63 research outputs found
Linear response theory around a localized impurity in the pseudogap regime of an anisotropic superconductor: precursor pairing vs the d-density-wave scenario
We derive the polarizability of an electron system in (i) the superconducting
phase, with d-wave symmetry, (ii) the pseudogap regime, within the precursor
pairing scenario, and (iii) the d-density-wave (dDW) state, characterized by a
d-wave hidden order parameter, but no pairing. Such a calculation is motivated
by the recent proposals that imaging the effects of an isolated impurity may
distinguish between precursor pairing and dDW order in the pseudogap regime of
the high-Tc superconductors. In all three cases, the wave-vector dependence of
the polarizability is characterized by an azymuthal modulation, consistent with
the d-wave symmetry of the underlying state. However, only the dDW result shows
the fingerprints of nesting, with nesting wave-vector Q=(pi,pi), albeit
imperfect, due to a nonzero value of the hopping ratio t'/t in the band
dispersion relation. As a consequence of nesting, the presence of hole pockets
is also exhibited by the (q,omega) dependence of the retarded polarizability.Comment: accepted in Phys. Rev.
Size shrinking of composite bosons for increasing density in the BCS to Bose-Einstein crossover
We consider a system of fermions in the continuum case at zero temperature,
in the strong-coupling limit of a short-range attraction when composite bosons
form as bound-fermion pairs. We examine the density dependence of the size of
the composite bosons at leading order in the density ("dilute limit"), and show
on general physical grounds that this size should decrease with increasing
density, both in three and two dimensions. We then compare with the analytic
zero-temperature mean-field solution, which indeed exhibits the size shrinking
of the composite bosons both in three and two dimensions. We argue,
nonetheless, that the two-dimensional mean-field solution is not consistent
with our general result in the "dilute limit", to the extent that mean field
treats the scattering between composite bosons in the Born approximation which
is known to break down at low energy in two dimensions.Comment: Revised version to be published on Eur. Phys. Jour. B, 7 pages, 1
figur
Internal structure of preformed Cooper pairs
In order to obtain information about the internal structure of the preformed
pairs in the pseudogap state of high superconductors, we calculate the
propagator of a singlet pair with center of mass coordinate , and
relative distance , by solving the Bethe-Salpeter equation,
representing the sum over repeated two-particle scattering events due to a
distance dependent attraction. We define then a ``pair structure function''
that depends on the internal distance
between the partners and on the momentum of the pair.
We calculate this function both for a local potential and wave symmetry of
the order parameter and for a separable potential and wave symmetry of the
order parameter. The influence of the center of mass momentum, strenght of the
interaction, temperature, density of particles and of the pseudogap in the
one-electron spectrum is studied for both cases.Comment: 12 pages, REVTeX4, 8 EPS figure
Internal structure of fluctuating Cooper pairs
Abstract.: In order to obtain information about the internal structure of fluctuating Cooper pairs in the pseudogap state and below the transition temperature of high Tc superconductors, we solve the Bethe-Salpeter equation for the two-electron propagator in order to calculate a "pair structure function” that depends on the internal distance between the partners and on the center of mass momentum P of the pair. We use an attractive Hubbard model with a local potential for s-wave and a separable potential for d-wave symmetry. The amplitude of gP for small ρ depends on temperature, chemical potential and interaction symmetry, but the ρ dependence itself is rather insensitive to the interaction strength. Asymptotically gP decreases as an inverse power of ρ for weak coupling, but exponentially when a pseudogap develops for stronger interaction. Some possibilities of observing the pair structure experimentally are mentione
Size-shrinking of deuterons in very dilute superfluid nuclear matter
It is shown within the strong-coupling BCS approach that, starting from the
zero-density limit of superfluid nuclear matter, with increasing density
deuterons first shrink before they start expanding.Comment: 2 pages, Latex, 1 figure included, submitted to Phys. Rev.
