639 research outputs found
Accurate photoionisation cross section for He at non-resonant photon energies
The total single-photon ionisation cross section was calculated for helium
atoms in their ground state. Using a full configuration-interaction approach
the photoionisation cross section was extracted from the complex-scaled
resolvent. In the energy range from ionisation threshold to 59\,eV our results
agree with an earlier -spline based calculation in which the continuum is
box discretised within a relative error of in the non-resonant part of
the spectrum. Above the \He^{++} threshold our results agree on the other
hand very well to a recent Floquet calculation. Thus our calculation confirms
the previously reported deviations from the experimental reference data outside
the claimed error estimate. In order to extend the calculated spectrum to very
high energies, an analytical hydrogenic-type model tail is introduced that
should become asymptotically exact for infinite photon energies. Its
universality is investigated considering also H, Li, and HeH. With
the aid of the tail corrections to the dipole approximation are estimated.Comment: 20 pages, 7 figures, 2 table
State-dependent TMS reveals representation of affective body movements in the anterior intraparietal cortex
In humans, recognition of othersâ actions involves a cortical network that comprises, among other cortical regions, the posterior superior temporal sulcus (pSTS), where biological motion is coded and the anterior intraparietal suclus (aIPS), where movement information is elaborated in terms of meaningful goal directed actions. This action observation system (AOS) is thought to encode neutral voluntary actions, and possibly some aspects of affective motor repertoire, but the role of the AOSâ areas in processing affective kinematic information has never been examined. Here we investigated whether the action observation system plays a role in representing dynamic emotional bodily expressions. In the first experiment, we assessed behavioural adaptation effects of observed affective movements. Participants watched series of happy or fearful whole-body point-light displays (PLDs) as adapters and were then asked to perform an explicit categorization of the emotion expressed in test PLDs. Participants were slower when categorizing any of the two emotions as long as it was congruent with the emotion in the adapter sequence. We interpreted this effect as adaptation to the emotional content of PLDs. In the second experiment, we combined this paradigm with TMS applied over either the right aIPS, pSTS and the right half of the occipital pole (corresponding to Brodmannâs area 17 and serving as control) to examine the neural locus of the adaptation effect. TMS over the aIPS (but not over the other sites) reversed the behavioural cost of adaptation, specifically for fearful contents. This demonstrates that aIPS contains an explicit representation of affective body movements
Exact infinite-time statistics of the Loschmidt echo for a quantum quench
The equilibration dynamics of a closed quantum system is encoded in the
long-time distribution function of generic observables. In this paper we
consider the Loschmidt echo generalized to finite temperature, and show that we
can obtain an exact expression for its long-time distribution for a closed
system described by a quantum XY chain following a sudden quench. In the
thermodynamic limit the logarithm of the Loschmidt echo becomes normally
distributed, whereas for small quenches in the opposite, quasi-critical regime,
the distribution function acquires a universal double-peaked form indicating
poor equilibration. These findings, obtained by a central limit theorem-type
result, extend to completely general models in the small-quench regime.Comment: 4 pages, 2 figure
Finding critical points using improved scaling Ansaetze
Analyzing in detail the first corrections to the scaling hypothesis, we
develop accelerated methods for the determination of critical points from
finite size data. The output of these procedures are sequences of
pseudo-critical points which rapidly converge towards the true critical points.
In fact more rapidly than previously existing methods like the Phenomenological
Renormalization Group approach. Our methods are valid in any spatial
dimensionality and both for quantum or classical statistical systems. Having at
disposal fast converging sequences, allows to draw conclusions on the basis of
shorter system sizes, and can be extremely important in particularly hard cases
like two-dimensional quantum systems with frustrations or when the sign problem
occurs. We test the effectiveness of our methods both analytically on the basis
of the one-dimensional XY model, and numerically at phase transitions occurring
in non integrable spin models. In particular, we show how a new Homogeneity
Condition Method is able to locate the onset of the
Berezinskii-Kosterlitz-Thouless transition making only use of ground-state
quantities on relatively small systems.Comment: 16 pages, 4 figures. New version including more general Ansaetze
basically applicable to all case
Universal sub-leading terms in ground state fidelity
The study of the (logarithm of the) {\em fidelity} i.e., of the overlap
amplitude, between ground states of Hamiltonians corresponding to different
coupling constants, provides a valuable insight on critical phenomena. When the
parameters are infinitesimally close, it is known that the leading term behaves
as ( system size) where is equal to the spatial
dimension for gapped systems, and otherwise depends on the critical
exponents. Here we show that when parameters are changed along a critical
manifold, a sub-leading O(1) term can appear. This term, somewhat similar to
the topological entanglement entropy, depends only on the system's universality
class and encodes non-trivial information about the topology of the system. We
relate it to universal factors and partition functions of (boundary)
conformal field theory in and dimensions. Numerical checks are
presented on the simple example of the XXZ chain.Comment: revtex4, 2 pdf figure
The victorious English language: hegemonic practices in the management academy
This study explores hegemonic linguistic processes, that is, the dominant and unreflective use of the English language in the production of textual knowledge accounts. The authors see the production of management knowledge as situated in central or peripheral locations, which they examine from an English language perspective. Their inquiry is based on an empirical study based on the perspectives of 33 management academics (not English language speakers) in (semi) peripheral locations, who have to generate and disseminate knowledge in and through the English language. Although the hegemony of the center in the knowledge production process has long been acknowledged, the specific contribution of this study is to explore how the English language operates as part of the âideological complexâ that produces and maintains this hegemony, as well as how this hegemony is manifested at the local level of publication practices in peripherally located business and management schools
UV variability and accretion dynamics in the young open cluster NGC 2264
Context. Photometric variability is a distinctive feature of young stellar objects; exploring variability signatures at different wavelengths provides insight into the physical processes at work in these sources.
