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
Accurate tight-binding and continuum models for the $π$ bands of bilayer grapheneSep 21 2013We derive an {\em ab initio} $\pi$-band tight-binding model for $AB$ stacked bilayer graphene based on maximally localized Wannier wave functions (MLWFs) centered on the carbon sites, finding that both intralayer and interlayer hopping is longer in range ... More
Enhancement of non-local exchange near isolated band-crossings in grapheneMay 03 2011The physics of non-local exchange interactions in graphene sheets is studied within a $\pi$-orbital tight-binding model using a Hartree-Fock approximation and Coulomb interactions modified at short distances by lattice effects and at large distances by ... More
Critical Tunneling Currents in Quantum Hall Superfluids: Pseudospin-Transfer Torque TheoryJan 18 2010At total filling factor $\nu=1$ 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; ... More
Many-Body Theory of Trion Absorption Features in Two-Dimensional SemiconductorsSep 20 2016Recent optical studies of monolayer transition metal dechalcogenides have demonstrated that their excitonic absorption feature splits into two widely separated peaks at finite carrier densities. The additional peak is usually attributed to the presence ... More
Spatially-indirect Exciton Condensate Phases in Double Bilayer GrapheneNov 19 2016We present a theory of spatially indirect exciton condensate states in systems composed of a pair of electrically isolated Bernal graphene bilayers. The ground state phase diagram in a two-dimensional displacement-field/inter-bilayer-bias space includes ... 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
Effect of spin fluctuations on quasiparticle excitations: first-principles theory and application to sodium and lithiumFeb 14 2014We present first-principles calculations for quasiparticle excitations in sodium and lithium including the effects of charge and spin fluctuations. We employ the Overhauser-Kukkonen form for the electron self energy arising from spin fluctuations and ... 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
Quantum transport simulation of exciton condensate transport physics in a double layer graphene systemSep 09 2015Spatially indirect electron-hole exciton condensates stabilized by interlayer Fock exchange interactions have been predicted in systems containing a pair of two-dimensional semiconductor or semimetal layers separated by a thin tunnel dielectric. The layer ... More
Anatomy of the magnetic anisotropy energy mediated by tight-binding Rashba electronsJan 31 2018The magnetic anisotropy is a fundamental quantity, defining the orientational stability of the magnetic state. Due to its importance, many different approaches have been put forth to explain its properties and behavior, ranging from simple models to fully ... More
Enhanced spin Seebeck effect signal due to spin-momentum locked topological surface statesMar 31 2016Spin-momentum locking in protected surface states enables efficient electrical detection of magnon decay at a magnetic-insulator/topological-insulator heterojunction. Here we demonstrate this property using the spin Seebeck effect, i.e. measuring the ... More
Observation of a Nematic Quantum Hall Liquid on the Surface of BismuthOct 24 2016Nematic quantum fluids with wavefunctions that break the underlying crystalline symmetry can form in interacting electronic systems. We examine the quantum Hall states that arise in high magnetic fields from anisotropic hole pockets on the Bi(111) surface. ... More
Emergence of Topologically Protected Helical States in Minimally Twisted Bilayer GrapheneFeb 08 2018Bilayer graphene samples in which inversion symmetry is broken have quantum valley Hall ground states that support counterpropogating topologically protected helical (TPH) edge states localized along domain walls between AB and BA stacking regions. Moreover, ... 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
Tunable $Γ- K$ Valley Populations in Hole-Doped Trilayer WSe$_2$Jan 10 2018We present a combined experimental and theoretical study of valley populations in the valence bands of trilayer WSe$_2$. Shubnikov$-$de Haas oscillations show that trilayer holes populate two distinct subbands associated with the $K$ and $\Gamma$ valleys, ... 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
Revealing the electronic band structure of trilayer graphene on SiC: An angle-resolved photoemission studyFeb 28 2014In recent times, trilayer graphene has attracted wide attention owing to its stacking and electric field dependent electronic properties. However, a direct and well-resolved experimental visualization of its band structure has not yet been reported. In ... More
Resistance spikes and domain wall loops in Ising quantum Hall ferromagnetsApr 18 2001We explain the recent observation of resistance spikes and hysteretic transport properties in Ising quantum Hall ferromagnets in terms of the unique physics of their domain walls. Self-consistent RPA/Hartree-Fock theory is applied to microscopically determine ... More
