Search

We present a theory of electronic transport in graphene in the presence of randomly placed adsorbates. Our analysis predicts a marked asymmetry of the conductivity about the Dirac point, as well as a negative weak-localization magnetoresistivity. In the region of strong scattering, renormalization group corrections drive the system further towards insulating behavior. These results explain key features of recent experiments, and are validated by numerical transport computations.

The ability to build electronic structures from graphene sheets has progressed significantly. Two theoretical studies suggest that graphene nanostructures could be used for quantum information processing.

The authors model the optical visibility of monolayer and bilayer graphene deposited on a SiO2/Si substrate or thermally annealed on the surface of SiC. Visibility is much stonger in reflection than in transmission, reaching the optimum conditions when the bare substrate transmits light resonantly. In the optical range of frequencies a bilayer is approximately twice as visible as a monolayer thereby making the two types of graphene distinguishable from each other. ©2007 American Institute of Physics

We analyze the spectroscopic features of bilayer graphene determined by the formation of pairs of low-energy and split bands in this material. We show that the inter-Landau-level absorption spectrum in bilayer graphene at high magnetic field is much denser in the far-infrared range than that in monolayer material and that the polarization dependence of its lowest-energy peak can be used to test the form of the bilayer ground state in the quantum Hall-effect regime.

We show that in granular normal-ferromagnetic metals the giant magnetothermopower is related to the giant magnetoresistance as it is a result of the interplay between the spin-dependent elastic scattering (responsible for magnetoresistance effect) and the inelastic spin mixing scattering on magnetic clusters. For a small change of resistance of sample in an applied magnetic field the variation of the thermopower is connected linearly with the giant magnetoresistance and both are proportional to the square of the sample magnetization.

We develop a theory of the critical current in superconductor–ferromagnetic alloy–superconductor trilayers, which takes into account the strong spin dependence of electron scattering on compositional disorder in a diluted ferromagnetic alloy. We show that in such a system the critical current oscillations as a function of the thickness of the ferromagnetic layer, with period of vF/2I, vF and I being the Fermi velocity and exchange splitting, respectively, decay exponentially with a characteristic length of the order of the mean free path.

We describe a peculiar fine structure acquired by the in-plane optical phonon at the point in graphene when it is brought into resonance with one of the inter-Landau-level transitions in this material. The effect is most pronounced when this lattice mode (associated with the G band in graphene Raman spectrum) is in resonance with inter-Landau-level transitions 0+, 1 and -,10, at a magnetic field B030 T. It can be used to measure the strength of the electron-phonon coupling directly, and its filling-factor dependence can be used experimentally to detect circularly polarized lattice vibrations.

The focusing of electric current by a single p-n junction in graphene is theoretically predicted. Precise focusing may be achieved by fine-tuning the densities of carriers on the n- and p-sides of the junction to equal values. This finding may be useful for the engineering of electronic lenses and focused beam splitters using gate-controlled n-p-n junctions in graphene-based transistors.

Bilayer graphene (two coupled graphitic monolayers arranged according to Bernal stacking) is a two-dimensional gapless semiconductor with a peculiar electronic spectrum different from the Dirac spectrum in the monolayer material. In particular, the electronic Fermi line in each of its valleys has a strong p -> -p asymmetry due to trigonal warping, which suppresses the weak localization effect. We show that weak localization in bilayer graphene may be present only in devices with pronounced intervalley scattering, and we evaluate the corresponding magnetoresistance.

We show that Friedel oscillations (FO) in grapehene are strongly affected by the chirality of electrons in this material. In particular, the FO of the charge density around an impurity show a faster (~r-3) decay than in conventional 2D electron systems and do not contribute to a linear temperature-dependent correction to the resistivity. In contrast, the FO of the exchange field which surrounds atomically sharp defects breaking the hexagonal symmetry of the honeycomb lattice lead to a negative linear T dependence of the resistivity.

We propose a kinetic theory of transport in metals embedded with ferromagnetic nanoclusters, predicting a quadratic low-field magnetoresistance in an ensemble of clusters in thermodynamic equilibrium. We show that this effect is strongest when all the clusters have the same intrinsic easy-axis anisotropy.

We study the acoustoelectric effect generated by surface acoustic waves (SAW) in a high-mobility two-dimensional electron gas with isotropic and especially small-angle impurity scattering. In both cases the acoustoelectric effect exhibits Weiss oscillations periodic in B–1 due to the commensurability of the SAW period with the size of the cyclotron orbit and resonances at the SAW frequency =kc multiple of the cyclotron frequency. We describe how oscillations in the acoustoelectric effect are damped in low fields where c*1 (with the time scale * dependent on the type of scattering) and find its nonoscillatory part, which remains finite to the lowest fields.

