Results for "Allan H. MacDonald"
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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 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 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 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 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 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 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 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 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 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 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 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 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 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 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 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 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 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 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 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