3,870 research outputs found
Nonlinear sigma model approach for phase disorder transitions and the pseudogap phase in chiral Gross-Neveu, Nambu-Jona-Lasinio models and strong-coupling superconductors
We briefly review the nonlinear sigma model approach for the subject of
increasing interest: "two-step" phase transitions in the Gross-Neveu and the
modified Nambu-Jona-Lasinio models at low and condensation from pseudogap
phase in strong-coupling superconductors. Recent success in describing
"Bose-type" superconductors that possess two characterstic temperatures and a
pseudogap above is the development approximately comparable with the BCS
theory. One can expect that it should have influence on high-energy physics,
similar to impact of the BCS theory on this subject. Although first
generalizations of this concept to particle physics were made recently, these
results were not systematized. In this review we summarize this development and
discuss similarities and differences of the appearence of the pseudogap phase
in superconductors and the Gross-Neveu and Nambu-Jona-Lasinio - like models. We
discuss its possible relevance for chiral phase transition in QCD and color
superconductors. This paper is organized in three parts: in the first section
we briefly review the separation of temperatures of pair formation and pair
condensation in strong - coupling and low carrier density superconductors (i.e.
the formation of the {\it pseudogap phase}).
Second part is a review of nonlinear sigma model approach to an analogous
phenomenon in the Chiral Gross-Neveu model at small N. In the third section we
discuss the modified Nambu-Jona-Lasinio model where the chiral phase transition
is accompanied by a formation of a phase analogous to the pseudogap phase.Comment: A brief review. Replaced with journal version (some grammatical
corrections). The latest updates of this and related papers are also
available at the author home page http://www.teorfys.uu.se/PEOPLE/egor
Phase structure and phase transitions in a three dimensional SU(2) superconductor
We study the three dimensional SU(2)-symmetric noncompact CP1 model, with two
charged matter fields coupled minimally to a noncompact Abelian gauge-field.
The phase diagram and the nature of the phase transitions in this model have
attracted much interest after it was proposed to describe an unusual continuous
transition associated with deconfinement of spinons. Previously, it has been
demonstrated for various two-component gauge theories that weakly first-order
transitions may appear as continuous ones of a new universality class in
simulations of relatively large, but finite systems. We have performed
Monte-Carlo calculations on substantially larger systems sizes than those in
previous works. We find that in some area of the phase diagram where at finite
sizes one gets signatures consistent with a single first-order transition, in
fact there is a sequence of two phase transitions with an O(3) paired phase
sandwiched in between. We report (i) a new estimate for the location of a
bicritical point and (ii) the first resolution of bimodal distributions in
energy histograms at relatively low coupling strengths. We perform a flowgram
analysis of the direct transition line with rescaling of the linear system size
in order to obtain a data collapse. The data collapses up to coupling constants
where we find bimodal distributions in energy histograms.Comment: 16 pages, 11 figures. Submitted to Physical Review
Length scales, collective modes, and type-1.5 regimes in three-band superconductors
The recent discovery of iron pnictide superconductors has resulted in a
rapidly growing interest in multiband models with more than two bands. In this
work we specifically focus on the properties of three-band Ginzburg-Landau
models which do not have direct counterparts in more studied two-band models.
First we derive normal modes and characteristic length scales in the
conventional U(1) three-band Ginzburg-Landau model as well as in its time
reversal symmetry broken counterpart with symmetry. We show
that in the latter case, the normal modes are mixed phase/density collective
excitations. A possibility of the appearance of a massless phase-difference
mode associated with fluctuations of the phase difference is also discussed.
