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John Schliemann

Publications and source records attributed to John Schliemann.

At least 73 records · Page 4Linked to original sources
arXiv2010

Unexpected systematic degeneracy in a system of two coupled Gaudin models with homogeneous couplings

We report an unexpected systematic degeneracy between different multiplets in an inversion symmetric system of two coupled Gaudin models with homogeneous couplings, as occurring for example in the context of solid state quantum information processing. We construct the full degenerate subspace (being of macroscopic dimension), which turns out to lie in the kernel of the commutator between the two Gaudin models and the coupling term. Finally we investigate to what extend the degeneracy is related to the inversion symmetry of the system and find that indeed there is a large class of systems showing the same type of degeneracy.

cond-mat.mes-hall↗
arXiv2010

Finite Conductivity Minimum in Bilayer Graphene without Charge Inhomogeneities

Boltzmann transport theory fails near the linear band-crossing of single-layer graphene and near the quadratic band-crossing of bilayer graphene. We report on a numerical study which assesses the role of inter-band coherence in transport when the Fermi level lies near the band-crossing energy of bilayer graphene. We find that interband coherence enhances conduction, and that it plays an essential role in graphene's minimum conductivity phenomena. This behavior is qualitatively captured by an approximate theory which treats inter-band coherence in a relaxation-time approximation. On the basis of this short-range-disorder model study, we conclude that electron-hole puddle formation is not a necessary condition for finite conductivity in graphene at zero average carrier density.

cond-mat.mtrl-sci↗
arXiv2010

Dielectric function of the semiconductor hole gas

We study the dielectric function of the homogeneous hole gas in p-doped zinc-blende III-V bulk semiconductors within random phase approximation with the valence band being modeled by Luttinger's Hamiltonian in the spherical approximation. In the static limit we find a beating of Friedel oscillations between the two Fermi momenta for heavy and light holes, while at large frequencies dramatic corrections to the plasmon dispersion occur.

cond-mat.str-el↗
arXiv2010

Hyperfine induced spin and entanglement dynamics in Double Quantum Dots: A homogeneous coupling approach

We investigate hyperfine induced electron spin and entanglement dynamics in a system of two quantum dot spin qubits. We focus on the situation of zero external magnetic field and concentrate on approximation-free theoretical methods. We give an exact solution of the model for homogeneous hyperfine coupling constants (with all coupling coefficients being equal) and varying exchange coupling, and we derive the dynamics therefrom. After describing and explaining the basic dynamical properties, the decoherence time is calculated from the results of a detailed investigation of the short time electron spin dynamics. The result turns out to be in good agreement with experimental data.

cond-mat.mes-hall↗
arXiv2010

Zero-field magnetization reversal of two-body Stoner particles with dipolar interaction

Nanomagnetism has recently attracted explosive attention, in particular, because of the enormous potential applications in information industry, e.g. new harddisk technology, race-track memory[1], and logic devices[2]. Recent technological advances[3] allow for the fabrication of single-domain magnetic nanoparticles (Stoner particles), whose magnetization dynamics have been extensively studied, both experimentally and theoretically, involving magnetic fields[4-9] and/or by spin-polarized currents[10-20]. From an industrial point of view, important issues include lowering the critical switching field $H_c$, and achieving short reversal times. Here we predict a new technological perspective: $H_c$ can be dramatically lowered (including $H_c=0$) by appropriately engineering the dipole-dipole interaction (DDI) in a system of two synchronized Stoner particles. Here, in a modified Stoner-Wohlfarth (SW) limit, both of the above goals can be achieved. The experimental feasibility of realizing our proposal is illustrated on the example of cobalt nanoparticles.

cond-mat.mes-hall↗
arXiv2010

Spins coupled to a Spin Bath: From Integrability to Chaos

Motivated by the hyperfine interaction of electron spins with surrounding nuclei, we investigate systems of central spins coupled to a bath of noninteracting spins in the framework of random matrix theory. With increasing number of central spins a transition from Poissonian statistics to the Gaussian orthogonal ensemble occurs which can be described by a generalized Brody distribution. These observations are unaltered upon applying an external magnetic field. In the transition region, the classical counterparts of the models studied have mixed phase space.

