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Allan H. MacDonald

Publications and source records attributed to Allan H. MacDonald.

At least 181 records · Page 10Linked to original sources
arXiv2014

Non-Local Transport Mediated by Spin-Supercurrents

In 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 supercurrents couple spatially remote spin-mixing vertical transport channels, even when easy-plane anisotropy is imperfect, and discuss the possibility that this effect can be used to fabricate new types of electronic devices.

cond-mat.mes-hall↗
arXiv2014

Origin of band gaps in graphene on hexagonal boron nitride

Recent 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 boron nitride substrate, demonstrating that they are produced by an interesting interplay between structural and electronic properties, including electronic many-body exchange interactions. Our theory is able to explain the observed gap behavior by accounting first for the structural relaxation of graphene's carbon atoms when placed on a boron nitride substrate and then for the influence of the substrate on low-energy $π$-electrons located at relaxed carbon atom sites. The methods we employ can be applied to many other van der Waals heterojunctions.

cond-mat.mes-hall↗
arXiv2014

Half-metallic magnetism and the search for better spin valves

We 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 by uncorrelated thermal fluctuations of magnetization orientations on opposite sides of a tunnel junction is in good agreement with recent studies of the temperature-dependent magnetoresistance of high quality tunnel junctions with MgO barriers. Using this insight, we propose a simple formula which captures the advantages for spin-valve optimization of using materials with a high spin polarization of Fermi-level tunneling electrons, and of using materials with high ferromagnetic transition temperatures. We conclude that half-metallic ferromagnets can yield better spin-value performance than current elemental transition metal ferromagnet/MgO systems only if their ferromagnetic transition temperatures exceed $\sim 950~\mathrm{K}$.

cond-mat.mes-hall↗
arXiv2014

Weak Localization, Spin Relaxation, and Spin-Diffusion: The Crossover Between Weak and Strong Rashba Coupling Limits

Disorder 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 and spin-independent disorder, focusing on the role of Rashba spin-orbit coupling in transport. Spin-orbit coupling contributes to spin relaxation, transforming the quantum interference contribution to conductivity from a negative weak localization (WL) correction to a positive weak anti-localization (WAL) correction. The importance of spin channel mixing in transport is largest in the regime where the Bloch state energy uncertainty $\hbar/τ$ and the Rashba spin-orbit splitting $Δ_\mathrm{SO}$ are comparable. We find that as a consequence of this spin channel mixing, the WL-WAL crossover is non-monotonic in this intermediate regime, and use our results to address recent experimental studies of transport at two-dimensional oxide interfaces.

cond-mat.mes-hall↗
arXiv2014

A stable path to ferromagnetic hydrogenated graphene growth

In this paper, we propose a practical way to stabilize half-hydrogenated graphene (graphone). We show that the dipole moments induced by an hexagonal-boron nitride (h-BN) substrate on graphene stabilize the hydrogen atoms on one sublattice of the graphene layer and suppress the migration of the absorbed hydrogen atoms. Based upon first principle spin polarized density of states (DOS) calculations, we show that the half hydrogenated graphene (graphone) obtained in different graphene-h-BN heterostructures exhibits a half metallic state. We propose to use this new exotic material for spin valve and other spintronics devices and applications.

cond-mat.mes-hall↗
arXiv2014

Broken SU(4) Symmetry and The Fractional Quantum Hall Effect in Graphene

We 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. Our approach explains the experimental finding that gaps at odd numerators are weak for 1<|ν|<2 and strong for 0<|ν|< 1. We find that in the SU(4) invariant case the incompressible ground state at |ν|=1/3 is a three-component incompressible state, not the Laughlin state, and discuss the competition between these two states in the presence of SU(4) spin-valley symmetry breaking terms.

cond-mat.str-el↗
arXiv2014

Revealing the electronic band structure of trilayer graphene on SiC: An angle-resolved photoemission study

In 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 this work, we present angle resolved photoemission spectroscopy (ARPES) data which show with high resolution the electronic band structure of trilayer graphene obtained on α-SiC(0001) and β-SiC(111) via hydrogen intercalation. Electronic bands obtained from tight-binding calculations are fitted to the experimental data to extract the interatomic hopping parameters for Bernal and rhombohedral stacked trilayers. Low energy electron microscopy (LEEM) measurements demonstrate that the trilayer domains extend over areas of tens of square micrometers, suggesting the feasibility of exploiting this material in electronic and photonic devices. Furthermore, our results suggest that on SiC substrates the occurrence of rhombohedral stacked trilayer is significantly higher than in natural bulk graphite.

cond-mat.mtrl-sci↗
arXiv2014

Effect of spin fluctuations on quasiparticle excitations: first-principles theory and application to sodium and lithium

We 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 demonstrate that the coupling of electrons to spin fluctuations gives an important contribution to the quasiparticle lifetime, but does not significantly reduce the occupied bandwidth. Including correlation effects beyond the random-phase approximation in the screening from charge fluctuations yields good agreement with experiment.

cond-mat.mtrl-sci↗
arXiv2013

Ab-Initio Theory of Moiré Superlattice Bands in Layered Two-Dimensional Materials

