Sub-electron Charge Relaxation via 2D Hopping Conductors

We have extended Monte Carlo simulations of hopping transport in completely disordered 2D conductors to the process of external charge relaxation. In this situation, a conductor of area $L \times W$ shunts an external capacitor $C$ with initial charge $Q_i$. At low temperatures, the charge relaxation process stops at some "residual" charge value corresponding to the effective threshold of the Coulomb blockade of hopping. We have calculated the r.m.s$.$ value $Q_R$ of the residual charge for a statistical ensemble of capacitor-shunting conductors with random distribution of localized sites in space and energy and random $Q_i$, as a function of macroscopic parameters of the system. Rather unexpectedly, $Q_{R}$ has turned out to depend only on some parameter combination: $X_0 \equiv L W \nu_0 e^2/C$ for negligible Coulomb interaction and $X_{\chi} \equiv LW \kappa^2/C^{2}$ for substantial interaction. (Here $\nu_0$ is the seed density of localized states, while $\kappa$ is the dielectric constant.) For sufficiently large conductors, both functions $Q_{R}/e =F(X)$ follow the power law $F(X)=DX^{-\beta}$, but with different exponents: $\beta = 0.41 \pm 0.01$ for negligible and $\beta = 0.28 \pm 0.01$ for significant Coulomb interaction. We have been able to derive this law analytically for the former (most practical) case, and also explain the scaling (but not the exact value of the exponent) for the latter case. In conclusion, we discuss possible applications of the sub-electron charge transfer for "grounding" random background charge in single-electron devices.Comment: 12 pages, 5 figures. In addition to fixing minor typos and updating references, the discussion has been changed and expande

A Numerical Study of Coulomb Interaction Effects on 2D Hopping Transport

We have extended our supercomputer-enabled Monte Carlo simulations of hopping transport in completely disordered 2D conductors to the case of substantial electron-electron Coulomb interaction. Such interaction may not only suppress the average value of hopping current, but also affect its fluctuations rather substantially. In particular, the spectral density $S_I (f)$ of current fluctuations exhibits, at sufficiently low frequencies, a $1/f$-like increase which approximately follows the Hooge scaling, even at vanishing temperature. At higher $f$, there is a crossover to a broad range of frequencies in which $S_I (f)$ is nearly constant, hence allowing characterization of the current noise by the effective Fano factor F\equiv S_I(f)/2e \left. For sufficiently large conductor samples and low temperatures, the Fano factor is suppressed below the Schottky value (F=1), scaling with the length $L$ of the conductor as $F = (L_c / L)^{\alpha}$. The exponent $\alpha$ is significantly affected by the Coulomb interaction effects, changing from $\alpha = 0.76 \pm 0.08$ when such effects are negligible to virtually unity when they are substantial. The scaling parameter $L_c$, interpreted as the average percolation cluster length along the electric field direction, scales as $L_c \propto E^{-(0.98 \pm 0.08)}$ when Coulomb interaction effects are negligible and $L_c \propto E^{-(1.26 \pm 0.15)}$ when such effects are substantial, in good agreement with estimates based on the theory of directed percolation.Comment: 19 pages, 7 figures. Fixed minor typos and updated reference

A Numerical Study of Transport and Shot Noise at 2D Hopping

We have used modern supercomputer facilities to carry out extensive Monte Carlo simulations of 2D hopping (at negligible Coulomb interaction) in conductors with the completely random distribution of localized sites in both space and energy, within a broad range of the applied electric field $E$ and temperature $T$, both within and beyond the variable-range hopping region. The calculated properties include not only dc current and statistics of localized site occupation and hop lengths, but also the current fluctuation spectrum. Within the calculation accuracy, the model does not exhibit $1/f$ noise, so that the low-frequency noise at low temperatures may be characterized by the Fano factor $F$. For sufficiently large samples, $F$ scales with conductor length $L$ as $(L_c/L)^{\alpha}$, where $\alpha=0.76\pm 0.08 < 1$, and parameter $L_c$ is interpreted as the average percolation cluster length. At relatively low $E$, the electric field dependence of parameter $L_c$ is compatible with the law $L_c\propto E^{-0.911}$ which follows from directed percolation theory arguments.Comment: 17 pages, 8 figures; Fixed minor typos and updated reference

Variable range hopping conduction in semiconductor nanocrystal solids

The temperature and electrical field dependent conductivity of n-type CdSe nanocrystal thin films is investigated. In the low electrical field regime, the conductivity follows G ~ exp(-(T*/T)^0.5) in the temperature range 10K<T<120K. At high electrical field, the conductivity is strongly field dependent. At 4K, the conductance increases by eight orders of magnitude over one decade of bias. At very high field, conductivity is temperature-independent with G ~ exp(-(E*/E)^0.5). The complete behavior is very well described by variable range hopping with Coulomb gap.Comment: Submitted to Physical Review Letters on Dec. 22, 200

The Wind Energy Potential of Iceland

AbstractDownscaling simulations performed with the Weather Research and Forecasting (WRF) model were used to determine the large-scale wind energy potential of Iceland. Local wind speed distributions are represented by Weibull statistics. The shape parameter across Iceland varies between 1.2 and 3.6, with the lowest values indicative of near-exponential distributions at sheltered locations, and the highest values indicative of normal distributions at exposed locations in winter. Compared with summer, average power density in winter is increased throughout Iceland by a factor of 2.0–5.5. In any season, there are also considerable spatial differences in average wind power density. Relative to the average value within 10 km of the coast, power density across Iceland varies between 50 and 250%, excluding glaciers, or between 300 and 1500 W m−2 at 50 m above ground level in winter. At intermediate elevations of 500–1000 m above mean sea level, power density is independent of the distance to the coast. In addition to seasonal and spatial variability, differences in average wind speed and power density also exist for different wind directions. Along the coast in winter, power density of onshore winds is higher by 100–700 W m−2 than that of offshore winds. Based on these results, 14 test sites were selected for more detailed analyses using the Wind Atlas Analysis and Application Program (WAsP)

WAVEGUIDE AND SURFACE PLASMON COUPLED INFRARED DEVICES USING SEMICONDUCTOR QUANTUM WELLS

In this paper we propose and present detailed calculations on a new method for using the intersubband transition in modulation doped semiconductor quantum wells grown from AlxGa1-xAs and GaAs in infra red devices (modulators and detectors) working at 10 um wavelength. The quantum wells are embedded near the the surface of a thick AlAs layer (1-9 microns) and capped by a metal electrode and coupling to the intersubband transition is mediated by either a "leaky" guided mode or a surface plasmon. Our calculations show that coupling of radiation to intersubband transitions can be strong under such circumstances