Pseudospin Anisotropy Classification of Quantum Hall FerromagnetsMar 27 2000Broken symmetry ground states with uniform electron density are common in quantum Hall systems when two Landau levels simultaneously approach the chemical potential at integer filling factor $\nu$. The close analogy between these two-dimensional electron ... More
EPR and ferromagnetism in diluted magnetic semiconductor quantum wellsJun 25 2003Aug 22 2003Motivated by recent measurements of electron paramagnetic resonance (EPR) spectra in modulation-doped CdMnTe quantum wells, [F.J. Teran {\it et al.}, Phys. Rev. Lett. {\bf 91}, 077201 (2003)], we develop a theory of collective spin excitations in quasi-two-dimensional ... More
Influence of a Transport Current on Magnetic Anisotropy in Gyrotropic FerromagnetsMay 23 2009Current-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 ... More
The Quantum Hall Effect of Interacting Electrons in a Periodic PotentialOct 01 1996Oct 15 1996We consider the influence of an external periodic potential on the fractional quantum Hall effect of two-dimensional interacting electron systems. For many electrons on a torus, we find that the splitting of incompressible ground state degeneracies by ... More
Gilbert Damping in Conducting Ferromagnets I: Kohn-Sham Theory and Atomic-Scale InhomogeneityAug 09 2008We derive an approximate expression for the Gilbert damping coefficient \alpha_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 ... 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
Interactions suppress Quasiparticle Tunneling at Hall Bar ConstrictionsMar 10 2004Tunneling of fractionally charged quasiparticles across a two-dimensional electron system on a fractional quantum Hall plateau is expected to be strongly enhanced at low temperatures. This theoretical prediction is at odds with recent experimental studies ... 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
Spin-Superfluidity and Spin-Current Mediated Non-Local TransportApr 08 2016Some strategies for reducing energy consumption in information processing devices involve the use of spin rather than charge to carry information. This idea is especially attractive when the spin current is a collective one carried by the condensate of ... 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
Distinguishing Spontaneous Quantum Hall States in Graphene BilayersJul 24 2011Chirally 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 ... More
Broken SU(4) Symmetry and The Fractional Quantum Hall Effect in GrapheneOct 06 2013Mar 28 2014We construct a variational description for incompressible ground states and charge gaps in the N = 0 LL of graphene which accounts for the 4-fold Landau level degeneracy and for the short range interactions that break the SU(4) spin-valley invariance. ... 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
Ferromagnetism in Diluted Magnetic Semiconductor Heterojunction SystemsMar 05 2002Diluted magnetic semiconductors (DMSs), in which magnetic elements are substituted for a small fraction of host elements in a semiconductor lattice, can become ferromagnetic when doped. In this article we discuss the physics of DMS ferromagnetism in systems ... More
A molecular line survey of the candidate massive Class 0 prototostar IRAS 23385+6053May 27 2003Jun 13 2003We have carried out a molecular line survey of the candidate massive protostar IRAS 23385+6053, covering a 27.2 GHz frequency range in the 330--360 GHz atmospheric window. We detected 27 lines originating from a total of 11 species.Over a third of the ... 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
Field Effect Magnetization Reversal in Ferromagnetic Semiconductor Quantum WellsJun 26 2001We predict that a novel bias-voltage assisted magnetization reversal process will occur in Mn doped II-VI semiconductor quantum wells or heterojunctions with carrier induced ferromagnetism. The effect is due to strong exchange-coupling induced subband ... More
Electronic Interface Reconstruction at Polar-Nonpolar Mott Insulator HeterojunctionsMay 24 2007Aug 22 2007We report on a theoretical study of the electronic interface reconstruction (EIR) induced by polarity discontinuity at a heterojunction between a polar and a nonpolar Mott insulators, and of the two-dimensional strongly-correlated electron systems (2DSCESs) ... 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
Theory of Magnetic Properties and Spin-Wave Dispersion for Ferromagnetic (Ga,Mn)AsMar 05 2001Oct 18 2001We present a microscopic theory of the long-wavelength magnetic properties of the ferromagnetic diluted magnetic semiconductor (Ga,Mn)As. Details of the host semiconductor band structure, described by a six-band Kohn-Luttinger Hamiltonian, are taken into ... More
Broken Symmetry Ground States in ν=2 Bilayer Quantum Hall SystemsMar 20 1999We report on a study of a bilayer two-dimensional electron gas at Landau level filling factor $\nu=2$. The system exhibits both magnetic and spontaneous interlayer phase coherence broken symmetries. We propose a 3-parameter Slater determinant variational ... 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