Aims. We explore the variability signatures at ultraviolet (UV) and optical wavelengths for several hundred accreting and non-accreting members of the star-forming region NGC 2264 (~3 Myr).
Methods. We performed simultaneous monitoring of u- and r-band variability for the cluster population with CFHT/MegaCam. The survey extended over two full weeks, with several flux measurements per observing night. A sample of about 750 young stars is probed in our study, homogeneously calibrated and reduced, with internally consistently derived stellar parameters. Objects span the mass range 0.1â2 Mâ; about 40% of them show evidence for active accretion based on various diagnostics (H_α, UV, and IR excesses).
Results. Statistically distinct variability properties are observed for accreting and non-accreting cluster members. The accretors exhibit a significantly higher level of variability than the non-accretors, in the optical and especially in the UV. The amount of u-band variability is found to correlate statistically with the median amount of UV excess in disk-bearing objects, which suggests that mass accretion and star-disk interaction are the main sources of variability in the u band. Spot models are applied to account for the amplitudes of variability of accreting and non-accreting members, which yields different results for each group. Cool magnetic spots, several hundred degrees colder than the stellar photosphere and covering from 5 to 30% of the stellar surface, appear to be the leading factor of variability for the non-accreting stars. In contrast, accretion spots with a temperature a few thousand degrees higher than the photospheric temperature and that extend over a few percent of the stellar surface best reproduce the variability of accreting objects. The color behavior is also found to be different between accreting and non-accreting stars. While objects commonly become redder when fainter, typical amplitudes of variability for accreting members rapidly increase from the r to the u band, which indicates a much stronger contrast at short wavelengths; a lower color dependence in the photometric amplitudes is instead measured for diskless stars. Finally, we compare the u-band variability monitored here on two-week timescales with that measured on both shorter (hours) and longer (years) timescales. We find that variability on timescales of hours is typically ~10% of the peak-to-peak variability on day timescales, while longer term variability on a timescale of years is consistent with amplitudes measured over weeks.
Conclusions. We conclude that for both accreting and non-accreting stars, the mid-term rotational modulation by hot and cold spots is the leading timescale for a variability of up to several years. In turn, this suggests that the accretion process is essentially stable over years, although it exhibits low-level shorter term variations in single accretion events
CSI 2264: Accretion process in classical T Tauri stars in the young cluster NGC 2264
Context. NGC 2264 is a young stellar cluster (~3 Myr) with hundreds of low-mass accreting stars that allow a detailed analysis of the accretion process taking place in the pre-main sequence.
Aims. Our goal is to relate the photometric and spectroscopic variability of classical T Tauri stars to the physical processes acting in the stellar and circumstellar environment, within a few stellar radii from the star.
Methods. NGC 2264 was the target of a multiwavelength observational campaign with CoRoT, MOST, Spitzer, and Chandra satellites and photometric and spectroscopic observations from the ground. We classified the CoRoT light curves of accreting systems according to their morphology and compared our classification to several accretion diagnostics and disk parameters.
Results. The morphology of the CoRoT light curve reflects the evolution of the accretion process and of the inner disk region. Accretion burst stars present high mass-accretion rates and optically thick inner disks. AA Tau-like systems, whose light curves are dominated by circumstellar dust obscuration, show intermediate mass-accretion rates and are located in the transition of thick to anemic disks. Classical T Tauri stars with spot-like light curves correspond mostly to systems with a low mass-accretion rate and low mid-IR excess. About 30% of the classical T Tauri stars observed in the 2008 and 2011 CoRoT runs changed their light-curve morphology. Transitions from AA Tau-like and spot-like to aperiodic light curves and vice versa were common. The analysis of the Hα emission line variability of 58 accreting stars showed that 8 presented a periodicity that in a few cases was coincident with the photometric period. The blue and red wings of the Hα line profiles often do not correlate with each other, indicating that they are strongly influenced by different physical processes. Classical T Tauri stars have a dynamic stellar and circumstellar environment that can be explained by magnetospheric accretion and outflow models, including variations from stable to unstable accretion regimes on timescales of a few years
The fidelity approach to the Hubbard model
We use the fidelity approach to quantum critical points to study the zero
temperature phase diagram of the one-dimensional Hubbard model. Using a variety
of analytical and numerical techniques, we analyze the fidelity metric in
various regions of the phase diagram, with particular care to the critical
points. Specifically we show that close to the Mott transition, taking place at
on-site repulsion U=0 and electron density n=1, the fidelity metric satisfies
an hyper-scaling form which we calculate. This implies that in general, as one
approaches the critical point U=0, n=1, the fidelity metric tends to a limit
which depends on the path of approach. At half filling, the fidelity metric is
expected to diverge as U^{-4} when U is sent to zero.Comment: 8 pages, 4 figures, added results on the hyper-scaling form of the
fidelity metri
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