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arXiv2009

UV-photon and electrically driven resistance switching in ZnO nanotube arrays

Vertically aligned ZnO nanotube arrays fabricated on an ITO substrate are found to exhibit strong persistent photoconductivity (PPC) effect and electrically driven conductance switching behavior, though the latter shows a gradual decay from high conductance state to a low conductance state. Unlike the electrical switching, the PPC cannot be reset or reversed by an electrical pulse. Excitation wavelength dependent conductance measurement indicates the presence of the defect localized states (DLS) ~ 240meV above the valence band edge, in support of the hypothesis that the doubly ionization of these DLS are responsible for the PPC effect.

cond-mat.mes-hall↗
arXiv2008

Electron drift velocity control in GaAs-in-Al2O3 quantum wire transistor structure due to the electron scattering rate alteration

Electron transport in the transistor structure based on thin undoped GaAs-in-Al2O3 quantum wire is simulated by ensemble Monte-Carlo method taking into account electron scattering by the phonons and surface roughness. The influence of surface roughness height on electron drift velocity at 77 and 300 K is investigated for the values of longitudinal electric field strength of 0.1 and 1.0 kV/cm. A possibility of electron drift velocity control due to variation of the bias applied to the gates, which results in the electron scattering rate alteration, is ascertained.

cond-mat.mes-hall↗
arXiv2008

Relativistic BCS Theory in Quasi-(2+1)-Dimensions: Effects of an Inter-layer Transfer in the Honeycomb Lattice Symmetry

Motivated by recent huge interests on graphene sheet and graphite as "relativistic" systems, a BCS superconductivity in a quasi-(2+1)-dimensional relativistic model is investigated. The intra-layer particle dynamics is described by a (2+1)-dimensional Gross-Neveu-type four-body contact interaction model, while inter-layer particle motions will be caused by a hopping term with a transfer parameter $t$. Especially, we examine the effects of non-vanishing $t$, chemical potential $μ$, and a mass parameter $m$ which will give a gap at a conical intersection point of the relativistic band dispersion, in the BCS s-wave (scalar) superconducting gap function $Δ$.

cond-mat.supr-con↗
arXiv2008

Analytical Tight-binding Approach for Ballistic Transport through Armchair Graphene Ribbons: Exact Solutions for Propagation through Step-like and Barrier-like Potentials

Based on a tight-binding approximation, we present analytical solutions for the wavefunction and propagation velocity of an electron in armchair graphene ribbons. The derived expressions are used for computing the transmission coefficients through step-like and barrier-like potentials. Our analytical solutions predict a new kind of transmission resonances for one-mode propagation in semiconducting ribbons. Contrary to the Klein paradox in graphene, this approach shows that backscattering for gapless mode is possible. In consistence with a higher order $\bf{k\cdot p}$ method, the backscattering probabilities vary with the square of the applied potential in the low-energy limit. We also demonstrate that gapless-mode propagation through a potential step in armchair ribbons can be described by the same through-step relation as that for an undimerized 1D chain of identical atoms.

cond-mat.mes-hall↗
arXiv2008

Decoherence of coupled qubit system

In this study, we examine decoherence of qubits system coupled independently by using influence functional. We especially concentrated on the effect of qubit flip process. We examine the zero-dimensional qubit and one-dimensional qubits system coupled with multi-band one-dimensional system. The qubit flip process leads to oscillation and self-excitation. We also obtained the time evolution of von Neumann entropy.The pure dephasing channel indicates entropy. These results show quantum brown motion. We present the numerical calculation of the entropy for random telegraph noise. This indicates that the slower random telegraph noise leads to more mixed state.Thus for charge qubit, the background charge fluctuation is important.

quant-ph↗
arXiv2008

Exciton many-body effects through infinite series of composite-exciton operators

We revisit the approach proposed by Mukamel and coworkers to describe interacting excitons through infinite series of composite-boson operators for both, the system Hamiltonian and the exciton commutator -- which, in this approach, is properly kept different from its elementary boson value. Instead of free electron-hole operators, as used by Mukamel's group, we here work with composite-exciton operators which are physically relevant operators for excited semiconductors. This allows us to get \emph{all} terms of these infinite series explicitly, the first terms of each series agreeing with the ones obtained by Mukamel's group when written with electron-hole pairs. All these terms nicely read in terms of Pauli and interaction scatterings of the composite-exciton many-body theory we have recently proposed. However, even if knowledge of these infinite series now allows to tackle $N$-body problems, not just 2-body problems like third order nonlinear susceptibility $χ^{(3)}$, the necessary handling of these two infinite series makes this approach far more complicated than the one we have developed and which barely relies on just four commutators.