cond-mat.mes-hall↗
arXiv2009

Landauer-Büttiker Study of the Anomalous Hall Effect

We report on Landauer-Büttiker studies of anomalous Hall transport in a two-dimensional electron gas with Rashba spin-orbit coupling and a magnetization provided by localized magnetic moments. Our system is described by a discretized tight-binding model in a four-terminal geometry. We consider both the case of magnetically disordered systems as well as ballistic transport in disorder-free systems with spatially homoge- neous magnetization. In the latter case we investigate both out-of-plane and in-plane magnetizations. We numerically establish a close connection between singularities in the density of states and peaks in the Hall conductance close to the lower band edge. Consistent with previous theoretical studies based on diagrammatic perturbation expansions, these peaks occur at Fermi energies where only the lower dispersion branch is occupied. Moreover, for large magnetization the Hall conductance is, along with the density of states, sup- pressed. This numerical finding can be understood from analytical properties of the underlying model in the limit of an infinite system.

cond-mat.mes-hall↗
arXiv2009

Anisotropic magnetoresistance of spin-orbit coupled carriers scattered from polarized magnetic impurities

Anisotropic magnetoresistance (AMR) is a relativistic magnetotransport phenomenon arising from combined effects of spin-orbit coupling and broken symmetry of a ferromagnetically ordered state of the system. In this work we focus on one realization of the AMR in which spin-orbit coupling enters via specific spin-textures on the carrier Fermi surfaces and ferromagnetism via elastic scattering of carriers from polarized magnetic impurities. We report detailed heuristic examination, using model spin-orbit coupled systems, of the emergence of positive AMR (maximum resistivity for magnetization along current), negative AMR (minimum resistivity for magnetization along current), and of the crystalline AMR (resistivity depends on the absolute orientation of the magnetization and current vectors with respect to the crystal axes) components. We emphasize potential qualitative differences between pure magnetic and combined electro-magnetic impurity potentials, between short-range and long-range impurities, and between spin-1/2 and higher spin-state carriers. Conclusions based on our heuristic analysis are supported by exact solutions to the integral form of the Boltzmann transport equation in archetypical two-dimensional electron systems with Rashba and Dresselhaus spin-orbit interactions and in the three-dimensional spherical Kohn-Littinger model. We include comments on the relation of our microscopic calculations to standard phenomenology of the full angular dependence of the AMR, and on the relevance of our study to realistic, two-dimensional conduction-band carrier systems and to anisotropic transport in the valence band of diluted magnetic semiconductors.

cond-mat.mtrl-sci↗
arXiv2009

Electronic properties of graphene and graphene nanoribbons with "pseudo-Rashba" spin-orbit coupling

We discuss the electronic properties of graphene and graphene nanoribbons including "pseudo-Rashba" spin-orbit coupling. After summarizing the bulk properties, we first analyze the scattering behavior close to an infinite mass and zigzag boundary. For low energies, we observe strong deviations from the usual spin-conserving behavior at high energies such as reflection acting as spin polarizer or switch. This results in a spin polarization along the direction of the boundary due to the appearance of evanescent modes in the case of non-equilibrium or when there is no coherence between the two one-particle branches. We then discuss the spin and density distribution of graphene nanoribbons.

cond-mat.mes-hall↗
arXiv2008

Graphene in a Strong Magnetic Field: Massless Dirac Particles vs. Skyrmions

We discuss models for massless Dirac fermions being subject to a perpendicular magnetic field in spherical geometry. These models are analogues of Haldane`s spherical construction for massful charge carriers. The single particle states constructed here are easily implemented in existing numerical code for many-body problems in conventional quantum Hall systems. Moreover, the many-body states of fully filled sublevels in the subspace of lowest Landau level index are skyrmions with respect to the layer spin.

