arXiv subjects

A. H. MacDonald

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

At least 145 records · Page 8Linked to original sources

arXiv2008

Density-Functional Theory of Graphene Sheets

We outline a Kohn-Sham-Dirac density-functional-theory (DFT) scheme for graphene sheets that treats slowly-varying inhomogeneous external potentials and electron-electron interactions on an equal footing. The theory is able to account for the the unusual property that the exchange-correlation contribution to chemical potential increases with carrier density in graphene. Consequences of this property, and advantages and disadvantages of using the DFT approach to describe it, are discussed. The approach is illustrated by solving the Kohn-Sham-Dirac equations self-consistently for a model random potential describing charged point-like impurities located close to the graphene plane. The influence of electron-electron interactions on these non-linear screening calculations is discussed at length, in the light of recent experiments reporting evidence for the presence of electron-hole puddles in nearly-neutral graphene sheets.

cond-mat.str-el↗

arXiv2008

How to make a bilayer exciton condensate flow

Bose 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 Cooper-pairs are responsible for superconductivity, Helium atoms for superfluidity, and ultracold alkali atoms in vapors for coherent matter waves. Electron-hole pair (exciton) condensation has maintained special interest because it has been difficult to realize experimentally, and because exciton phase coherence is never perfectly spontaneous. Although ideal condensates can support an exciton supercurrent, it has not been clear how such a current could be induced or detected, or how its experimental manifestation would be altered by the phase-fixing exciton creation and annhilation processes which are inevitably present. In this article we explain how to induce an exciton supercurrent in separately contacted bilayer condensates, and predict electrical effects which enable unambiguous detection.

cond-mat.mes-hall↗

arXiv2008

Pseudospin Magnetism in Graphene

We 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 which is responsible for unusual competition between band and kinetic energies leading to symmetry breaking in the vortex core. We discuss the possibility of realizing a pseudospin version of ferromagnetic metal spintronics in graphene bilayers based on hysteresis associated with this broken symmetry.

cond-mat.str-el↗

arXiv2007

Hysteretic behavior in weakly coupled double-dot transport in the spin blockade regime

Double quantum dot systems in the spin blockade regime exhibit leakage currents that have been attributed to the Hyperfine interaction. We model weakly coupled double-dot transport using a rate equation approach which accounts for Hyperfine flip-flop transitions. The rate equations allow us to obtain self-consistently the time evolution for electronic charge occupations and for the nuclei polarizations in each dot. We analyze the current in the spin blockade region as a function of magnetic field and observe hysteretic behavior for fields corresponding to the crossing between triplet and singlet states.

cond-mat.mes-hall↗

arXiv2007

Effect of Edge Roughness on Electronic Transport in Graphene Nanoribbon Channel Metal Oxide Semiconductor Field-Effect Transistors

Results of quantum mechanical simulations of the influence of edge disorder on transport in graphene nanoribbon metal oxide semiconductor field-effect transistors (MOSFETs) are reported. The addition of edge disorder significantly reduces ON-state currents and increases OFF-state currents, and introduces wide variability across devices. These effects decrease as ribbon widths increase and as edges become smoother. However the bandgap decreases with increasing width, thereby increasing the band-to-band tunneling mediated subthreshold leakage current even with perfect nanoribbons. These results suggest that without atomically precise edge control during fabrication, MOSFET performance gains through use of graphene will be difficult to achieve.

cond-mat.mes-hall↗

arXiv2007

Current-Induced Torques in Magnetic Metals: Beyond Spin Transfer

Current-induced torques on ferromagnetic nanoparticles and on domain walls in ferromagnetic nanowires are normally understood in terms of transfer of conserved spin angular momentum between spin-polarized currents and the magnetic condensate. In a series of recent articles we have discussed a microscopic picture of current-induced torques in which they are viewed as following from exchange fields produced by the misaligned spins of current carrying quasiparticles. This picture has the advantage that it can be applied to systems in which spin is not approximately conserved. More importantly, this point of view makes it clear that current-induced torques can also act on the order parameter of an antiferromagnetic metal, even though this quantity is not related to total spin. In this informal and intentionally provocative review we explain this picture and discuss its application to antiferromagnets.

cond-mat.mes-hall↗

arXiv2007

Effect of the long-range interaction in transport through one-dimensional nanoparticle arrays