Lattice Theory of Pseudospin Ferromagnetism in Bilayer Graphene: Competing Orders and Interaction Induced Quantum Hall StatesOct 09 2010Nov 29 2010In 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 ... More
Weak Localization, Spin Relaxation, and Spin-Diffusion: The Crossover Between Weak and Strong Rashba Coupling LimitsJun 10 2014Jun 12 2014Disorder scattering and spin-orbit coupling are together responsible for the diffusion and relaxation of spin-density in time-reversal invariant systems. We study spin-relaxation and diffusion in a two-dimensional electron gas with Rashba spin-orbit coupling ... More
Comment on ``Coexistence of superconductivity and ferromagnetism in ferromagnetic metals"Oct 29 2003We argue that a single-band itinerant electron model with short-range interactions, proposed by Karchev et al. (cond-mat/9911489) and investigated further by Jackiewicz et al. (cond-mat/0302449), cannot describe the coexistence of superconducting and ... More
Order parameter suppression in double layer quantum Hall ferromagnetsSep 03 1999Double-layer quantum Hall systems at Landau level filling factor $\nu=1$ have a broken symmetry ground state with spontaneous interlayer phase coherence and a gap between symmetric and antisymmetric subbands in the absence of interlayer tunneling. We ... More
Microscopic Functional Integral Theory of Quantum Fluctuations in Double-Layer Quantum Hall FerromagnetsMay 31 2001We present a microscopic theory of zero-temperature order parameter and pseudospin stiffness reduction due to quantum fluctuations in the ground state of double-layer quantum Hall ferromagnets. Collective excitations in this systems are properly described ... 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
Is there a d.c. Josephson Effect in Bilayer Quantum Hall Systems?Mar 22 2001We argue on the basis of phenomenological and microscopic considerations that there is no d.c. Josephson effect in ordered bilayer quantum Hall systems, even at T=0. Instead the tunnel conductance is strongly enhanced, approaching a finite value proportional ... 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
Bias-voltage induced phase-transition in bilayer quantum Hall ferromagnetsNov 04 2001Apr 10 2002We consider bilayer quantum Hall systems at total filling factor $\nu=1$ in presence of a bias voltage $\Delta_v$ which leads to different filling factors in each layer. We use auxiliary field functional integral approach to study mean-field solutions ... More
Generalized Random-Phase Approximation Theory of Quasiparticle Spectral Functions: Application to Bilayer Quantum Hall FerromagnetsAug 04 2001We present a microscopic theory of ground-state spectral function of bilayer quantum Hall systems that includes interactions between Hartree-Fock quasiparticles and quantum fluctuations of the order parameter field. The collective modes in these systems ... 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
Intrinsic Curie temperature bistability in ferromagnetic semiconductor resonant tunneling diodesMay 04 2005May 05 2005We predict bistability in the Curie temperature-voltage characteristic of double barrier resonant-tunneling structures with dilute ferromagnetic semiconductor quantum wells. Our conclusions are based on simulations of electrostatics and ballistic quantum ... 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
Observability of counterpropagating modes at fractional-quantum-Hall edgesJan 17 1998Aug 04 1998When the bulk filling factor is equal to 1 - 1/m with m odd, at least one counterpropagating chiral collective mode occurs simultaneously with magnetoplasmons at the edge of fractional-quantum-Hall samples. Initial experimental searches for an additional ... 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
Bias voltage controlled magnetization switch in ferromagnetic semiconductor resonant tunneling diodesSep 05 2004Oct 25 2004We predict that the Curie temperature of a ferromagnetic resonant tunneling diode will decrease abruptly, by approximately a factor of two, when the downstream chemical potential falls below the quantum well resonance energy. This property follows from ... 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
Quantum Dots in Strong Magnetic Fields: Stability Criteria for the Maximum Density DropletJan 15 1993In this article we discuss the ground state of a parabolically confined quantum dots in the limit of very strong magnetic fields where the electron system is completely spin-polarized and all electrons are in the lowest Landau level. Without electron-electron ... More
Thermodynamic and Tunneling Density of States of the Integer Quantum Hall Critical StateSep 26 2001Dec 14 2001We examine the long wave length limit of the self-consistent Hartree-Fock approximation irreducible static density-density response function by evaluating the charge induced by an external charge. Our results are consistent with the compressibility sum ... More
Theory of interlayer tunneling in bi-layer quantum Hall ferromagnetsJun 28 2000Spielman et al. have recently observed a large zero-bias peak in the tunnel conductance of a bi-layer system in a quantum Hall ferromagnet state. We argue that disorder-induced topological defects in the pseudospin order parameter limit the peak size ... More
