869 research outputs found
Magnetic incommensurability in -type cuprate perovskites
For the superconducting phase with a d-wave order parameter and zero
temperature the magnetic susceptibility of the t-J model is calculated using
the Mori projection operator technique. Conditions for the appearance of an
incommensurate magnetic response below the resonance frequency are identified.
A fast decay of the tails of the hole coherent peaks and a weak intensity of
the hole incoherent continuum near the Fermi level are enough to produce an
incommensurate response using different hole dispersions established for
-type cuprates, in which such response was observed. In this case, the
nesting of the itinerant-electron theory or the charge modulation of the stripe
theory is unnecessary for the incommensurability. The theory reproduces the
hourglass dispersion of the susceptibility maxima with their location in the
momentum space similar to that observed experimentally. The upper branch of the
dispersion stems from the excitations of localized spins, while the lower one
is due to the incommensurate maxima of their damping. The narrow and intensive
resonance peak arises if the frequency of these excitations at the
antiferromagnetic momentum lies below the edge of the two-fermion continuum;
otherwise the maximum is broad and less intensive.Comment: 22 pages, 7 figure
The accessibility of research-based knowledge for nurses in United Kingdom acute care settings
Background. The successful dissemination of the results of the National Health Service (NHS) research and development strategy and the development of evidence based approaches to health care rely on clinicians having access to the best available evidence; evidence fit for the purpose of reducing the uncertainties associated with clinical decisions. Aim. To reveal the accessibility of those sources of information actually used by nurses, as well as those which they say they use. Design. Mixed method case site, using interview, observational, Q sort and documentary audit data in medical, surgical and coronary care units (CCUs) in three acute hospitals. Results. Three perspectives on accessibility were identified: (a) the humanist-in which human sources of information were the most accessible; (b) local information for local needs-in which locally produced resources were seen as the most accessible and (c) moving towards technology-in which information technology begins to be seen as accessible. Nurses' experience in a clinical specialty is positively associated with a perception that human sources such clinical nurse specialists, link nurses, doctors and experienced clinical colleagues are more accessible than text based sources. Clinical specialization is associated with different approaches to accessing research knowledge. Coronary care unit nurses were more likely perceive local guidelines, protocols and on-line databases as more accessible than their counterparts in general medical and surgical wards. Only a third of text-based resources available to nurses oil the wards had any explicit research base. These, and the remainder were Out of date (mean age of textbooks 11 years), and authorship hard to ascertain. Conclusion. A strategy to increase the use of research evidence by nurses should harness the influence of clinical nurse specialists, link nurses and those engaged in practice development. These roles Could act as 'conduits' through which research-based messages for practice, and information for clinical decision making, could flow. This role should be explored and enhanced
Acute care nurses' perceptions of barriers to using research information in clinical decision-making
Aim. To examine the barriers that nurses feel prevent them from using research in the decisions they make. Background. A sizeable research literature focusing on research utilization in nursing has developed over the past 20 years. However, this literature is characterized by a number of weaknesses: self-reported utilization behaviour; poor response rates and small, nonrandom sampling strategies. Design. Cross-case analysis involving anonymised qualitative interviews, observation, documentary audit and Q methodological modelling of shared subjectivities amongst nurses. The case sites were three large acute hospitals in the north of England. One hundred and eight nurses were interviewed, 61 of whom were also observed for a total of 180 h, and 122 nurses were involved in the Q modelling exercise (response rate of 64%). Results. Four perspectives were isolated that encompassed the characteristics associated with barriers to research use. These related to the individual, organization, nature of research information itself and environment. Nurses clustered around four main perspectives on the barriers to research use: (1) Problems in interpreting and using research products, which were seen as too complex, 'academic' and overly statistical; (2) Nurses who felt confident with research-based information perceived a lack of organizational support as a significant block; (3) Many nurses felt that researchers and research products lack clinical credibility and that they fail to offer the desired level of clinical direction; (4) Some nurses lacked the skills and, to a lesser degree, the motivation to use research themselves. These individuals liked research messages passed on to them by a third party and sought to foster others' involvement in research-based practice, rather than becoming directly involved themselves. Conclusions. Rejection of research knowledge is not a barrier to its application. Rather, the presentation and management of research knowledge in the workplace represent significant challenges for clinicians, policy-makers and the research community
Ordered and periodic chaos of the bounded one dimensinal multibarrier potential
Numerical analysis indicates that there exists an unexpected new ordered
chaos for the bounded one-dimensional multibarrier potential. For certain
values of the number of barriers, repeated identical forms (periods) of the
wavepackets result upon passing through the multibarrier potential.Comment: 16 pages, 9 figures, 1 Table. Some former text removed and other
introduce
Local electronic nematicity in the one-band Hubbard model
Nematicity is a well known property of liquid crystals and has been recently
discussed in the context of strongly interacting electrons. An electronic
nematic phase has been seen by many experiments in certain strongly correlated
materials, in particular, in the pseudogap phase generic to many hole-doped
cuprate superconductors. Recent measurements in high superconductors has
shown even if the lattice is perfectly rotationally symmetric, the ground state
can still have strongly nematic local properties. Our study of the
two-dimensional Hubbard model provides strong support of the recent
experimental results on local rotational symmetry breaking. The
variational cluster approach is used here to show the possibility of an
electronic nematic state and the proximity of the underlying symmetry-breaking
ground state within the Hubbard model. We identify this nematic phase in the
overdoped region and show that the local nematicity decreases with increasing
electron filling. Our results also indicate that strong Coulomb interaction may
drive the nematic phase into a phase similar to the stripe structure. The
calculated spin (magnetic) correlation function in momentum space shows the
effects resulting from real-space nematicity
The Optimal Inhomogeneity for Superconductivity: Finite Size Studies
We report the results of exact diagonalization studies of Hubbard models on a
square lattice with periodic boundary conditions and various
degrees and patterns of inhomogeneity, which are represented by inequivalent
hopping integrals and . We focus primarily on two patterns, the
checkerboard and the striped cases, for a large range of values of the on-site
repulsion and doped hole concentration, . We present evidence that
superconductivity is strongest for of order the bandwidth, and intermediate
inhomogeneity, . The maximum value of the ``pair-binding
energy'' we have found with purely repulsive interactions is for the checkerboard Hubbard model with and .
