202 research outputs found
The stochastic gravitational wave background from turbulence and magnetic fields generated by a first-order phase transition
We analytically derive the spectrum of gravitational waves due to
magneto-hydrodynamical turbulence generated by bubble collisions in a
first-order phase transition. In contrast to previous studies, we take into
account the fact that turbulence and magnetic fields act as sources of
gravitational waves for many Hubble times after the phase transition is
completed. This modifies the gravitational wave spectrum at large scales. We
also model the initial stirring phase preceding the Kolmogorov cascade, while
earlier works assume that the Kolmogorov spectrum sets in instantaneously. The
continuity in time of the source is relevant for a correct determination of the
peak position of the gravitational wave spectrum. We discuss how the results
depend on assumptions about the unequal-time correlation of the source and
motivate a realistic choice for it. Our treatment gives a similar peak
frequency as previous analyses but the amplitude of the signal is reduced due
to the use of a more realistic power spectrum for the magneto-hydrodynamical
turbulence. For a strongly first-order electroweak phase transition, the signal
is observable with the space interferometer LISA.Comment: 46 pages, 17 figures. Replaced with revised version accepted for
publication in JCA
Symmetric coupling of four spin-1/2 systems
We address the non-binary coupling of identical angular momenta based upon
the representation theory for the symmetric group. A correspondence is pointed
out between the complete set of commuting operators and the
reference-frame-free subsystems. We provide a detailed analysis of the coupling
of three and four spin-1/2 systems and discuss a symmetric coupling of four
spin-1/2 systems.Comment: 20 pages, no figure
Supersymmetric Axion-Neutrino Merger
The recently proposed supersymmetric model of the neutrino mass matrix
is modified to merge with a previously proposed axionic solution of the strong
CP problem. The resulting model has only one input scale, i.e. that of
symmetry breaking, which determines both the seesaw neutrino mass scale and the
axion decay constant. It also solves the problem and conserves R parity
automatically.Comment: 7 pages, no figur
The annealing mechanism of AuGe/Ni/Au ohmic contacts to a two-dimensional electron gas in GaAs/AlGaAs heterostructures
Ohmic contacts to a two-dimensional electron gas (2DEG) in GaAs/AlGaAs
heterostructures are often realized by annealing of AuGe/Ni/Au that is
deposited on its surface. We studied how the quality of this type of ohmic
contact depends on the annealing time and temperature, and how optimal
parameters depend on the depth of the 2DEG below the surface. Combined with
transmission electron microscopy and energy-dispersive X-ray spectrometry
studies of the annealed contacts, our results allow for identifying the
annealing mechanism and proposing a model that can predict optimal annealing
parameters for a certain heterostructure.Comment: 9 pages, 4 figure
Gluon mass generation in the PT-BFM scheme
In this article we study the general structure and special properties of the
Schwinger-Dyson equation for the gluon propagator constructed with the pinch
technique, together with the question of how to obtain infrared finite
solutions, associated with the generation of an effective gluon mass.
Exploiting the known all-order correspondence between the pinch technique and
the background field method, we demonstrate that, contrary to the standard
formulation, the non-perturbative gluon self-energy is transverse
order-by-order in the dressed loop expansion, and separately for gluonic and
ghost contributions. We next present a comprehensive review of several subtle
issues relevant to the search of infrared finite solutions, paying particular
attention to the role of the seagull graph in enforcing transversality, the
necessity of introducing massless poles in the three-gluon vertex, and the
incorporation of the correct renormalization group properties. In addition, we
present a method for regulating the seagull-type contributions based on
dimensional regularization; its applicability depends crucially on the
asymptotic behavior of the solutions in the deep ultraviolet, and in particular
on the anomalous dimension of the dynamically generated gluon mass. A
linearized version of the truncated Schwinger-Dyson equation is derived, using
a vertex that satisfies the required Ward identity and contains massless poles
belonging to different Lorentz structures. The resulting integral equation is
then solved numerically, the infrared and ultraviolet properties of the
obtained solutions are examined in detail, and the allowed range for the
effective gluon mass is determined. Various open questions and possible
connections with different approaches in the literature are discussed.Comment: 54 pages, 24 figure
Order in glassy systems
A directly measurable correlation length may be defined for systems having a
two-step relaxation, based on the geometric properties of density profile that
remains after averaging out the fast motion. We argue that the length diverges
if and when the slow timescale diverges, whatever the microscopic mechanism at
the origin of the slowing down. Measuring the length amounts to determining
explicitly the complexity from the observed particle configurations. One may
compute in the same way the Renyi complexities K_q, their relative behavior for
different q characterizes the mechanism underlying the transition. In
particular, the 'Random First Order' scenario predicts that in the glass phase
K_q=0 for q>x, and K_q>0 for q<x, with x the Parisi parameter. The hypothesis
of a nonequilibrium effective temperature may also be directly tested directly
from configurations.Comment: Typos corrected, clarifications adde
Super-Hubble de Sitter Fluctuations and the Dynamical RG
Perturbative corrections to correlation functions for interacting theories in
de Sitter spacetime often grow secularly with time, due to the properties of
fluctuations on super-Hubble scales. This growth can lead to a breakdown of
perturbation theory at late times. We argue that Dynamical Renormalization
Group (DRG) techniques provide a convenient framework for interpreting and
resumming these secularly growing terms. In the case of a massless scalar field
in de Sitter with quartic self-interaction, the resummed result is also less
singular in the infrared, in precisely the manner expected if a dynamical mass
is generated. We compare this improved infrared behavior with large-N
expansions when applicable.Comment: 33 pages, 4 figure
Simulation of Ionic Surfaces from an Absolutely Convergent Solution of the Madelung Problem
The classic Madelung problem is cast into an absolutely convergent form that is readily evaluated by direct lattice summation, revealing a net r{sup {minus}5} range of the net Coulomb potential in ionic crystals and liquids. The realization that Coulomb interactions in condensed systems can actually be rather short ranged (provided the system is overall neutral) leads to the prediction, verified by computer simulations for rocksalt-structure surfaces, that all surfaces in predominantly ionic crystals should be fundamentally reconstructed. The work also provides a conceptual framework for the theoretical treatment of polar surfaces, as demonstrated for the case of the (111) surfaces of NaCl and MgO
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