-electron interactions, and apply it to the ... More Origin of band gaps in graphene on hexagonal boron nitrideMar 03 2014Jul 27 2014Recent progress in preparing well controlled 2D van der Waals heterojunctions has opened up a new frontier in materials physics. In this paper we address the intriguing energy gaps that are sometimes observed when a graphene sheet is placed on a hexagonal ... More Integer Quantum Hall Effect in Double-Layer SystemsJun 03 1996Jun 13 1996We consider the localization of independent electron orbitals in double-layer two-dimensional electron systems in the strong magnetic field limit. Our study is based on numerical Thouless number calculations for realistic microscopic models and on transfer ... More