9,087 research outputs found
Equilibrium Properties of Quantum Spin Systems with Non-additive Long-Range Interactions
We study equilibrium states of quantum spin systems with non-additive
long-range interactions by adopting an appropriate scaling of the interaction
strength, i.e., the so called Kac prescription. In classical spin systems, it
is known that the equilibrium free energy is obtained by minimizing the free
energy functional over the coarse-grained magnetization. Here we show that it
is also true for quantum spin systems. From this observation, it is found that
when the canonical ensemble and the microcanonical ensemble are not equivalent
in some parameter region, it is not necessarily justified to replace the actual
long-range interaction by the infinite-range interaction (Curie-Weiss type
interaction). On the other hand, in the parameter region where the two
ensembles are equivalent, this replacement is always justified. We examine the
Heisenberg XXZ model as an illustrative example, and discuss the relation to
experiments.Comment: 13 pages, two columns; to appear in Phys. Rev.
How does gravity save or kill Q-balls?
We explore stability of gravitating Q-balls with potential
via catastrophe
theory, as an extension of our previous work on Q-balls with potential
. In flat spacetime
Q-balls with in the thick-wall limit are unstable and there is a minimum
charge , where Q-balls with are nonexistent.
If we take self-gravity into account, on the other hand, there exist stable
Q-balls with arbitrarily small charge, no matter how weak gravity is. That is,
gravity saves Q-balls with small charge. We also show how stability of Q-balls
changes as gravity becomes strong.Comment: 10 pages, 10 figure
Determining the Equation of State of the Expanding Universe Using a New Independent Variable
To determine the equation of state of the universe, we propose to use a new
independent variable , where and
are the present Hubble parameter and the luminosity distance, respectively. For
the flat universe suggested from the observation of the anisotropy of cosmic
microwave background, the density and the pressure are expressed as
and where
is the present density and . In plane the sign
as well as the strength of the pressure is in proportion to the curvature of
the curve . We propose to adopt a Pade-like expression of
with . For flat
model the expansion up to N=7 has at most an error for and any value of . We also propose a general method to determine
the equation of state of the universe which has free parameters. If the
number of parameters are smaller than , there is a consistency check of
the equation of state so that we may confirm or refute each model.Comment: 12 pages, to be published in the Astrophysical Journa
All Optical Measurement Proposed for the Photovoltaic Hall Effect
We propose an all optical way to measure the recently proposed "photovoltaic
Hall effect", i.e., a DC Hall effect induced by a circularly polarized light in
the absence of static magnetic fields. For this, we have calculated the Faraday
rotation angle induced by the photovoltaic Hall effect with the Kubo formula
extended for photovoltaic optical response in the presence of strong AC
electric fields treated with the Floquet formalism. We also point out the
possibility of observing the effect in three-dimensional graphite, and more
generally in multi-band systems such as materials described by the dp-model.Comment: 5 page
Solitonic generation of five-dimensional black ring solution
Using the solitonic solution-generating technique we rederived the
one-rotational five-dimensional black ring solution found by Emparan and Reall.
The seed solution is not the Minkowski metric, which is the seed of
-rotating black ring. The obtained solution has more parameters than the
Emparan and Reall's -rotating black ring. We found the conditions of
parameters to reduce the solution to the -rotating black ring. In addition
we examined the relation between the expressions of the metric in the
prolate-spheroidal coordinates and in the canonical coordinates.Comment: 5 pages, 4 figures ; accepted version, several details are remove
Unified pictures of Q-balls and Q-tubes
While Q-balls have been investigated intensively for many years, another type
of nontopological solutions, Q-tubes, have not been understood very well. In
this paper we make a comparative study of Q-balls and Q-tubes. First, we
investigate their equilibrium solutions for four types of potentials. We find,
for example, that in some models the charge-energy relation is similar between
Q-balls and Q-tubes while in other models the relation is quite different
between them. To understand what determines the charge-energy relation, which
is a key of stability of the equilibrium solutions, we establish an analytical
method to obtain the two limit values of the energy and the charge. Our
prescription indicates how the existent domain of solutions and their stability
depends on their shape as well as potentials, which would also be useful for a
future study of Q-objects in higher-dimensional spacetime.Comment: 11 pages, 14 figure
Unified picture of Q-balls and boson stars via catastrophe theory
We make an analysis of Q-balls and boson stars using catastrophe theory, as
an extension of the previous work on Q-balls in flat spacetime. We adopt the
potential for Q-balls and
that with for boson stars. For solutions with at
its peak, stability of Q-balls has been lost regardless of the potential
parameters. As a result, phase relations, such as a Q-ball charge versus a
total Hamiltonian energy, approach those of boson stars, which tell us an
unified picture of Q-balls and boson stars.Comment: 10 pages, 13 figure
Nonperturbative infrared effects for light scalar fields in de Sitter space
We study the phi^4 scalar field theory in de Sitter space using the 2PI
effective action formalism. This formalism enables us to investigate the
nonperturbative quantum effects. We use the mean field and gap equations and
calculate the physical mass and effective potential. We find that
nonperturbative infrared effects on de Sitter space produce a curvature-induced
mass and work to restore the broken Z_2 symmetry.Comment: 14 pages, 3 figures, section 2 revised, discussion in section 4
changed, results not change
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