173 research outputs found
Exact renormalization group equation for the Lifshitz critical point
An exact renormalization equation (ERGE) accounting for an anisotropic
scaling is derived. The critical and tricritical Lifshitz points are then
studied at leading order of the derivative expansion which is shown to involve
two differential equations. The resulting estimates of the Lifshitz critical
exponents compare well with the calculations. In the case
of the Lifshitz tricritical point, it is shown that a marginally relevant
coupling defies the perturbative approach since it actually makes the fixed
point referred to in the previous perturbative calculations
finally unstable.Comment: Final versio
Flow Equations for U_k and Z_k
By considering the gradient expansion for the wilsonian effective action S_k
of a single component scalar field theory truncated to the first two terms, the
potential U_k and the kinetic term Z_k, I show that the recent claim that
different expansion of the fluctuation determinant give rise to different
renormalization group equations for Z_k is incorrect. The correct procedure to
derive this equation is presented and the set of coupled differential equations
for U_k and Z_k is definitely established.Comment: 5 page
Wegner-Houghton equation and derivative expansion
We study the derivative expansion for the effective action in the framework
of the Exact Renormalization Group for a single component scalar theory. By
truncating the expansion to the first two terms, the potential and the
kinetic coefficient , our analysis suggests that a set of coupled
differential equations for these two functions can be established under certain
smoothness conditions for the background field and that sharp and smooth
cut-off give the same result. In addition we find that, differently from the
case of the potential, a further expansion is needed to obtain the differential
equation for , according to the relative weight between the kinetic and
the potential terms. As a result, two different approximations to the
equation are obtained. Finally a numerical analysis of the coupled equations
for and is performed at the non-gaussian fixed point in
dimensions to determine the anomalous dimension of the field.Comment: 15 pages, 3 figure
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
Ubiquitin ligase UBR3 regulates cellular levels of the essential DNA repair protein APE1 and is required for genome stability
APE1 (Ref-1) is an essential human protein involved in DNA damage repair and regulation of transcription. Although the cellular functions and biochemical properties of APE1 are well characterized, the mechanism involved in regulation of the cellular levels of this important DNA repair/transcriptional regulation enzyme, remains poorly understood. Using an in vitro ubiquitylation assay, we have now purified the human E3 ubiquitin ligase UBR3 as a major activity that polyubiquitylates APE1 at multiple lysine residues clustered on the N-terminal tail. We further show that a knockout of the Ubr3 gene in mouse embryonic fibroblasts leads to an up-regulation of the cellular levels of APE1 protein and subsequent genomic instability. These data propose an important role for UBR3 in the control of the steady state levels of APE1 and consequently error free DNA repair
Resolved Psychosis after Liver Transplantation in a Patient with Wilson’s Disease
A psychiatric involvement is frequently present in Wilson’s disease. Psychiatric symptoms are sometimes the first and only manifestation of Wilson’s disease. More often a psychiatric involvement is present beside a neurologic or hepatic disease
Three-loop renormalization group analysis of a complex model with stable fixed point: Critical exponents up to and
The complete analysis of a model with three quartic coupling constants
associated with an O(2N)--symmetric, a cubic, and a tetragonal interactions is
carried out within the three-loop approximation of the renormalization-group
(RG) approach in dimensions. Perturbation expansions for RG
functions are calculated using dimensional regularization and the minimal
subtraction (MS) scheme. It is shown that for the model does possess a
stable fixed point in three dimensional space of coupling constants, in
accordance with predictions made earlier on the base of the lower-order
approximations. Numerical estimate for critical (marginal) value of the order
parameter dimensionality is given using Pad\'e-Borel summation of the
corresponding --expansion series obtained. It is observed that
two-fold degeneracy of the eigenvalue exponents in the one-loop approximation
for the unique stable fixed point leads to the substantial decrease of the
accuracy expected within three loops and may cause powers of
to appear in the expansions. The critical exponents and are
calculated for all fixed points up to and ,
respectively, and processed by the Borel summation method modified with a
conformal mapping. For the unique stable fixed point the magnetic
susceptibility exponent for N=2 is found to differ in third order in
from that of an O(4)--symmetric point. Qualitative comparison of the
results given by --expansion, three-dimensional RG analysis,
non-perturbative RG arguments, and experimental data is performed.Comment: 30 pages, LaTeX, no figures. To be published in Phys. Rev. B, V.57,
Jan. issue (1998
Goal-directed and habitual control in the basal ganglia: implications for Parkinson's disease
Progressive loss of the ascending dopaminergic projection in the basal ganglia is a fundamental pathological feature of Parkinson's disease. Studies in animals and humans have identified spatially segregated functional territories in the basal ganglia for the control of goal-directed and habitual actions. In patients with Parkinson's disease the loss of dopamine is predominantly in the posterior putamen, a region of the basal ganglia associated with the control of habitual behaviour. These patients may therefore be forced into a progressive reliance on the goal-directed mode of action control that is mediated by comparatively preserved processing in the rostromedial striatum. Thus, many of their behavioural difficulties may reflect a loss of normal automatic control owing to distorting output signals from habitual control circuits, which impede the expression of goal-directed action. © 2010 Macmillan Publishers Limited. All rights reserved
- …