Magnetoresistance measurements of graphene at the charge neutrality point

We report on transport measurements of the insulating state that forms at the charge neutrality point of graphene in a magnetic field. Using both conventional two-terminal measurements, sensitive to bulk and edge conductance, and Corbino measurements, sensitive only to the bulk conductance, we observed a vanishing conductance with increasing magnetic fields. By examining the resistance changes of this insulating state with varying perpendicular and in-plane fields, we probe the spin-active components of the excitations in total fields of up to 45?T. Our results indicate that the ν=0 quantum Hall state in single layer graphene is not spin-polarized.     

Justification of the local space charge neutrality hypothesis in metal oxidation

The important local charge neutrality hypothesis in metal oxidation theory is shown numerically to be self-consistent with Poisson's equation and the coupled transport equations for diffusion of oppositely-charged ionic and electronic defect species deep within very thick growing oxides.     

General transport properties of superconducting quantum point contacts: a Green functions approach

We discuss the general transport properties of superconducting quantum point contacts. We show how these properties can be obtained from a microscopic model using nonequilibrium Green’s function techniques. For the case of a one-channel contact we analyze the response under different biasing conditions: constant applied voltage, current bias and microwave-induced transport. Current fluctuations are also analyzed with particular emphasis on thermal and shot-noise. Finally, the case of superconducting transport through a resonant level is discussed. The calculated properties show a remarkable agreement with the available experimental data from atomic-size contacts measurements. We suggest the possibility of extending this comparison to several other predictions of the theory.     

Dissipative quantum hall effect in graphene near the Dirac point

We report on the unusual nature of the nu=0 state in the integer quantum Hall effect (QHE) in graphene and show that electron transport in this regime is dominated by counterpropagating edge states. Such states, intrinsic to massless Dirac quasiparticles, manifest themselves in a large longitudinal resistivity rho(xx) > or approximately h/e(2), in striking contrast to rho(xx) behavior in the standard QHE. The nu=0 state in graphene is also predicted to exhibit pronounced fluctuations in rho(xy) and rho(xx) and a smeared zero Hall plateau in sigma(xy), in agreement with experiment. The existence of gapless edge states puts stringent constraints on possible theoretical models of the nu=0 state.     

Carrier relaxation in epitaxial graphene photoexcited near the Dirac point

We study the carrier dynamics in epitaxially grown graphene in the range of photon energies from 10 to 250 meV. The experiments complemented by microscopic modeling reveal that the carrier relaxation is significantly slowed down as the photon energy is tuned to values below the optical-phonon frequency; however, owing to the presence of hot carriers, optical-phonon emission is still the predominant relaxation process. For photon energies about twice the value of the Fermi energy, a transition from pump-induced transmission to pump-induced absorption occurs due to the interplay of interband and intraband processes.     

Quantum Hall states near the charge-neutral Dirac point in graphene

We investigate the quantum Hall (QH) states near the charge-neutral Dirac point of a high mobility graphene sample in high magnetic fields. We find that the QH states at filling factors nu=+/-1 depend only on the perpendicular component of the field with respect to the graphene plane, indicating that they are not spin related. A nonlinear magnetic field dependence of the activation energy gap at filling factor nu=1 suggests a many-body origin. We therefore propose that the nu=0 and +/-1 states arise from the lifting of the spin and sublattice degeneracy of the n=0 Landau level, respectively.     

Negative and positive magnetoresistance in bilayer graphene: Effects of weak localization and charge inhomogeneity

We report measurements of magnetoresistance in bilayer graphene as a function of gate voltage (carrier density) and temperature. We examine multiple contributions to the magnetoresistance, including those of weak localization (WL), universal conductance fluctuations (UCF), and inhomogeneous charge transport. A clear WL signal is evident at all measured gate voltages (in the hole doped regime) and temperature ranges (from 0.25 to 4.3 K), and the phase coherence length extracted from the WL data does not saturate at low temperatures. The WL data is fit to demonstrate that the electron-electron Nyquist scattering is the major source of phase decoherence. A decrease in UCF amplitude with increase in gate voltage and temperature is shown to be consistent with a corresponding decrease in the phase coherence length. In addition, a weak positive magnetoresistance at higher magnetic fields is observed, and attributed to inhomogeneous charge transport.     

Self-energy of a charge near a metal surface

A quantum mechanical expression for the self-energy of a charged particle near a metal surface has been derived. The expression incorporates the interaction of the charge with the surface and bulk plasmons including their dispersions in the hydrodynamic model. It is found that the inclusion of the plasmons dispersions gives result for the saturated value of the image potential for copper closer to experiment.     