Moreover, for near optimal values, our results are insensitive to changes in
boundary conditions, suggesting that the correlation length is sufficiently
short that finite size effects are already unimportant.Comment: 8 pages, 9 figures; minor revisions; more references adde
Theory of momentum resolved tunneling into a short quantum wire
Motivated by recent tunneling experiments in the parallel wire geometry, we
calculate results for momentum resolved tunneling into a short one-dimensional
wire, containing a small number of electrons. We derive some general theorems
about the momentum dependence, and we carry out exact calculations for up to
N=4 electrons in the final state, for a system with screened Coulomb
interactions that models the situation of the experiments. We also investigate
the limit of large using a Luttinger-liquid type analysis. We consider the
low-density regime, where the system is close to the Wigner crystal limit, and
where the energy scale for spin excitations can be much lower than for charge
excitations, and we consider temperatures intermediate between the relevant
spin energies and charge excitations, as well as temperatures below both energy
scales.Comment: 19 pages, 13 figures, clarified text in a few points, added 1 figure,
updated reference
Lanczos algorithm with Matrix Product States for dynamical correlation functions
The density-matrix renormalization group (DMRG) algorithm can be adapted to
the calculation of dynamical correlation functions in various ways which all
represent compromises between computational efficiency and physical accuracy.
In this paper we reconsider the oldest approach based on a suitable
Lanczos-generated approximate basis and implement it using matrix product
states (MPS) for the representation of the basis states. The direct use of
matrix product states combined with an ex-post reorthogonalization method
allows to avoid several shortcomings of the original approach, namely the
multi-targeting and the approximate representation of the Hamiltonian inherent
in earlier Lanczos-method implementations in the DMRG framework, and to deal
with the ghost problem of Lanczos methods, leading to a much better convergence
of the spectral weights and poles. We present results for the dynamic spin
structure factor of the spin-1/2 antiferromagnetic Heisenberg chain. A
comparison to Bethe ansatz results in the thermodynamic limit reveals that the
MPS-based Lanczos approach is much more accurate than earlier approaches at
minor additional numerical cost.Comment: final version 11 pages, 11 figure
Randomised, controlled trial of alternating pressure mattresses compared with alternating pressure overlays for the prevention of pressure ulcers : PRESSURE (pressure relieving support surfaces) trial
Objective To compare whether differences exist between alternating pressure overlays and alternating pressure mattresses in the development of new pressure ulcers, healing of existing pressure ulcers, and patient acceptability. Design Pragmatic, open, multicentre, randomised controlled trial. Setting 11 hospitals in six NHS trusts. Participants 1972 people admitted to hospital as acute or elective patients. Interventions Participants were randomised to an alternating pressure mattress (n = 982) of- an alternating pressure overlay (n = 990). Main outcome measures The proportion of participants developing a new pressure ulcer of grade 2 or worse; time to development of new pressure ulcers; proportions of participants developing a new ulcer within 30 days; healing of existing pressure ulcers; and patient acceptability Results Intention to treat analysis found no difference in the proportions of participants developing a new pressure ulcer of grade 2 or worse (10.7% overlay patients, 10.3% mattress patients; difference 0.4%, 95% confidence interval - 23% to 3.1%, P = 0.75). More overlay patients requested change owing to dissatisfaction (23.3%) than mattress patients (18.9%, P = 0.02). Conclusion No difference was found between alternating pressure mattresses and alternating pressure overlays in the proportion of people who develop a pressure ulcer
High Speed Phase-Resolved 2-d UBV Photometry of the Crab pulsar
We report a phase-resolved photometric and morphological analysis of UBV data
of the Crab pulsar obtained with the 2-d TRIFFID high speed optical photometer
mounted on the Russian 6m telescope. By being able to accurately isolate the
pulsar from the nebular background at an unprecedented temporal resolution (1
\mu s), the various light curve components were accurately fluxed via
phase-resolved photometry. Within the range, our datasets are consistent
with the existing trends reported elsewhere in the literature. In terms of flux
and phase duration, both the peak Full Width Half Maxima and Half Width Half
Maxima decrease as a function of photon energy. This is similarly the case for
the flux associated with the bridge of emission. Power-law fits to the various
light curve components are as follows; \alpha = 0.07 \pm 0.19 (peak 1), \alpha
= -0.06 \pm 0.19 (peak 2) and \alpha = -0.44 \pm 0.19 (bridge) - the
uncertainty here being dominated by the integrated CCD photometry used to
independently reference the TRIFFID data. Temporally, the main peaks are
coincident to \le 10 \mu s although an accurate phase lag with respect to the
radio main peak is compromised by radio timing uncertainties. The plateau on
the Crab's main peak was definitively determined to be \leq 55 \mu s in extent
and may decrease as a function of photon energy. There is no evidence for
non-stochastic activity over the light curves or within various phase regions,
nor is there evidence of anything akin to the giant pulses noted in the radio.
Finally, there is no evidence to support the existence of a reported 60 second
modulation suggested to be as a consequence of free precession.Comment: 13 pages, 12 figures, accepted for publication in Astronomy &
Astrophysic
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