When 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 obtained from {\it ab initio} calculations performed on short-period crystalline structures to derive effective Hamiltonians that are able to efficiently describe the influence of the moir{\' e} pattern superlattices on electronic properties. We apply our approach to the cases of graphene on graphene (G/G) and graphene on hexagonal boron nitride (G/BN), deriving explicit effective Hamiltonians that have the periodicity of the moir{\' e} pattern and can be used to calculate electronic properties of interest for arbitrary twist angles and lattice constants.

cond-mat.mes-hall↗
arXiv2013

Accurate tight-binding and continuum models for the $π$ bands of bilayer graphene

We derive an {\em ab initio} $π$-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 than assumed in commonly used phenomenological tight-binding models. Starting from this full tight-binding model, we derive two effective models that are intended to provide a convenient starting point for theories of $π$-band electronic properties by achieving accuracy over the full width of the $π$-bands, and especially at the Dirac points, in models with a relatively small number of hopping parameters. The simplified models are then compared with phenomenological Slonczewski-Weiss-McClure type tight-binding models in an effort to clarify confusions that exists in the literature concerning tight-binding model parameter signs.

cond-mat.mes-hall↗
arXiv2013

Theory of Native Orientational Pinning in Quantum Hall Nematics

The 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 over more than a decade of research, follows from the presence of both Rashba and Dresselhaus spin-orbit interactions. The hard transport direction of the nematic state is determined by the relative sign of the Rashba and Dresselhaus coefficients, and coincides with either the $[110]$ or the $[1\bar{1}0]$ crystallographic direction. Our theoretical estimate of the pinning energy is in agreement with experimental studies of the competition between native pinning and intentional pinning by an in-plane magnetic field.

cond-mat.str-el↗
arXiv2013

Current partition at topological zero-line intersections

An intersection between one-dimensional chiral acts as a topological current splitter. We find that the splitting of a chiral zero-line mode obeys very simple, yet highly counterintuitive, partition laws which relate current paths to the geometry of the intersection. Our results have far reaching implications for device proposals based on chiral zero-line transport in the design of electron beam splitters and interferometers, and for understanding transport properties in systems where multiple topological domains lead to a statistical network of chiral channels.

cond-mat.mes-hall↗
arXiv2013

Gate-Tunable Exchange Coupling Between Cobalt Clusters on Graphene

We 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 supercells we demonstrate that the exchange interaction is strongly dependent on gate voltage, but find that it is also sensitive to the relative sublattice registration of the cobalt clusters. We use our results to discuss strategies for achieving strong and reproducible magneto-electric effects in graphene/transition-metal hybrid systems.

cond-mat.mtrl-sci↗
arXiv2012

Electron-electron interactions in non-equilibrium bilayer graphene

Conducting 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. We show that because of the strength of interlayer tunneling, electron-electron interactions nevertheless have a negligible influence on the conductivity which vanishes as the carrier number density goes to zero. The influence of interactions is qualitatively weaker than in the comparable cases of single-layer graphene or topological insulators, because the momentum-space layer pseudospin vorticity is 2 rather than 1. Our study relies on the quantum Liouville equation in the first Born approximation with respect to the scattering potential, with electron-electron interactions taken into account self-consistently in the Hartree-Fock approximation and screening in the random phase approximation. Within this framework the result we obtain is exact.

cond-mat.mes-hall↗
arXiv2012

Gapped broken symmetry states in ABC trilayer graphene

We use a self-consistent Hartree-Fock approximation with realistic Coulomb interactions for $π$-band electrons to explore the possibility of broken symmetry states in weakly disordered ABC stacked trilayer graphene. The competition between gapped and gapless broken symmetry states, and normal states is studied by comparing total energies. We find that gapped states are favored and that, unlike the bilayer case, gapless nematic broken symmetry states are not metastable. Among the gapped states the layer antiferromagnetic state is favored over anomalous and spin Hall states.

cond-mat.mes-hall↗
arXiv2012

Drude weight, cyclotron resonance, and the Dicke model of graphene cavity QED

The 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 the Dicke model of cavity cyclotron resonance must be supplemented by a term that is quadratic in the cavity photon field and suppresses the model's transition to a super-radiant state. We develop the theory of graphene cavity cyclotron resonance and show that the quadratic term, which is absent in graphene's low-energy Dirac model Hamiltonian, is in this case dynamically generated by virtual inter-band transitions.

cond-mat.mes-hall↗
arXiv2012

Photonic Analogue of Two-dimensional Topological Insulators and Helical One-Way Edge Transport in Bi-Anisotropic Metamaterials

Recent progress in understanding the topological properties of condensed matter has led to the discovery of time-reversal invariant topological insulators. Because of limitations imposed by nature, topologically non-trivial electronic order seems to be uncommon except in small-band-gap semiconductors with strong spin-orbit interactions. In this Article we show that artificial electromagnetic structures, known as metamaterials, provide an attractive platform for designing photonic analogues of topological insulators. We demonstrate that a judicious choice of the metamaterial parameters can create photonic phases that support a pair of helical edge states, and that these edge states enable one-way photonic transport that is robust against disorder.

cond-mat.mtrl-sci↗