442 research outputs found
Negative Differential Spin Conductance by Population Switching
An examination of the properties of many-electron conduction through
spin-degenerate systems can lead to situations where increasing the bias
voltage applied to the system is predicted to decrease the current flowing
through it, for the electrons of a particular spin. While this does not
necessarily constitute negative differential conductance (NDC) per se, it is an
example of negative differential conductance per spin (NDSC) which to our
knowledge is discussed here for the first time. Within a many-body master
equation approach which accounts for charging effects in the Coulomb Blockade
regime, we show how this might occur.Comment: 6 page, 2 figure
Memory Effects In Nonequilibrium Quantum Impurity Models
Memory effects play a key role in the dynamics of strongly correlated systems
driven out of equilibrium. In the present study, we explore the nature of
memory in the nonequilibrium Anderson impurity model. The
Nakajima--Zwanzig--Mori formalism is used to derive an exact generalized
quantum master equation for the reduced density matrix of the interacting
quantum dot, which includes a non-Markovian memory kernel. A real-time path
integral formulation is developed, in which all diagrams are stochastically
sampled in order to numerically evaluate the memory kernel. We explore the
effects of temperature down to the Kondo regime, as well as the role of
source--drain bias voltage and band width on the memory. Typically, the memory
decays on timescales significantly shorter than the dynamics of the reduced
density matrix itself, yet under certain conditions it develops a smaller long
tail. In addition we address the conditions required for the existence,
uniqueness and stability of a steady-state.Comment: 4 pages, 3 figure
Dynamics and kinetics of molecular high Rydberg states in the presence of an electrical field an experimental and classical computational study
Constructing Spin Interference Devices from Nanometric Rings
The study of nanospintronic devices utilizing coherent transport through
molecular scale multiply-connected geometries in the presence of moderate
magnetic fields is presented. It is shown how two types of simple devices, spin
filters and spin splitters (or Stern-Gerlach devices) may be constructed from
molecular nanometric rings utilizing the Aharonov-Bohm effect. The current is
calculated within a single electron approximation and within a many-body master
equation approach where charging effects are accounted for in the Coulomb
Blockade regime. We provide rules and tools to develop and analyze efficient
spintronic devices based on nanometric interferometers.Comment: 16 pages, 8 figures, submitted to Phys. Rev.
The torsional waveguide viscosity probe: design and anomalous behavior
This paper is concerned with the design of viscosity sensors based on a torsional waveguide. The advantages of using guided wave attenuation instead of speed for viscosity estimation are established. The effects of probe material, dimensions and operating frequency on viscosity measurement are discussed in the context of a requirement to match the measured attenuation to the range of viscosity values that are required to be measured, given the constraints on measurability imposed by the overall signal and noise conditions. A prototype probe is shown to work well with Newtonian liquids but to appreciably underestimate the viscosities of polymeric oils; these anomalies are explained quantitatively on the basis of a model of intramolecular relaxation. The probe was unsuccessful when applied to slurries, and a basic explanation is given
Rate of shear of an ultrasonic oscillating rod viscosity probe
Ultrasonic oscillating rod probes have recently been used by researchers to measure viscosity and/or density in fluids. However, in order to use such probes to characterise the rheological properties of fluids, it is necessary to define the shear rate produced by the probe. This paper proposes an analytical solution to estimate the shear rate of ultrasonic oscillating rod viscosity probes and a method to measure their maximum operational shear rate. A relationship is developed which relates the torsional surface velocity of an oscillating cylindrical rigid body to the rate of shear in its vicinity. The surface displacement and torsional surface velocity of a torsional probe of length 1000 mm and diameter 1 mm were measured over the frequency range from 525 to 700 kHz using a laser interferometer and the maximum shear rate estimated. The reported work provides the basis for characterising shear rate for such probes, enabling their application for rheological investigations
Synchronization in Scale Free networks: The role of finite size effects
Synchronization problems in complex networks are very often studied by
researchers due to its many applications to various fields such as
neurobiology, e-commerce and completion of tasks. In particular, Scale Free
networks with degree distribution , are widely used in
research since they are ubiquitous in nature and other real systems. In this
paper we focus on the surface relaxation growth model in Scale Free networks
with , and study the scaling behavior of the fluctuations, in
the steady state, with the system size . We find a novel behavior of the
fluctuations characterized by a crossover between two regimes at a value of
that depends on : a logarithmic regime, found in previous
research, and a constant regime. We propose a function that describes this
crossover, which is in very good agreement with the simulations. We also find
that, for a system size above , the fluctuations decrease with
, which means that the synchronization of the system improves as
increases. We explain this crossover analyzing the role of the
network's heterogeneity produced by the system size and the exponent of the
degree distribution.Comment: 9 pages and 5 figures. Accepted in Europhysics Letter
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