1,616 research outputs found
Correlated few-photon transport in one-dimensional waveguides: linear and nonlinear dispersions
We address correlated few-photon transport in one-dimensional waveguides
coupled to a two-level system (TLS), such as an atom or a quantum dot. We
derive exactly the single-photon and two-photon current (transmission) for
linear and nonlinear (tight-binding sinusoidal) energy-momentum dispersion
relations of photons in the waveguides and compare the results for the
different dispersions. A large enhancement of the two-photon current for the
sinusoidal dispersion has been seen at a certain transition energy of the TLS
away from the single-photon resonances.Comment: 6 pages, 5 figures, revised version, to appear in Phys. Rev.
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Herd behaviour in extreme market conditions: The case of the Athens stock exchange
This paper examines herd behaviour in extreme market conditions using data from the Athens Stock Exchange. We test for the presence of herding as suggested by Christie and Huang (1995) and Chang, Cheng, and Khorana (2000). Results based on daily, weekly and monthly data indicate the existence of herd behaviour for the years 1998-2007. Evidence of herd behaviour over daily time intervals is much stronger, revealing the short-term nature of the phenomenon. When the testing period is broken into semi-annual sub-periods, herding is found during the stock market crisis of 1999. Investor behaviour seems to have become more rational since 2002, owing to the regulatory and institutional reforms of the Greek equity market and the intense presence of foreign institutional investors
Van Hove singularities in the paramagnetic phase of the Hubbard model: a DMFT study
Using the dynamical mean-field theory (DMFT) we study the paramagnetic phase
of the Hubbard model with the density of states (DOS) corresponding to the
three-dimensional cubic lattice and the two-dimensional square lattice, as well
as a DOS with inverse square root singularity. We show that the electron
correlations rapidly smooth out the square-root van Hove singularities (kinks)
in the spectral function for the 3D lattice and that the Mott metal-insulator
transition (MIT) as well as the magnetic-field-induced MIT differ only little
from the well-known results for the Bethe lattice. The consequences of the
logarithmic singularity in the DOS for the 2D lattice are more dramatic. At
half filling, the divergence pinned at the Fermi level is not washed out, only
its integrated weight decreases as the interaction is increased. While the Mott
transition is still of the usual kind, the magnetic-field-induced MIT falls
into a different universality class as there is no field-induced localization
of quasiparticles. In the case of a power-law singularity in the DOS at the
Fermi level, the power-law singularity persists in the presence of interaction,
albeit with a different exponent, and the effective impurity model in the DMFT
turns out to be a pseudo-gap Anderson impurity model with a hybridization
function which vanishes at the Fermi level. The system is then a generalized
Fermi liquid. At finite doping, regular Fermi liquid behavior is recovered.Comment: 7 pages, 9 figure
Local control of entanglement in a spin chain
In a ferromagnetic spin chain, the control of the local effective magnetic
field allows to manipulate the static and dynamical properties of entanglement.
In particular, the propagation of quantum correlations can be driven to a great
extent so as to achieve an entanglement transfer on demand toward a selected
site
Electric coupling to the magnetic resonance of split ring resonators
We study both theoretically and experimentally the transmission properties of
a lattice of split ring resonators (SRRs) for different electromagnetic (EM)
field polarizations and propagation directions. We find unexpectedly that the
incident electric field E couples to the magnetic resonance of the SRR when the
EM waves propagate perpendicular to the SRR plane and the incident E is
parallel to the gap-bearing sides of the SRR. This is manifested by a dip in
the transmission spectrum. A simple analytic model is introduced to explain
this interesting behavior.Comment: 4 pages, 4 figure
Nonlinear surface impurity in a semi-infinite 2D square lattice
We examine the formation of localized states on a generalized nonlinear
impurity located at, or near the surface of a semi-infinite 2D square lattice.
Using the formalism of lattice Green functions, we obtain in closed form the
number of bound states as well as their energies and probability profiles, for
different nonlinearity parameter values and nonlinearity exponents, at
different distances from the surface. We specialize to two cases: impurity
close to an "edge" and impurity close to a "corner". We find that, unlike the
case of a 1D semi-infinite lattice, in 2D, the presence of the surface helps
the formation of a localized state.Comment: 6 pages, 7 figures, submitted to PR
Effect of hydrogen adsorption on the quasiparticle spectra of graphene
We use the non-interacting tight-binding model to study the effect of
isolated hydrogen adsorbates on the quasiparticle spectra of single-layer
graphene. Using the Green's function approach, we obtain analytic expressions
for the local density of states and the spectral function of hydrogen-doped
graphene, which are also numerically evaluated and plotted. Our results are
relevant for the interpretation of scanning tunneling microscopy and
angle-resolved photoemission spectroscopy data of functionalized graphene.Comment: 4 pages, 3 figures, minor corrections to tex
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