2,633 research outputs found
Spin Coulomb drag in the two-dimensional electron liquid
We calculate the spin-drag transresistivity
in a two-dimensional electron gas at temperature in the random phase
approximation. In the low-temperature regime we show that, at variance with the
three-dimensional low-temperature result [], the spin transresistivity of a two-dimensional {\it spin unpolarized}
electron gas has the form . In the
spin-polarized case the familiar form is
recovered, but the constant of proportionality diverges logarithmically as
the spin-polarization tends to zero. In the high-temperature regime we obtain
(where
is the effective Rydberg energy) {\it independent} of the density.
Again, this differs from the three-dimensional result, which has a logarithmic
dependence on the density. Two important differences between the spin-drag
transresistivity and the ordinary Coulomb drag transresistivity are pointed
out: (i) The singularity at low temperature is smaller, in the Coulomb
drag case, by a factor where is the Fermi wave vector and
is the separation between the layers. (ii) The collective mode contribution
to the spin-drag transresistivity is negligible at all temperatures. Moreover
the spin drag effect is, for comparable parameters, larger than the ordinary
Coulomb drag effect.Comment: 6 figures; various changes; version accepted for publicatio
Compounds affecting cholesterol absorption
A class of novel compounds is described for use in affecting lymphatic absorption of cholesterol. Compounds of particular interest are defined by Formula I: ##STR1## or a pharmaceutically acceptable salt thereof
Load distribution in weighted complex networks
We study the load distribution in weighted networks by measuring the
effective number of optimal paths passing through a given vertex. The optimal
path, along which the total cost is minimum, crucially depend on the cost
distribution function . In the strong disorder limit, where , the load distribution follows a power law both in the
Erd\H{o}s-R\'enyi (ER) random graphs and in the scale-free (SF) networks, and
its characteristics are determined by the structure of the minimum spanning
tree. The distribution of loads at vertices with a given vertex degree also
follows the SF nature similar to the whole load distribution, implying that the
global transport property is not correlated to the local structural
information. Finally, we measure the effect of disorder by the correlation
coefficient between vertex degree and load, finding that it is larger for ER
networks than for SF networks.Comment: 4 pages, 4 figures, final version published in PR
Dielectric constants of Ir, Ru, Pt, and IrO2: Contributions from bound charges
We investigated the dielectric functions () of Ir, Ru, Pt,
and IrO, which are commonly used as electrodes in ferroelectric thin film
applications. In particular, we investigated the contributions from bound
charges (), since these are important scientifically as
well as technologically: the (0) of a metal electrode is one of
the major factors determining the depolarization field inside a ferroelectric
capacitor. To obtain (0), we measured reflectivity spectra of
sputtered Pt, Ir, Ru, and IrO2 films in a wide photon energy range between 3.7
meV and 20 eV. We used a Kramers-Kronig transformation to obtain real and
imaginary dielectric functions, and then used Drude-Lorentz oscillator fittings
to extract (0) values. Ir, Ru, Pt, and IrO produced
experimental (0) values of 4810, 8210, 5810, and
295, respectively, which are in good agreement with values obtained using
first-principles calculations. These values are much higher than those for
noble metals such as Cu, Ag, and Au because transition metals and IrO have
such strong d-d transitions below 2.0 eV. High (0) values will
reduce the depolarization field in ferroelectric capacitors, making these
materials good candidates for use as electrodes in ferroelectric applications.Comment: 26 pages, 6 figures, 2 table
Dynamic behavior of driven interfaces in models with two absorbing states
We study the dynamics of an interface (active domain) between different
absorbing regions in models with two absorbing states in one dimension;
probabilistic cellular automata models and interacting monomer-dimer models.
These models exhibit a continuous transition from an active phase into an
absorbing phase, which belongs to the directed Ising (DI) universality class.
In the active phase, the interface spreads ballistically into the absorbing
regions and the interface width diverges linearly in time. Approaching the
critical point, the spreading velocity of the interface vanishes algebraically
with a DI critical exponent. Introducing a symmetry-breaking field that
prefers one absorbing state over the other drives the interface to move
asymmetrically toward the unpreferred absorbing region. In Monte Carlo
simulations, we find that the spreading velocity of this driven interface shows
a discontinuous jump at criticality. We explain that this unusual behavior is
due to a finite relaxation time in the absorbing phase. The crossover behavior
from the symmetric case (DI class) to the asymmetric case (directed percolation
class) is also studied. We find the scaling dimension of the symmetry-breaking
field .Comment: 5 pages, 5 figures, Revte
Topological insulators in the quaternary chalcogenide compounds and ternary famatinite compounds
We present first-principles calculations to predict several three dimensional
(3D) topological insulators in quaternary chalcogenide compounds which are made
of I-II-IV-VI compositions and in ternary compositions of
I-V-VI famatinite compounds. Among the large members of these two
families, we give examples of naturally occurring compounds which are mainly
Cu-based chalcogenides. We show that these materials are candidates of 3D
topological insulators or can be tuned to obtain topological phase transition
by manipulating the atomic number of the other cation and anion elements. A
band inversion can occur at a single point with considerably large
inversion strength, in addition to the opening of a bulk band gap throughout
the Brillouin zone. We also demonstrate that both of these families are related
to each other by cross-substitutions of cations in the underlying tetragonal
structure and that one can suitably tune their topological properties in a
desired manner.Comment: 7 pages, 4 figure
Exact scaling properties of a hierarchical network model
We report on exact results for the degree , the diameter , the
clustering coefficient , and the betweenness centrality of a
hierarchical network model with a replication factor . Such quantities are
calculated exactly with the help of recursion relations. Using the results, we
show that (i) the degree distribution follows a power law with , (ii) the diameter grows
logarithmically as with the number of nodes , (iii) the
clustering coefficient of each node is inversely proportional to its degree, , and the average clustering coefficient is nonzero in the infinite
limit, and (iv) the betweenness centrality distribution follows a power law
. We discuss a classification scheme of scale-free networks
into the universality class with the clustering property and the betweenness
centrality distribution.Comment: 4 page
- …