8,907 research outputs found
Non Local Electron-Phonon Correlations in a Dispersive Holstein Model
Due to the dispersion of optical phonons, long range electron-phonon
correlations renormalize downwards the coupling strength in the Holstein model.
We evaluate the size of this effect both in a linear chain and in a square
lattice for a time averaged {\it e-ph} potential, where the time variable is
introduced according to the Matsubara formalism. Mapping the Holstein
Hamiltonian onto the time scale we derive the perturbing source current which
appears to be non time retarded. This property permits to disentangle phonon
and electron coordinates in the general path integral for an electron coupled
to dispersive phonons. While the phonon paths can be integrated out
analytically, the electron path integrations have to be done numerically. The
equilibrium thermodynamic properties of the model are thus obtained as a
function of the electron hopping value and of the phonon spectrum parameters.
We derive the {\it e-ph} corrections to the phonon free energy and show that
its temperature derivatives do not depend on the {\it e-ph} effective coupling
hence, the Holstein phonon heat capacity is strictly harmonic. A significant
upturn in the low temperature total heat capacity over ratio is attributed
to the electron hopping which largely contributes to the action.Comment: Phys.Rev.B (2005
Autosomal recessive primary microcephaly: an analysis of locus heterogeneity and phenotypic variation
BACKGROUND AND OBJECTIVES: Locus heterogeneity is well established in autosomal recessive primary
microcephaly (MCPH) and to date five loci have been mapped. However, the relative contributions of
these loci have not been assessed and genotype-phenotype correlations have not been investigated.
DESIGN: A study population of 56 consanguineous families resident in or originating from northern
Pakistan was ascertained and assessed by the authors. A panel of microsatellite markers spanning
each of the MCPH loci was designed, against which the families were genotyped.
RESULTS: The head circumference of the 131 affected subjects ranged from 4 to 14 SD below the mean,
but there was little intrafamilial variation among affecteds (± 1 SD). MCPH5 was the most prevalent,
with 24/56 families consistent with linkage; 2/56 families were compatible with linkage to MCPH1,
10/56 to MCPH2, 2/56 to MCPH3, none to MCPH4, and 18/56 did not segregate with any of the
loci.
CONCLUSIONS: MCPH5 is the most common locus in this population. On clinical grounds alone, the
phenotype of families linked to each MCPH locus could not be distinguished. We have also shown that
further MCPH loci await discovery with a number of families as yet unlinked
Energy Level Statistics of Quantum Dots
We investigate the charging energy level statistics of disordered interacting
electrons in quantum dots by numerical calculations using the Hartree
approximation. The aim is to obtain a global picture of the statistics as a
function of disorder and interaction strengths. We find Poisson statistics at
very strong disorder, Wigner- Dyson statistics for weak disorder and
interactions, and a Gaussian intermediate regime. These regimes are as expected
from previous studies and fundamental considerations, but we also find
interesting and rather broad crossover regimes. In particular, intermediate
between the Gaussian and Poisson regimes we find a two-sided exponential
distribution for the energy level spacings. In comparing with experiment, we
find that this distribution may be realized in some quantum dots.Comment: 21 pages 10 figure
The experience of family carers attending a joint reminiscence group with people with dementia: A thematic analysis
Reminiscence therapy has the potential to improve quality of life for people with dementia. In recent years reminiscence groups have extended to include family members, but carers' experience of attending joint sessions is undocumented. This qualitative study explored the experience of 18 family carers attending 'Remembering Yesterday Caring Today' groups. Semi-structured interviews were transcribed and subjected to thematic analysis. Five themes were identified: experiencing carer support; shared experience; expectations (met and unmet), carer perspectives of the person with dementia's experience; and learning and comparing. Family carers' experiences varied, with some experiencing the intervention as entirely positive whereas others had more mixed feelings. Negative aspects included the lack of respite from their relative, the lack of emphasis on their own needs, and experiencing additional stress and guilt through not being able to implement newly acquired skills. These findings may explain the failure of a recent trial of joint reminiscence groups to replicate previous findings of positive benefit. More targeted research within subgroups of carers is required to justify the continued use of joint reminiscence groups in dementia care
Coulomb crystals in the magnetic field
The body-centered cubic Coulomb crystal of ions in the presence of a uniform
magnetic field is studied using the rigid electron background approximation.
