Results for "Allan H. MacDonald"

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Vortex Lattice Structure and Topological Superconductivity in the Quantum Hall RegimeMar 28 2019Apr 15 2019Chiral topological superconductors are expected to appear as intermediate states when a quantum anomalous Hall system is proximity coupled to an s-wave superconductor and the magnetization direction is reversed. In this paper we address the edge state ... More
Electrical Reservoirs for Bilayer ExcitonsDec 07 2017Aug 08 2018The ground state of two-dimensional (2D) electron systems with equal low densities of electrons and holes in nearby layers is an exciton fluid. We show that a reservoir for excitons can be established by contacting the two layers separately and maintaining ... 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
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
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
Topological antiferromagnetic spintronics: Part of a collection of reviews on antiferromagnetic spintronicsMay 30 2017The recent demonstrations of electrical manipulation and detection of antiferromagnetic spins have opened up a chapter in the spintronics story. In this article, we review the emerging research field that is exploring synergies between antiferromagnetic ... 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
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
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
Quantum theory of bilayer quantum Hall smecticsSep 06 2002Mean-field theory predicts that bilayer quantum Hall systems at odd integer total filling factors can have stripe ground states in which the top Landau level is occupied alternately by electrons in one of the two layers. We report on an analysis of the ... 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
Collective transport properties of bilayer-quantum-Hall excitonic condensatesOct 30 2003Double-layer electron systems in the quantum Hall regime have excitonic condensate ground states when the layers are close together and the total Landau level filling factor is close to an odd integer. In this paper we discuss the microscopic physics ... More
Tunneling and Fluctuating Electron-Hole Cooper Pairs in Double Bilayer GrapheneMar 18 2019A strong low-temperature enhancement of the tunneling conductance between graphene bilayers has been reported recently, and interpreted as a signature of electron-hole pairing. The pairing in electron-hole double layers was first predicted more than forty ... More
Carrier and strain tunable intrinsic magnetism in two-dimensional MAX$_3$ transition metal chalcogenidesSep 27 2017We present a density functional theory study of the carrier-density and strain dependence of magnetic order in two-dimensional (2D) MAX$_3$ (M= V, Cr, Mn, Fe, Co, Ni; A= Si, Ge, Sn, and X= S, Se, Te) transition metal trichalcogenides. Our {\em ab initio} ... 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
Accurate Gap Determination in Monolayer and Bilayer Graphene/h-BN Moire SuperlatticesAug 20 2018High mobility single and few-layer graphene sheets are in many ways attractive as nanoelectronic circuit hosts but lack energy gaps, which are essential to the operation of field-effect transistors. One of the methods used to create gaps in the spectrum ... 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
Direct Measurement of Exciton Valley Coherence in Monolayer WSe$_2$Sep 29 2015In crystals, energy band extrema in momentum space can be identified by their valley index. The internal quantum degree of freedom associated with valley pseudospin indices can act as a useful information carrier analogous to electronic charge or spin. ... More
Ferroelectric quantum Hall phase revealed by visualizing Landau level wavefunction interferenceMay 11 2018Novel broken symmetry states can spontaneously form due to Coulomb interactions in electronic systems with multiple internal degrees of freedom. Multi-valley materials offer an especially rich setting for the emergence of such states, which have potential ... More
Observation of Majorana Fermions in Ferromagnetic Atomic Chains on a SuperconductorOct 02 2014Majorana fermions are predicted to localize at the edge of a topological superconductor, a state of matter that can form when a ferromagnetic system is placed in proximity to a conventional superconductor with strong spin-orbit interaction. With the goal ... More
Topological Dirac states beyond $π$ orbitals for silicene on SiC(0001) surfaceOct 01 2017The discovery of intriguing properties related to the Dirac states in graphene has spurred huge interest in exploring its two-dimensional group-IV counterparts, such as silicene, germanene, and stanene. However, these materials have to be obtained via ... 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
