9,129 research outputs found
Selfsimilar solutions in a sector for a quasilinear parabolic equation
We study a two-point free boundary problem in a sector for a quasilinear
parabolic equation. The boundary conditions are assumed to be spatially and
temporally "self-similar" in a special way. We prove the existence, uniqueness
and asymptotic stability of an expanding solution which is self-similar at
discrete times. We also study the existence and uniqueness of a shrinking
solution which is self-similar at discrete times.Comment: 23 page
On the role of kinetic and interfacial anisotropy in the crystal growth theory
A planar anisotropic curvature flow equation with constant driving force term is considered when the interfacial energy is crystalline. The driving force term is given so that a closed convex set grows if it is sufficiently large. If initial shape is convex, it is shown that a flat part called a facet (with admissible orientation) is instantaneously formed. Moreover, if the initial shape is convex and slightly bigger than the critical size, the shape becomes fully faceted in a finite time provided that the Frank diagram of interfacial energy density is a regular polygon centered at the origin. The proofs of these statements are based on approximation by crystalline algorithm whose foundation was established a decade ago. Our results indicate that the anisotropy of intefacial energy plays a key role when crystal is small in the theory of crystal growth. In particular, our theorems explain a reason why snow crystal forms a hexagonal prism when it is very small
Photoactive thin silver films by atmospheric pressure CVD
We report the visible and UV activity of thin silver films. The films are grown using a CVD process employing aqueous-based silver precursors, flame-assisted chemical vapour deposition. This approach overcomes many of the previously encountered limitations to silver deposition by employing an atmospheric pressure process, low-cost and low-toxicity precursors. The resultant films are assessed for activity using stearic acid destruction as a model compound. We also report on the addition of titania to these silver films to increase the potential functionality. This activity is also demonstrated, where the films appear largely transparent to the eye, further widening the potential application of this work. It is speculated that the nanoparticulate nature, of the CVD silver, is crucial in determining photoactivity
Atom-light crystallization of BECs in multimode cavities: Nonequilibrium classical and quantum phase transitions, emergent lattices, supersolidity, and frustration
The self-organization of a Bose-Einstein condensate in a transversely pumped
optical cavity is a process akin to crystallization: when pumped by a laser of
sufficient intensity, the coupled matter and light fields evolve,
spontaneously, into a spatially modulated pattern, or crystal, whose lattice
structure is dictated by the geometry of the cavity. In cavities having
multiple degenerate modes, the quasi-continuum of possible lattice
arrangements, and the continuous symmetry breaking associated with the adoption
of a particular lattice arrangement, give rise to phenomena such as phonons,
defects, and frustration, which have hitherto been unexplored in ultracold
atomic settings involving neutral atoms. The present work develops a
nonequilibrium field-theoretic approach to explore the self-organization of a
BEC in a pumped, lossy optical cavity. We find that the transition is well
described, in the regime of primary interest, by an effective equilibrium
theory. At nonzero temperatures, the self-organization occurs via a
fluctuation-driven first-order phase transition of the Brazovskii class; this
transition persists to zero temperature, and crosses over into a quantum phase
transition of a new universality class. We make further use of our
field-theoretic description to investigate the role of nonequilibrium
fluctuations on the self-organization transition, as well as to explore the
nucleation of ordered-phase droplets, the nature and energetics of topological
defects, supersolidity in the ordered phase, and the possibility of frustration
controlled by the cavity geometry. In addition, we discuss the range of
experimental parameters for which we expect the phenomena described here to be
observable, along with possible schemes for detecting ordering and fluctuations
via either atomic correlations or the correlations of the light emitted from
the cavity.Comment: 34 pages, 13 figures; follow up to Nat. Phys. 5, 845 (2009
Dual gauge field theory of quantum liquid crystals in two dimensions
We present a self-contained review of the theory of dislocation-mediated
quantum melting at zero temperature in two spatial dimensions. The theory
describes the liquid-crystalline phases with spatial symmetries in between a
quantum crystalline solid and an isotropic superfluid: quantum nematics and
smectics. It is based on an Abelian-Higgs-type duality mapping of phonons onto
gauge bosons ("stress photons"), which encode for the capacity of the crystal
to propagate stresses. Dislocations and disclinations, the topological defects
of the crystal, are sources for the gauge fields and the melting of the crystal
can be understood as the proliferation (condensation) of these defects, giving
rise to the Anderson-Higgs mechanism on the dual side. For the liquid crystal
phases, the shear sector of the gauge bosons becomes massive signaling that
shear rigidity is lost. Resting on symmetry principles, we derive the
phenomenological imaginary time actions of quantum nematics and smectics and
analyze the full spectrum of collective modes. The quantum nematic is a
superfluid having a true rotational Goldstone mode due to rotational symmetry
breaking, and the origin of this 'deconfined' mode is traced back to the
crystalline phase. The two-dimensional quantum smectic turns out to be a
dizzyingly anisotropic phase with the collective modes interpolating between
the solid and nematic in a non-trivial way. We also consider electrically
charged bosonic crystals and liquid crystals, and carefully analyze the
electromagnetic response of the quantum liquid crystal phases. In particular,
the quantum nematic is a real superconductor and shows the Meissner effect.
Their special properties inherited from spatial symmetry breaking show up
mostly at finite momentum, and should be accessible by momentum-sensitive
spectroscopy.Comment: Review article, 137 pages, 32 figures. Accepted versio
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