7,878 research outputs found
Stochastic Resonance in Nonpotential Systems
We propose a method to analytically show the possibility for the appearance
of a maximum in the signal-to-noise ratio in nonpotential systems. We apply our
results to the FitzHugh-Nagumo model under a periodic external forcing, showing
that the model exhibits stochastic resonance. The procedure that we follow is
based on the reduction to a one-dimensional dynamics in the adiabatic limit,
and in the topology of the phase space of the systems under study. Its
application to other nonpotential systems is also discussed.Comment: Submitted to Phys. Rev.
Experimental elucidation of the origin of the `double spin resonances' in Ba(FeCo)As
We report a combined study of the spin resonances and superconducting gaps
for underdoped ( K), optimally doped ( K), and overdoped
( K) Ba(FeCo)As single crystals with inelastic
neutron scattering and angle resolved photoemission spectroscopy. We find a
quasi two dimensional spin resonance whose energy scales with the
superconducting gap in all three compounds. In addition, anisotropic low energy
spin excitation enhancements in the superconducting state have been deduced and
characterized for the under and optimally doped compounds. Our data suggest
that the quasi two dimensional spin resonance is a spin exciton that
corresponds to the spin singlet-triplet excitations of the itinerant electrons.
However, the intensity enhancements of the anisotropic spin excitations are
dominated by the out-of-plane spin excitations of the ordered moments due to
the suppression of damping in the superconducting state. Hence we offer a new
interpretation of the double energy scales differing from previous
interpretations based on anisotropic superconducting energy gaps, and
systematically explain the doping-dependent trend across the phase diagram.Comment: 8 pages, 7 figures, 1 table. Accepted for publication on Physical
Review
Global versus local billiard level dynamics: The limits of universality
Level dynamics measurements have been performed in a Sinai microwave billiard
as a function of a single length, as well as in rectangular billiards with
randomly distributed disks as a function of the position of one disk. In the
first case the field distribution is changed globally, and velocity
distributions and autocorrelation functions are well described by universal
functions derived by Simons and Altshuler. In the second case the field
distribution is changed locally. Here another type of universal correlations is
observed. It can be derived under the assumption that chaotic wave functions
may be described by a random superposition of plane waves
Lattice-Boltzmann and finite-difference simulations for the permeability for three-dimensional porous media
Numerical micropermeametry is performed on three dimensional porous samples
having a linear size of approximately 3 mm and a resolution of 7.5 m. One
of the samples is a microtomographic image of Fontainebleau sandstone. Two of
the samples are stochastic reconstructions with the same porosity, specific
surface area, and two-point correlation function as the Fontainebleau sample.
The fourth sample is a physical model which mimics the processes of
sedimentation, compaction and diagenesis of Fontainebleau sandstone. The
permeabilities of these samples are determined by numerically solving at low
Reynolds numbers the appropriate Stokes equations in the pore spaces of the
samples. The physical diagenesis model appears to reproduce the permeability of
the real sandstone sample quite accurately, while the permeabilities of the
stochastic reconstructions deviate from the latter by at least an order of
magnitude. This finding confirms earlier qualitative predictions based on local
porosity theory. Two numerical algorithms were used in these simulations. One
is based on the lattice-Boltzmann method, and the other on conventional
finite-difference techniques. The accuracy of these two methods is discussed
and compared, also with experiment.Comment: to appear in: Phys.Rev.E (2002), 32 pages, Latex, 1 Figur
Atomic Hole Doping of Graphene
Graphene is an excellent candidate for the next generation of electronic
materials due to the strict two-dimensionality of its electronic structure as
well as the extremely high carrier mobility. A prerequisite for the development
of graphene based electronics is the reliable control of the type and density
of the charge carriers by external (gate) and internal (doping) means. While
gating has been successfully demonstrated for graphene flakes and epitaxial
graphene on silicon carbide, the development of reliable chemical doping
methods turns out to be a real challenge. In particular hole doping is an
unsolved issue. So far it has only been achieved with reactive molecular
adsorbates, which are largely incompatible with any device technology. Here we
show by angle-resolved photoemission spectroscopy that atomic doping of an
epitaxial graphene layer on a silicon carbide substrate with bismuth, antimony
or gold presents effective means of p-type doping. Not only is the atomic
doping the method of choice for the internal control of the carrier density. In
combination with the intrinsic n-type character of epitaxial graphene on SiC,
the charge carriers can be tuned from electrons to holes, without affecting the
conical band structure
Testing the sign-changing superconducting gap in iron-based superconductors with quasiparticle interference and neutron scattering
We present a phenomenological calculation of the quasiparticle-interference
(QPI) pattern and inelastic Neutron scattering (INS) spectra in iron-pnictide
and layered iron-selenide compounds by using materials specific band-structure
and superconducting (SC) gap properties. As both the QPI and the INS spectra
arise due to scattering of the Bogolyubov quasiaprticles, they exibit an
one-to-one correspondence of the scattering vectors and the energy scales. We
show that these two spectroscopies complement each other in such a way that a
comparative study allows one to extract the quantitative and unambiguous
information about the underlying pairing structure and the phase of the SC gap.
Due to the nodeless and isotropic nature of the SC gaps, both the QPI and INS
maps are concentrated at only two energies in pnictide (two SC gaps) and one
energy in iron-selenide, while the associated scattering vectors q for
scattering of sign-changing and same-sign of the SC gaps change between these
spectroscopies. The results presented, particularly for newly iron-selenide
compounds, can be used to test the nodeless d-wave pairing in this class of
high temperature superconductors.Comment: 8 pages, 5 figures, J. Phys.: Cond. Mat. (2012) v2: Experimental data
include
Structural-configurated magnetic plasmon bands in connected ring chains
Magnetic resonance coupling between connected split ring resonators (SRRs)
and magnetic plasmon (MP) excitations in the connected SRR chains were
theoretically studied. By changing the connection configuration, two different
coupling behaviors were observed, and therefore two kinds of MP bands were
formed in the connected ring chains, accordingly. These MPs were revealed with
positive and negative dispersion for the homo- and anti-connected chain,
respectively. Notably, these two MP modes both have wide bandwidth due to the
conductive coupling. Moreover, the anti-connected chain is found supporting a
novel negative propagating wave with a wide band starting from zero frequency,
which is a fancy phenomenon in one-dimensional system.Comment: 7 pages, 4 figures Band structures of magnetic plasmons in
one-dimenstional metamaterail
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