6,126 research outputs found
Multicritical behavior in models with two competing order parameters
We employ the nonperturbative functional Renormalization Group to study
models with an O(N_1)+O(N_2) symmetry. Here, different fixed points exist in
three dimensions, corresponding to bicritical and tetracritical behavior
induced by the competition of two order parameters. We discuss the critical
behavior of the symmetry-enhanced isotropic, the decoupled and the biconical
fixed point, and analyze their stability in the N_1, N_2 plane. We study the
fate of non-trivial fixed points during the transition from three to four
dimensions, finding evidence for a triviality problem for coupled two-scalar
models in high-energy physics. We also point out the possibility of
non-canonical critical exponents at semi-Gaussian fixed points and show the
emergence of Goldstone modes from discrete symmetries.Comment: 16 pages, 7 figures, 5 tables, minor changes in updated version,
identical to published one in Phys. Rev.
Discovering and quantifying nontrivial fixed points in multi-field models
We use the functional renormalization group and the -expansion
concertedly to explore multicritical universality classes for coupled
vector-field models in three Euclidean dimensions.
Exploiting the complementary strengths of these two methods we show how to make
progress in theories with large numbers of interactions, and a large number of
possible symmetry-breaking patterns. For the three- and four-field models we
find a new fixed point that arises from the mutual interaction between
different field sectors, and we establish the absence of infrared-stable fixed
point solutions for the regime of small . Moreover, we explore these
systems as toy models for theories that are both asymptotically safe and
infrared complete. In particular, we show that these models exhibit complete
renormalization group trajectories that begin and end at nontrivial fixed
points.Comment: 10 pages, 6 figures; minor changes, as published in EPJ
Platelet kinetics in the pulmonary microcirculation in vivo assessed by intravital microscopy
Growing evidence supports the substantial pathophysiological impact of platelets on the development of acute lung injury. Methods for studying these cellular mechanisms in vivo are not present yet. The aim of this study was to develop a model enabling the quantitative analysis of platelet kinetics and platelet-endothelium interaction within consecutive segments of the pulmonary microcirculation in vivo. New Zealand White rabbits were anesthetized and ventilated. Autologous platelets were separated from blood and labeled ex vivo with rhodamine 6G. After implantation of a thoracic window, microhemodynamics and kinetics of platelets were investigated by intravital microscopy. Velocities of red blood cells (RBCs) and platelets were measured in arterioles, capillaries and venules, and the number of platelets adhering to the microvascular endothelium was counted. Kinetics of unstimulated platelets was compared with kinetics of thrombin-activated platelets. Velocity of unstimulated platelets was comparable to RBC velocity in all vessel segments. Unstimulated platelets passed the pulmonary microcirculation without substantial platelet-endothelial interaction. In contrast, velocity of activated platelets was decreased in all vascular segments indicating platelet margination and temporal platelet-endothelium interaction. Thrombin-activated platelets adhered to arteriolar endothelium; in capillaries and venules adherence of platelets was increased 8-fold and 13-fold, respectively. In conclusion, using intravital microscopy platelet kinetics were directly analyzed in the pulmonary microcirculation in vivo for the first time. In contrast to leukocytes, no substantial platelet-endothelium interaction occurs in the pulmonary microcirculation without any further stimulus. In response to platelet activation, molecular mechanisms enable adhesion of platelets in arterioles and venules as well as retention of platelets within capillaries. Copyright (C) 2002 S. Karger AG, Basel
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