cond-mat.mes-hall↗
arXiv2008

Numerical Studies of Quantum Hall Ferromagnetism in Two-Subband Systems

We carry out a numerical study of the quantum Hall ferromagnetism in a two-subband system using a set of experimental parameters in a recently experiment [X. C. Zhang, I. Martin, and H. W. Jiang, Phys. Rev. B \textbf{74}, 073301 (2006)]. Employing the self-consistence local density approximation for growth direction wave function and the Hartree-Fock theory for the pseudospin anisotropy energy, we are able to account for the easy-axis and easy-plane quantum Hall ferromagnetism observed at total filling factor $ν= 3$ and $ν= 4$, respectively. Our study provides some insight of how the anisotropy energy, which highly depends upon the distribution of growth direction wave functions, determines the symmetry of the quantum Hall ferromagnets.

cond-mat.mes-hall↗
arXiv2008

Intense Terahertz Laser Fields Induced and Manipulated Pseudospin Polarization in Graphene

We investigate the pseudospin-dependent density-energy relation (whose differential with respect to energy is density of states) of a monolayer graphene under intense terahertz laser field by exactly solving the time-dependent Schrödinger equation with help of Floquet's theorem. We find that psedospin polarization can be induced by a circular polarized terahertz laser field. The psedospin polarization in K and K$^{\prime}$ valleys can be exactly opposite sign when the electron densities in these two calleys are equivalent. Further more, we find that the psedospin polarization can be manipulated by the strength, frequency, and especially the polarization orientation of the field.

cond-mat.mes-hall↗
arXiv2009

Spin rotation, spin filtering, and spin transfer in directional tunneling through non-centrosymmetric semiconductor barriers

We consider spin-dependent tunneling through a gallium arsenide barrier, a material which has no inversion symmetry. We are dealing with free electrons, with one effective mass and a spin-splitting in the barrier material. When we take into account both the spin-orbit interaction and the absence of the inversion symmetry, the evanescent states in the barrier are spin split and the tunneling process can become rather involved. Along some crystallographic directions, the incident wave experiences spin filtering during the tunneling. These results open stimulating perspectives for spin manipulation in tunnel devices.

cond-mat.mes-hall↗
arXiv2011

Temperature effects on magnetoplasmon spectrum of a weakly modulated graphene monolayer

In this work, we determine the effects of temperature on the magnetoplasmon spectrum of an electrically modulated graphene monolayer as well as the two-dimensional electron gas (2DEG). The intra-Landau-band magnetoplasmon spectrum within the Self Consistent Field (SCF) approach is investigated for both the aforementioned systems. Results obtained not only exhibit Shubnikov-de Hass (SdH) oscillations but also commensurability oscillations (Weiss oscillations). These oscillations are periodic as a function of inverse magnetic field. We find that both the magnetic oscillations, SdH and Weiss, have a greater amplitude and are more robust against temperature in graphene compared to conventional 2DEG. Furthermore, there is a $π$ phase shift between the magnetoplasmon oscillations in the two systems which can be attributed to Dirac electrons in graphene acquiring a Berry's phase as they traverse a closed path in a magnetic field.

cond-mat.mes-hall↗
arXiv2009

Absence of diffusion in certain random lattices: Numerical evidence

We demonstrate, by solving numerically the time-dependent Schroedinger equation, the physical character of electron localization in a disordered two-dimensional lattice. We show, in agreement with the prediction of P. W. Anderson, that the disorder prevents electron diffusion. The electron becomes spatially localized in a specific area of the system. Our numerical analysis confirms that the electron localization is a quantum effect caused by the wave character of electron propagation and has no analogy in classical mechanics.

cond-mat.dis-nn↗
arXiv2008

Decay of a metastable state activated by non-Gaussian noise: A critical review of the generalized Kramers problem

