167 research outputs found
Critical thermodynamics of three-dimensional MN-component field model with cubic anisotropy from higher-loop \epsilon expansion
The critical thermodynamics of an -component field model with cubic
anisotropy relevant to the phase transitions in certain crystals with
complicated ordering is studied within the four-loop \ve expansion using the
minimal subtraction scheme. Investigation of the global structure of RG flows
for the physically significant cases M=2, N=2 and M=2, N=3 shows that the model
has an anisotropic stable fixed point with new critical exponents. The critical
dimensionality of the order parameter is proved to be equal to
, that is exactly half its counterpart in the real hypercubic
model.Comment: 9 pages, LaTeX, no figures. Published versio
The stability of the O(N) invariant fixed point in three dimensions
We study the stability of the O(N) fixed point in three dimensions under
perturbations of the cubic type. We address this problem in the three cases
by using finite size scaling techniques and high precision Monte
Carlo simulations. It is well know that there is a critical value
below which the O(N) fixed point is stable and above which the cubic fixed
point becomes the stable one. While we cannot exclude that , as recently
claimed by Kleinert and collaborators, our analysis strongly suggests that
coincides with 3.Comment: latex file of 18 pages plus three ps figure
The stability of a cubic fixed point in three dimensions from the renormalization group
The global structure of the renormalization-group flows of a model with
isotropic and cubic interactions is studied using the massive field theory
directly in three dimensions. The four-loop expansions of the \bt-functions
are calculated for arbitrary . The critical dimensionality and the stability matrix eigenvalues estimates obtained on the basis of
the generalized Pad-Borel-Leroy resummation technique are shown
to be in a good agreement with those found recently by exploiting the five-loop
\ve-expansions.Comment: 18 pages, LaTeX, 5 PostScript figure
Developing the content of two behavioural interventions : using theory-based interventions to promote GP management of upper respiratory tract infection without prescribing antibiotics #1
Background: Evidence shows that antibiotics have limited effectiveness in the management of upper respiratory tract infection (URTI) yet GPs continue to prescribe antibiotics. Implementation research does not currently provide a strong evidence base to guide the choice of interventions to promote the uptake of such evidence-based practice by health professionals. While systematic reviews demonstrate that interventions to change clinical practice can be effective, heterogeneity between studies hinders generalisation to routine practice. Psychological models of behaviour change that have been used successfully to predict variation in behaviour in the general population can also predict the clinical behaviour of healthcare professionals. The purpose of this study was to design two theoretically-based interventions to promote the management of upper respiratory tract infection (URTI) without prescribing antibiotics. Method: Interventions were developed using a systematic, empirically informed approach in which we: selected theoretical frameworks; identified modifiable behavioural antecedents that predicted GPs intended and actual management of URTI; mapped these target antecedents on to evidence-based behaviour change techniques; and operationalised intervention components in a format suitable for delivery by postal questionnaire. Results: We identified two psychological constructs that predicted GP management of URTI: "Self-efficacy," representing belief in one's capabilities, and "Anticipated consequences," representing beliefs about the consequences of one's actions. Behavioural techniques known to be effective in changing these beliefs were used in the design of two paper-based, interactive interventions. Intervention 1 targeted self-efficacy and required GPs to consider progressively more difficult situations in a "graded task" and to develop an "action plan" of what to do when next presented with one of these situations. Intervention 2 targeted anticipated consequences and required GPs to respond to a "persuasive communication" containing a series of pictures representing the consequences of managing URTI with and without antibiotics. Conclusion: It is feasible to systematically develop theoretically-based interventions to change professional practice. Two interventions were designed that differentially target generalisable constructs predictive of GP management of URTI. Our detailed and scientific rationale for the choice and design of our interventions will provide a basis for understanding any effects identified in their evaluation. Trial registration: Clinicaltrials.gov NCT00376142This study is funded by the European Commission Research Directorate as part of a multi-partner program: Research Based Education and Quality Improvement (ReBEQI): A Framework and tools to develop effective quality improvement programs in European healthcare. (Proposal No: QLRT-2001-00657)
