146 research outputs found
Modelling local order in organic and metal-organic ferroic materials using the reverse Monte Carlo method and total neutron scattering
PhDThe ordering processes of ferroelectric and multiferroic materials were investigated via neutron
total scattering and the reverse Monte Carlo method.
The results presented in this thesis are from three materials where ferroelectric behaviour
is a result of ordering of molecular groups rather than individual atoms. Two of the materials
are metal-organic frameworks, three dimensional cage-like structures with guest ions inside
the pores; the third material, is a room temperature ferroelectric.
In the high-temperature phase of dimethylammonium manganese formate, the framework
distorts around the disordered cation, and the cations form shorter hydrogen bonds with the
formate framework than the average structure suggests. Framework deformations became
increasingly unfavourable as the material cooled. The cations continue to order as the material
was cooled below Tc. Analysis of the high-temperature phase atomistic configurations showed
that in addition to the three known orientations about the threefold axis, a significant minority
of the cations lie mid-way between these positions, a feature which could not have been
observed via standard crystallographic techniques.
The mechanisms for thermal expansion of potassium imidazolium hexacyanoferrate change
between the intermediate-temperature phase and the high-temperature phase. In the hightemperature
phase the framework distorts around the disordered guest, but in the intermediatetemperature
phase the framework stiffens. I propose that the temperature of the dielectric
transition is dependent of the volume inside the framework, but that the temperature range of
the intermediate-temperature phase is dependent on the rate of contraction of the framework
around the guest cation..
For triglycine sulfate no correlation was observed between the orientation of the polar
molecules and the motion of the intermediate deuterium. Furthermore, in the high temperature
phase the atomistic configurations produced models with macroscopic polarisation. I propose
that this material forms domains of aligned polar molecules above Tc and that these domains
are larger than the atomistic configurations
Non-perturbative Heavy Quark Effective Theory
We explain how to perform non-perturbative computations in HQET on the
lattice. In particular the problem of the subtraction of power-law divergences
is solved by a non-perturbative matching of HQET and QCD. As examples, we
present a full calculation of the mass of the b-quark in the combined static
and quenched approximation and outline an alternative way to obtain the B-meson
decay constant at lowest order. Since no excessively large lattices are
required, our strategy can also be applied including dynamical fermions.Comment: 27 pages including figures and tables, latex2e; version published in
JHEP, typos corrected and 1 reference adde
Adjoint "quarks" on coarse anisotropic lattices: Implications for string breaking in full QCD
A detailed study is made of four dimensional SU(2) gauge theory with static
adjoint ``quarks'' in the context of string breaking. A tadpole-improved action
is used to do simulations on lattices with coarse spatial spacings ,
allowing the static potential to be probed at large separations at a
dramatically reduced computational cost. Highly anisotropic lattices are used,
with fine temporal spacings , in order to assess the behavior of the
time-dependent effective potentials. The lattice spacings are determined from
the potentials for quarks in the fundamental representation. Simulations of the
Wilson loop in the adjoint representation are done, and the energies of
magnetic and electric ``gluelumps'' (adjoint quark-gluon bound states) are
calculated, which set the energy scale for string breaking. Correlators of
gauge-fixed static quark propagators, without a connecting string of spatial
links, are analyzed. Correlation functions of gluelump pairs are also
considered; similar correlators have recently been proposed for observing
string breaking in full QCD and other models. A thorough discussion of the
relevance of Wilson loops over other operators for studies of string breaking
is presented, using the simulation results presented here to support a number
of new arguments.Comment: 22 pages, 14 figure
Concerning the quark condensate
A continuum expression for the trace of the massive dressed-quark propagator
is used to explicate a connection between the infrared limit of the QCD Dirac
operator's spectrum and the quark condensate appearing in the operator product
expansion, and the connection is verified via comparison with a lattice-QCD
simulation. The pseudoscalar vacuum polarisation provides a good approximation
to the condensate over a larger range of current-quark masses.Comment: 7 pages, LaTeX2e, revtex
Matter degrees of freedom and string breaking in Abelian projected quenched SU(2) QCD
In the Abelian projection the Yang--Mills theory contains Abelian gauge
fields (diagonal degrees of freedom) and the Abelian matter fields
(off-diagonal degrees) described by a complicated action. The matter fields are
essential for the breaking of the adjoint string. We obtain numerically the
effective action of the Abelian gauge and the Abelian matter fields in quenched
SU(2) QCD and show that the Abelian matter fields provide an essential
contribution to the total action even in the infrared region. We also observe
the breaking of an Abelian analog of the adjoint string using Abelian
operators. We show that the adjoint string tension is dominated by the Abelian
and the monopole contributions similarly to the case of the fundamental
particles. We conclude that the adjoint string breaking can successfully be
described in the Abelian projection formalism.Comment: 16 pages, 10 figures, 2 table
Lattice Calculation of Heavy-Light Decay Constants with Two Flavors of Dynamical Quarks
We present results for , , , and their ratios in
the presence of two flavors of light sea quarks (). We use Wilson light
valence quarks and Wilson and static heavy valence quarks; the sea quarks are
simulated with staggered fermions. Additional quenched simulations with
nonperturbatively improved clover fermions allow us to improve our control of
the continuum extrapolation. For our central values the masses of the sea
quarks are not extrapolated to the physical , masses; that is, the
central values are "partially quenched." A calculation using "fat-link clover"
valence fermions is also discussed but is not included in our final results. We
find, for example,
MeV, , MeV, and , where in each case the first error is
statistical and the remaining three are systematic: the error within the
partially quenched approximation, the error due to the missing strange
