1,269 research outputs found
A systematic study of non-ideal contacts in integer quantum Hall systems
In the present article we investigate the influence of the contact region on
the distribution of the chemical potential in integer quantum Hall samples, as
well as the longitudinal and Hall resistance as a function of the magnetic
field. First we use a standard quantum Hall sample geometry and analyse the
influence of the length of the leads where current enters/leaves the sample and
the ratio of the contact width to the width of these leads. Furthermore we
investigate potential barriers in the current injecting leads and the
measurement arms in order to simulate non-ideal contacts. Second we simulate
nonlocal quantum Hall samples with applied gating voltage at the metallic
contacts. For such samples it has been found experimentally that both the
longitudinal and Hall resistance as a function of the magnetic field can change
significantly. Using the nonequilibrium network model we are able to reproduce
most qualitative features of the experiments.Comment: 29 pages, 16 Figure
Phase space spinor amplitudes for spin 1/2 systems
The concept of phase space amplitudes for systems with continuous degrees of
freedom is generalized to finite-dimensional spin systems. Complex amplitudes
are obtained on both a sphere and a finite lattice, in each case enabling a
more fundamental description of pure spin states than that previously given by
Wigner functions. In each case the Wigner function can be expressed as the star
product of the amplitude and its conjugate, so providing a generalized Born
interpretation of amplitudes that emphasizes their more fundamental status. The
ordinary product of the amplitude and its conjugate produces a (generalized)
spin Husimi function. The case of spin-\half is treated in detail, and it is
shown that phase space amplitudes on the sphere transform correctly as spinors
under under rotations, despite their expression in terms of spherical
harmonics. Spin amplitudes on a lattice are also found to transform as spinors.
Applications are given to the phase space description of state superposition,
and to the evolution in phase space of the state of a spin-\half magnetic
dipole in a time-dependent magnetic field.Comment: 19 pages, added new results, fixed typo
Molecular and mass spectroscopic analysis of isotopically labeled organic residues
Experimental studies aimed at understanding the evolution of complex organic molecules on interstellar grains were performed. The photolysis of frozen gas mixtures of various compositions containing H2O, CO, NH3, and CH4 was studied. These species were chosen because of their astrophysical importance as deducted from observational as well as theoretical studies of ice mantles on interstellar grains. These ultraviolet photolyzed ices were warmed up in order to produce refractory organic molecules like the ones formed in molecular clouds when the icy mantles are being irradiated and warmed up either by a nearby stellar source or impulsive heating. The laboratory studies give estimates of the efficiency of production of such organic material under interstellar conditions. It is shown that the gradual carbonization of organic mantles in the diffuse cloud phase leads to higher and higher visual absorptivity - yellow residues become brown in the laboratory. The obtained results can be applied to explaining the organic components of comets and their relevance to the origin of life
Adaptive query-based sampling of distributed collections
As part of a Distributed Information Retrieval system a de-scription of each remote information resource, archive or repository is usually stored centrally in order to facilitate resource selection. The ac-quisition ofprecise resourcedescriptionsistherefore animportantphase in Distributed Information Retrieval, as the quality of such represen-tations will impact on selection accuracy, and ultimately retrieval per-formance. While Query-Based Sampling is currently used for content discovery of uncooperative resources, the application of this technique is dependent upon heuristic guidelines to determine when a suïŹciently accurate representation of each remote resource has been obtained. In this paper we address this shortcoming by using the Predictive Likelihood to provide both an indication of thequality of an acquired resource description estimate, and when a suïŹciently good representation of a resource hasbeen obtained during Query-Based Sampling
Transport Theory beyond Binary Collisions
Using the Schwinger-Keldysh technique, we derive the transport equations for
a system of quantum scalar fields. We first discuss the general structure of
the equations and then their collision terms. Taking into account up to
three-loop diagrams in \phi^3 model and up to four-loop diagrams in \phi^4
model, we obtain the transport equations which include the contributions of
multi-particle collisions and particle production processes, in addition to
mean-field effects and binary interactions.Comment: 30 pages, 21 figures, minor changes, to appear in Phys. Rev.
Spin Accumulation at Ferromagnet/Non-magnetic Material Interfaces
Many proposed and realized spintronic devices involve spin injection and
accumulation at an interface between a ferromagnet and a non-magnetic material.
