27,713 research outputs found
Simulating `Complex' Problems with Quantum Monte Carlo
We present a new quantum Monte Carlo algorithm suitable for generically
complex problems, such as systems coupled to external magnetic fields or anyons
in two spatial dimensions. We find that the choice of gauge plays a nontrivial
role, and can be used to reduce statistical noise in the simulation.
Furthermore, it is found that noise can be greatly reduced by approximate
cancellations between the phases of the (gauge dependent) statistical flux and
the external magnetic flux.Comment: Revtex, 11 pages. 3 postscript files for figures attache
Analytically solvable driven time-dependent two-level quantum systems
Analytical solutions to the time-dependent Schrodinger equation describing a
driven two-level system are invaluable to many areas of physics, but they are
also extremely rare. Here, we present a simple algorithm that generates an
unlimited number of exact analytical solutions. We show that a general
single-axis driving term and its corresponding evolution operator are
determined by a single real function which is constrained only by a certain
inequality and initial conditions. Any function satisfying these constraints
yields an exact analytical solution. We demonstrate this method by presenting
several new exact solutions to the time-dependent Schrodinger equation. Our
general method and many of the new solutions we present are particularly
relevant to qubit control in quantum computing applications.Comment: 4.5 pages, 4 figures, PRL versio
All Keynesians now? Public support for countercyclical government borrowing
In the wake of the 2008 financial crisis, macroeconomic policy returned to the political agenda, and the influence of Keynesian ideas about fiscal stimulus rose (and then fell) in expert circles. Much less is known, however, about whether and when Keynesian prescriptions for countercyclical spending have any support among the general public. We use a survey experiment, fielded twice, to recover the extent to which UK respondents hold such countercyclical attitudes. Our results indicate that public opinion was countercyclical—Keynesian—in 2016. We then use Eurobarometer data to estimate the same basic parameter for the population for the period 2010–2017. The observational results validate our experimental findings for the later period, but also provide evidence that the UK population held procyclical views at the start of the period. Thus, there appear to be important dynamics in public opinion on a key macroeconomic policy issue
Are Policy Analogies Persuasive? The Household Budget Analogy and Public Support for Austerity
Public opinion on complex policy questions is shaped by the ways in which elites simplify the issues. Given the prevalence of metaphor and analogy as tools for cognitive problem solving, the deployment of analogies is often proposed as a tool for this kind of influence. For instance, a prominent explanation for the acceptance of austerity is that voters understand government deficits through an analogy to household borrowing. Indeed, there are theoretical reasons to think the household finance analogy represents a most likely case for the causal influence of analogical reasoning on policy preferences. This article examines this best-case scenario using original survey data from the United Kingdom. It reports observational and experimental analyses that find no evidence of causation running from the household analogy to preferences over the government budget. Rather, endorsement of the analogy is invoked ex post to justify support for fiscal consolidation
Weak double layers in the auroral ionosphere
Previous work on the evolution of weak double layers in a hydrogen plasma was extended to include H(+) and O(+) with relative drift. The relative drift between hydrogen and oxygen ions due to a quasi-static parallel electric field gives rise to a strong linear fluid instability which dominates the ion-acoustic mode at the bottom of the auroral acceleration region. This ion-ion instability can modify ion distributions at lower altitudes and the subsequent nonlinear evolution of weak double layers at higher altitudes in the ion-acoustic regime. Ion hole formation can occur for smaller relative electron-ion drifts than seen in previous simulations, due to the hydrogen-oxygen two-stream instability. This results in local modification of the ion distributions in phase space, and a partial filling of the valley between the hydrogen and oxygen peaks, which would be expected at higher altitudes on auroral field lines. The observed velocity diffusion does not necessarily preclude ion hole and double layer formation in hydrogen in the ion-acoustic regime. These simulation results are consistent with the experimentally measured persistence of separate hydrogen and oxygen peaks, and the observation of weak double layers above an altitude of 3000 km on auroral field lines
Effective chiral-spin Hamiltonian for odd-numbered coupled Heisenberg chains
An system of odd number of coupled Heisenberg spin chains
is studied using a degenerate perturbation theory, where is the number of
coupled chains. An effective chain Hamiltonian is derived explicitly in terms
of two spin half degrees of freedom of a closed chain of sites, valid in
the regime the inter-chain coupling is stronger than the intra-chain coupling.
