56,495 research outputs found
Negative-Index Refraction in a Lamellar Composite with Alternating Single Negative Layers
Negative-index refraction is achieved in a lamellar composite with
epsilon-negative (ENG) and mu-negative (MNG) materials stacked alternatively.
Based on the effective medium approximation, simultaneously negative effective
permittivity and permeability of such a lamellar composite are obtained
theoretically and further proven by full-wave simulations. Consequently, the
famous left-handed metamaterial comprising split ring resonators and wires is
interpreted as an analogy of such an ENG-MNG lamellar composite. In addition,
beyond the effective medium approximation, the propagating field squeezed near
the ENG/MNG interface is demonstrated to be left-handed surface waves with
backward phase velocity.Comment: 18 pages, 6 figure
Quantum information transfer and models for black hole mechanics
General features of information transfer between quantum subsystems, via
unitary evolution, are investigated, with applications to the problem of
information transfer from a black hole to its surroundings. A particularly
direct form of quantum information transfer is "subspace transfer," which can
be characterized by saturation of a subadditivity inequality. We also describe
more general unitary quantum information transfer, and categorize different
models for black hole evolution. Evolution that only creates paired excitations
inside/outside the black hole is shown not to extract information, but
information-transferring models exist both in the "saturating" and
"non-saturating" category. The former more closely capture thermodynamic
behavior; the latter generically have enhanced energy flux, beyond that of
Hawking.Comment: 31 pages, harvmac. v2: nomenclature change, minor added explanation.
v3: small corrections/rewordings; improved figure; version to match
publication in PR
Clustering Coefficients of Protein-Protein Interaction Networks
The properties of certain networks are determined by hidden variables that
are not explicitly measured. The conditional probability (propagator) that a
vertex with a given value of the hidden variable is connected to k of other
vertices determines all measurable properties. We study hidden variable models
and find an averaging approximation that enables us to obtain a general
analytical result for the propagator. Analytic results showing the validity of
the approximation are obtained. We apply hidden variable models to
protein-protein interaction networks (PINs) in which the hidden variable is the
association free-energy, determined by distributions that depend on
biochemistry and evolution. We compute degree distributions as well as
clustering coefficients of several PINs of different species; good agreement
with measured data is obtained. For the human interactome two different
parameter sets give the same degree distributions, but the computed clustering
coefficients differ by a factor of about two. This shows that degree
distributions are not sufficient to determine the properties of PINs.Comment: 16 pages, 3 figures, in Press PRE uses pdflate
Shaping the waveform of entangled photons
We demonstrate experimentally the tunable control of the joint spectrum, i.e.
waveform and degree of frequency correlations, of paired photons generated in
spontaneous parametric downconversion. This control is mediated by the spatial
shape of the pump beam in a type-I noncollinear configuration. We discuss the
applicability of this technique to other sources of frequency entangled
photons, such as electromagnetically induced Raman transitions.Comment: 5 Pages, 4 Figure
Stationary untrapped boundary conditions in general relativity
A class of boundary conditions for canonical general relativity are proposed
and studied at the quasi-local level. It is shown that for untrapped or
marginal surfaces, fixing the area element on the 2-surface (rather than the
induced 2-metric) and the angular momentum surface density is enough to have a
functionally differentiable Hamiltonian, thus providing definition of conserved
quantities for the quasi-local regions. If on the boundary the evolution vector
normal to the 2-surface is chosen to be proportional to the dual expansion
vector, we obtain a generalization of the Hawking energy associated with a
generalized Kodama vector. This vector plays the role for the stationary
untrapped boundary conditions which the stationary Killing vector plays for
stationary black holes. When the dual expansion vector is null, the boundary
conditions reduce to the ones given by the non-expanding horizons and the null
trapping horizons.Comment: 11 pages, improved discussion section, a reference added, accepted
for publication in Classical and Quantum Gravit
Classical simulation of quantum many-body systems with a tree tensor network
We show how to efficiently simulate a quantum many-body system with tree
structure when its entanglement is bounded for any bipartite split along an
edge of the tree. This is achieved by expanding the {\em time-evolving block
decimation} simulation algorithm for time evolution from a one dimensional
lattice to a tree graph, while replacing a {\em matrix product state} with a
{\em tree tensor network}. As an application, we show that any one-way quantum
computation on a tree graph can be efficiently simulated with a classical
computer.Comment: 4 pages,7 figure
Optical characterization of BiSe in a magnetic field: infrared evidence for magnetoelectric coupling in a topological insulator material
We present an infrared magneto-optical study of the highly thermoelectric
narrow-gap semiconductor BiSe. Far-infrared and mid-infrared (IR)
reflectance and transmission measurements have been performed in magnetic
fields oriented both parallel and perpendicular to the trigonal axis of
this layered material, and supplemented with UV-visible ellipsometry to obtain
the optical conductivity . With lowering of temperature we
observe narrowing of the Drude conductivity due to reduced quasiparticle
scattering, as well as the increase in the absorption edge due to direct
electronic transitions. Magnetic fields dramatically
renormalize and asymmetrically broaden the strongest far-IR optical phonon,
indicating interaction of the phonon with the continuum free-carrier spectrum
and significant magnetoelectric coupling. For the perpendicular field
orientation, electronic absorption is enhanced, and the plasma edge is slightly
shifted to higher energies. In both cases the direct transition energy is
softened in magnetic field.Comment: Final versio
Observation of strong-coupling pairing with weakened Fermi-surface nesting at optimal hole doping in CaNaFeAs
We report an angle-resolved photoemission investigation of optimally-doped
CaNaFeAs. The Fermi surface topology of this compound
is similar to that of the well-studied BaKFeAs
material, except for larger hole pockets resulting from a higher hole
concentration per Fe atoms. We find that the quasi-nesting conditions are
weakened in this compound as compared to BaKFeAs. As
with BaKFeAs though, we observe nearly isotropic
superconducting gaps with Fermi surface-dependent magnitudes. A small variation
in the gap size along the momentum direction perpendicular to the surface is
found for one of the Fermi surfaces. Our superconducting gap results on all
Fermi surface sheets fit simultaneously very well to a global gap function
derived from a strong coupling approach, which contains only 2 global
parameters.Comment: 5 pages, 4 figure
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