This paper develops a mathematical model that is based on the absorbing Markov chain approach to describe taxi movements, taking into account the stochastic searching processes of taxis in a network. The local searching behavior of taxis is specified by a logit form, and the O-D demand of passengers is estimated as a logit model with a choice of taxi meeting point. The relationship between customer and taxi waiting times is modeled by a double-ended queuing system. The problem is solved with a set of non-linear equations, and some interesting results are presented. The research provides a novel and potentially useful formulation for describing the urban taxi services in a network.
Microwave irradiation of Nb-InAs-Nb junctions reveals frequency-doubled Josephson currents which persist to high temperatures, in the absence of a critical current. A nonequilibrium dynamical model, based on time-dependent Andreev bound states, successfully accounts for the resulting half-integer Shapiro step and an enhancement in the conductance near zero bias.
The growth of modulation-doped InAs/(Al,Ga)Sb quantum wells on GaAs substrates employing molecular beam epitaxy requires care in the nucleation and the use of buffer layers to achieve high quality material. Despite a 7% lattice mismatch between the substrate and the active layers, fully relaxed epitaxial growth can be accomplished, and quantum wells with electron sheet concentrations of 7×1012 cm−2 having low-temperature mobilities as high as 300 000 cm2/V s have been routinely fabricated recently in our laboratory. In the present work the combination of atomic force microscopy and van der Pauw measurements is used to investigate and explain the strong influence of the buffer layers on the morphology in the quantum well that is shown to be responsible for the great differences in the observed low-temperature mobilities.
We have studied the current-voltage characteristics of superconducting weak links in which the coupling medium is the 2-D electron gas in InAs-based semiconductor quantum wells, with relatively large (typically 0.5µm) separations between niobium electrodes. The devices exhibit Josephson-like current-voltage characteristics; however, the falloff of the differential resistance with decreasing temperature is thermally activated, and is orders of magnitude slower than for more conventional weak links. Most unexpectedly, the thermal activation energies are found to be proportional to the width of the device, taken perpendicular to the current flow. This behavior falls outside the range of established theories; we propose that it is a fluctuation effect caused by giant shot noise associated with multiple Andreev reflections. The possibility of non-equilibrium effects is discussed.
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