49,548 research outputs found
Critical Casimir effect in classical binary liquid mixtures
If a fluctuating medium is confined, the ensuing perturbation of its
fluctuation spectrum generates Casimir-like effective forces acting on its
confining surfaces. Near a continuous phase transition of such a medium the
corresponding order parameter fluctuations occur on all length scales and
therefore close to the critical point this effect acquires a universal
character, i.e., to a large extent it is independent of the microscopic details
of the actual system. Accordingly it can be calculated theoretically by
studying suitable representative model systems.
We report on the direct measurement of critical Casimir forces by total
internal reflection microscopy (TIRM), with femto-Newton resolution. The
corresponding potentials are determined for individual colloidal particles
floating above a substrate under the action of the critical thermal noise in
the solvent medium, constituted by a binary liquid mixture of water and
2,6-lutidine near its lower consolute point. Depending on the relative
adsorption preferences of the colloid and substrate surfaces with respect to
the two components of the binary liquid mixture, we observe that, upon
approaching the critical point of the solvent, attractive or repulsive forces
emerge and supersede those prevailing away from it. Based on the knowledge of
the critical Casimir forces acting in film geometries within the Ising
universality class and with equal or opposing boundary conditions, we provide
the corresponding theoretical predictions for the sphere-planar wall geometry
of the experiment. The experimental data for the effective potential can be
interpreted consistently in terms of these predictions and a remarkable
quantitative agreement is observed.Comment: 30 pages, 17 figure
Peaks in the Hartle-Hawking Wave Function from Sums over Topologies
Recent developments in ``Einstein Dehn filling'' allow the construction of
infinitely many Einstein manifolds that have different topologies but are
geometrically close to each other. Using these results, we show that for many
spatial topologies, the Hartle-Hawking wave function for a spacetime with a
negative cosmological constant develops sharp peaks at certain calculable
geometries. The peaks we find are all centered on spatial metrics of constant
negative curvature, suggesting a new mechanism for obtaining local homogeneity
in quantum cosmology.Comment: 16 pages,LaTeX, no figures; v2: some changes coming from revision of
a math reference: wave function peaks sharp but not infinite; v3: added
paragraph in intro on interpretation of wave functio
Crossover of conductance and local density of states in a single-channel disordered quantum wire
The probability distribution of the mesoscopic local density of states (LDOS)
for a single-channel disordered quantum wire with chiral symmetry is computed
in two different geometries. An approximate ansatz is proposed to describe the
crossover of the probability distributions for the conductance and LDOS between
the chiral and standard symmetry classes of a single-channel disordered quantum
wire. The accuracy of this ansatz is discussed by comparison with a
large-deviation ansatz introduced by Schomerus and Titov in Phys. Rev. B
\textbf{67}, 100201(R) (2003).Comment: 19 pages, 5 eps figure
Dynamically Triangulating Lorentzian Quantum Gravity
Fruitful ideas on how to quantize gravity are few and far between. In this
paper, we give a complete description of a recently introduced non-perturbative
gravitational path integral whose continuum limit has already been investigated
extensively in d less than 4, with promising results. It is based on a
simplicial regularization of Lorentzian space-times and, most importantly,
possesses a well-defined, non-perturbative Wick rotation. We present a detailed
analysis of the geometric and mathematical properties of the discretized model
in d=3,4. This includes a derivation of Lorentzian simplicial manifold
constraints, the gravitational actions and their Wick rotation. We define a
transfer matrix for the system and show that it leads to a well-defined
self-adjoint Hamiltonian. In view of numerical simulations, we also suggest
sets of Lorentzian Monte Carlo moves. We demonstrate that certain pathological
phases found previously in Euclidean models of dynamical triangulations cannot
be realized in the Lorentzian case.Comment: 41 pages, 14 figure
Quantum Matrix Models for Simple Current Orbifolds
An algebraic formulation of the stringy geometry on simple current orbifolds
of the WZW models of type A_N is developed within the framework of Reflection
Equation Algebras, REA_q(A_N). It is demonstrated that REA_q(A_N) has the same
set of outer automorphisms as the corresponding current algebra A^{(1)}_N which
is crucial for the orbifold construction. The CFT monodromy charge is naturally
identified within the algebraic framework. The ensuing orbifold matrix models
are shown to yield results on brane tensions and the algebra of functions in
agreement with the exact BCFT data.Comment: 31 pages, LaTeX; typos corrected, new elements added, the contents
restructure
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