Stability of 3D Cubic Fixed Point in Two-Coupling-Constant \phi^4-Theory
For an anisotropic euclidean -theory with two interactions [u
(\sum_{i=1^M {\phi}_i^2)^2+v \sum_{i=1}^M \phi_i^4] the -functions are
calculated from five-loop perturbation expansions in
dimensions, using the knowledge of the large-order behavior and Borel
transformations. For , an infrared stable cubic fixed point for
is found, implying that the critical exponents in the magnetic phase
transition of real crystals are of the cubic universality class. There were
previous indications of the stability based either on lower-loop expansions or
on less reliable Pad\'{e approximations, but only the evidence presented in
this work seems to be sufficently convincing to draw this conclusion.Comment: Author Information under
http://www.physik.fu-berlin.de/~kleinert/institution.html . Paper also at
http://www.physik.fu-berlin.de/~kleinert/kleiner_re250/preprint.htm
Translating clinicians' beliefs into implementation interventions (TRACII) : a protocol for an intervention modeling experiment to change clinicians' intentions to implement evidence-based practice
Background: Biomedical research constantly produces new findings, but these are not routinely incorporated into health care practice. Currently, a range of interventions to promote the uptake of emerging evidence are available. While their effectiveness has been tested in pragmatic trials, these do not form a basis from which to generalise to routine care settings. Implementation research is the scientific study of methods to promote the uptake of research findings, and hence to reduce inappropriate care. As clinical practice is a form of human behaviour, theories of human behaviour that have proved to be useful in other settings offer a basis for developing a scientific rationale for the choice of interventions. Aims: The aims of this protocol are 1) to develop interventions to change beliefs that have already been identified as antecedents to antibiotic prescribing for sore throats, and 2) to experimentally evaluate these interventions to identify those that have the largest impact on behavioural intention and behavioural simulation.
Design: The clinical focus for this work will be the management of uncomplicated sore throat in general practice. Symptoms of upper respiratory tract infections are common presenting features in primary care. They are frequently treated with antibiotics, and research evidence is clear that antibiotic treatment offers little or no benefit to otherwise healthy adult patients.
Reducing antibiotic prescribing in the community by the "prudent" use of antibiotics is seen as one way to slow the rise in antibiotic resistance, and appears safe, at least in children. However, our understanding of how to do this is limited. Participants will be general medical practitioners. Two theory-based interventions will be designed to address the discriminant beliefs in the prescribing of antibiotics for sore throat, using empirically derived resources. The interventions will be evaluated in a 2 × 2 factorial randomised controlled trial delivered in a postal questionnaire survey. Two outcome measures will be assessed: behavioural intention and behavioural simulation.This study is funded by the European Commission Research Directorate as part of a multi-partner program: Research Based Education and Quality Improvement (ReBEQI): A Framework and tools to develop effective quality improvement programs in European healthcare. (Proposal No: QLRT-2001-00657)
The N-component Ginzburg-Landau Hamiltonian with cubic anisotropy: a six-loop study
We consider the Ginzburg-Landau Hamiltonian with a cubic-symmetric quartic
interaction and compute the renormalization-group functions to six-loop order
in d=3. We analyze the stability of the fixed points using a Borel
transformation and a conformal mapping that takes into account the
singularities of the Borel transform. We find that the cubic fixed point is
stable for N>N_c, N_c = 2.89(4). Therefore, the critical properties of cubic
ferromagnets are not described by the Heisenberg isotropic Hamiltonian, but
instead by the cubic model at the cubic fixed point. For N=3, the critical
exponents at the cubic and symmetric fixed points differ very little (less than
the precision of our results, which is in the case of
