14,225 research outputs found
Interplay between strong correlations and magnetic field in the symmetric periodic Anderson model
Magnetic field effects in Kondo insulators are studied theoretically, using a
local moment approach to the periodic Anderson model within the framework of
dynamical mean-field theory. Our main focus is on field-induced changes in
single-particle dynamics and the associated hybridization gap in the density of
states. Particular emphasis is given to the strongly correlated regime, where
dynamics are found to exhibit universal scaling in terms of a field-dependent
low energy coherence scale. Although the bare applied field is globally
uniform, the effective fields experienced by the conduction electrons and the
-electrons differ because of correlation effects. A continuous
insulator-metal transition is found to occur on increasing the applied field,
closure of the hybridization gap reflecting competition between Zeeman
splitting and screening of the -electron local moments. For intermediate
interaction strengths the hybridization gap depends non-linearly on the applied
field, while in strong coupling its field dependence is found to be linear. For
the classic Kondo insulator YbB, good agreement is found upon direct
comparison of the field evolution of the experimental transport gap with the
theoretical hybridization gap in the density of states.Comment: 8 pages, 8 figure
Wakes from arrays of buildings
Experiments were carried out in a small wind tunnel in which atmospheric flow around buildings was simulated. Arrays of one, two, three, and four model buildings were tested, and wake profiles of velocity and turbulence were measured. The data indicate the effect of the buildings on the wind environment encountered by aircraft during landing or takeoff operations. It was possible to use the results to locate the boundaries of the air regions affected by the obstacles and to recommend preferred arrangements of buildings to maximize light safety
Wind tunnel measurements of three-dimensional wakes of buildings
Measurements relevant to the effect of buildings on the low level atmospheric boundary layer are presented. A wind tunnel experiment was undertaken to determine the nature of the flow downstream from a gap between two transversely aligned, equal sized models of rectangular cross section. These building models were immersed in an equilibrium turbulent boundary layer which was developed on a smooth floor in a zero longitudinal pressure gradient. Measurements with an inclined (45 degree) hot-wire were made at key positions downstream of models arranged with a large, small, and no gap between them. Hot-wire theory is presented which enables computation of the three mean velocity components, U, V and W, as well as Reynolds stresses. These measurements permit understanding of the character of the wake downstream of laterally spaced buildings. Surface streamline patterns obtained by the oil film method were used to delineate the separation region to the rear of the buildings for a variety of spacings
Dynamics and scaling in the periodic Anderson model
The periodic Anderson model (PAM) captures the essential physics of heavy
fermion materials. Yet even for the paramagnetic metallic phase, a practicable
many-body theory that can simultaneously handle all energy scales while
respecting the dictates of Fermi liquid theory at low energies, and all
interaction strengths from the strongly correlated Kondo lattice through to
weak coupling, has remained quite elusive. Aspects of this problem are
considered in the present paper where a non-perturbative local moment approach
(LMA) to single-particle dynamics of the asymmetric PAM is developed within the
general framework of dynamical mean-field theory. All interaction strengths and
energy scales are encompassed, although our natural focus is the Kondo lattice
regime of essentially localized -spins but general conduction band filling,
characterised by an exponentially small lattice coherence scale .
Particular emphasis is given to the resultant universal scaling behaviour of
dynamics in the Kondo lattice regime as an entire function of , including its dependence on conduction band filling,
-level asymmetry and lattice type.A rich description arises, encompassing
both coherent Fermi liquid behaviour at low- and the crossover
to effective single-impurity scaling physics at higher energies -- but still in
the -scaling regime, and as such incompatible with the
presence of two-scale `exhaustion' physics, which is likewise discussed.Comment: 22 pages in EPJB format, 14 figures; accepted for publication in
EPJB; (small change in the comments section, no change in manuscript
Spectral scaling and quantum critical behaviour in the pseudogap Anderson model
The pseudogap Anderson impurity model provides a classic example of an
essentially local quantum phase transition. Here we study its single-particle
dynamics in the vicinity of the symmetric quantum critical point (QCP)
separating generalized Fermi liquid and local moment phases, via the local
moment approach. Both phases are shown to be characterized by a low-energy
scale that vanishes at the QCP; and the universal scaling spectra, on all
energy scales, are obtained analytically. The spectrum precisely at the QCP is
also obtained; its form showing clearly the non-Fermi liquid, interacting
nature of the fixed point.Comment: 7 pages, 2 figure
Discovering an Invisibly Decaying Higgs at Hadron Colliders
A Higgs boson lighter than 2 m_W that decays mostly into invisible channels
(e.g., dark matter particles) is theoretically well-motivated. We study the
prospects for discovery of such an invisible Higgs, h_inv, at the LHC and the
Tevatron in three production modes: (1) in association with a Z, (2) through
Weak Boson Fusion (WBF), and (3) accompanied by a jet. In the Z+h_inv channel,
we show that the LHC can yield a discovery signal above 5 sigma with 10 fb-1 of
integrated luminosity for a Higgs mass of 120 GeV. With 30 fb-1 the discovery
reach extends up to a Higgs mass of 160 GeV. We also study the extraction of
the h_inv mass from production cross sections at the LHC, and find that
combining WBF and Z+h_inv allows a relatively model-independent determination
of the h_inv mass with an uncertainty of 35-50 GeV (15-20 GeV) with 10 (100)
fb-1. At the Tevatron, a 3 sigma observation of a 120 GeV h_inv in any single
channel is not possible with less than 12 fb-1 per detector. However, we show
that combining the signal from WBF with the previously-studied Z+h_inv channel
allows a 3 sigma observation of h_inv with 7 fb-1 per detector. Because of
overwhelming irreducible backgrounds, h_inv+j is not a useful search channel at
either the Tevatron or the LHC, despite the larger production rate.Comment: 21 pages, 1 figure; v2: added refs, added discussion of invisible
Higgs mass extraction from cross sections at LHC; v3: minor clarifications in
text, version to appear in PR
Communications network design and costing model programmers manual
Otpimization algorithms and techniques used in the communications network design and costing model for least cost route and least cost network problems are examined from the programmer's point of view. All system program modules, the data structures within the model, and the files which make up the data base are described
Communications network design and costing model technical manual
This computer model provides the capability for analyzing long-haul trunking networks comprising a set of user-defined cities, traffic conditions, and tariff rates. Networks may consist of all terrestrial connectivity, all satellite connectivity, or a combination of terrestrial and satellite connectivity. Network solutions provide the least-cost routes between all cities, the least-cost network routing configuration, and terrestrial and satellite service cost totals. The CNDC model allows analyses involving three specific FCC-approved tariffs, which are uniquely structured and representative of most existing service connectivity and pricing philosophies. User-defined tariffs that can be variations of these three tariffs are accepted as input to the model and allow considerable flexibility in network problem specification. The resulting model extends the domain of network analysis from traditional fixed link cost (distance-sensitive) problems to more complex problems involving combinations of distance and traffic-sensitive tariffs
A spin-dependent local moment approach to the Anderson impurity model
We present an extension of the local moment approach to the Anderson impurity
model with spin-dependent hybridization. By employing the two-self-energy
description, as originally proposed by Logan and co-workers, we applied the
symmetry restoration condition for the case with spin-dependent hybridization.
Self-consistent ground states were determined through variational minimization
of the ground state energy. The results obtained with our spin-dependent local
moment approach applied to a quantum dot system coupled to ferromagnetic leads
are in good agreement with those obtained from previous work using numerical
renormalization group calculations
- …