18,746 research outputs found
Frustrated spin- Heisenberg magnet on a square-lattice bilayer: High-order study of the quantum critical behavior of the ---- model
The zero-temperature phase diagram of the spin-
---- model on an -stacked square-lattice
bilayer is studied using the coupled cluster method implemented to very high
orders. Both nearest-neighbor (NN) and frustrating next-nearest-neighbor
Heisenberg exchange interactions, of strengths and , respectively, are included in each layer. The two layers are
coupled via a NN interlayer Heisenberg exchange interaction with a strength
. The magnetic order parameter (viz.,
the sublattice magnetization) is calculated directly in the thermodynamic
(infinite-lattice) limit for the two cases when both layers have
antiferromagnetic ordering of either the N\'{e}el or the striped kind, and with
the layers coupled so that NN spins between them are either parallel (when
) to one another. Calculations
are performed at th order in a well-defined sequence of approximations,
which exactly preserve both the Goldstone linked cluster theorem and the
Hellmann-Feynman theorem, with . The sole approximation made is to
extrapolate such sequences of th-order results for to the exact limit,
. By thus locating the points where vanishes, we calculate
the full phase boundaries of the two collinear AFM phases in the
-- half-plane with . In particular, we provide the
accurate estimate, (), for the
position of the quantum triple point (QTP) in the region . We also
show that there is no counterpart of such a QTP in the region ,
where the two quasiclassical phase boundaries show instead an ``avoided
crossing'' behavior, such that the entire region that contains the nonclassical
paramagnetic phases is singly connected
Word contexts enhance the neural representation of individual letters in early visual cortex
Visual context facilitates perception, but how this is neurally implemented remains unclear. One example of contextual facilitation is found in reading, where letters are more easily identified when embedded in a word. Bottom-up models explain this word advantage as a post-perceptual decision bias, while top-down models propose that word contexts enhance perception itself. Here, we arbitrate between these accounts by presenting words and nonwords and probing the representational fidelity of individual letters using functional magnetic resonance imaging. In line with top-down models, we find that word contexts enhance letter representations in early visual cortex. Moreover, we observe increased coupling between letter information in visual cortex and brain activity in key areas of the reading network, suggesting these areas may be the source of the enhancement. Our results provide evidence for top-down representational enhancement in word recognition, demonstrating that word contexts can modulate perceptual processing already at the earliest visual regions
Performance of a First-Level Muon Trigger with High Momentum Resolution Based on the ATLAS MDT Chambers for HL-LHC
Highly selective first-level triggers are essential to exploit the full
physics potential of the ATLAS experiment at High-Luminosity LHC (HL-LHC). The
concept for a new muon trigger stage using the precision monitored drift tube
(MDT) chambers to significantly improve the selectivity of the first-level muon
trigger is presented. It is based on fast track reconstruction in all three
layers of the existing MDT chambers, made possible by an extension of the
first-level trigger latency to six microseconds and a new MDT read-out
electronics required for the higher overall trigger rates at the HL-LHC. Data
from -collisions at is used to study the
minimal muon transverse momentum resolution that can be obtained using the MDT
precision chambers, and to estimate the resolution and efficiency of the
MDT-based trigger. A resolution of better than is found in all sectors
under study. With this resolution, a first-level trigger with a threshold of
becomes fully efficient for muons with a transverse momentum
above in the barrel, and above in the
end-cap region.Comment: 6 pages, 11 figures; conference proceedings for IEEE NSS & MIC
conference, San Diego, 201
Next Generation Higgs Bosons: Theory, Constraints and Discovery Prospects at the Large Hadron Collider
Particle physics model building within the context of string theory suggests
that further copies of the Higgs boson sector may be expected. Concerns
regarding tree-level flavor changing neutral currents are easiest to allay if
little or no couplings of next generation Higgs bosons are allowed to Standard
Model fermions. We detail the resulting general Higgs potential and mass
spectroscopy in both a Standard Model extension and a supersymmetric extension.