sea quark and to partial quenching, and an estimate of the effects of chiral
logarithms at small quark mass. The last error, though quite significant in
decay constant ratios, appears to be smaller than has been recently suggested
by Kronfeld and Ryan, and Yamada. We emphasize, however, that as in other
lattice computations to date, the lattice quark masses are not very light
and chiral log effects may not be fully under control.Comment: Revised version includes an attempt to estimate the effects of chiral
logarithms at small quark mass; central values are unchanged but one more
systematic error has been added. Sections III E and V D are completely new;
some changes for clarity have also been made elsewhere. 82 pages; 32 figure
Model-based analyses: Promises, pitfalls, and example applications to the study of cognitive control
We discuss a recent approach to investigating cognitive control, which has the potential to deal with some of the challenges inherent in this endeavour. In a model-based approach, the researcher defines a formal, computational model that performs the task at hand and whose performance matches that of a research participant. The internal variables in such a model might then be taken as proxies for latent variables computed in the brain. We discuss the potential advantages of such an approach for the study of the neural underpinnings of cognitive control and its pitfalls, and we make explicit the assumptions underlying the interpretation of data obtained using this approach
Impact of Chlamydia trachomatis in the reproductive setting: British Fertility Society Guidelines for practice
Chlamydia trachomatis infection of the genital tract is the most common sexually transmitted infection and has a world-wide distribution. The consequences of infection have an adverse effect on the reproductive health of women and are a common cause of infertility. Recent evidence also suggests an adverse effect on male reproduction. There is a need to standardise the approach in managing the impact of C. trachomatis infection on reproductive health. We have surveyed current UK practice towards screening and management of Chlamydia infections in the fertility setting. We found that at least 90% of clinicians surveyed offered screening. The literature on this topic was examined and revealed a paucity of solid evidence for estimating the risks of long-term reproductive sequelae following lower genital tract infection with C. trachomatis. The mechanism for the damage that occurs after Chlamydial infections is uncertain. However, instrumentation of the uterus in women with C. trachomatis infection is associated with a high risk of pelvic inflammatory disease, which can be prevented by appropriate antibiotic treatment and may prevent infected women from being at increased risk of the adverse sequelae, such as ectopic pregnancy and tubal factor infertility. Recommendations for practice have been proposed and the need for further studies is identified
Analysis of shared heritability in common disorders of the brain
ience, this issue p. eaap8757 Structured Abstract INTRODUCTION Brain disorders may exhibit shared symptoms and substantial epidemiological comorbidity, inciting debate about their etiologic overlap. However, detailed study of phenotypes with different ages of onset, severity, and presentation poses a considerable challenge. Recently developed heritability methods allow us to accurately measure correlation of genome-wide common variant risk between two phenotypes from pools of different individuals and assess how connected they, or at least their genetic risks, are on the genomic level. We used genome-wide association data for 265,218 patients and 784,643 control participants, as well as 17 phenotypes from a total of 1,191,588 individuals, to quantify the degree of overlap for genetic risk factors of 25 common brain disorders. RATIONALE Over the past century, the classification of brain disorders has evolved to reflect the medical and scientific communities' assessments of the presumed root causes of clinical phenomena such as behavioral change, loss of motor function, or alterations of consciousness. Directly observable phenomena (such as the presence of emboli, protein tangles, or unusual electrical activity patterns) generally define and separate neurological disorders from psychiatric disorders. Understanding the genetic underpinnings and categorical distinctions for brain disorders and related phenotypes may inform the search for their biological mechanisms. RESULTS Common variant risk for psychiatric disorders was shown to correlate significantly, especially among attention deficit hyperactivity disorder (ADHD), bipolar disorder, major depressive disorder (MDD), and schizophrenia. By contrast, neurological disorders appear more distinct from one another and from the psychiatric disorders, except for migraine, which was significantly correlated to ADHD, MDD, and Tourette syndrome. We demonstrate that, in the general population, the personality trait neuroticism is significantly correlated with almost every psychiatric disorder and migraine. We also identify significant genetic sharing between disorders and early life cognitive measures (e.g., years of education and college attainment) in the general population, demonstrating positive correlation with several psychiatric disorders (e.g., anorexia nervosa and bipolar disorder) and negative correlation with several neurological phenotypes (e.g., Alzheimer's disease and ischemic stroke), even though the latter are considered to result from specific processes that occur later in life. Extensive simulations were also performed to inform how statistical power, diagnostic misclassification, and phenotypic heterogeneity influence genetic correlations. CONCLUSION The high degree of genetic correlation among many of the psychiatric disorders adds further evidence that their current clinical boundaries do not reflect distinct underlying pathogenic processes, at least on the genetic level. This suggests a deeply interconnected nature for psychiatric disorders, in contrast to neurological disorders, and underscores the need to refine psychiatric diagnostics. Genetically informed analyses may provide important "scaffolding" to support such restructuring of psychiatric nosology, which likely requires incorporating many levels of information. By contrast, we find limited evidence for widespread common genetic risk sharing among neurological disorders or across neurological and psychiatric disorders. We show that both psychiatric and neurological disorders have robust correlations with cognitive and personality measures. Further study is needed to evaluate whether overlapping genetic contributions to psychiatric pathology may influence treatment choices. Ultimately, such developments may pave the way toward reduced heterogeneity and improved diagnosis and treatment of psychiatric disorders
- …