Next we show that gradients of densities and phase differences can be
inextricably intertwined in vortex excitations in three-band models. This can
lead to very long-range attractive intervortex interactions and appearance of
type-1.5 regimes even when the intercomponent Josephson coupling is large. In
some cases it also results in the formation of a domain-like structures in the
form of a ring of suppressed density around a vortex across which one of the
phases shifts by . We also show that field-induced vortices can lead to a
change of broken symmetry from U(1) to in the system. In the
type-1.5 regime, it results in a semi-Meissner state where the system has a
macroscopic phase separation in domains with broken U(1) and
symmetries.Comment: Version 3: Corrected som inconstancies in the parameter set in Fig.2
Also som minor typos corrected. No changes to results or conclusion
THE BUSINESS EDUCATIONAL IN UKRAINE: KEY PROBLEMS & STRATEGIC PERSPECTIVES (on the example of O.M. Beketov National University of Urban Economy)
BUSINESS AND ECONOMICS EDUCATION IN UKRAINE: KEY PROBLEMS & STRATEGIC PERSPECTIVES (on the example of O.M. Beketov National University of Urban Economy in Kharkiv)
Phase transitions in a three dimensional lattice London superconductor
We consider a three-dimensional lattice superconductor in
the London limit, with two individually conserved condensates. The problem,
generically, has two types of intercomponent interactions of different
characters. First, the condensates are interacting via a minimal coupling to
the same fluctuating gauge field. A second type of coupling is the direct
dissipationless drag represented by a local intercomponent current-current
coupling term in the free energy functional. The interplay between these two
types of interactions produces a number of physical effects not present in
previously investigated models with only one kind of
intercomponent interaction. In this work, we present a study of the phase
diagram of a superconductor which includes both of these
interactions. We study phase transitions and two types of competing paired
phases which occur in this general model: (i) a metallic superfluid phase
(where there is order only in the gauge invariant phase difference of the order
parameters), (ii) a composite superconducting phase where there is order in the
phase sum of the order parameters which has many properties of a
single-component superconductor but with a doubled value of electric charge. We
investigate the phase diagram with particular focus on what we call "preemptive
phase transitions". These are phase transitions {\it unique to multicomponent
condensates with competing topological objects}. A sudden proliferation of one
kind of topological defects may come about due to a fluctuating background of
topological defects in other sectors of the theory.Comment: 17 pages, 6 figures. Submitted to Physical Review
Freezing of an unconventional two-dimensional plasma
We study an unconventional two-dimensional, two-component classical plasma on
a sphere, with emphasis on detecting signatures of melting transitions. This
system is relevant to Ising-type quantum Hall states, and is unconventional in
the sense that it features particles interacting via two different
two-dimensional Coulomb interactions. One species of particles in the plasma
carries charge of both types (Q_1,Q_2), while the other species carries only
charge of the second type (0,-Q_2). We find signatures of a freezing transition
at Q_1^2 approximately 140. This means that the species with charge of both
types will form a Wigner crystal, whereas the species with charge of the second
type also shows signatures of being a Wigner crystal, due to the attractive
inter-component interaction of the second type. Moreover, there is also a
Berezinskii-Kosterlitz-Thouless phase transition at Q_2^2 approximately 4, at
which the two species of particles bind to form molecules that are neutral with
respect to the second Coulomb interaction. These two transitions appear to be
independent of each other, giving a rectangular phase diagram. As a special
case, Q_2=0 describes the (conventional) two-dimensional one-component plasma.
Our study is consistent with previous studies of this plasma, and sheds new
light on the freezing transition of this system.Comment: 8 pages, 8 figures. Submitted to Physical Review
Vortex matter and generalizations of dipolar superfluidity concept in layered systems
In the first part of this letter we discuss electrodynamics of an excitonic
condensate in a bilayer. We show that under certain conditions the system has a
dominant energy scale and is described by the effective electrodynamics with
"planar magnetic charges". In the second part of the paper we point out that a
vortex liquid state in bilayer superconductors also possesses dipolar
superfluid modes and establish equivalence mapping between this state and a
dipolar excitonic condensate. We point out that a vortex liquid state in an
N-layer superconductor possesses multiple topologically coupled dipolar
superfluid modes and therefore represents a generalization of the dipolar
superfluidity concept.Comment: v2: references added. v3: discussion extended, references adde
Screening properties and phase transitions in unconventional plasmas for Ising-type quantum Hall states
Utilizing large-scale Monte-Carlo simulations, we investigate an
unconventional two-component classical plasma in two dimensions which controls
the behavior of the norms and overlaps of the quantum-mechanical wavefunctions
of Ising-type quantum Hall states. The plasma differs fundamentally from that
which is associated with the two-dimensional XY model and Abelian fractional
quantum Hall states. We find that this unconventional plasma undergoes a
Berezinskii-Kosterlitz-Thouless phase transition from an insulator to a metal.
The parameter values corresponding to Ising-type quantum Hall states lie on the
metallic side of this transition. This result verifies the required properties
of the unconventional plasma used to demonstrate that Ising-type quantum Hall
states possess quasiparticles with non-Abelian braiding statistics.Comment: 16 pages, 14 figures. Submitted to Physical Review
Dual neutral variables and knot solitons in triplet superconductors
In this paper we derive a dual presentation of free energy functional for
spin-triplet superconductors in terms of gauge-invariant variables. The
resulting equivalent model in ferromagnetic phase has a form of a version of
the Faddeev model. This allows one in particular to conclude that spin-triplet
superconductors allow formation of stable finite-length closed vortices (the
knotted solitons).Comment: Replaced with version published in PRL (added a discussion of the
effect of the coupling of the fields {\vec s} and {\vec C} on knot
stability). Latest updates of the paper and miscellaneous links related to
knotted solitons are also available at the homepage of the author
http://www.teorfys.uu.se/PEOPLE/egor/ . Animations of knotted solitons by
Hietarinta and Salo are available at
http://users.utu.fi/h/hietarin/knots/c45_p2.mp
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