2,993 research outputs found
Anomalous Lattice Response at the Mott Transition in a Quasi-2D Organic Conductor
Discontinuous changes of the lattice parameters at the Mott metal-insulator
transition are detected by high-resolution dilatometry on deuterated crystals
of the layered organic conductor -(BEDT-TTF)Cu[N(CN)]Br.
The uniaxial expansivities uncover a striking and unexpected anisotropy,
notably a zero-effect along the in-plane c-axis along which the electronic
interactions are relatively strong. A huge thermal expansion anomaly is
observed near the end-point of the first-order transition line enabling to
explore the critical behavior with very high sensitivity. The analysis yields
critical fluctuations with an exponent 0.8 0.15
at odds with the novel criticality recently proposed for these materials
[Kagawa \textit{et al.}, Nature \textbf{436}, 534 (2005)]. Our data suggest an
intricate role of the lattice degrees of freedom in the Mott transition for the
present materials.Comment: 4 pages, 4 figure
Tunneling edges at strong disorder
Scattering between edge states that bound one-dimensional domains of opposite
potential or flux is studied, in the presence of strong potential or flux
disorder. A mobility edge is found as a function of disorder and energy, and we
have characterized the extended phase. "paper_FINAL.tex" 439 lines, 20366
characters In the presence of flux and/or potential disorder, the localization
length scales exponentially with the width of the barrier. We discuss
implications for the random-flux problem.Comment: RevTeX, 4 page
First-principles prediction of a decagonal quasicrystal containing boron
We interpret experimentally known B-Mg-Ru crystals as quasicrystal
approximants. These approximant structures imply a deterministic decoration of
tiles by atoms that can be extended quasiperiodically. Experimentally observed
structural disorder corresponds to phason (tile flip) fluctuations.
First-principles total energy calculations reveal that many distinct tilings
lie close to stability at low temperatures. Transfer matrix calculations based
on these energies suggest a phase transition from a crystalline state at low
temperatures to a high temperature state characterized by tile fluctuations. We
predict BMgRu forms a decagonal quasicrystal that is
metastable at low temperatures and may be thermodynamically stable at high
temperatures.Comment: 4 pages, 3 figures, submitted to Phys. Rev. Let
The pion mass dependence of the nucleon form-factors of the energy momentum tensor in the chiral quark-soliton model
The nucleon form factors of the energy-momentum tensor are studied in the
large-Nc limit in the framework of the chiral quark-soliton model for model
parameters that simulate physical situations in which pions are heavy. This
allows for a direct comparison to lattice QCD results.Comment: 17 pages, 12 figure
Finite-temperature properties of the two-orbital Anderson model
The metallic phase of the two-orbital Anderson lattice is study in the limit
of infinite spatial dimensions, where a second order perturbation treatment is
used to solve the single-site problem. Using this approximation, in the Kondo
regime, we find that the finite temperature properties of the conduction
electrons exhibit the same behaviour as observed in the metallic phase of the
two-channel Kondo lattice. Possible connections between these two models are
discussed.Comment: 4 pages, 2 figures, to appear in Journal of Physics: Condensed Matte
Boundary multifractality in critical 1D systems with long-range hopping
Boundary multifractality of electronic wave functions is studied analytically
and numerically for the power-law random banded matrix (PRBM) model, describing
a critical one-dimensional system with long-range hopping. The peculiarity of
the Anderson localization transition in this model is the existence of a line
of fixed points describing the critical system in the bulk. We demonstrate that
the boundary critical theory of the PRBM model is not uniquely determined by
the bulk properties. Instead, the boundary criticality is controlled by an
additional parameter characterizing the hopping amplitudes of particles
reflected by the boundary.Comment: 7 pages, 4 figures, some typos correcte
An Agent-Based Model of Collective Emotions in Online Communities
We develop a agent-based framework to model the emergence of collective
emotions, which is applied to online communities. Agents individual emotions
are described by their valence and arousal. Using the concept of Brownian
agents, these variables change according to a stochastic dynamics, which also
considers the feedback from online communication. Agents generate emotional
information, which is stored and distributed in a field modeling the online
medium. This field affects the emotional states of agents in a non-linear
manner. We derive conditions for the emergence of collective emotions,
observable in a bimodal valence distribution. Dependent on a saturated or a
superlinear feedback between the information field and the agent's arousal, we
further identify scenarios where collective emotions only appear once or in a
repeated manner. The analytical results are illustrated by agent-based computer
simulations. Our framework provides testable hypotheses about the emergence of
collective emotions, which can be verified by data from online communities.Comment: European Physical Journal B (in press), version 2 with extended
introduction, clarification
Spitzer Quasar and ULIRG evolution study (QUEST): I. The origin of the far infrared continuum of QSOs
This paper addresses the origin of the far-infrared (FIR) continuum of QSOs,
based on the Quasar and ULIRG Evolution Study (QUEST) of nearby QSOs and ULIRGs
using observations with the Spitzer Space Telescope. For 27 Palomar-Green QSOs
at z <~ 0.3, we derive luminosities of diagnostic lines ([NeII]12.8um,
[NeV]14.3um, [OIV]25.9um) and emission features (PAH7.7um emission which is
related to star formation), as well as continuum luminosities over a range of
mid- to far-infrared wavelengths between 6 and 60um. We detect star-formation
related PAH emission in 11/26 QSOs and fine-structure line emission in all of
them, often in multiple lines. The detection of PAHs in the average spectrum of
sources which lack individual PAH detections provides further evidence for the
widespread presence of PAHs in QSOs. Similar PAH/FIR and [NeII]/FIR ratios are
found in QSOs and in starburst-dominated ULIRGs and lower luminosity
starbursts. We conclude that the typical QSO in our sample has at least 30% but
likely most of the far-infrared luminosity (~ 10^(10...12)Lsun) arising from
star formation, with a tendency for larger star formation contribution at the
largest FIR luminosities. In the QSO sample, we find correlations between most
of the quantities studied including combinations of AGN tracers and starburst
tracers. The common scaling of AGN and starburst luminosities (and fluxes) is
evidence for a starburst-AGN connection in luminous AGN. Strong correlations of
far-infrared continuum and starburst related quantities (PAH, low excitation
[NeII]) offer additional support for the starburst origin of far-infrared
emission.Comment: 39 pages, 8 figures, accepted for publication in Ap
Systemic Risk in a Unifying Framework for Cascading Processes on Networks
We introduce a general framework for models of cascade and contagion
processes on networks, to identify their commonalities and differences. In
particular, models of social and financial cascades, as well as the fiber
bundle model, the voter model, and models of epidemic spreading are recovered
as special cases. To unify their description, we define the net fragility of a
node, which is the difference between its fragility and the threshold that
determines its failure. Nodes fail if their net fragility grows above zero and
their failure increases the fragility of neighbouring nodes, thus possibly
triggering a cascade. In this framework, we identify three classes depending on
the way the fragility of a node is increased by the failure of a neighbour. At
the microscopic level, we illustrate with specific examples how the failure
spreading pattern varies with the node triggering the cascade, depending on its
position in the network and its degree. At the macroscopic level, systemic risk
is measured as the final fraction of failed nodes, , and for each of
the three classes we derive a recursive equation to compute its value. The
phase diagram of as a function of the initial conditions, thus allows
for a prediction of the systemic risk as well as a comparison of the three
different model classes. We could identify which model class lead to a
first-order phase transition in systemic risk, i.e. situations where small
changes in the initial conditions may lead to a global failure. Eventually, we
generalize our framework to encompass stochastic contagion models. This
indicates the potential for further generalizations.Comment: 43 pages, 16 multipart figure
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