505 research outputs found
Lower bound for energies of harmonic tangent unit-vector fields on convex polyhedra
We derive a lower bound for energies of harmonic maps of convex polyhedra in
to the unit sphere with tangent boundary conditions on the
faces. We also establish that maps, satisfying tangent boundary
conditions, are dense with respect to the Sobolev norm, in the space of
continuous tangent maps of finite energy.Comment: Acknowledgment added, typos removed, minor correction
Fluctuation, time-correlation function and geometric Phase
We establish a fluctuation-correlation theorem by relating the quantum
fluctuations in the generator of the parameter change to the time integral of
the quantum correlation function between the projection operator and force
operator of the ``fast'' system. By taking a cue from linear response theory we
relate the quantum fluctuation in the generator to the generalised
susceptibility. Relation between the open-path geometric phase, diagonal
elements of the quantum metric tensor and the force-force correlation function
is provided and the classical limit of the fluctuation-correlation theorem is
also discussed.Comment: Latex, 12 pages, no figures, submitted to J. Phys. A: Math & Ge
Landau-de Gennes Corrections to the Oseen-Frank Theory of Nematic Liquid Crystals
We study the asymptotic behavior of the minimisers of the Landau-de Gennes model for nematic liquid crystals in a two-dimensional domain in the regime of small elastic constant. At leading order in the elasticity constant, the minimum-energy configurations can be described by the simpler Oseen-Frank theory. Using a refined notion of Î-development we recover Landau-de Gennes corrections to the Oseen-Frank energy. We provide an explicit characterisation of minimizing Q-tensors at this order in terms of optimal Oseen-Frank directors and observe the emerging biaxiality. We apply our results to distinguish between optimal configurations in the class of conformal director fields of fixed topological degree saturating the lower bound for the Oseen-Frank energy
Energies of S^2-valued harmonic maps on polyhedra with tangent boundary conditions
A unit-vector field n:P \to S^2 on a convex polyhedron P \subset R^3
satisfies tangent boundary conditions if, on each face of P, n takes values
tangent to that face. Tangent unit-vector fields are necessarily discontinuous
at the vertices of P. We consider fields which are continuous elsewhere. We
derive a lower bound E^-_P(h) for the infimum Dirichlet energy E^inf_P(h) for
such tangent unit-vector fields of arbitrary homotopy type h. E^-_P(h) is
expressed as a weighted sum of minimal connections, one for each sector of a
natural partition of S^2 induced by P. For P a rectangular prism, we derive an
upper bound for E^inf_P(h) whose ratio to the lower bound may be bounded
independently of h. The problem is motivated by models of nematic liquid
crystals in polyhedral geometries. Our results improve and extend several
previous results.Comment: 42 pages, 2 figure
Quantum pumping and dissipation: from closed to open systems
Current can be pumped through a closed system by changing parameters (or
fields) in time. The Kubo formula allows to distinguish between dissipative and
non-dissipative contributions to the current. We obtain a Green function
expression and an matrix formula for the associated terms in the
generalized conductance matrix: the "geometric magnetism" term that corresponds
to adiabatic transport; and the "Fermi golden rule" term which is responsible
to the irreversible absorption of energy. We explain the subtle limit of an
infinite system, and demonstrate the consistency with the formulas by Landauer
and Buttiker, Pretre and Thomas. We also discuss the generalization of the
fluctuation-dissipation relation, and the implications of the Onsager
reciprocity.Comment: 4 page paper, 1 figure (published version) + 2 page appendi
Time-Dependent Warping, Fluxes, and NCYM
We describe the supergravity solutions dual to D6-branes with both
time-dependent and time-independent B-fields. These backgrounds generalize the
Taub-NUT metric in two key ways: they have asymmetric warp factors and
background fluxes. In the time-dependent case, the warping takes a novel form.
Kaluza-Klein reduction in these backgrounds is unusual, and we explore some of
the new features. In particular, we describe how a localized gauge-field
emerges with an analogue of the open string metric and coupling. We also
describe a gravitational analogue of the Seiberg-Witten map. This provides a
framework in supergravity both for studying non-commutative gauge theories, and
for constructing novel warped backgrounds.Comment: 32 pages, LaTeX, references adde
Maslov indices and monodromy
We prove that for a Hamiltonian system on a cotangent bundle that is Liouville-integrable and has monodromy the vector of Maslov indices is an eigenvector of the monodromy matrix with eigenvalue 1. As a corollary the resulting restrictions on the monodromy matrix are derived
Non-commutative gauge theory on D-branes in Melvin Universes
Non-commutative gauge theory with a non-constant non-commutativity parameter
can be formulated as a decoupling limit of open strings ending on D3-branes
wrapping a Melvin universe. We construct the action explicitly and discuss
various physical features of this theory. The decoupled field theory is not
supersymmetric. Nonetheless, the Coulomb branch appears to remain flat at least
in the large N and large 't Hooft coupling limit. We also find the analogue of
Prasad-Sommerfield monopoles whose size scales with the non-commutativity
parameter and is therefore position dependent.Comment: 15 pages, 1 figure, reference adde
On Supergravity Solutions of Branes in Melvin Universes
We study supergravity solutions of type II branes wrapping a Melvin universe.
These solutions provide the gravity description of non-commutative field
theories with non-constant non-commutative parameter. Typically these theories
are non-supersymmetric, though they exhibit some feature of their corresponding
supersymmetric theories. An interesting feature of these non-commutative
theories is that there is a critical length in the theory in which for
distances larger than this length the effects of non-commutativity become
important and for smaller distances these effects are negligible. Therefore we
would expect to see this kind of non-commutativity in large distances which
might be relevant in cosmology. We also study M5-brane wrapping on
11-dimensional Melvin universe and its descendant theories upon compactifying
on a circle.Comment: 25 pages, latex file; v2: typos corrected, Refs. adde
Supersymmteric Null-like Holographic Cosmologies
We construct a new class of 1/4-BPS time dependent domain-wall solutions with
null-like metric and dilaton in type II supergravities, which admit a null-like
big bang singularity. Based on the domain-wall/QFT correspondence, these
solutions are dual to 1/4-supersymmetric quantum field theories living on a
boundary cosmological background with time dependent coupling constant and UV
cutoff. In particular we evaluate the holographic function for the
2-dimensional dual field theory living on the corresponding null-like
cosmology. We find that this function runs in accordance with the
-theorem as the boundary universe evolves, this means that the number of
degrees of freedom is divergent at big bang and suggests the possible
resolution of big bang singularity.Comment: 26 pages;v2 references adde
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