The phonon mode spectra are calculated for a wide range of magnetic field
strengths and for several orientations of the field in the crystal. The phonon
spectra are used to calculate the phonon contribution to the crystal energy,
entropy, specific heat, Debye-Waller factor of ions, and the rms ion
displacements from the lattice nodes for a broad range of densities,
temperatures, chemical compositions, and magnetic fields. Strong magnetic field
dramatically alters the properties of quantum crystals. The phonon specific
heat increases by many orders of magnitude. The ion displacements from their
equilibrium positions become strongly anisotropic. The results can be relevant
for dusty plasmas, ion plasmas in Penning traps, and especially for the crust
of magnetars (neutron stars with superstrong magnetic fields G). The effect of the magnetic field on ion displacements in a
strongly magnetized neutron star crust can suppress the nuclear reaction rates
and make them extremely sensitive to the magnetic field direction.Comment: 25 pages, 8 figures; accepted to Phys. Rev.
‘Trying to pin down jelly’ - exploring intuitive processes in quality assessment for meta-ethnography
Background:
Studies that systematically search for and synthesise qualitative research are becoming more evident in health care, and they can make an important contribution to patient care. However, there is still no agreement as to whether, or how we should appraise studies for inclusion. We aimed to explore the intuitive processes that determined the ‘quality’ of qualitative research for inclusion in qualitative research syntheses. We were particularly interested to explore the way that knowledge was constructed.
Methods:
We used qualitative methods to explore the process of quality appraisal within a team of seven qualitative researchers funded to undertake a meta-ethnography of chronic non-malignant musculoskeletal pain. Team discussions took place monthly between October 2010 and June 2012 and were recorded and transcribed. Data was coded and organised using constant comparative method. The development of our conceptual analysis was both iterative and collaborative. The strength of this team approach to quality came from open and honest discussion, where team members felt free to agree, disagree, or change their position within the safety of the group.
Results:
We suggest two core facets of quality for inclusion in meta-ethnography - (1) Conceptual clarity; how clearly has the author articulated a concept that facilitates theoretical insight. (2) Interpretive rigour; fundamentally, can the interpretation ‘be trusted?’ Our findings showed that three important categories help the reader to judge interpretive rigour: (ii) What is the context of the interpretation? (ii) How inductive is the interpretation? (iii) Has the researcher challenged their interpretation?
Conclusions:
We highlight that methods alone do not determine the quality of research for inclusion into a meta-ethnography. The strength of a concept and its capacity to facilitate theoretical insight is integral to meta-ethnography, and arguably to the quality of research. However, we suggest that to be judged ‘good enough’ there also needs to be some assurance that qualitative findings are more than simply anecdotal. Although our conceptual model was developed specifically for meta-ethnography, it may be transferable to other research methodologies
Small Universal Accepting Networks of Evolutionary Processors with Filtered Connections
In this paper, we present some results regarding the size complexity of
Accepting Networks of Evolutionary Processors with Filtered Connections
(ANEPFCs). We show that there are universal ANEPFCs of size 10, by devising a
method for simulating 2-Tag Systems. This result significantly improves the
known upper bound for the size of universal ANEPFCs which is 18.
We also propose a new, computationally and descriptionally efficient
simulation of nondeterministic Turing machines by ANEPFCs. More precisely, we
describe (informally, due to space limitations) how ANEPFCs with 16 nodes can
simulate in O(f(n)) time any nondeterministic Turing machine of time complexity
f(n). Thus the known upper bound for the number of nodes in a network
simulating an arbitrary Turing machine is decreased from 26 to 16
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