5,467 research outputs found
External Fields and Chiral Symmetry Breaking in the Sakai-Sugimoto Model
Using the Sakai-Sugimoto model we study the effect of an external magnetic
field on the dynamics of fundamental flavours in both the confined and
deconfined phases of a large N_c gauge theory. We find that an external
magnetic field promotes chiral symmetry breaking, consistent with the "magnetic
catalysis" observed in the field theory literature, and seen in other studies
using holographic duals. The external field increases the separation between
the deconfinement temperature and the chiral symmetry restoring temperature. In
the deconfined phase we investigate the temperature-magnetic field phase
diagram and observe, for example, there exists a maximum critical temperature
(at which symmetry is restored) for very large magnetic field. We find that
this and certain other phenomena persist for the Sakai-Sugimoto type models
with probe branes of diverse dimensions. We comment briefly on the dynamics in
the presence of an external electric field.Comment: 26 pages. LaTeX, minor change
Electron transport and thermoelectric properties of layered perovskite LaBaCo2O5.5
We have investigated the systematic transport properties of the layered
112-type cobaltite LaBaCo2O5.5 by means of electrical resistivity,
magnetoresistance, electroresistance and thermoelectric measurements in various
conditions. In order to understand the complex conduction mechanism of
LaBaCo2O5.5, the transport data have been analyzed using different theoretical
models. The system shows semiconductor-semiconductor like transition (TSC)
around 326K, corresponding to ferromagnetic transition and in the low
temperature region resistivity data follows the Motts variable range hopping
model. Interestingly, near and below the room temperature this compound depicts
significant change in electro- and magnetoresistance behavior, the latter one
is noteworthy near the magnetic phase boundary. The temperature dependence of
thermopower, S(T), exhibits p-type polaronic conductivity in the temperature
range of 60-320K and reaches a maximum value of 303 uV/K (at 120K). In the low
temperature AFM region, the unusual S(T) behavior, generally observed for the
cobaltite series LnBaCo2O5.5 (Ln = Rare Earth), is explained by the electron
magnon scattering mechanism as previously described for perovskite manganites.Comment: 18 pages including fig
Yang-Baxter algebra and generation of quantum integrable models
An operator deformed quantum algebra is discovered exploiting the quantum
Yang-Baxter equation with trigonometric R-matrix. This novel Hopf algebra along
with its limit appear to be the most general Yang-Baxter algebra
underlying quantum integrable systems. Three different directions of
application of this algebra in integrable systems depending on different sets
of values of deforming operators are identified. Fixed values on the whole
lattice yield subalgebras linked to standard quantum integrable models, while
the associated Lax operators generate and classify them in an unified way.
Variable values construct a new series of quantum integrable inhomogeneous
models. Fixed but different values at different lattice sites can produce a
novel class of integrable hybrid models including integrable matter-radiation
models and quantum field models with defects, in particular, a new quantum
integrable sine-Gordon model with defect.Comment: 13 pages, revised and bit expanded with additional explanations,
accepted for publication in Theor. Math. Phy
Global Currents, Phase Transitions, and Chiral Symmetry Breaking in Large N_c Gauge Theory
We study, using a gravity dual, the finite temperature dynamics of
gauge theory for large , with fundamental quark flavours in a quenched
approximation, in the presence of a fixed R--charge under a global R--current.
We observe several notable phenomena. There is a first order phase transition
where the quark condensate jumps discontinuously at finite quark mass,
generalizing similar transitions seen at zero charge. Our tool in these studies
is holography, the string dual of the gauge theory being the geometry of
spinning D3--branes at finite temperature, probed by a D7--brane.Comment: (corrected version that appeared in JHEP
Quarks in an External Electric Field in Finite Temperature Large N Gauge Theory
We use a ten dimensional dual string background to aspects of the physics
large N four dimensional SU(N) gauge theory, where its fundamental quarks are
charged under a background electric field. The theory is N=2 supersymmetric for
vanishing temperature and electric field. At zero temperature, we observe that
the electric field induces a phase transition associated with the dissociation
of the mesons into their constituent quarks. This is an analogue of an
insulator-metal transition, since the system goes from being an insulator with
zero current (in the applied field) to a conductor with free charge carriers
(the quarks). At finite temperature this phenomenon persists, with the
dissociation transition become subsumed into the more familiar meson melting
transition. Here, the dissociation phenomenon reduces the critical melting
temperature.Comment: 20 pages, multiple figures. Corrected typo
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