21,736 research outputs found
Superconductivity in striped and multi-Fermi-surface Hubbard models: From the cuprates to the pnictides
Single- and multi-band Hubbard models have been found to describe many of the
complex phenomena that are observed in the cuprate and iron-based
high-temperature superconductors. Simulations of these models therefore provide
an ideal framework to study and understand the superconducting properties of
these systems and the mechanisms responsible for them. Here we review recent
dynamic cluster quantum Monte Carlo simulations of these models, which provide
an unbiased view of the leading correlations in the system. In particular, we
discuss what these simulations tell us about superconductivity in the
homogeneous 2D single-orbital Hubbard model, and how charge stripes affect this
behavior. We then describe recent simulations of a bilayer Hubbard model, which
provides a simple model to study the type and nature of pairing in systems with
multiple Fermi surfaces such as the iron-based superconductors.Comment: Published as part of Superstripes 2011 (Rome) conference proceeding
Extended Variational Cluster Approximation
The variational cluster approximation (VCA) proposed by M. Potthoff {\it et
al.} [Phys. Rev. Lett. {\bf 91}, 206402 (2003)] is extended to electron or spin
systems with nonlocal interactions. By introducing more than one source field
in the action and employing the Legendre transformation, we derive a
generalized self-energy functional with stationary properties. Applying this
functional to a proper reference system, we construct the extended VCA (EVCA).
In the limit of continuous degrees of freedom for the reference system, EVCA
can recover the cluster extension of the extended dynamical mean-field theory
(EDMFT). For a system with correlated hopping, the EVCA recovers the cluster
extension of the dynamical mean-field theory for correlated hopping. Using a
discrete reference system composed of decoupled three-site single impurities,
we test the theory for the extended Hubbard model. Quantitatively good results
as compared with EDMFT are obtained. We also propose VCA (EVCA) based on
clusters with periodic boundary conditions. It has the (extended) dynamical
cluster approximation as the continuous limit. A number of related issues are
discussed.Comment: 23 pages, 5 figures, statements about DCA corrected; published
versio
The Dynamical Cluster Approximation (DCA) versus the Cellular Dynamical Mean Field Theory (CDMFT) in strongly correlated electrons systems
We are commenting on the article Phys. Rev. {\bf B 65}, 155112 (2002) by G.
Biroli and G. Kotliar in which they make a comparison between two cluster
techniques, the {\it Cellular Dynamical Mean Field Theory} (CDMFT) and the {\it
Dynamical Cluster Approximation} (DCA). Based upon an incorrect implementation
of the DCA technique in their work, they conclude that the CDMFT is a faster
converging technique than the DCA. We present the correct DCA prescription for
the particular model Hamiltonian studied in their article and conclude that the
DCA, once implemented correctly, is a faster converging technique for the
quantities averaged over the cluster. We also refer to their latest response to
our comment where they argue that instead of averaging over the cluster, local
observables should be calculated in the bulk of the cluster which indeed makes
them converge much faster in the CDMFT than in the DCA. We however show that in
their original work, the authors themselves use the cluster averaged quantities
to draw their conclusions in favor of using the CDMFT over the DCA.Comment: Comment on Phys. Rev. B 65, 155112 (2002). 3 pages, 2 figure
Biaxial order parameter in the homologous series of orthogonal bent-core smectic liquid crystals
The fundamental parameter of the uniaxial liquid crystalline state that governs nearly all of its physical properties is the primary orientational order parameter (S) for the long axes of molecules with respect to the director. The biaxial liquid crystals (LCs) possess biaxial order parameters depending on the phase symmetry of the system. In this paper we show that in the first approximation a biaxial orthogonal smectic phase can be described by two primary order parameters: S for the long axes and C for the ordering of the short axes of molecules. The temperature dependencies of S and C are obtained by the Haller's extrapolation technique through measurements of the optical birefringence and biaxiality on a nontilted polar antiferroelectric (Sm-APA) phase of a homologous series of LCs built from the bent-core achiral molecules. For such a biaxial smectic phase both S and C, particularly the temperature dependency of the latter, are being experimentally determined. Results show that S in the orthogonal smectic phase composed of bent cores is higher than in Sm-A calamatic LCs and C is also significantly large
