3,585 research outputs found
Stochastic vortex dynamics in two-dimensional easy-plane ferromagnets: Multiplicative versus additive noise
We study how thermal fluctuations affect the dynamics of vortices in the
two-dimensional classical, ferromagnetic, anisotropic Heisenberg model
depending on their additive or multiplicative character. Using a collective
coordinate theory, we analytically show that multiplicative noise, arising from
fluctuations in the local field term of the Landau-Lifshitz equations, and
Langevin-like additive noise both have the same effect on vortex dynamics
(within a very plausible assumption consistent with the collective coordinate
approach). This is a non-trivial result, as multiplicative and additive noises
usually modify the dynamics quite differently. We also carry out numerical
simulations of both versions of the model finding that they indeed give rise to
very similar vortex dynamics.Comment: 10 pages, 6 figure
Finite temperature dynamics of vortices in the two dimensional anisotropic Heisenberg model
We study the effects of finite temperature on the dynamics of non-planar
vortices in the classical, two-dimensional anisotropic Heisenberg model with
XY- or easy-plane symmetry. To this end, we analyze a generalized
Landau-Lifshitz equation including additive white noise and Gilbert damping.
Using a collective variable theory with no adjustable parameters we derive an
equation of motion for the vortices with stochastic forces which are shown to
represent white noise with an effective diffusion constant linearly dependent
on temperature. We solve these stochastic equations of motion by means of a
Green's function formalism and obtain the mean vortex trajectory and its
variance. We find a non-standard time dependence for the variance of the
components perpendicular to the driving force. We compare the analytical
results with Langevin dynamics simulations and find a good agreement up to
temperatures of the order of 25% of the Kosterlitz-Thouless transition
temperature. Finally, we discuss the reasons why our approach is not
appropriate for higher temperatures as well as the discreteness effects
observed in the numerical simulations.Comment: 12 pages, 8 figures, accepted for publication in European Physical
Journal B (uses EPJ LaTeX
Ab initio calculations of edge-functionalized armchair graphene nanoribbons: Structural, electronic, and vibrational effects
We present a theoretical study on narrow armchair graphene nanoribbons
(AGNRs) with hydroxyl functionalized edges. Although this kind of passivation
strongly affects the structure of the ribbon, a high degree of edge
functionalization proves to be particularly stable. An important consequence of
the geometric deviations is a severe reduction of the band-gap of the
investigated 7-AGNR. This shift follows a linear dependence on the number of
added hydroxyl groups per unit cell and thus offers the prospect of a tunable
band-gap by edge functionalization. We furthermore cover the behavior of
characteristic phonons for the ribbon itself as well as fingerprint modes of
the hydroxyl groups. A large down-shift of prominent Raman active modes allows
the experimental determination of the degree of edge functionalization.Comment: 6 pages, 9 figure
Structural analysis of the GH43 enzyme Xsa43E from Butyrivibrio proteoclasticus
The rumen of dairy cattle can be thought of as a large, stable fermentation vat and as such it houses a large and diverse community of microorganisms. The bacterium Butyrivibrio proteoclasticus is a representative of a significant component of this microbial community. It is a xylan-degrading organism whose genome encodes a large number of open reading frames annotated as fibre-degrading enzymes. This suite of enzymes is essential for the organism to utilize the plant material within the rumen as a fuel source, facilitating its survival in this competitive environment. Xsa43E, a GH43 enzyme from B. proteoclasticus, has been structurally and functionally characterized. Here, the structure of selenomethionine-derived Xsa43E determined to 1.3 Ă
resolution using single-wavelength anomalous diffraction is reported. Xsa43E possesses the characteristic five-bladed β-propeller domain seen in all GH43 enzymes. GH43 enzymes can have a range of functions, and the functional characterization of Xsa43E shows it to be an arabinofuranosidase capable of cleaving arabinose side chains from short segments of xylan. Full functional and structural characterization of xylan-degrading enzymes will aid in creating an enzyme cocktail that can be used to completely degrade plant material into simple sugars. These molecules have a range of applications as starting materials for many industrial processes, including renewable alternatives to fossil fuels
Wer den Kodex nicht einhält, den bestraft der Kapitalmarkt?