Influence of density inhomogeneity on the quantum capacitance of graphene nanoribbon field effect transistors

A semi-analytical model for the capacitance–voltage characteristics of graphene nanoribbon field-effect transistors (GNR-FETs), in the quantum capacitance limit, is presented. The model incorporates the presence of electron–hole puddles induced by local potential fluctuations assuming a Gaussian distribution associated with these puddles. Our numerical results show that the multi-peaks in the non-monotonic quantum capacitance–voltage characteristics are broadened as the potential fluctuation strength increases and the broadening effect is much more pronounced in wide GNRs. The influence of both gate-insulator thickness and dielectric constant scaling on the total gate-capacitance characteristics is also explored. Gate capacitance has non-monotonic behavior with ripples for thin gate-insulators. However, as we go beyond the quantum capacitance limit by increasing insulator thickness or decreasing dielectric constant, the ripples are suppressed and smooth monotonic characteristics are obtained.     

The effect of electric charge on the mechanical properties of graphene

The effect of electric charge on the mechanical properties of graphene under tensile loading is investigated by using molecular dynamics method.A modified atomistic moment method based on the classical electrostatics theory is proposed to obtain the distribution of extra charges induced by an external electric field and net electric charges stored in graphene.The electrostatic interactions between charged atoms are calculated using the coulomb law.The results show that the Young’s modulus and the critical fracture stress under uniaxial tension decrease with the increase of electric potential and net charges on graphene.The failure of graphene induced by electric charges is found to be controlled by charge level.The results indicate that the carbon-carbon bonds at the edge of graphene will break first.     

Counting statistics and detector properties of quantum point contacts

Quantum detector properties of the quantum point contact (QPC) are analyzed for an arbitrary electron transparency and coupling strength to the measured system and are shown to be determined by the electron counting statistics. Conditions of the quantum-limited operation of the QPC detector, which prevent information loss through the scattering time and scattering phases, are found for arbitrary coupling. We show that the phase information can be restored and used for the quantum-limited detection by inclusion of the QPC detector in the electronic Mach-Zehnder interferometer.     

Transition metal and rare-earth metal atoms on single-layer graphene: Estimations of the charge transfer and adsorption energy

A Hamiltonian accounting for s- and d(f)-states of adatoms has been proposed to describe adsorption of atoms of d- and f-metals on single-sheet graphene. It has been shown that s-electrons mainly contribute to the charge transfer Δn _a between the adatom and graphene substrate. Analytical formulas proposed previously within the M-model of the graphene density of states are used for Δn _a calculations. To estimate the adsorption energy, a simple analytical expression is proposed. Calculations are performed for 3d-, 4d-, 5d-, and 4f-adatoms. The results of calculations are compared with the data obtained by other authors.     

Shot noise fluctuations in disordered graphene nanoribbons near the Dirac point

Random fluctuations of the shot-noise power in disordered graphene nanoribbons are studied. In particular, we calculate the distribution of the shot noise of nanoribbons with zigzag and armchair edge terminations. We show that the shot noise statistics is different for each type of these two graphene structures, which is a consequence of the presence of different electron localizations: while in zigzag nanoribbons electronic edge states are Anderson localized, in armchair nanoribbons edge states are absent, but electrons are anomalously localized. Our analytical results are verified by tight binding numerical simulations with random hopping elements, i.e., off diagonal disorder, which preserves the symmetry of the graphene sublattices.     

Crossover of the conductivity of zigzag graphene nanoribbon connected by normal metal contacts

The transport property of zigzag graphene nanoribbon (ZGNR) connected by two normal metal contacts is investigated by Landauer-Buttiker formula combined with transfer matrix method. In addition to even-odd parity, we found that the conductivity is completely determined by the width-to-length ratio. For certain wide ZGNR with even number carbon atoms in the width direction, the conductivity dependence on the length changes from linearly to inversely, when the length approaches the thermodynamic limit, as the transport property is quite different for different aspect of ZGNR.     

Metal-molecule contacts and charge transport across monomolecular layers: measurement and theory

Charge transport studies across molecular length scales under symmetric and asymmetric metal-molecule contact conditions using a simple crossed-wire tunnel junction technique are presented. It is demonstrated that oligo(phenylene ethynylene), a conjugated organic molecule, acts like a molecular wire under symmetric contact conditions, but exhibits characteristics of a molecular diode when the connections are asymmetric. To understand this behavior, we have calculated current-voltage (I-V) characteristics using extended Huckel theory coupled with a Green's function approach. The experimentally observed I-V characteristics are in excellent qualitative agreement with the theory.     

Resistivity of ferromagnetic metal near the reorientation phase transition point

Anomalies of the resistivity near the reorientation phase transition point are studied theoretically. The resistivity is shown to have an asymmetrical peak at the point of the transition. The derivatives of the resistivity with respect to temperature as well as magnetic field diverge at this point.     

Influence of laser light on electronic transport through atomic-size contacts

This Letter reports on the influence of laser irradiation onto the electrical conductance of gold nanocontacts established with the mechanically controllable break-junction technique. We concentrate on the study of reversible conductance changes which can be as high as 200%. We investigate the dependence on the initial conductance of the contacts, and on the wavelength, the intensity, and the position of the laser spot with respect to the sample. Under most conditions an enhancement of the conductance is observed. Several physical mechanisms which might contribute to the observed effect including thermal expansion, rectification, plasmon excitation, and photon-assisted transport are discussed, among which the two latter ones are most likely the dominating ones.