No abstract

The resistivity of gated graphene is studied taking into account electron and hole scattering by short-and long-range structural imperfections (the characteristics of disorder were taken from the scanning tunneling microscopy data) and by acoustic phonons. The calculations are based on the quasiclassical kinetic equation with the normalization condition fixed by surface charge. The gatevoltage and temperature effects on the resistance peak, which is centered at the point of intrinsic conductivity, are found to be in agreement with the transport measurements.

We examine the photoconductivity of an intrinsic graphene associated with far-and mid-infrared irradiation at low temperatures. The model under consideration accounts for the excitation of the electron-hole pairs by incident radiation, the interband generation-recombination transitions due to thermal radiation, and the intraband energy relaxation due to acoustic phonon scattering. The momentum relaxation is assumed to be caused by elastic scattering. The pertinent collision integrals are adapted for the case of the massless energy spectrum of carriers that interact with the longitudinal acoustic mode and the thermal radiation. It is found that the photoconductivity is determined by interplay between weak energy relaxation and generation-recombination processes. Due to this the threshold of nonlinear response is fairly low.

We study nonequilibrium carriers (electrons and holes) in an intrinsic graphene at low temperatures under far-and mid-infrared (IR) radiation in a wide range of its intensities. The energy distributions of carriers are calculated using a quasiclassic kinetic equation which accounts for the energy relaxation due to acoustic phonons and the radiative generation-recombination processes associated with thermal radiation and the carrier photoexcitation by incident radiation. It is found that the nonequilibrium distributions are determined by an interplay between weak energy relaxation on acoustic phonons and generation-recombination processes as well as by the effect of pumping saturation. Due to the effect of saturation, the carrier distribution functions can exhibit plateaus around the pumping region at elevated intensities. As shown, at sufficiently strong mid-IR pumping, the population inversion can occur below the pumping energy. The graphene dc conductivity as a function of the pumping intensity exhibits a pronounced nonlinearity with a sub-linear region at fairly low intensities and a saturation at a strong pumping. However, an increase in the pumping intensity in very wide range leads only to a modest increase in the carrier concentration and, particularly, the dc conductivity. The graphene conductivity at mid-IR irradiation exhibit strong sensitivity to mechanisms of carrier momentum relaxation.

Heating of carriers in an intrinsic graphene under dc electric field is considered taking into account the intraband energy relaxation due to acoustic phonon scattering and the interband generationrecombination transitions due to thermal radiation. The distribution of nonequilibrium carriers is obtained for the cases when the intercarrier scattering is unessential and when the carrier-carrier Coulomb scattering effectively establishes the quasiequilibrium distribution with the temperature and the density of carriers that are determined by the balance equations. Because of an interplay between weak energy relaxation and generation-recombination processes a very low threshold of nonlinear response takes place. The nonlinear current-voltage characteristics are calculated for the case of the momentum relaxation caused by the elastic scattering. Obtained current-voltage characteristics show low threshold of nonlinear behavior and appearance of the second ohmic region, for strong fields.

In the last couple of decades, the world has seen several stunning instances of quantum algorithms that provably outperform the best classical algorithms.For most problems, however, it is currently unknown whether quantum algorithms can provide an advantage, and if so by how much, or how to design quantum algorithms that realize such advantages. Many of the most challenging computational problems arising in the practical world are tackled today by heuristic algorithms that have not been mathematically proven to outperform other approaches but have been shown to be effective empirically. While quantum heuristic algorithms have been proposed, empirical testing becomes possible only as quantum computation hardware is built. The next few years will be exciting as empirical testing of quantum heuristic algorithms becomes more and more feasible. While large-scale universal quantum computers are likely decades away, special-purpose quantum computational hardware has begun to emerge that will become more powerful over time, as well as some small-scale universal quantum computers. * Rupak Biswas

The tunneling current between independently contacted graphene sheets separated by boron nitride insulator is calculated. Both dissipative tunneling transitions, with momentum transfer due to disorder scattering, and non-dissipative regime of tunneling, which appears due to intersection of electron and hole branches of energy spectrum, are described. Dependencies of tunneling current on concentrations in top and bottom graphene layers, which are governed by the voltages applied through independent contacts and gates, are considered for the back-and double-gated structures. The current-voltage characteristics of the back-gated structure are in agreement with the recent experiment [Science 335, 947 (2012)]. For the double-gated structures, the resonant dissipative tunneling causes a ten times enhancement of response which is important for transistor applications.

The transient response of an intrinsic graphene, which is caused by the ultrafast interband transitions, is studied theoretically for the range of pumping correspondent to the saturated absorption regime. Spectral and temporal dependencies of the photoexcited concentration as well as the transmission and relitive absotption coefficients are considered for mid-IR and visible (or near-IR) spectral regions at different durations of pulse and broadening energies. The characteristic intencities of saturation are calculated and the results are compared with the experimental data measured for the near-IR lasers with a saturable absorber. The negative absorption of a probe radiation during cascade emission of optical phonons is obtained.