1,957 research outputs found
Symmetry-projected variational approach for ground and excited states of the two-dimensional Hubbard model
We present a symmetry-projected configuration mixing scheme to describe
ground and excited states, with well defined quantum numbers, of the
two-dimensional Hubbard model with nearestneighbor hopping and periodic
boundary conditions. Results for the half-filled 2{\times}4, 4{\times}4, and
6{\times}6 lattices, as well as doped 4 {\times} 4 systems, compare well with
available results, both exact and from other state-of-the-art approximations.
We report spectral functions and density of states obtained from a
well-controlled ansatz for the (Ne {\pm} 1)-electron system. Symmetry projected
methods have been widely used for the many-body nuclear physics problem but
have received little attention in the solid state community. Given their
relatively low (mean-field) computational cost and the high quality of results
here reported, we believe that they deserve further scrutiny
A proposal for a new type of thin-film field-emission display by edge breakdown of MIS structure
A new type of field emission display(FED) based on an edge-enhance electron
emission from metal-insulator-semiconductor (MIS) thin film structure is
proposed. The electrons produced by an avalanche breakdown in the semiconductor
near the edge of a top metal electrode are initially injected to the thin film
of an insulator with a negative electron affinity (NEA), and then are injected
into vacuum in proximity to the top electrode edge. The condition for the
deep-depletition breakdown near the edge of the top metal electrode is
analytically found in terms of ratio of the insulator thickness to the maximum
(breakdown) width of the semiconductor depletition region: this ratio should be
less than 2/(3 \pi - 2) = 0.27. The influence of a neighboring metal electrode
and an electrode thickness on this condition are analyzed. Different practical
schemes of the proposed display with a special reference to M/CaF_2/Si
structure are considered.Comment: 11 pages, 5 figure
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Cue-Induced Dopamine Release Predicts Cocaine Preference: Positron Emission Tomography Studies in Freely Moving Rodents
Positron emission tomography studies in drug-addicted patients have shown that exposure to drug-related cues increases striatal dopamine, which displaces binding of the D2 ligand, [11C]-raclopride. However, it is not known if animals will also show cue-induced displacement of [11C]-raclopride binding. In this study, we use [11C]-raclopride imaging in awake rodents to capture cue-induced changes in dopamine release associated with the conditioned place preference model of drug craving. Ten animals were conditioned to receive cocaine in a contextually distinct environment from where they received saline. Following conditioning, each animal was tested for preference and then received two separate [11C]-raclopride scans. For each scan, animals were confined to the cocaine and/or the saline-paired environment for the first 25 min of uptake, after which they were anesthetized and scanned. [11C]-raclopride uptake in the saline-paired environment served as a within-animal control for uptake in the cocaine-paired environment. Cocaine produced a significant place preference (p = 0.004) and exposure to the cocaine-paired environment decreased [11C]-raclopride binding relative to the saline-paired environment in both the dorsal (20%; p < 0.002) and ventral striatum (22%; p < 0.05). The change in [11C]-raclopride binding correlated with preference in the ventral striatum (R2 = −0.87; p = 0.003). In this region, animals who showed little or no preference exhibited little or no change in [11C]-raclopride binding in the cocaine-paired environment. This noninvasive procedure of monitoring neurochemical events in freely moving, behaving animals advances preclinical molecular imaging by interrogating the degree to which animal models reflect the human condition on multiple dimensions, both biological and behavioral.Chemistry and Chemical Biolog
Versatile, Cheap, Readily Modifiable Sample Delivery Method for Analysis of Air-/Moisture-Sensitive Samples Using Atmospheric Pressure Solids Analysis Probe Mass Spectrometry
A cheap, versatile, readily modified, and reusable glass probe system enabling delivery of solid air-/moisture-sensitive samples for mass spectrometric (MS) analysis using an Atmospheric pressure Solids Analysis Probe (ASAP) is described. The simplicity of the design allows quick and easy ASAP MS analyses of sensitive solid and liquid samples without the need for any modifications to commercially available vertically loaded ASAP mass spectrometers. A comparison of ASAP mass spectra obtained for metal complexes under air and an inert atmosphere is given
The effect of social media communication on consumer perceptions of brands
Researchers and brand managers have limited understanding of the effects social media communication has on how consumers perceive brands. We investigated 504 Facebook users in order to observe the impact of firm-created and user-generated social media communication on brand equity, brand attitude and purchase intention by using a standardized online survey throughout Poland. To test the conceptual model, we analyzed 60 brands across three different industries: non-alcoholic beverages, clothing and mobile network operators. When analyzing the data, we applied the structural equation modeling technique to both investigate the interplay of firm-created and user-generated social media communication and examine industry-specific differences. The results of the empirical studies showed that user-generated social media communication had a positive influence on both brand equity and brand attitude, whereas firm-created social media communication affected only brand attitude. Both brand equity and brand attitude were shown to have a positive influence on purchase intention. In addition, we assessed measurement invariance using a multi-group structural modeling equation. The findings revealed that the proposed measurement model was invariant across the researched industries. However, structural path differences were detected across the models
SO/Sp Monopoles and Branes with Orientifold 3 Plane
We study BPS monopoles in 4 dimensional N=4 SO(N) and super
Yang-Mills theories realized as the low energy effective theory of
(physical and its mirror) parallel D3 branes and an {\it Orientifold 3 plane}
with D1 branes stretched between them in type IIB string theory. Monopoles on
D3 branes give the natural understanding by embedding in SU(N) through the
constraints on both the asymptotic Higgs field (corresponding to the horizontal
positions of D3 branes) and the magnetic charges (corresponding to the number
of D1 branes) imposed by the O3 plane. The compatibility conditions of Nahm
data for monopoles for these groups can be interpreted very naturally through
the D1 branes in the presence of O3 plane.Comment: 18 pages, Latex with RevTex, 1 table, 4 figures, v2: Clarified the
introduction and improved on the supersymmetric theory on D1 branes in page 7
and 8 and this final version to appear in Phys.Rev.