and ). Moreover, the irrelevant interaction bringing from the symmetric to
the cubic fixed point gives rise to slowly-decaying scaling corrections with
exponent . For N=2, the isotropic fixed point is stable and
the cubic interaction induces scaling corrections with exponent . These conclusions are confirmed by a similar analysis of the
five-loop -expansion. A constrained analysis which takes into account
that in two dimensions gives .Comment: 29 pages, RevTex, new refs added, Phys. Rev. B in pres
Critical Behavior of the Supersolid transition in Bose-Hubbard Models
We study the phase transitions of interacting bosons at zero temperature
between superfluid (SF) and supersolid (SS) states. The latter are
characterized by simultaneous off-diagonal long-range order and broken
translational symmetry. The critical phenomena is described by a
long-wavelength effective action, derived on symmetry grounds and verified by
explicit calculation. We consider two types of supersolid ordering:
checkerboard (X) and collinear (C), which are the simplest cases arising in two
dimensions on a square lattice. We find that the SF--CSS transition is in the
three-dimensional XY universality class. The SF--XSS transition exhibits
non-trivial new critical behavior, and appears, within a
expansion to be driven generically first order by fluctuations. However, within
a one--loop calculation directly in a strong coupling fixed point with
striking ``non-Bose liquid'' behavior is found. At special isolated
multi-critical points of particle-hole symmetry, the system falls into the 3d
Ising universality class.Comment: RevTeX, 24 pages, 16 figures. Also available at
http://www.cip.physik.tu-muenchen.de/tumphy/d/T34/Mitarbeiter/frey.htm
Stability of a cubic fixed point in three dimensions. Critical exponents for generic N
The detailed analysis of the global structure of the renormalization-group
(RG) flow diagram for a model with isotropic and cubic interactions is carried
out in the framework of the massive field theory directly in three dimensions
(3D) within an assumption of isotropic exchange. Perturbative expansions for RG
functions are calculated for arbitrary up to the four-loop order and
resummed by means of the generalized Pad-Borel-Leroy technique.
Coordinates and stability matrix eigenvalues for the cubic fixed point are
found under the optimal value of the transformation parameter. Critical
dimensionality of the model is proved to be equal to that
agrees well with the estimate obtained on the basis of the five-loop
\ve-expansion [H. Kleinert and V. Schulte-Frohlinde, Phys. Lett. B342, 284
(1995)] resummed by the above method. As a consequence, the cubic fixed point
should be stable in 3D for , and the critical exponents controlling
phase transitions in three-dimensional magnets should belong to the cubic
universality class. The critical behavior of the random Ising model being the
nontrivial particular case of the cubic model when N=0 is also investigated.
For all physical quantities of interest the most accurate numerical estimates
with their error bounds are obtained. The results achieved in the work are
discussed along with the predictions given by other theoretical approaches and
experimental data.Comment: 33 pages, LaTeX, 7 PostScript figures. Final version corrected and
added with an Appendix on the six-loop stud
Critical behavior of weakly-disordered anisotropic systems in two dimensions
The critical behavior of two-dimensional (2D) anisotropic systems with weak
quenched disorder described by the so-called generalized Ashkin-Teller model
(GATM) is studied. In the critical region this model is shown to be described
by a multifermion field theory similar to the Gross-Neveu model with a few
independent quartic coupling constants. Renormalization group calculations are
used to obtain the temperature dependence near the critical point of some
thermodynamic quantities and the large distance behavior of the two-spin
correlation function. The equation of state at criticality is also obtained in
this framework. We find that random models described by the GATM belong to the
same universality class as that of the two-dimensional Ising model. The
critical exponent of the correlation length for the 3- and 4-state
random-bond Potts models is also calculated in a 3-loop approximation. We show
that this exponent is given by an apparently convergent series in
(with the central charge of the Potts model) and
that the numerical values of are very close to that of the 2D Ising
model. This work therefore supports the conjecture (valid only approximately
for the 3- and 4-state Potts models) of a superuniversality for the 2D
disordered models with discrete symmetries.Comment: REVTeX, 24 pages, to appear in Phys.Rev.
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