cond-mat.mes-hall↗
arXiv2008

Cyclotron motion in graphene

We investigate cyclotron motion in graphene monolayers considering both the full quantum dynamics and its semiclassical limit reached at high carrier energies. Effects of zitterbewegung due to the two dispersion branches of the spectrum dominate the irregular quantum motion at low energies and are obtained as a systematic correction to the semiclassical case. Recent experiments are shown to operate in the semiclassical regime.

cond-mat.mes-hall↗
arXiv2008

Cyclotron motion and magnetic focusing in semiconductor quantum wells with spin-orbit coupling

We investigate the ballistic motion of electrons in III-V semiconductor quantum wells with Rashba spin-orbit coupling in a perpendicular magnetic field. Taking into account the full quantum dynamics of the problem, we explore the modifications of classical cyclotron orbits due to spin-orbit interaction. As a result, for electron energies comparable with the cyclotron energy the dynamics are particularly rich and not adequately described by semiclassical approximations. Our study is complementary to previous semiclassical approaches concentrating on the regime of weaker fields.

cond-mat.mes-hall↗
arXiv2007

Minimum Electrical and Thermal Conductivity of Graphene: A Quasiclassical Approach

We investigate the minimum conductivity of graphene within a quasiclassical approach taking into account electron-hole coherence effects which stem from the chiral nature of low energy excitations. Relying on an analytical solution of the kinetic equation in the electron-hole coherent and incoherent cases we study both the electrical and thermal conductivity whose relation fullfills Wiedemann-Franz law. We found that the most of the previous findings based on the Boltzmann equation are restricted to only high mobility samples where electron-hole coherence effects are not sufficient.

cond-mat.mes-hall↗
arXiv2007

Spin orbit interaction and zitterbewegung in symmetric wells

Recently, we have introduced a novel inter-subband-induced spin-orbit (s-o) coupling [Phys. Rev. Lett. 99, 076603 (2007); cond-mat/0607218] arising in \textit{symmetric} wells with at least two subbands. This new s-o coupling gives rise to an usual zitterbewegung -- i.e. the semiconductor analog to the relativistic trembling motion of electrons -- with cycloidal motion without magnetic fields. Here we complement these findings by explicitly deriving expressions for the corresponding zitterbewegung in spin space.

cond-mat.mes-hall↗
arXiv2007

Spin-orbit interaction in symmetric wells with two subbands

We investigate the spin-orbit (s-o) interaction in two-dimensional electron gases (2DEGs) in quantum wells with two subbands. From the $8\times 8$ Kane model, we derive a new inter-subband-induced s-o term which resembles the functional form of the Rashba s-o -- but is non-zero even in \emph{symmetric} structures. This follows from the distinct parity of the confined states (even/odd) which obliterates the need for asymmetric potentials. We self-consistently calculate the new s-o coupling strength for realistic wells and find it comparable to the usual Rashba constant. Our new s-o term gives rise to a non-zero ballistic spin-Hall conductivity, which changes sign as a function of the Fermi energy ($ε_F$), and can induce an unusual \emph{zitterbewegung} with cycloidal trajectories \textit{without} magnetic fields.

cond-mat.mes-hall↗
arXiv2007

Spin dynamics in rolled-up two dimensional electron gases

A curved two dimensional electron gas with spin-orbit interactions due to the radial confinement asymmetry is considered. At certain relation between the spin-orbit coupling strength and curvature radius the tangential component of the electron spin becomes a conserved quantity for any spin-independent scattering potential that leads to a number of interesting effects such as persistent spin helix and strong anisotropy of spin relaxation times. The effect proposed can be utilized in the non-ballistic spin-field-effect transistors.

cond-mat.mes-hall↗
arXiv2007

Anisotropic current-induced spin accumulation in the two-dimensional electron gas with spin-orbit coupling

We investigate the magnetoelectric (or inverse spin-galvanic) effect in the two dimensional electron gases with both Rashba and Dresselhaus spin-orbit coupling using an exact solution of the Boltzmann equation for electron spin and momentum. The spin response to an in-plane electric field turns out to be highly anisotropic while the usual charge conductivity remains isotropic, contrary to earlier statements.

cond-mat.mes-hall↗