Infrared magneto-optical properties of (III,Mn)V ferromagetic semiconductorsJan 22 2003We present a theoretical study of the infrared magneto-optical properties of ferromagnetic (III,Mn)V semiconductors. Our analysis combines the kinetic exchange model for (III,Mn)V ferromagnetism with Kubo linear response theory and Born approximation ... 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
Drude weight, cyclotron resonance, and the Dicke model of graphene cavity QEDJul 30 2012The Dicke model of cavity quantum electrodynamics is approximately realized in condensed matter when the cyclotron transition of a two-dimensional electron gas is nearly resonant with a cavity photon mode. We point out that in the strong coupling limit ... More
Ab-Initio Theory of Moiré Superlattice Bands in Layered Two-Dimensional MaterialsDec 30 2013When atomically thin two-dimensional (2D) materials are layered they often form incommensurate non-crystalline structures that exhibit long-period moir{\' e} patterns when examined by scanning probes. In this paper we present an approach which uses information ... 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
Noise spectroscopy and interlayer phase-coherence in bilayer quantum Hall systemsOct 29 2003Bilayer quantum Hall systems develop strong interlayer phase-coherence when the distance between layers is comparable to the typical distance between electrons within a layer. The phase-coherent state has until now been investigated primarily via transport ... 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
Transport and particle-hole asymmetry in graphene on boron nitrideMar 14 2015May 29 2015All local electronic properties of graphene on a hexagonal boron nitride (hBN) substrate exhibit spatial moir\'e patterns related to lattice constant and orientation differences between shared triangular Bravais lattices. We apply a previously derived ... More
Energy gaps, magnetism, and electric field effects in bilayer graphene nanoribbonsJan 13 2008Jul 02 2008Using a first principles density functional electronic structure method, we study the energy gaps and magnetism in bilayer graphene nanoribbons as a function of the ribbon width and the strength of an external electric field between the layers. We assume ... More
Interaction Correction to the Magneto-Electric Polarizability of $Z_{2}$ Topological InsulatorsAug 13 2015When time-reversal symmetry is weakly broken and interactions are neglected, the surface of a $Z_{2}$ topological insulator supports a half-quantized Hall conductivity $\sigma_{S} = e^{2}/(2h)$. A surface Hall conductivity in an insulator is equivalent ... 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
Limits on the Curie temperature of (III,Mn)V ferromagnetic semiconductorsOct 02 2000Jan 19 2001Mean-field-theory predicts that the Curie temperature T_c of a (III,Mn)V ferromagnet will be proportional to the valence band density-of-states of its host (III,V) semiconductor, suggesting a route toward room-temperature ferromagnetism in this materials ... More
Reply to Comment of Yang, Sun, and Chang on "Theory of Diluted Magnetic Semiconductor Ferromagnetism"Sep 28 2000We reply to the comment cond-mat/0008098 by Yang, Sun, and Chang on our paper "Theory of Diluted Magnetic Semiconductor Ferromagnetism", Phys. Rev. Lett. 84, 5628 (2000).
Electron-electron interactions in non-equilibrium bilayer grapheneDec 21 2012Conducting steady-states of doped bilayer graphene have a non-zero sublattice pseudospin polarization. Electron-electron interactions renormalize this polarization even at zero temperature, when the phase space for electron-electron scattering vanishes. ... 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
Transport Properties of Graphene Nanoroads in Boron-Nitride SheetsApr 23 2012We 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 ... 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
Renormalized Landau Levels and Particle-Hole Symmetry in GrapheneOct 05 2009In 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 ... More
Fractional Hofstadter States in Graphene on Hexagonal Boron NitrideMay 23 2016Jun 25 2016In fractionally filled Landau levels there is only a small energy difference between broken translational symmetry electron-crystal states and exotic correlated quantum fluid states. We show that the spatially periodic substrate interaction associated ... More
Extensive eigenvalues in spin-spin correlations: a tool for counting pure states in Ising spin glassesOct 19 2000We study the nature of the broken ergodicity in the low temperature phase of Ising spin glass systems, using as a diagnostic tool the spectrum of eigenvalues of the spin-spin correlation function. We show that multiple extensive eigenvalues of the correlation ... More
Gate-Tunable Exchange Coupling Between Cobalt Clusters on GrapheneJan 21 2013We use spin-density-functional theory (SDFT) ab initio calculations to theoretically explore the possibility of achieving useful gate control over exchange coupling between cobalt clusters placed on a graphene sheet. By applying an electric field across ... 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