Linear Response Theory Around a Localized Impurity in the Pseudogap Regime of an Anisotropic Superconductor
We compare and contrast the polarizability of a d-wave superconductor in the pseudogap regime, within the precursor pairing scenario (dPG), and of a d-density-wave (dDW) state, characterized by a d-wave hidden order parameter, but no pairing. Our study is motivated by STM imaging experiments around an isolated impurity, which may in principle distinguish between precursor pairing and dDW order in the pseudogap regime of the high- superconductors. In both cases, the -dependence of the polarizability is characterized by an azimuthal modulation, consistent with the d-wave symmetry of the underlying state. However, only the dDW result shows the fingerprints of nesting, with nesting wave vector , albeit imperfect, due to a nonzero value of the hopping ratio in the band dispersion relation. As a consequence of nesting, the presence of hole pockets is also reflected by the dependence of the retarded polarizabilit
Good trouble in the academy: inventing design-focused case studies about public management as an archetype of policy design research
Speaking archetypically, public organizations are practical means for implementing policy interventions. In this regard, their purposeful roles include furnishing operational capacity, while also sustaining support and legitimacy for the interventions as implemented. Contributions to fulfilling these roles are made by myriad practices and systems that are situated organizationally. Organizationally situated practices and systems are matters of concern for professional practitioners concerned with public organizations and their management. As they engage in creating and adapting such working phenomena, design-oriented professional practitioners bring professional knowledge into play. From this standpoint, there's a need for professional practitioners to acquire such professional knowledge, which implies a need for researchers to furnish it. At present, there's no good off the shelf solution for meeting that particular need. This chapter deals with the question of what to do about that gap. Dealing with it makes for good trouble
Density-induced BCS to Bose-Einstein crossover
We investigate the zero-temperature BCS to Bose-Einstein crossover at the mean-field level, by driving it
with the attractive potential and the particle density. We emphasize specifically the role played by the particle
density in this crossover. Three different interparticle potentials are considered for the continuum model in
three spatial dimensions, while both s- and d-wave solutions are analyzed for the attractive ~extended! Hubbard
model on a two-dimensional square lattice. For this model the peculiar behavior of the crossover for the
d-wave solution is discussed. In particular, in the strong-coupling limit when approaching half-filling we
evidence the occurrence of strong correlations among antiparallel-spin fermions belonging to different composite
bosons, which give rise to a quasi-long-range antiferromagnetic order in this limit
NB-IoT via LEO satellites: An efficient resource allocation strategy for uplink data transmission
In this paper, we focus on the use of Low-Eart Orbit (LEO) satellites
providing the Narrowband Internet of Things (NB-IoT) connectivity to the
on-ground user equipment (UEs). Conventional resource allocation algorithms for
the NBIoT systems are particularly designed for terrestrial infrastructures,
where devices are under the coverage of a specific base station and the whole
system varies very slowly in time. The existing methods in the literature
cannot be applied over LEO satellite-based NB-IoT systems for several reasons.
First, with the movement of the LEO satellite, the corresponding channel
parameters for each user will quickly change over time. Delaying the scheduling
of a certain user would result in a resource allocation based on outdated
parameters. Second, the differential Doppler shift, which is a typical
impairment in communications over LEO, directly depends on the relative
distance among users. Scheduling at the same radio frame users that overcome a
certain distance would violate the differential Doppler limit supported by the
NB-IoT standard. Third, the propagation delay over a LEO satellite channel is
around 4-16 times higher compared to a terrestrial system, imposing the need
for message exchange minimization between the users and the base station. In
this work, we propose a novel uplink resource allocation strategy that jointly
incorporates the new design considerations previously mentioned together with
the distinct channel conditions, satellite coverage times and data demands of
various users on Earth. The novel methodology proposed in this paper can act as
a framework for future works in the field.Comment: Tis work has been submitted to the IEEE IoT Journal for possible
publication. Copyright may be transferred without notice, after which this
version may no longer be accessibl
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