Seit der EinfĂźhrung des Deutschen Corporate Governance Kodex (Kodex) im Jahr 2002 sind deutsche bĂśrsennotierte Unternehmen zur Abgabe der Entsprechenserklärung gemäà § 161 AktG verpflichtet (Comply-or-Explain-Prinzip). Auf der Basis dieser Information soll durch den Druck des Kapitalmarkts die Einhaltung des Kodex Ăźberwacht und gegebenenfalls sanktioniert werden. Dabei wird regelmäĂig postuliert, dass bei Ăźberdurchschnittlicher Befolgung bzw. Nichtbefolgung der Kodex-Empfehlungen eine Belohnung durch Kurszuschläge bzw. eine Sanktionierung durch Kursabschläge erfolgt. Die Ergebnisse einer Ereignisstudie zeigen, dass die Abgabe der Entsprechenserklärung keine erhebliche Kursbeeinflussung auslĂśst und die fĂźr das Enforcement des Kodex angenommene (und erforderliche) Selbstregulierung durch den Kapitalmarkt nicht stattfindet. Es wird daher kritisch hinterfragt, ob der fĂźr den Kodex gewählte und grundsätzlich zu begrĂźĂende flexible Regulierungsansatz im System des zwingenden deutschen Gesellschaftsrechts einen geeigneten Enforcement-Mechanismus darstellt. This paper studies the short-run announcement effects of compliance with the German Corporate Governance Code (âthe Codeâ) on firm value. Event study results suggest that firm value is unaffected by the announcement, although such market reactions to the first time disclosure of the declaration of conformity were widely assumed by the private and public promoters of the Code. This result from acceptance of the German Code add evidence to the hypothesis that regulatory corporate governance initiatives that rely on mandatory disclosure without monitoring and enforcement are ineffective in civil law countries.This paper studies the short-run announcement effects of compliance with the German Corporate Governance Code (âthe Codeâ) on firm value. Event study results suggest that firm value is unaffected by the announcement, although such market reactions to the first time disclosure of the declaration of conformity were widely assumed by the private and public promoters of the Code. This result from acceptance of the German Code add evidence to the hypothesis that regulatory corporate governance initiatives that rely on mandatory disclosure without monitoring and enforcement are ineffective in civil law countries
Family studies in acute leukaemia in childhood: a possible association with autoimmune disease.
Medical histories of themselves and their first-degree relatives were obtained from parents of 82 leukaemic children (54 acute lymphoblastic (ALL), 28 acute myeloblastic (AML)) and from control couples matched for age. The possibility of a primary familial immunological abnormality as an aetiological factor in childhood leukaemia was suggested by binding some infections significantly more frequently reported in parents than in controls, but more strongly supported by the finding of a significantly (P less than 0.02) increased prevalence of disorders associated with autoimmunity (but not of other conditions such as peptic ulceration, infective hepatitis, tuberculosis or malignancy) amongst members of ALL families compared to those of controls. Analogy with Down's syndrome and the strain of NZB mice, in which diminished T-cell function is associated with autoimmune disease and lymphoid neoplasia, is discussed. Varicella and herpes zoster occurred respectively in 2 ALL mothers during their pregnancies involving the patients and in none of the other 388 pregnancies here reported. This supports previous evidence that antenatal varicella infections may be of aetiological importance in some cases of ALL
Dermatoglyphics in childhood leukaemia: a guide to prognosis and aetiology?
The results of analysis of the dermatoglyphics of 152 children with acute lymphoblastic leukaemia (ALL) (and the first-degree relatives of 54 of them) contrast with those of 31 children with acute myeloblastic leukaemia (AML) (and the first-degree relatives of 25 of them). In ALL our findings suggest that neither genetic susceptibility nor an environmental factor, effective during the early antenatal period, is of aetiological importance; but the response to treatment, assessed as length of first remission, was found to be related to the amount of fingertip pattern. This may have clinical application. In AML there is evidence of a genetically determined factor carrying a high risk of the development of the disease, in that a member of each of 5 different families of the 25 studied bore a rare hypothenar pattern, compared with none in 75 control families. No dermatoglyphic features were of prognostic significance in AML
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