We examine the electric field, voltage profile, charge distribution, spin
fluxes, and spin accumulation at such an interface. We include the effects of
both screening and spin scattering. We also include both the spin-dependent
chemical potentials {\mu}_{\uparrow,\downarrow} and the effective magnetic
field H* that is zero in equilibrium. For a Co/Cu interface, we find that the
spin accumulation in the copper is an order of magnitude larger when both
chemical potential and effective magnetic field are included. We also show that
screening contributes to the spin accumulation in the ferromagnet; this
contribution can be significant.Comment: 11 pages, 4 figures, 2 table
Error estimates for solid-state density-functional theory predictions: an overview by means of the ground-state elemental crystals
Predictions of observable properties by density-functional theory
calculations (DFT) are used increasingly often in experimental condensed-matter
physics and materials engineering as data. These predictions are used to
analyze recent measurements, or to plan future experiments. Increasingly more
experimental scientists in these fields therefore face the natural question:
what is the expected error for such an ab initio prediction? Information and
experience about this question is scattered over two decades of literature. The
present review aims to summarize and quantify this implicit knowledge. This
leads to a practical protocol that allows any scientist - experimental or
theoretical - to determine justifiable error estimates for many basic property
predictions, without having to perform additional DFT calculations. A central
role is played by a large and diverse test set of crystalline solids,
containing all ground-state elemental crystals (except most lanthanides). For
several properties of each crystal, the difference between DFT results and
experimental values is assessed. We discuss trends in these deviations and
review explanations suggested in the literature. A prerequisite for such an
error analysis is that different implementations of the same first-principles
formalism provide the same predictions. Therefore, the reproducibility of
predictions across several mainstream methods and codes is discussed too. A
quality factor Delta expresses the spread in predictions from two distinct DFT
implementations by a single number. To compare the PAW method to the highly
accurate APW+lo approach, a code assessment of VASP and GPAW with respect to
WIEN2k yields Delta values of 1.9 and 3.3 meV/atom, respectively. These
differences are an order of magnitude smaller than the typical difference with
experiment, and therefore predictions by APW+lo and PAW are for practical
purposes identical.Comment: 27 pages, 20 figures, supplementary material available (v5 contains
updated supplementary material
Phenomenological approach to the critical dynamics of the QCD phase transition revisited
The phenomenological dynamics of the QCD critical phenomena is revisited.
Recently, Son and Stephanov claimed that the dynamical universality class of
the QCD phase transition belongs to model H. In their discussion, they employed
a time-dependent Ginzburg-Landau equation for the net baryon number density,
which is a conserved quantity. We derive the Langevin equation for the net
baryon number density, i.e., the Cahn-Hilliard equation. Furthermore, they
discussed the mode coupling induced through the {\it irreversible} current.
Here, we show the {\it reversible} coupling can play a dominant role for
describing the QCD critical dynamics and that the dynamical universality class
does not necessarily belong to model H.Comment: 13 pages, the Curie principle is discussed in S.2, to appear in
J.Phys.
Channel kets, entangled states, and the location of quantum information
The well-known duality relating entangled states and noisy quantum channels
is expressed in terms of a channel ket, a pure state on a suitable tripartite
system, which functions as a pre-probability allowing the calculation of
statistical correlations between, for example, the entrance and exit of a
channel, once a framework has been chosen so as to allow a consistent set of
probabilities. In each framework the standard notions of ordinary (classical)
information theory apply, and it makes sense to ask whether information of a
particular sort about one system is or is not present in another system.
Quantum effects arise when a single pre-probability is used to compute
statistical correlations in different incompatible frameworks, and various
constraints on the presence and absence of different kinds of information are
expressed in a set of all-or-nothing theorems which generalize or give a
precise meaning to the concept of ``no-cloning.'' These theorems are used to
discuss: the location of information in quantum channels modeled using a
mixed-state environment; the (classical-quantum) channels introduced by
Holevo; and the location of information in the physical carriers of a quantum
code. It is proposed that both channel and entanglement problems be classified
in terms of pure states (functioning as pre-probabilities) on systems of parts, with mixed bipartite entanglement and simple noisy channels belonging
to the category , a five-qubit code to the category , etc.; then by
the dimensions of the Hilbert spaces of the component parts, along with other
criteria yet to be determined.Comment: Latex 32 pages, 4 figures in text using PSTricks. Version 3: Minor
typographical errors correcte
The cytoplasm of living cells: A functional mixture of thousands of components
Inside every living cell is the cytoplasm: a fluid mixture of thousands of
different macromolecules, predominantly proteins. This mixture is where most of
the biochemistry occurs that enables living cells to function, and it is
perhaps the most complex liquid on earth. Here we take an inventory of what is
actually in this mixture. Recent genome-sequencing work has given us for the
first time at least some information on all of these thousands of components.
Having done so we consider two physical phenomena in the cytoplasm: diffusion
and possible phase separation. Diffusion is slower in the highly crowded
cytoplasm than in dilute solution. Reasonable estimates of this slowdown can be
obtained and their consequences explored, for example, monomer-dimer equilibria
are established approximately twenty times slower than in a dilute solution.
Phase separation in all except exceptional cells appears not to be a problem,
despite the high density and so strong protein-protein interactions present. We
suggest that this may be partially a byproduct of the evolution of other
properties, and partially a result of the huge number of components present.Comment: 11 pages, 1 figure, 1 tabl
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