107 research outputs found
Decoherence of domains and defects at phase transitions
In this further letter on the onset of classical behaviour in field theory
due to a phase transition, we show that it can be phrased easily in terms of
the decoherence functional, without having to use the master equation. To
demonstrate this, we consider the decohering effects due to the displacement of
domain boundaries, with implications for the displacement of defects, in
general. We see that decoherence arises so quickly in this event, that it is
negligible in comparison to decoherence due to field fluctuations in the way
defined in our previous papers.Comment: Version published in Phys. Lett.
The Formation of Classical Defects After a Slow Quantum Phase Transition
Classical defects (monopoles, vortices, etc.) are a characteristic
consequence of many phase transitions of quantum fields. We show a model in
which the onset of classical probability distributions, for the long-wavelength
modes at early times, allows the identification of line-zeroes of the field
with vortex separation. We obtain a refined version of Kibble's causal results
for defect separation, but from a completely different approach. It is apparent
that vortices are not created from thermal fluctuations in the Ginzburg regime.Comment: 10 pages, RevTex file. No figures. To appear in Phys. Lett.
Real time thermal propagtors for massive gauge bosons
We derive Feynman rules for gauge theories exhibiting spontaneous symmetry
breaking using the real-time formalism of finite temperature field theory. We
also derive the thermal propagators where only the physical degrees of freedom
are given thermal boundary conditions. We analyse the abelian Higgs model and
find that these new propagators simplify the calculation of the thermal
contribution to the self energy.Comment: 7 pages, late
Initial Vortex Densities after a Temperature Quench
We calculate the initial density of relativistic global vortices (strings)
produced at a quench and contrast it with the predictions of Kibble and Zurek.Comment: Latex file, 8 pages, No figures. The text contains minor changes that
elucidate the onset of classical behaviour more thoroughly. Also including
the word temperature in the title makes the nature of the transition more
explici
Dual PT-Symmetric Quantum Field Theories
Some quantum field theories described by non-Hermitian Hamiltonians are
investigated. It is shown that for the case of a free fermion field theory with
a mass term the Hamiltonian is -symmetric. Depending on the
mass parameter this symmetry may be either broken or unbroken. When the symmetry is unbroken, the spectrum of the quantum field theory is real. For
the -symmetric version of the massive Thirring model in
two-dimensional space-time, which is dual to the -symmetric scalar
Sine-Gordon model, an exact construction of the operator is given. It
is shown that the -symmetric massive Thirring and Sine-Gordon models
are equivalent to the conventional Hermitian massive Thirring and Sine-Gordon
models with appropriately shifted masses.Comment: 9 pages, 1 figur
Classical behaviour after a phase transition
We analyze the onset of classical behaviour after a second-order phase
transition by considering a scalar field theory in which the system-field
interacts with its environment, represented both by further fields and by its
own short-wavelength modes. Within our approximations we see that the
long-wavelength modes have become classical by the time that the transition has
been first implemented (the spinodal time).Comment: 12 pages, final version to appear in Phys. Lett.
Quantum Extremism: Effective Potential and Extremal Paths
The reality and convexity of the effective potential in quantum field
theories has been studied extensively in the context of Euclidean space-time.
It has been shown that canonical and path-integral approaches may yield
different results, thus resolving the `convexity problem'. We discuss the
transferral of these treatments to Minkowskian space-time, which also
necessitates a careful discussion of precisely which field configurations give
the dominant contributions to the path integral. In particular, we study the
effective potential for the N=1 linear sigma model.Comment: 11 pages, 4 figure
Chiral Symmetry restoration in the massive Thirring model at finite T and : Dimensional reduction and the Coulomb gas
We show that in certain limits the (1+1)-dimensional massive Thirring model
at finite temperature is equivalent to a one-dimensional Coulomb gas of
charged particles at the same . This equivalence is then used to explore the
phase structure of the massive Thirring model. For strong coupling and
(the fermion mass) the system is shown to behave as a free gas of "molecules"
(charge pairs in the Coulomb gas terminology) made of pairs of chiral
condensates. This binding of chiral condensates is responsible for the
restoration of chiral symmetry as . In addition, when a fermion
chemical potential is included, the analogy with a Coulomb gas
still holds with playing the role of a purely imaginary external electric
field. For small and we find a typical massive Fermi gas behaviour
for the fermion density, whereas for large it shows chiral restoration by
means of a vanishing effective fermion mass. Some similarities with the chiral
properties of low-energy QCD at finite and baryon chemical potential are
discussed.Comment: 28 pages, 6 figures, better resolution figures are available upon
reques
Finite Temperature Effective Potential for the Abelian Higgs Model to the Order
A complete calculation of the finite temperature effective potential for the
abelian Higgs model to the order is presented and the result is
expressed in terms of physical parameters defined at zero temperature. The
absence of a linear term is verified explicitly to the given order and proven
to survive to all orders. The first order phase transition has weakened in
comparison with lower order calculation, which shows up in a considerable
decrease of the surface tension. The only difference from the original version
is the splitting of some overlong lines causing problems with certain mailers.Comment: 13 pages LaTex ( figures not included , hardcopy available on request
: [email protected] or t00heb@dhhdesy3 ) , DESY 93-08
Energies and collapse times of symmetric and symmetry-breaking states of finite systems with a U(1) symmetry
We study quantum systems of volume V, which will exhibit the breaking of a
U(1) symmetry in the limit of V \to \infty, when V is large but finite. We
estimate the energy difference between the `symmetric ground state' (SGS),
which is the lowest-energy state that does not breaks the symmetry, and a `pure
phase vacuum' (PPV), which approaches a symmetry-breaking vacuum as V \to
\infty. Under some natural postulates on the energy of the SGS, it is shown
that PPVs always have a higher energy than the SGS, and we derive a lower bound
of the excess energy. We argue that the lower bound is O(V^0), which becomes
much larger than the excitation energies of low-lying excited states for a
large V. We also discuss the collapse time of PPVs for interacting many bosons.
It is shown that the wave function collapses in a microscopic time scale,
because PPVs are not energy eigenstates. We show, however, that for PPVs the
expectation value of any observable, which is a finite polynomial of boson
operators and their derivatives, does not collapse for a macroscopic time
scale. In this sense, the collapse time of PPVs is macroscopically long.Comment: In the revised manuscript, Eq. (22), Ref. [8], and Notes [13], [15]
and [17] have been adde
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