We review the problem of the noise activated escape from a metastable state in the presence of non-Gaussian noise, and present connections between various theoretical approaches. We also respond to criticism of our work by Tomáš Novotný [arXiv:0807.0387] concerning the weak damping limit. The discrepancy between our results is linked to uncontrolled approximations made by Novotný.

cond-mat.mes-hall↗
arXiv2009

Onset of an Insulating Zero-Plateau Quantum Hall State in Graphene

We analyze the dissipative conductance of the zero-plateau quantum Hall state appearing in undoped graphene in strong magnetic fields. Charge transport in this state is assumed to be carried by a magnetic domain wall, which forms by hybridization of two counter--propagating edge states of opposing spin due to interactions. The resulting non--chiral edge mode is a Luttinger liquid of parameter K, which enters a gapped, perfectly conducting state below a critical value K_c\approx 1/2. Backscattering in this system involves spin flip, so that interaction with localized magnetic moments generates a finite resistivity R_{xx} via a "chiral Kondo effect". At finite temperatures T, R_{xx}(T) exhibits a crossover from metallic to insulating behavior as K is tuned across a threshold K_{MI}. For T->0, R_{xx} in the intermediate regime K_{MI}<K<K_c is finite, but diverges as K approaches K_c. This model provides a natural interpretation of recent experiments.

cond-mat.mes-hall↗
arXiv2008

Molecular Electronics: Effect of external electric field

The effect of electric field, applied on systems in the nanoscale regime has attracted a lot of research in recent times. We highlight some of the recent results in the field of single molecule electronics and then move on to focus on some of our own results in this area. We have first shown how important it is to obtain the spatial profile of the external bias potential across the system, and how this would change in the presence of electron-electron interactions. We have also studied different kinds of insulators in the presence of the spatially varying external bias, and have explicitly shown that a two sublattice structure, caused either by a lattice distortion, or by the presence of substituents with strong dipolar nature, can result in negative differential resistance (NDR) in the transport characteristics. We also find this to be true in case of correlated insulators. Additionally, we have shown clear NDR behavior in a correlated double quantum dot by tuning the electron-electron interaction strength in the system.

cond-mat.mtrl-sci↗
arXiv2008

Dirac charge dynamics in graphene by infrared spectroscopy

A remarkable manifestation of the quantum character of electrons in matter is offered by graphene, a single atomic layer of graphite. Unlike conventional solids where electrons are described with the Schrodinger equation, electronic excitations in graphene are governed by the Dirac Hamiltonian. Some of the intriguing electronic properties of graphene, such as massless Dirac quasiparticles with linear energy-momentum dispersion, have been confirmed by recent observations. Here we report an infrared (IR) spectromicroscopy study of charge dynamics in graphene integrated in gated devices. Our measurements verify the expected characteristics of graphene and, owing to the previously unattainable accuracy of IR experiments, also uncover significant departures of the quasiparticle dynamics from predictions made for Dirac fermions in idealized, free standing graphene. Several observations, including a marked deviation of the energy bands from the simple linear dispersion, point to the relevance of many body interactions to the rather exotic electromagnetic response in graphene.

cond-mat.mes-hall↗
arXiv2008

Collective Excitations of Rotating Dipolar Fermi Gases in the Fractional Quantum Hall Regime

We apply the magneto-roton theory of the fractional quantum Hall effect to study the collective excitation spectrum of rotating dipolar Fermi gases. The predicted spectrum has a finite energy gap in the long wavelength limit and a roton minimum at finite wave vector. The roton minimum being deepened from filling factor 1/3 to filling factor 1/5 is a signature of incipient crystallization near filling factor 1/7. We also demonstrate that there are no low-lying single-particle excitations below the roton mode. The fractional-quantum-Hall fluid rotating dipolar fermions behaves as an incompressible superfluid at low temperature.

cond-mat.mes-hall↗
arXiv2008

Ab-initio band structure of doped graphene

We present an \emph{ab-initio} study of the graphene quasi-particle band structure as function of the doping in G_0 W_0 approximation. We show that the LDA Fermi velocity is substantially renormalized and this renormalization rapidly decreases as function of the doping. We found, in agreement with previous papers, that close to the Dirac point the linear dispersion of the bands is broken but this behaviour disappears with a small doping. We discuss our results in the light of recent experiments on graphene and intercalate graphite.

cond-mat.mes-hall↗