Electronic structure of multilayer grapheneJun 17 2008Feb 17 2011We study the electronic structure of multilayer graphene using a $\pi$-orbital continuum model with nearest-neighbor intralayer and interlayer tunneling. Using degenerate state perturbation theory, we show that the low-energy electronic structure of arbitrarily ... More
Chiral Decomposition in the Electronic Structure of Graphene MultilayersNov 27 2007Apr 10 2008We show that the low-energy electronic structure of arbitrarily stacked graphene multilayers with nearest-neighbor interlayer tunneling consists of chiral pseudospin doublets. Although the number of doublets in an $N$-layer system depends on the stacking ... More
Coulomb drag between disordered two-dimensional electron gas layersApr 01 1993We derive and evaluate expressions for the frictional Coulomb drag between disordered two-dimensional electron gas layers. Our derivation is based on the memory-function formalism and the expression for the drag reduces to previously known results in ... More
Moiré Assisted Fractional Quantum Hall State SpectroscopyNov 02 2016Dec 10 2016Intra-Landau level excitations in the fractional quantum Hall regime are not accessible via optical absorption measurements. We point out that optical probes are enabled by the periodic potentials produced by a moir\'e pattern. Our observation is motivated ... More
Nondissipative Drag Conductance as a Topological Quantum NumberApr 05 1999Jan 24 2001We show in this paper that the boundary condition averaged nondissipative drag conductance of two coupled mesoscopic rings with no tunneling, evaluated in a particular many-particle eigenstate, is a topological invariant characterized by a Chern integer. ... More
Vortex Lattice Structure of Fulde-Ferrell-Larkin-Ovchinnikov SuperconductorsApr 23 2004Sep 27 2004In superconductors with singlet pairing, the inhomogeneous Fulde-Ferrell-Larkin-Ovchinnikov (FFLO) state is expected to be stabilized by a large Zeeman splitting. We develop an efficient method to evaluate the Landau-Ginzburg free energies of FFLO-state ... More
On the Thermodynamics of Laughlin Liquid FreezingMay 20 1994The competition between liquid and solid states of strongly correlated electron systems occurs in a novel way in a strong magnetic field. The fact that certain Landau level filling factors are especially favorable for the formation of strongly correlated ... More
Carrier Density and Magnetism in Graphene Zigzag NanoribbonsApr 15 2009Jun 25 2009The influence of carrier density on magnetism in a zigzag graphene nanoribbon is studied in a $\pi$-orbital Hubbard-model mean-field approximation. Departures from half-filling alter the magnetism, leading to states with charge density variation across ... More
Long-Distance Spin Transport Through a Graphene Quantum Hall AntiferromagnetJan 22 2018Antiferromagnetic insulators (AFMI) are robust against stray fields, and their intrinsic dynamics could enable ultrafast magneto-optics and ultrascaled magnetic information processing. Low dissipation, long distance spin transport and electrical manipulation ... 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
Path-integral calculation of the third virial coefficient of quantum gases at low temperaturesMar 15 2011We derive path-integral expressions for the second and third virial coefficients of monatomic quantum gases. Unlike previous work that considered only Boltzmann statistics, we include exchange effects (Bose-Einstein or Fermi-Dirac statistics). We use ... More
History Based Coalition Formation in Hedonic Context Using TrustAug 15 2013In this paper we address the problem of coalition formation in hedonic context. Our modelling tries to be as realistic as possible. In previous models, once an agent joins a coalition it would not be able to leave the coalition and join the new one; in ... 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
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
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
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
Vortex Lattice Structure and Topological Superconductivity in the Quantum Hall RegimeMar 28 2019Chiral topological superconductors are expected to appear as intermediate states when a quantum anomalous Hall system is proximity coupled to an s-wave superconductor and the magnetization direction is reversed. In this paper we address the edge state ... More
On the nature of the correlated insulator states in twisted bilayer grapheneDec 11 2018We use self-consistent Hartree-Fock calculations performed in the full $\pi$-band Hilbert space to assess the nature of the recently discovered correlated insulator states in magic-angle twisted bilayer graphene (TBG). In single spin/valley-flavor models ... More
