2,035 research outputs found
Non-linear Dynamics in QED_3 and Non-trivial Infrared Structure
In this work we consider a coupled system of Schwinger-Dyson equations for
self-energy and vertex functions in QED_3. Using the concept of a
semi-amputated vertex function, we manage to decouple the vertex equation and
transform it in the infrared into a non-linear differential equation of
Emden-Fowler type. Its solution suggests the following picture: in the absence
of infrared cut-offs there is only a trivial infrared fixed-point structure in
the theory. However, the presence of masses, for either fermions or photons,
changes the situation drastically, leading to a mass-dependent non-trivial
infrared fixed point. In this picture a dynamical mass for the fermions is
found to be generated consistently. The non-linearity of the equations gives
rise to highly non-trivial constraints among the mass and effective (`running')
gauge coupling, which impose lower and upper bounds on the latter for dynamical
mass generation to occur. Possible implications of this to the theory of
high-temperature superconductivity are briefly discussed.Comment: 29 pages LATEX, 7 eps figures incorporated, uses axodraw style.
Discussion on the massless case (section 2) modified; no effect on
conclusions, typos correcte
The trans-activation domain of the sporulation response regulator Spo0A revealed by X-ray crystallography
Sporulation in Bacillus involves the induction of scores of genes in a temporally and spatially co-ordinated programme of cell development. Its initiation is under the control of an expanded two-component signal transduction system termed a phosphorelay. The master control element in the decision to sporulate is the response regulator, Spo0A, which comprises a receiver or phosphoacceptor domain and an effector or transcription activation domain. The receiver domain of Spo0A shares sequence similarity with numerous response regulators, and its structure has been determined in phosphorylated and unphosphorylated forms. However, the effector domain (C-Spo0A) has no detectable sequence similarity to any other protein, and this lack of structural information is an obstacle to understanding how DNA binding and transcription activation are controlled by phosphorylation in Spo0A. Here, we report the crystal structure of C-Spo0A from Bacillus stearothermophilus revealing a single alpha -helical domain comprising six alpha -helices in an unprecedented fold. The structure contains a helix-turn-helix as part of a three alpha -helical bundle reminiscent of the catabolite gene activator protein (CAP), suggesting a mechanism for DNA binding. The residues implicated in forming the sigma (A)-activating region clearly cluster in a flexible segment of the polypeptide on the opposite side of the structure from that predicted to interact with DNA. The structural results are discussed in the context of the rich array of existing mutational data
A dynamical gluon mass solution in a coupled system of the Schwinger-Dyson equations
We study numerically the Schwinger-Dyson equations for the coupled system of
gluon and ghost propagators in the Landau gauge and in the case of pure gauge
QCD. We show that a dynamical mass for the gluon propagator arises as a
solution while the ghost propagator develops an enhanced behavior in the
infrared regime of QCD. Simple analytical expressions are proposed for the
propagators, and the mass dependency on the scale and its
perturbative scaling are studied. We discuss the implications of our results
for the infrared behavior of the coupling constant, which, according to fits
for the propagators infrared behavior, seems to indicate that as .Comment: 17 pages, 7 figures - Revised version to be consistent with erratum
to appear in JHE
Meson masses in large Nf QCD from the Bethe-Salpeter equation
We solve the homogeneous Bethe-Salpeter (HBS) equation for the scalar,
pseudoscalar, vector, and axial-vector bound states of quark and anti-quark in
large Nf QCD with the improved ladder approximation in the Landau gauge. The
quark mass function in the HBS equation is obtained from the Schwinger-Dyson
(SD) equation in the same approximation for consistency with the chiral
symmetry. Amazingly, due to the fact that the two-loop running coupling of
large Nf QCD is explicitly written in terms of an analytic function, large Nf
QCD turns out to be the first example in which the SD equation can be solved in
the complex plane and hence the HBS equation directly in the time-like region.
We find that approaching the chiral phase transition point from the broken
phase, the scalar, vector, and axial-vector meson masses vanish to zero with
the same scaling behavior, all degenerate with the massless pseudoscalar meson.
This may suggest a new type of manifestation of the chiral symmetry restoration
in large Nf QCD.Comment: 33 pages, 16 figures. Typos are corrected. Minor corrections and
references are added. Version to appear in Phys. Rev.
Effective Action and Conformal Phase Transition in Three-Dimensional QED
The effective action for local composite operators in is considered.