GIS, Information Technology and Spatial Planning
Geographic information systems have been introduced local and regional planning several stages. They have influenced the technique of planning but only to a lesser extent the procedures of planning and the methodology of plan-making. More recently, information technology has challenged the whole concept of planning as an expert-and-government interplay. However, legal frameworks have not reflected the substantial change in the potentials of the technology.Any effort to reflect the new technology will face not only institutional inertia but increasingly also the human capacity of users of planning (i.e., decision-makers, administrators, stakeholders), namely the limited extent of overall IT literacy, which restricts the possible benefits of the technology. The dimension of access to and empowerment in planning may reappear in the context of new technologies, with new professional requirements for planners, beyond the computer, GIS and information technology
The role of regional information in the optimal composition of a committee
In this paper we present a model for the optimal composition of a federal or supra-national committee. The involvement of regional (national) entities in federal committees is typically motivated by their knowledge of regional information about the state of the economy. Using this argument we show that if the uncertainties regarding the state of the economy are not evenly distributed across the currency area, the optimal representation of regions in the federal committee increases with the overall uncertainty about their economic performance. Second, if certain parts of the economic area behave in a relatively synchronized way, it may not be necessary that all these regions are equally represented in the federal committee.Composition of a committee, currency union, optimal representation, information uncertainty
Signatures of the term in ultrastrongly-coupled oscillators
We study a bosonic matter excitation coupled to a single-mode cavity field
via electric dipole. Counter-rotating and terms are included in the
interaction model, being the vector potential of the cavity
field. In the ultrastrong coupling regime the vacuum of the bare modes is no
longer the ground state of the Hamiltonian and contains a nonzero population of
polaritons, the true normal modes of the system. If the parameters of the model
satisfy the Thomas-Reiche-Kuhn sum rule, we find that the two polaritons are
always equally populated. We show how this prediction could be tested in a
quenching experiment, by rapidly switching on the coupling and analyzing the
radiation emitted by the cavity. A refinement of the model based on a
microscopic minimal coupling Hamiltonian is also provided, and its consequences
on our results are characterized analytically.Comment: 11 pages, 5 figure
Noisy Classical Field Theories with Two Coupled Fields: Dependence of Escape Rates on Relative Field Stiffnesses
Exit times for stochastic Ginzburg-Landau classical field theories with two
or more coupled classical fields depend on the interval length on which the
fields are defined, the potential in which the fields deterministically evolve,
and the relative stiffness of the fields themselves. The latter is of
particular importance in that physical applications will generally require
different relative stiffnesses, but the effect of varying field stiffnesses has
not heretofore been studied. In this paper, we explore the complete phase
diagram of escape times as they depend on the various problem parameters. In
addition to finding a transition in escape rates as the relative stiffness
varies, we also observe a critical slowing down of the string method algorithm
as criticality is approached.Comment: 16 pages, 10 figure
Tunable negative permeability in a quantum plasmonic metamaterial
We consider the integration of quantum emitters into a negative permeability
metamaterial design in order to introduce tunability as well as nonlinear
behavior. The unit cell of our metamaterial is a ring of metamolecules, each
consisting of a metal nanoparticle and a two-level semiconductor quantum dot
(QD). Without the QDs, the ring of the unit cell is known to act as an
artificial optical magnetic resonator. By adding the QDs we show that a Fano
interference profile is introduced into the magnetic field scattered from the
ring. This induced interference is shown to cause an appreciable effect in the
collective magnetic resonance of the unit cell. We find that the interference
provides a means to tune the response of the negative permeability
metamaterial. The exploitation of the QD's inherent nonlinearity is proposed to
modulate the metamaterial's magnetic response with a separate control field.Comment: 11 pages, 6 figure
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