The spin gap has been calculated numerically using the effective Hamiltonian
for for a finite chain up to ten sites. It is suggested that the
ground state of the effective Hamiltonian is correlated, by examining
variational states for the effective chiral-spin chain Hamiltonian.Comment: 9 Pages, Latex, report ICTP-94-28
Executive function in first-episode schizophrenia
BACKGROUND: We tested the hypothesis that schizophrenia is primarily a frontostriatal disorder by examining executive function in first-episode patients. Previous studies have shown either equal decrements in many cognitive domains or specific deficits in memory. Such studies have grouped test results or have used few executive measures, thus, possibly losing information. We, therefore, measured a range of executive ability with tests known to be sensitive to frontal lobe function.
METHODS: Thirty first-episode schizophrenic patients and 30 normal volunteers, matched for age and NART IQ, were tested on computerized test of planning, spatial working memory and attentional set shifting from the Cambridge Automated Neuropsychological Test Battery. Computerized and traditional tests of memory were also administered for comparison. RESULTS: Patients were worse on all tests but the profile was non-uniform. A componential analysis indicated that the patients were characterized by a poor ability to think ahead and organize responses but an intact ability to switch attention and inhibit prepotent responses. Patients also demonstrated poor memory, especially for free recall of a story and associate learning of unrelated word pairs.
CONCLUSIONS: In contradistinction to previous studies, schizophrenic patients do have profound executive impairments at the beginning of the illness. However, these concern planning and strategy use rather than attentional set shifting, which is generally unimpaired. Previous findings in more chronic patients, of severe attentional set shifting impairment, suggest that executive cognitive deficits are progressive during the course of schizophrenia. The finding of severe mnemonic impairment at first episode suggests that cognitive deficits are not restricted to one cognitive domain
gamma Doradus pulsation in two pre-main sequence stars discovered by CoRoT
Pulsations in pre-main sequence stars have been discovered several times
within the last years. But nearly all of these pulsators are of delta
Scuti-type. gamma Doradus-type pulsation in young stars has been predicted by
theory, but lack observational evidence. We present the investigation of
variability caused by rotation and (gammaDoradus-type) pulsation in two
pre-main sequence members of the young open cluster NGC2264 using
high-precision time series photometry from the CoRoT satellite and dedicated
high-resolution spectroscopy. Time series photometry of NGC2264VAS20 and NGC
2264VAS87 was obtained by the CoRoT satellite during the dedicated short run
SRa01 in March 2008. NGC2264VAS87 was re-observed by CoRoT during the short run
SRa05 in December 2011 and January 2012. Frequency analysis was conducted using
Period04 and SigSpec. The spectral analysis was performed using equivalent
widths and spectral synthesis. The frequency analysis yielded 10 and 14
intrinsic frequencies for NGC2264VAS20 and NGC2264VAS 87, respectively, in the
range from 0 to 1.5c/d which are attributed to be caused by a combination of
rotation and pulsation. The effective temperatures were derived to be
6380150K for NGC2264VAS20 and 6220150K for NGC2264VAS87. Membership
of the two stars to the cluster is confirmed independently using X-ray fluxes,
radial velocity measurements and proper motions available in the literature.
The derived Li abundances of log n(Li)=3.34 and 3.54 for NGC2264VAS20 and
NGC2264VAS87, respectively, are in agreement with the Li abundance for other
stars in NGC2264 of similar Teff reported in the literature. We conclude that
the two objects are members of NGC2264 and therefore are in their pre-main
sequence evolutionary stage. Assuming that part of their variability is caused
by pulsation, these two stars might be the first pre-main sequence gamma
Doradus candidates.Comment: 11 pages, 10 figures, A&A accepte
Efficient coupling of surface plasmon polaritons to radiation using a bi-grating
Copyright © 2001 American Institute of Physics. This article may be downloaded for personal use only. Any other use requires prior permission of the author and the American Institute of Physics. The following article appeared in Applied Physics Letters 79 (2001) and may be found at http://link.aip.org/link/?APPLAB/79/3035/1A nanostructured surface in the form of a bi-grating is shown to efficiently couple surface plasmon polaritons to free-space radiation in the visible part of the spectrum. Coupling was achieved for all propagation directions of the surface mode and the efficiency found to be independent of the propagation direction, taking a mean value of 60% for the structure examined. The consequences of the findings for emissive devices that make use of surface plasmons are discussed
- …