arXiv subjects

A. H. MacDonald

Publications and source records attributed to A. H. MacDonald.

At least 109 records · Page 6Linked to original sources

arXiv2010

Compressibility of bilayer graphene

Bilayer graphene is a recently isolated and intriguing class of many-body systems with massive chiral quasiparticles. We present theoretical results for the electronic compressibility of bilayer graphene that are based on a four-band continuum band structure model combined with a random phase approximation treatment of electronic correlations. We find that the compressibility is strongly suppressed by electron-electron interactions at low carrier densities. Correlations do not lead to any qualitative new features, but are crucially important for a quantitative understanding of this fundamental thermodynamic property of graphene bilayers.

cond-mat.mes-hall↗

arXiv2010

Electron-electron interactions in decoupled graphene layers

Multi-layer graphene on the carbon face of silicon carbide is an intriguing electronic system which typically consists of a stack of ten or more layers. Rotational stacking faults in this system dramatically reduce inter-layer coherence. In this article we report on the influence of inter-layer interactions, which remain strong even when coherence is negligible, on the Fermi liquid properties of charged graphene layers. We find that inter-layer interactions increase the magnitudes of correlation energies and decrease quasiparticle velocities, even when remote-layer carrier densities are small, and that they lessen the influence of exchange and correlation on the distribution of carriers across layers.

cond-mat.mes-hall↗

arXiv2010

Orbital order in bilayer graphene at filling factor $ν=-1 $

In 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 index $K$ or $K^{\prime}$, and an orbital quantum number $n=0,1.$ The two-dimensional electron gas (2DEG) in the bilayer supports a wide variety of broken-symmetry states in which the pseudospins associated these three quantum numbers order in a manner that is dependent on both filling factor $ν$ and the electric potential difference between the layers. In this paper, we study the case of $ν=-1$ in an external field strong enough to freeze electronic spins. We show that an electric potential difference between layers drives a series of transitions, starting from interlayer-coherent states (ICS) at small potentials and leading to orbitally coherent states (OCS) that are polarized in a single layer. Orbital pseudospins carry electric dipoles with orientations that are ordered in the OCS and have Dzyaloshinskii-Moriya interactions that can lead to spiral instabilities. We show that the microwave absorption spectra of ICSs, OCSs, and the mixed states that occur at intermediate potentials are sharply distinct.

cond-mat.mes-hall↗

arXiv2010

Magnetic interactions of substitutional Mn pairs in GaAs

We 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 configuration is usually one in which the spins are ferromagnetically aligned along the vector connecting the Mn atoms. The ferromagnetic configuration is characterized by a splitting of the topmost unoccupied acceptor levels, which is visible in scanning tunneling microscope studies when the pair is close to the surface and is strongly dependent on pair orientation. The largest acceptor splittings occur when the Mn pair is oriented along the <110> symmetry direction, and the smallest when they are oriented along <100>. We show explicitly that the acceptor splitting is not simply related to the effective exchange interaction between the Mn local moments. The exchange interaction constant is instead more directly related to the width of the distribution of all impurity levels -- occupied and unoccupied. When the Mn pair is at the (110) GaAs surface, both acceptor splitting and effective exchange interaction are very small except for the smallest possible Mn separation.

cond-mat.mtrl-sci↗

arXiv2010

Blockade and Counterflow Supercurrent in exciton-condensate Josephson junctions

We demonstrate that perfect conversion between charged supercurrents in superconductors and neutral supercurrents in electron-hole pair condensates is possible via a new Andreev-like scattering mechanism. As a result, when two superconducting circuits are coupled through a bilayer exciton condensate, the superflow in both layers is drastically modified. Depending on the phase biases the supercurrents can be completely blocked or exhibit perfect drag.

cond-mat.mes-hall↗

arXiv2009

Velocity-modulation control of electron-wave propagation in graphene

Wave propagation control by spatial modulation of velocity has a long history in optics and acoustics. We address velocity-modulation control of electron wave propagation in graphene and other two-dimensional Dirac-electron systems, pointing out a key distinction of the Dirac-wave case. We also propose a strategy for pattern transfer from a remote metallic layer that is based on many-body velocity renormalization.