The effective potential is calculated in leading order in ( is the
number of fermion flavors) and used to describe the features of the phase
transition at , . It is shown that this
continuous phase transition satisfies the criteria of the conformal phase
transition, considered recently in the literature. In particular, there is an
abrupt change of the spectrum of light excitations at the critical point,
although the phase transition is continuous, and the structure of the equation
for the divergence of the dilatation current is essentially different in the
symmetric and nonsymmetric phases. The connection of this dynamics with the
dynamics in is briefly discussed.Comment: 17 pages, RevTex file, no figures. The discussion of the effective
action is extende
Quantum effects of a massive 3-form coupled to a Dirac field
We consider the coupling of A_{\mu\nu\rho} to the generic current of matter
field, later identified with the spin density current of a Dirac field. In
fact, one of the objectives of this paper is to investigate the impact of the
quantum fluctuations of A_{\mu\nu\rho} on the effective dynamics of the spinor
field. The consistency of the field equations, even at the classical level,
requires the introduction of a mass term for A_{\mu\nu\rho}. In this case, the
Casimir vacuum pressure includes a contribution that is explicitly dependent on
the mass of A_{\mu\nu\rho} and leads us to conclude that the mass term plays
the same role as the infrared cutoff needed to regularize the finite volume
partition functional previously calculated in the massless case. Remarkably,
even in the presence of a mass term, A_{\mu\nu\rho} contains a mixture of
massless and massive spin-0 fields so that the resulting equation is still
gauge invariant. This is yet another peculiar, but physically relevant property
of A_{\mu\nu\rho} since it is reflected in the effective dynamics of the spinor
fields and confirms the confining property of A_{\mu\nu\rho} already expected
from the earlier calculation of the Wilson loop.Comment: 10 pages, Revtex, no figures; in print on Phys.Rev.D; added new
reference
Spin Susceptibility and Superexchange Interaction in the Antiferromagnet CuO
Evidence for the quasi one-dimensional (1D) antiferromagnetism of CuO is
presented in a framework of Heisenberg model. We have obtained an experimental
absolute value of the paramagnetic spin susceptibility of CuO by subtracting
the orbital susceptibility separately from the total susceptibility through the
Cu NMR shift measurement, and compared directly with the theoretical
predictions. The result is best described by a 1D antiferromagnetic
Heisenberg (AFH) model, supporting the speculation invoked by earlier authors.
We also present a semi-quantitative reason why CuO, seemingly of 3D structure,
is unexpectedly a quasi 1D antiferromagnet.Comment: 7 pages including 4 tables and 9 figure
Non-linear response of a Kondo system: Perturbation approach to the time dependent Anderson impurity model
Nonlinear tunneling current through a quantum dot
(an Anderson impurity system) subject to both constant and alternating
electric fields is studied in the Kondo regime. A systematic diagram technique
is developed for perturbation study of the current in physical systems out of
equilibrium governed by time - dependent Hamiltonians of the Anderson and the
Kondo models. The ensuing calculations prove to be too complicated for the
Anderson model, and hence, a mapping on an effective Kondo problem is called
for. This is achieved by constructing a time - dependent version of the
Schrieffer - Wolff transformation. Perturbation expansion of the current is
then carried out up to third order in the Kondo coupling J yielding a set of
remarkably simple analytical expressions for the current. The zero - bias
anomaly of the direct current differential conductance is shown to be
suppressed by the alternating field while side peaks develop at finite source -
drain voltage. Both the direct component and the first harmonics of the time -
dependent response are equally enhanced due to the Kondo effect, while
amplitudes of higher harmonics are shown to be relatively small. A zero
alternating bias anomaly is found in the alternating current differential
conductance, that is, it peaks around zero alternating bias. This peak is
suppressed by the constant bias. No side peaks show up in the differential
alternating - conductance but their counterpart is found in the derivative of
the alternating current with respect to the direct bias. The results pertaining
to nonlinear response are shown to be valid also below the Kondo temperature.Comment: 55 latex pages 11 ps figure
Structure and Stability of an Amorphous Metal
Using molecular dynamics simulations, with a realistic many-body
embedded-atom potential, and a novel method to characterize local order, we
study the structure of pure nickel during the rapid quench of the liquid and in
the resulting glass. In contrast with previous simulations with pair
potentials, we find more crystalline order and fewer icosahedra for slower
quenching rates, resulting in a glass less stable against crystallization. It
is shown that there is not a specific amorphous structure, only the arrest of
the transition from liquid to crystal, resulting in small crystalline clusters
immersed in an amorphous matrix with the same structure of the liquid.Comment: 4 pages, 4 ps figs., to appear in Phys. Rev. Let
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