We study theoretically the geometrical and temporal commensurability oscillations induced in the resistivity of two-dimensional electrons in a perpendicular magnetic field by surface acoustic waves (SAWs). We show that there is a positive anisotropic dynamical classical contribution and an isotropic nonequilibrium quantum contribution to the resistivity. We describe how the commensurability oscillations modulate the resonances in the SAW-induced resistivity at multiples of the cyclotron frequency. We study the effects of both short-range and long-range disorder on the resistivity corrections for both the classical and quantum nonequilibrium cases. We predict that the quantum correction will give rise to zero-resistance states with associated geometrical commensurability oscillations at large SAW amplitude for sufficiently large inelastic scattering times. These zero resistance states are qualitatively similar to those observed under microwave illumination, and their nature depends crucially on whether the disorder is short or long range. Finally, we discuss the implications of our results for current and future experiments on two-dimensional electron gases.

Inelastic spin relaxation and spin splitting s in lateral quantum dots are studied in the regime of strong in-plane magnetic field. Because of both the g-factor energy dependence and spin-orbit coupling, s demonstrates a substantial nonlinear magnetic field dependence similar to that observed by Hanson et al. [Phys. Rev. Lett. 91, 196802 (2003)]. It also varies with the in-plane orientation of the magnetic field due to crystalline anisotropy of the spin-orbit coupling. The spin relaxation rate is also anisotropic, the anisotropy increasing with the field. When the magnetic length is less than the "thickness" of the GaAs dot, the relaxation can be an order of magnitude faster for B||[100] than for B||[110].

We establish the consistency of the Fermi liquid description and find a relation between the Fermi liquid constants for a two-dimensional electron system near the point of full polarization due to a parallel magnetic field H. Our results enable us to predict connections between different thermodynamic properties of the system. In particular, we find that near the point of full polarization Hc, the thermodynamic compressibility of the system experiences a jump with a subleading (Hc-H)1/2 dependence on the magnetic field. Also, the magnetization has a cusp with dependence of the type (H-Hc)+(Hc-H)3/2 at H

We use the effective mass model to describe spinless electrons near the Fermi level in metallic, single-wall carbon nanotubes. Taking into account two nonequivalent valleys (K-points) produces a four component Dirac equation for massless fermions, with the role of spin assumed by pseudospin due to the relative amplitude of the wavefunction on the two nonequivalent sublattice atoms. We show that the position of a short-ranged impurity within the hexagonal graphite unit cell produces a particular 4×4 matrix structure of the corresponding effective Hamiltonian. The symmetry of this Hamiltonian with respect to pseudospin flip is related to degeneracy breaking and, for an armchair tube, symmetry with respect to mirror reflection in the nanotube axis is related to pseudospin mixing. In a nanotube of finite length, we predict a sinusoidal oscillation of energy level shift as a function of energy with a period determined by the position of the impurity along the tube axis.

A Comment on the Letter by Pritiraj Mohanty and Richard A. Webb, Phys. Rev. Lett. 88, 146601 (2002). The authors of the Letter offer a Reply.

The effects of interplay between spin-orbit coupling and Zeeman splitting on weak localization and universal conductance fluctuations in lateral semiconductor quantum dots are analyzed: All possible symmetry classes of corresponding random matrix theories are listed and crossovers between them achievable by sweeping magnetic field and changing the dot parameters are described. We also suggest experiments to measure the spin-orbit coupling constants.

A spectroscopic method is applied to measure the inelastic quasiparticle relaxation rate in a disordered Fermi liquid. The quasiparticle relaxation rate g is deduced from the magnitude of fluctuations in the local density of states, which are probed using resonant tunneling through a localized impurity state. We study its dependence on the excitation energy E measured from the Fermi level. In a disordered metal (heavily doped GaAs) we find that g ~ E32 within the experimentally accessible energy interval, in agreement with the Altshuler-Aronov theory for electron-electron interactions in diffusive conductors.

The local density of states of heavily doped GaAs is explored at high magnetic fields, where only a single or few Landau bands are occupied. Our experiment is based on resonant tunneling through impurity states and images the local density of states both below and above the Fermi level. Fan-type mesoscopic fluctuations are observed in the energy-magnetic-field plane, which we attribute to the interplay of Landau quantization and quantum interference of scattered electron waves in the disordered conductor. Our conclusion is supported by the suppression of the fluctuations high above the Fermi level, where dephasing due to inelastic processes is as fast as elastic scattering.