We present the important experimental constraints from meson-meson mixing,
loop-induced decays and LEP2 direct production limits. We
investigate the energy range of valid perturbation theory of these ideas. In
the supersymmetric context we present a class of examples that marginally aids
the fine-tuning problem for parameter space where the lightest Higgs boson mass
is greater than the Standard Model limit of 114 GeV. Finally, we study collider
physics signatures generic to next generation Higgs bosons, with special
emphasis on signal events, and describe the capability of
discovery at the Large Hadron Collider.Comment: 43 pages, 12 figures; v3: minor corrections, published in Physical
Review
The effects of regional insolation differences upon advanced solar thermal electric power plant performance and energy costs
The performance and cost of four 10 MWe advanced solar thermal electric power plants sited in various regions of the continental United States was studied. Each region has different insolation characteristics which result in varying collector field areas, plant performance, capital costs and energy costs. The regional variation in solar plant performance was assessed in relation to the expected rise in the future cost of residential and commercial electricity supplied by conventional utility power systems in the same regions. A discussion of the regional insolation data base is presented along with a description of the solar systems performance and costs. A range for the forecast cost of conventional electricity by region and nationally over the next several decades is given
The effects of regional insolation differences upon advanced solar thermal electric power plant performance and energy costs
The performance and cost of the 10 MWe advanced solar thermal electric power plants sited in various regions of the continental United States were determined. The regional insolation data base is discussed. A range for the forecast cost of conventional electricity by region and nationally over the next several cades are presented
The density of states of chaotic Andreev billiards
Quantum cavities or dots have markedly different properties depending on
whether their classical counterparts are chaotic or not. Connecting a
superconductor to such a cavity leads to notable proximity effects,
particularly the appearance, predicted by random matrix theory, of a hard gap
in the excitation spectrum of quantum chaotic systems. Andreev billiards are
interesting examples of such structures built with superconductors connected to
a ballistic normal metal billiard since each time an electron hits the
superconducting part it is retroreflected as a hole (and vice-versa). Using a
semiclassical framework for systems with chaotic dynamics, we show how this
reflection, along with the interference due to subtle correlations between the
classical paths of electrons and holes inside the system, are ultimately
responsible for the gap formation. The treatment can be extended to include the
effects of a symmetry breaking magnetic field in the normal part of the
billiard or an Andreev billiard connected to two phase shifted superconductors.
Therefore we are able to see how these effects can remold and eventually
suppress the gap. Furthermore the semiclassical framework is able to cover the
effect of a finite Ehrenfest time which also causes the gap to shrink. However
for intermediate values this leads to the appearance of a second hard gap - a
clear signature of the Ehrenfest time.Comment: Refereed version. 23 pages, 19 figure
Frustrated Heisenberg antiferromagnet on the honeycomb lattice: Spin gap and low-energy parameters
We use the coupled cluster method implemented to high orders of approximation
to investigate the frustrated spin- ----
antiferromagnet on the honeycomb lattice with isotropic Heisenberg interactions
of strength between nearest-neighbor pairs, between
next-nearest-neighbor pairs, and between next-next-neareast-neighbor
pairs of spins. In particular, we study both the ground-state (GS) and
lowest-lying triplet excited-state properties in the case , in the window of the frustration
parameter, which includes the (tricritical) point of maximum classical
frustration at . We present GS results for the
spin stiffness, , and the zero-field uniform magnetic susceptibility,
, which complement our earlier results for the GS energy per spin, ,
and staggered magnetization, , to yield a complete set of accurate
low-energy parameters for the model. Our results all point towards a phase
diagram containing two quasiclassical antiferromagnetic phases, one with N\'eel
order for , and the other with collinear striped order
for . The results for both and the spin gap
provide compelling evidence for a quantum paramagnetic phase that is
gapped over a considerable portion of the intermediate region , especially close to the two quantum critical points
at and . Each of our fully independent sets of
results for the low-energy parameters is consistent with the values
and , and with
the transition at being of continuous (and probably of the
deconfined) type and that at being of first-order type
The Heisenberg antiferromagnet on the kagome lattice with arbitrary spin: A high-order coupled cluster treatment
Starting with the sqrt{3} x sqrt{3} and the q=0 states as reference states we
use the coupled cluster method to high orders of approximation to investigate
the ground state of the Heisenberg antiferromagnet on the kagome lattice for
spin quantum numbers s=1/2,1,3/2,2,5/2, and 3. Our data for the ground-state
energy for s=1/2 are in good agreement with recent large-scale density-matrix
renormalization group and exact diagonalization data. We find that the
ground-state selection depends on the spin quantum number s. While for the
extreme quantum case, s=1/2, the q=0 state is energetically favored by quantum
fluctuations, for any s>1/2 the sqrt{3} x sqrt{3} state is selected. For both
the sqrt{3} x sqrt{3} and the q=0 states the magnetic order is strongly
suppressed by quantum fluctuations. Within our coupled cluster method we get
vanishing values for the order parameter (sublattice magnetization) M for s=1/2
and s=1, but (small) nonzero values for M for s>1. Using the data for the
ground-state energy and the order parameter for s=3/2,2,5/2, and 3 we also
estimate the leading quantum corrections to the classical values.Comment: 7 pages, 6 figure
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