We analyze the effect of the long-range interaction on the transport properties through ordered and disordered one-dimensional metallic nanoparticle arrays. We discuss how the threshold voltage, the I-V curves and the voltage drop through the array are modified as compared to the case in which interactions are restricted to charges placed on the same island. We show that some of these modifications are due to finite interactions between charges in different nanoparticles while other ones are due to interactions between charges in the islands and those at the electrodes, what produces a polarization potential drop through the array. We study the screening of the disorder potential due to charges impurities trapped in the substrate and find that long-range interactions introduce correlations between the disorder potentials of neighboring islands.

cond-mat.mes-hall↗

arXiv2007

Electronic correlations and disorder in transport through one-dimensional nanoparticle arrays

We analyze and clarify the transport properties of a one-dimensional metallic nanoparticle array with interaction between charges restricted to charges placed in the same conductor. We study the threshold voltage, the I-V curves and the potential drop through the array and their dependence on the array parameters including the effect of charge and resistance disorder. We show that very close to threshold the current depends linearly on voltage with a slope independent on the array size. At intermediate bias voltages, for which a Coulomb staircase is observed we find that the average potential drop through the array oscillates with position. At higher voltages I-V curves are linear but have a finite offset voltage. We show that the slope is given by the inverse of the resistances added in series and estimate the voltage at which this linear regime is reached. We also calculate the offset voltage and relate it to the potential drop through the array.

cond-mat.mes-hall↗

arXiv2007

Electronic transport through a double quantum dot in the spin blockade regime: Theoretical models

We analyzed the electronic transport through a double quantum dot in the spin blockade regime. Experiments of current rectification by Pauli exclusion principle in double quantum dots were discussed. The electron and nuclei spin dynamics and their interplay due to the Hyperfine interaction were self-consistently analyzed within the framework of rate equations. Our results show that the current leakage experimentally observed in the spin-blockade region, is due to spin-flip processes induced by Hyperfine interaction through Overhauser effect. We show as well how a magnetic field applied parallel to the current allows excited states to participate in the electronic current and removes spin blockade. Our model includes also a self-consistent description of inelastic transitions where the energy is exchanged through interactions with acoustic phonons in the environment. It accounts for spontaneous emission of phonons which results in additional features in the current characteristics. We develop a microscopical model to treat the Hyperfine interaction in each dot. Using this model we study the dynamical nuclear polarization as a function of the applied voltage.

cond-mat.mes-hall↗

arXiv2007

Electronic Interface Reconstruction at Polar-Nonpolar Mott Insulator Heterojunctions

We 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) which accompany the reconstruction. We derive an expression for the minimum number of polar layers required to drive the EIR, and discuss key parameters of the heterojunction system which control 2DSCES properties. The role of strong correlations in enhancing confinement at the interface is emphasized.

cond-mat.str-el↗

arXiv2007

Hubbard-Thomas-Fermi Theory of Transition Metal Oxide Heterostructures

We demonstrate that the charge distributions in Hubbard-model representations of transition metal oxide heterojucntions can be described by a Thomas-Fermi theory in which the energy is approximated as the sum of the electrostatic energy and the uniform three-dimensional Hubbard model energy per site at the local density equals to a constant. When charged atomic layers in the oxides are approximated as two-dimensional sheets with uniform charge density, the electrostatic energy is simply evaluated. We find that this Thomas-Fermi theory can reproduce results obtained from full Hartree-Fock theory for various different heterostructures. We also show explicitly how Thomas-Fermi theory can be used to estimate some key properties qualitatively.

cond-mat.str-el↗

arXiv2007

The Role of Electron-electron Interactions in Graphene ARPES Spectra

We report on a theoretical study of the influence of electron-electron interactions on ARPES spectra in graphene that is based on the random-phase-approximation and on graphene's massless Dirac equation continuum model. We find that level repulsion between quasiparticle and plasmaron resonances gives rise to a gap-like feature at small k. ARPES spectra are sensitive to the electron-electron interaction coupling strength $α_{\rm gr}$ and might enable an experimental determination of this material parameter.