Work functions, ionization potentials, and in-between: Scaling relations based on the image charge model
We revisit a model in which the ionization energy of a metal particle is
associated with the work done by the image charge force in moving the electron
from infinity to a small cut-off distance just outside the surface. We show
that this model can be compactly, and productively, employed to study the size
dependence of electron removal energies over the range encompassing bulk
surfaces, finite clusters, and individual atoms. It accounts in a
straightforward manner for the empirically known correlation between the atomic
ionization potential (IP) and the metal work function (WF), IP/WF2. We
formulate simple expressions for the model parameters, requiring only a single
property (the atomic polarizability or the nearest neighbor distance) as input.
Without any additional adjustable parameters, the model yields both the IP and
the WF within 10% for all metallic elements, as well as matches the size
evolution of the ionization potentials of finite metal clusters for a large
fraction of the experimental data. The parametrization takes advantage of a
remarkably constant numerical correlation between the nearest-neighbor distance
in a crystal, the cube root of the atomic polarizability, and the image force
cutoff length. The paper also includes an analytical derivation of the relation
of the outer radius of a cluster of close-packed spheres to its geometric
structure.Comment: Original submission: 8 pages with 7 figures incorporated in the text.
Revised submission (added one more paragraph about alloy work functions): 18
double spaced pages + 8 separate figures. Accepted for publication in PR
BPS R-balls in N=4 SYM on R X S^3, Quantum Hall Analogy and AdS/CFT Holography
In this paper, we propose a new approach to study the BPS dynamics in N=4
supersymmetric U(N) Yang-Mills theory on R X S^3, in order to better understand
the emergence of gravity in the gauge theory. Our approach is based on
supersymmetric, space-filling Q-balls with R-charge, which we call R-balls. The
usual collective coordinate method for non-topological scalar solitons is
applied to quantize the half and quarter BPS R-balls. In each case, a different
quantization method is also applied to confirm the results from the collective
coordinate quantization. For finite N, the half BPS R-balls with a U(1)
R-charge have a moduli space which, upon quantization, results in the states of
a quantum Hall droplet with filling factor one. These states are known to
correspond to the ``sources'' in the Lin-Lunin-Maldacena geometries in IIB
supergravity. For large N, we find a new class of quarter BPS R-balls with a
non-commutativity parameter. Quantization on the moduli space of such R-balls
gives rise to a non-commutative Chern-Simons matrix mechanics, which is known
to describe a fractional quantum Hall system. In view of AdS/CFT holography,
this demonstrates a profound connection of emergent quantum gravity with
non-commutative geometry, of which the quantum Hall effect is a special case.Comment: 42 pages, 2 figures; v3: a new paragraph on counting unbroken susy of
NC R-balls and references adde
Towards an Understanding of the Globular Cluster Over--abundance around the Central Giant Elliptical NGC 1399
We investigate the kinematics of a combined sample of 74 globular clusters
around NGC 1399. Their high velocity dispersion, increasing with radius,
supports their association with the gravitational potential of the galaxy
cluster rather than with that of NGC 1399 itself. We find no evidence for
rotation in the full sample, although some indication for rotation in the outer
regions. The data do not allow us to detect differences between the kinematics
of the blue and red sub-populations of globular clusters.
A comparison between the globular cluster systems of NGC 1399 and those of
NGC 1404 and NGC 1380 indicates that the globular clusters in all three
galaxies are likely to have formed via similar mechanisms and at similar
epochs. The only property which distinguishes the NGC 1399 globular cluster
system from these others is that it is ten times more abundant. We summarize
the evidence for associating these excess globulars with the galaxy cluster
rather than with NGC 1399 itself, and suggest that the over-abundance can be
explained by tidal stripping, at an early epoch, of neighboring galaxies and
subsequent accumulation of globulars in the gravitational potential of the
galaxy cluster.Comment: AJ accepted (March issue), 27 pages (6 figures included), AAS style,
two columns. Also available at http://www.eso.org/~mkissle
Fuzzy Sphere Dynamics and Non-Abelian DBI in Curved Backgrounds
We consider the non-Abelian action for the dynamics of -branes in the
background of -branes, which parameterises a fuzzy sphere using the SU(2)
algebra. We find that the curved background leads to collapsing solutions for
the fuzzy sphere except when we have branes in the background, which
is a realisation of the gravitational Myers effect. Furthermore we find the
equations of motion in the Abelian and non-Abelian theories are identical in
the large limit. By picking a specific ansatz we find that we can
incorporate angular momentum into the action, although this imposes restriction
upon the dimensionality of the background solutions. We also consider the case
of non-Abelian non-BPS branes, and examine the resultant dynamics using
world-volume symmetry transformations. We find that the fuzzy sphere always
collapses but the solutions are sensitive to the combination of the two
conserved charges and we can find expanding solutions with turning points. We
go on to consider the coincident 5-brane background, and again construct
the non-Abelian theory for both BPS and non-BPS branes. In the latter case we
must use symmetry arguments to find additional conserved charges on the
world-volumes to solve the equations of motion. We find that in the Non-BPS
case there is a turning solution for specific regions of the tachyon and radion
fields. Finally we investigate the more general dynamics of fuzzy
in the -brane background, and find collapsing solutions
in all cases.Comment: 49 pages, 3 figures, Latex; Version to appear in JHE
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