cond-mat.mes-hall↗

arXiv2009

Quantum vortex dynamics in two-dimensional neutral superfluids

We derive an effective action for the vortex position degree-of-freedom in a superfluid by integrating out condensate phase and density fluctuation environmental modes. When the quantum dynamics of environmental fluctuations is neglected, we confirm the occurrence of the vortex Magnus force and obtain an expression for the vortex mass. We find that this adiabatic approximation is valid only when the superfluid droplet radius $R$, or the typical distance between vortices, is very much larger than the coherence length $ξ$. We go beyond the adiabatic approximation numerically, accounting for the quantum dynamics of environmental modes and capturing their dissipative coupling to condensate dynamics. For the case of an optical-lattice superfluid we demonstrate that vortex motion damping can be adjusted by tuning the ratio between the tunneling energy $J$ and the on-site interaction energy $U$. We comment on the possibility of realizing vortex Landau level physics.

cond-mat.quant-gas↗

arXiv2009

Theory of the Magnetic-Field-Induced Insulator in Neutral Graphene

Recent experiments have demonstrated that neutral graphene sheets have an insulating ground state in the presence of an external magnetic field. We report on a $π$-band tight-binding-model Hartree-Fock calculation which examines the competition between distinct candidate insulating ground states. We conclude that for graphene sheets on substrates the ground state is most likely a field-induced spin-density-wave, and that a charge density wave state is possible for suspended samples. Neither of these density-wave states support gapless edge excitations.

cond-mat.mes-hall↗

arXiv2009

Anomalous Exciton-Condensation in Graphene Bilayers

In ordinary semiconductor bilayers, exciton condensates appear at total Landau level filling factor $ν_{T}=1$. We predict that similar states will occur in Bernal stacked graphene bilayers at many non-zero integer filling factors. For $ν_{T} = -3,1$ we find that the superfluid density of the exciton condensate vanishes and that a finite-temperature fluctuation induced first order isotropic-smectic phase transition occurs when the layer densities are not balanced. These anomalous properties of bilayer graphene exciton condensates are due to the degeneracy of Landau levels with $n=0$ and $n=1$ orbital character.

cond-mat.mes-hall↗

arXiv2009

Origin of Universal Optical Conductivity and Optical Stacking Sequence Identification in Multilayer Graphene

We show that the origin of the universal optical conductivity in a normal $N$-layer graphene multilayer is an emergent chiral symmetry which guarantees that $σ(ω)=Nσ_{uni}$ in both low and high frequency limits. [$σ_{uni}=(π/2) e^2/h$]. We use this physics to relate intermediate frequency conductivity trends to qualitative characteristics of the multilayer stacking sequence.

cond-mat.mtrl-sci↗

arXiv2009

Drude-Interband Coupling, Screening, and the Optical Conductivity of Doped Bilayer Graphene

We present a theory of the influence of band renormalization and excitonic electron-electron interaction effects on the optical conductivity $σ(ω)$ of doped bilayer graphene. Using the Keldysh formalism, we derive a kinetic equation from which we extract numerical and approximate analytic results for $σ(ω)$. Our calculations reveal a previously unrecognized mechanism which couples the Drude and interband response and renormalizes the plasmon frequency, and suggest that screening must play an essential role in explaining the weakly renormalized conductivity seen in recent experiments.

cond-mat.mes-hall↗

arXiv2009

Magnetic properties of substitutional Mn in (110) GaAs surface and subsurface layers

Motivated 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 the surface. We employ a kinetic-exchange tight-binding model in which the relaxation of the (110) surface is taken into account. The acceptor wave function is strongly anisotropic in space and its detailed features depend on the depth of the sublayer in which the Mn atom is located. The local-density-of-states (LDOS) on the (110) surface associated with the acceptor level is more sensitive to the direction of the Mn magnetic moment when the Mn atom is located further below the surface. We show that the total magnetic anisotropy energy of the system is due almost entirely to the dependence of the acceptor level energy on Mn spin orientation, and that this quantity is strongly dependent on the depth of the Mn atom.