cond-mat.mes-hall↗

arXiv2007

Functional Keldysh Theory of Spin Torques

We present a microscopic treatment of current-induced torques and thermal fluctuations in itinerant ferromagnets based on a functional formulation of the Keldysh formalism. We find that the nonequilibrium magnetization dynamics is governed by a stochastic Landau-Lifschitz-Gilbert equation with spin transfer torques. We calculate the Gilbert damping parameter $α$ and the non-adiabatic spin transfer torque parameter $β$ for a model ferromagnet. We find that $β\neq α$, in agreement with the results obtained using imaginary-time methods of Kohno, Tatara and Shibata [J. Phys. Soc. Japan 75, 113706 (2006)]. We comment on the relationship between $s-d$ and isotropic-Stoner toy models of ferromagnetism and more realistic density-functional-theory models, and on the implications of these relationships for predictions of the $β/α$ ratio which plays a central role in domain wall motion. Only for a single-parabolic-band isotropic-Stoner model with an exchange splitting that is small compared to the Fermi energy does $β/α$ approach one. In addition, our microscopic formalism incorporates naturally the fluctuations needed in a nonzero-temperature description of the magnetization. We find that to first order in the applied electric field, the usual form of thermal fluctuations via a phenomenological stochastic magnetic field holds.

cond-mat.mes-hall↗

arXiv2007

Optical Control of Topological Quantum Transport in Semiconductors

Intense coherent laser radiation red-detuned from absorption edge can reactively activate sizable Hall type charge and spin transport in n-doped paramagnetic semiconductors as a consequence of k-space Berry curvature transferred from valence band to photon-dressed conduction band. In the presence of disorder, the optically induced Hall conductance can change sign with laser intensity.

cond-mat.mes-hall↗

arXiv2007

On the character of states near the Fermi level in (Ga,Mn)As: impurity to valence band crossover

We discuss the character of states near the Fermi level in Mn doped GaAs, as revealed by a survey of dc transport and optical studies over a wide range of Mn concentrations. A thermally activated valence band contribution to dc transport, a mid-infrared peak at energy hbar omega approx 200 meV in the ac- conductivity, and the hot photoluminescence spectra indicate the presence of an impurity band in low doped (<<1% Mn) insulating GaAs:Mn materials. Consistent with the implications of this picture, both the impurity band ionization energy inferred from the dc transport and the position of the mid-infrared peak move to lower energies and the peak broadens with increasing Mn concentration. In metallic materials with > 2% doping, no traces of Mn-related activated contribution can be identified in dc-transport, suggesting that the impurity band has merged with the valence band. No discrepancies with this perception are found when analyzing optical measurements in the high-doped GaAs:Mn. A higher energy (hbar omega approx 250 meV) mid-infrared feature which appears in the metallic samples is associated with inter-valence band transitions. Its red-shift with increased doping can be interpreted as a consequence of increased screening which narrows the localized-state valence-band tails and weakens higher energy transition amplitudes. Our examination of the dc and ac transport characteristics of GaAs:Mn is accompanied by comparisons with its shallow acceptor counterparts, confirming the disordered valence band picture of high-doped metallic GaAs:Mn material.

cond-mat.mtrl-sci↗

arXiv2007

Theory of the Pseudospin Resonance in Semiconductor Bilayers

The pseudospin degree of freedom in a semiconductor bilayer gives rise to a collective mode analogous to the ferromagnetic resonance mode of a ferromagnet. We present a theory of the dependence of the energy and the damping of this mode on layer separation $d$. Based on these results, we discuss the possibility of realizing transport-current driven pseudospin-transfer oscillators in semiconductors.

cond-mat.mes-hall↗

arXiv2007

Chirality and Correlations in Graphene

Graphene is described at low-energy by a massless Dirac equation whose eigenstates have definite chirality. We show that the tendency of Coulomb interactions in lightly doped graphene to favor states with larger net chirality leads to suppressed spin and charge susceptibilities. Our conclusions are based on an evaluation of graphene's exchange and random-phase-approximation (RPA) correlation energies. The suppression is a consequence of the quasiparticle chirality switch which enhances quasiparticle velocities near the Dirac point.

cond-mat.str-el↗

arXiv2007

Anomalous Hall effect in a two-dimensional electron gas

The anomalous Hall effect in a magnetic two-dimensional electron gas with Rashba spin-orbit coupling is studied within the Kubo-Streda formalism in the presence of pointlike potential impurities. We find that all contributions to the anomalous Hall conductivity vanish to leading order in disorder strength when both chiral subbands are occupied. In the situation that only the majority subband is occupied, all terms are finite in the weak scattering limit and the total anomalous Hall conductivity is dominated by skew scattering. We compare our results to previous treatments and resolve some of the discrepancies present in the literature.

cond-mat.mtrl-sci↗