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
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
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
Time-Reversal Symmetry-Breaking Nematic Insulators near Quantum Spin Hall Phase TransitionsOct 01 2017Apr 27 2018We study the phase diagram of a model quantum spin Hall system as a function of band inversion and band-coupling strength, demonstrating that when band hybridization is weak, an interaction-induced nematic insulator state emerges over a wide range of ... More
Anomalous Drag in Double Bilayer Graphene Quantum-Hall SuperfluidsMar 06 2019Semiconductor double-layers in the quantum Hall regime tend to have superfluid exciton condensate ground states when the total filling factor is an odd integer, provided that the Landau orbitals at the Fermi level in the two layers have the same orbital ... 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
Theory of phonon-mediated superconductivity in twisted bilayer grapheneMay 22 2018Dec 17 2018We present a theory of phonon-mediated superconductivity in near magic angle twisted bilayer graphene. Using a microscopic model for phonon coupling to moir\'e band electrons, we find that phonons generate attractive interactions in both $s$ and $d$ wave ... More
Anomalous Hall effect arising from noncollinear antiferromagnetismSep 16 2013In most conductors current flow perpendicular to electric field direction (Hall current) can be explained in terms of the Lorentz forces present when charged particles flow in an external magnetic field. However, as established in the very early work ... 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
Theory of Ferromagnetism in Diluted Magnetic Semiconductor Quantum WellsJan 26 2000We present a mean field theory of ferromagnetism in diluted magnetic semiconductor quantum wells. When subband mixing due to exchange interactions between quantum well free carriers and magnetic impurities is neglected, analytic result can be obtained ... 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
Optical conductivity of the $t_{2g}$ two-dimensional electron gasAug 05 2013Motivated by recent interest in perovskite surfaces and heterostructures, we present an analysis of the Kubo conductivity of a two-dimensional electron gas (2DEG) formed in the $t_{2g}$ bands of an oxide with perovskite structure. We find that when the ... More
Topological Exciton Bands in Moiré HeterojunctionsOct 12 2016Mar 18 2017Moir\'e patterns are common in Van der Waals heterostructures and can be used to apply periodic potentials to elementary excitations. We show that the optical absorption spectrum of transition metal dichalcogenide bilayers is profoundly altered by long ... More
How to make a bilayer exciton condensate flowJan 24 2008Bose condensation is responsible for many of the most spectacular effects in physics because it can promote quantum behavior from the microscopic to the macroscopic world. Bose condensates can be distinguished by the condensing object; electron-electron ... More
Minimum Conductivity and Evidence for Phase Transitions in Ultra-clean Bilayer GrapheneFeb 15 2012Bilayer graphene (BLG) at the charge neutrality point (CNP) is strongly susceptible to electronic interactions, and expected to undergo a phase transition into a state with spontaneous broken symmetries. By systematically investigating a large number ... 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
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
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
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
Quantum Anomalous Hall Effect through Canted AntiferromagnetismFeb 27 2019Most canted antiferromagnets are also anomalous Hall antiferromagnets (AHE AFMs), i.e. they have an anomalous Hall response and other responses with the same symmetry requirements. We suggest that AHE AFMs are promising materials as hosts for high-temperature ... 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
Electronic properties of thin films of tensile strained HgTeApr 05 2018Tensile strained bulk HgTe is a three-dimensional topological insulator. Because of the energetic position of its surface state Dirac points relative to its small bulk gap, the electronic properties of the relatively thin MBE-grown films used to study ... More
Moiré Excitons in Van der Waals HeterostructuresJul 10 2018In van der Waals (vdW) heterostructures formed by stacking two monolayer semiconductors, lattice mismatch or rotational misalignment introduces an in-plane moir\'e superlattice. While it is widely recognized that a moir\'e superlattice can modulate the ... 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