cond-mat.mtrl-sci↗

arXiv2009

Electron-electron interactions in graphene bilayers

Electrons most often organize into Fermi-liquid states in which electron-electron interactions play an inessential role. A well known exception is the case of one-dimensional (1D) electron systems (1DES). In 1D the electron Fermi-surface consists of points, and divergences associated with low-energy particle-hole excitations abound when electron-electron interactions are described perturbatively. In higher space dimensions, the corresponding divergences occur only when Fermi lines or surfaces satisfy idealized nesting conditions. In this article we discuss electron-electron interactions in 2D graphene bilayer systems which behave in many ways as if they were one-dimensional, because they have Fermi points instead of Fermi lines and because their particle-hole energies have a quadratic dispersion which compensates for the difference between 1D and 2D phase space. We conclude, on the basis of a perturbative RG calculation similar to that commonly employed in 1D systems, that interactions in neutral graphene bilayers can drive the system into a strong-coupling broken symmetry state with layer-pseudospin ferromagnetism and an energy gap.

cond-mat.str-el↗

arXiv2009

Carrier Density and Magnetism in Graphene Zigzag Nanoribbons

The influence of carrier density on magnetism in a zigzag graphene nanoribbon is studied in a $π$-orbital Hubbard-model mean-field approximation. Departures from half-filling alter the magnetism, leading to states with charge density variation across the ribbon and parallel spin-alignment on opposite edges. Finite carrier densities cause the spin-density near the edges to decrease steadily, leading eventually to the absence of magnetism. At low doping densities the system shows a tendency to multiferroic order in which edge charges and spins are simultaneously polarized.

cond-mat.mes-hall↗

arXiv2009

Hydrodynamic theory of transport in doped graphene

We study non-linear dc transport in graphene using a hydrodynamic approach and conclude that in clean samples the drift velocity saturates at a weakly density-dependent value v_{sat} ~ 10^7 cm/s. We show that saturation results from the interactions between graphene's Dirac quasi-particles and both acoustic and optical phonons. Saturation is accompanied by substantial electron heating and is not reached at realistic driving fields in moderately or strongly disordered samples. We find that it is essential to account for interactions among graphene's Dirac quasi-particles, which increase the linear response resistivity at high temperatures or low densities.

cond-mat.other↗

arXiv2009

Dynamical response functions and collective modes of bilayer graphene

Bilayer graphene (BLG) has recently attracted a great deal of attention because of its electrically tunable energy gaps and its unusual electronic structure. In this Letter we present analytical and semi-analytical expressions, based on the four-band continuum model, for the layer-sum and layer-difference density response functions of neutral and doped BLG. These results demonstrate that BLG density-fluctuations can exhibit either single-component massive-chiral character or standard two-layer character, depending on energy and doping.

cond-mat.str-el↗

arXiv2009

Berry Phase Coupling and the Cuprate Neutron Scattering Resonance

We examine the influence of coupling between particle-hole and particle-particle spin fluctuations on the inelastic neutron scattering resonance (INSR) in cuprate superconductors in both weak and strong interaction limits. For weak-interactions in the particle-hole channel, we find that the interchannel coupling can eliminate the resonance. For strong interactions which drive the system close to a $\bm{Q}=(π,π)$ magnetic instablity, the resonance frequency always approaches zero but its value is influenced by the interchannel coupling. We comment on constraints imposed on cuprate physics by the INSR phenomenology, and a comparison between the cuprates and the newly-discovered iron pnictide superconductors is discussed.

cond-mat.str-el↗

arXiv2009

Spin-Orbit Interactions in Bilayer Exciton-Condensate Ferromagnets

Bilayer electron-hole systems with unequal electron and hole densities are expected to have exciton condensate ground states with spontaneous spin-polarization in both conduction and valence bands. In the absence of spin-orbit and electron-hole exchange interactions there is no coupling between the spin-orientations in the two quantum wells. In this article we show that Rashba spin-orbit interactions lead to unconventional magnetic anisotropies, whose strength we estimate, and to ordered states with unusual quasiparticle spectra.

cond-mat.mes-hall↗