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
Spin Hall effect without spin currents in magnetic insulatorsMar 04 2018The spin Hall effect (SHE) is normally discussed in terms of a spin current, which is ill-defined in strongly spin-orbit-coupled systems because of spin non-conservation. In this work we propose an alternative view of SHE phenomena by relating them to ... 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
Magnetic interactions of substitutional Mn pairs in GaAsJan 17 2010Jan 19 2010We employ a kinetic-exchange tight-binding model to calculate the magnetic interaction and anisotropy energies of a pair of substitutional Mn atoms in GaAs as a function of their separation distance and direction. We find that the most energetically stable ... More
Chern number spins of Mn acceptor magnets in GaAsAug 02 2010We determine the effective total spin $J$ of local moments formed from acceptor states bound to Mn ions in GaAs by evaluating their magnetic Chern numbers. We find that when individual Mn atoms are close to the sample surface, the total spin changes from ... More
Magnetic properties of substitutional Mn in (110) GaAs surface and subsurface layersJul 29 2009Motivated by recent STM experiments, we present a theoretical study of the electronic and magnetic properties of the Mn-induced acceptor level obtained by substituting a single Ga atom in the (110) surface layer of GaAs or in one of the atoms layers below ... More
Pseudospin Magnetism in GrapheneJul 10 2007Jan 18 2008We predict that neutral graphene bilayers are pseudospin magnets in which the charge density-contribution from each valley and spin spontaneously shifts to one of the two layers. The band structure of this system is characterized by a momentum-space vortex ... More
Interlayer Transport in Bilayer Quantum Hall SystemsAug 29 2005Dec 26 2005Bilayer quantum Hall systems have a broken symmetry ground state at filling factor $\nu=1$ which can be viewed either as an excitonic superfluid or as a pseudospin ferromagnet. We present a theory of inter-layer transport in quantum Hall bilayers that ... More
Collective Modes and Skyrmion Excitations in Graphene SU(4) Quantum Hall FerromagnetsMay 26 2006Aug 27 2006Graphene exhibits quantum Hall ferromagnetism in which an approximate SU(4) symmetry involving spin and valley degrees of freedom is spontaneously broken. We construct a set of integer and fractional quantum Hall states that break the SU(4) spin/valley ... More
Room-Temperature Superfluidity in Graphene BilayersFeb 23 2008Aug 19 2008Because graphene is an atomically two-dimensional gapless semiconductor with nearly identical conduction and valence bands, graphene-based bilayers are attractive candidates for high-temperature electron-hole pair condensation. We present estimates which ... 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
A Quantum Theory of Cold Bosonic Atoms in Optical LatticesMay 11 2011Ultracold atoms in optical lattices undergo a quantum phase transition from a superfluid to a Mott insulator as the lattice potential depth is increased. We describe an approximate theory of interacting bosons in optical lattices which provides a qualitative ... More
Theory of inter-edge superexchange in zigzag edge magnetismDec 04 2008A graphene nanoribbon with zigzag edges has a gapped magnetic ground state with an antiferromagnetic inter-edge superexchange interaction. We present a theory based on asymptotic properties of the Dirac-model ribbon wavefunction which predicts $W^{-2}$ ... More
Orbital order in bilayer graphene at filling factor $ν=-1 $Mar 29 2010In a graphene bilayer with Bernal stacking both $n=0$ and $n=1$ orbital Landau levels have zero kinetic energy. An electronic state in the N=0 Landau level consequently has three quantum numbers in addition to its guiding center label: its spin, its valley ... More
Hilbert space operators with compatible off-diagonal cornersSep 06 2017Given a complex, separable Hilbert space $\mathcal{H}$, we characterize those operators for which $\| P T (I-P) \| = \| (I-P) T P \|$ for all orthogonal projections $P$ on $\mathcal{H}$. When $\mathcal{H}$ is finite-dimensional, we also obtain a complete ... 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
Cartan TriplesOct 11 2018We introduce the class of Cartan triples as a generalization of the notion of a Cartan MASA in a von Neumann algebra. We obtain a one-to-one correspondence between Cartan triples and certain Clifford extensions of inverse semigroups. Moreover, there is ... 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).
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