335 research outputs found
Criticality in inhomogeneous magnetic systems: Application to quantum ferromagnets
We consider a -theory with a position-dependent distance from the
critical point. One realization of this model is a classical ferromagnet
subject to non-uniform mechanical stress. We find a sharp phase transition
where the envelope of the local magnetization vanishes uniformly. The
first-order transition in a quantum ferromagnet also remains sharp. The
universal mechanism leading to a tricritical point in an itinerant quantum
ferromagnet is suppressed, and in principle one can recover a quantum critical
point with mean-field exponents. Observable consequences of these results are
discussed.Comment: 4pp, 4 eps figs, contains additional information compared to PRL
version. PRl, in pres
Six-dimensional weak-strong simulations of head-on beam-beam compensation in RHIC
To compensate the large beam-beam tune spread and beam-beam resonance driving
terms in the polarized proton operation in the Relativistic Heavy Ion Collider
(RHIC), we will introduce a low-energy DC electron beam into each ring to
collide head-on with the opposing proton beam. The device to provide the
electron beam is called an electron lens. In this article, using a 6-D
weak-strong-beam-beam interaction simulation model, we investigate the effects
of head-on beam-beam compensation with electron lenses on the proton beam
dynamics in the RHIC 250 GeV polarized proton operation. This article is
abridged from the published article [1].Comment: 5 pages, contribution to the ICFA Mini-Workshop on Beam-Beam Effects
in Hadron Colliders, CERN, Geneva, Switzerland, 18-22 Mar 201
Optics modification of the electron collector for the Relativistic Heavy Ion Collider Electron Beam Ion Source
N/
Effects of weak anchoring on C1 and C2 chevron structures
We present a theoretical study of the effect of weak anchoring on the transition between C1 and C2 chevron structures in smectic C liquid crystals. We employ a continuum theory which allows for variable cone, azimuthal and layer tilt angles. Equilibrium profiles for the director cone and azimuthal angles in the C1 and C2 states are calculated from the standard Euler-Lagrange minimisation of the total energy of the system. By comparing the total energies of the C1 and C2 states we can determine the globally stable chevron profile and calculate the critical temperature for the C1-C2 transition, which depends on anchoring strength and pretilt angle variations
Circulating Marangoni flows within droplets in smectic films
We present theoretical study and numerical simulation of Marangoni convection
within ellipsoidal isotropic droplets embedded in free standing smectic films
(FSSF). The thermocapillary flows are analyzed for both isotropic droplets
spontaneously formed in FSSF overheated above the bulk smectic-isotropic
transition, and oil lenses deposited on the surface of the smectic film. The
realistic model, for which the upper drop interface is free from the smectic
layers, while at the lower drop surface the smectic layering still persists is
considered in detail. For isotropic droplets and oil lenses this leads
effectively to a sticking of fluid motion at the border with a smectic shell.
The above mentioned asymmetric configuration is realized experimentally when
the temperature of the upper side of the film is higher than at the lower one.
The full set of stationary solutions for Stokes stream functions describing the
Marangoni convection flows within the ellipsoidal drops were derived
analytically. The temperature distribution in the ellipsoidal drop and the
surrounding air was determined in the frames of the perturbation theory. As a
result the analytical solutions for the stationary thermocapillary convection
were derived for different droplet ellipticity ratios and the heat conductivity
of the liquid crystal and air. In parallel, the numerical hydrodynamic
calculations of the thermocapillary motion in the drops were performed. Both
the analytical and numerical simulations predict the axially-symmetric
circulatory convection motion determined by the Marangoni effect at the droplet
free surface. Due to a curvature of the drop interface a temperature gradient
along its free surface always persists. Thus, the thermocapillary convection
within the ellipsoidal droplets in overheated FSSF is possible for the
arbitrarily small Marangoni numbers
Marangoni instability in oblate droplets suspended on a circular frame
We study theoretically internal flows in a small oblate droplet suspended on
the circular frame. Marangoni convection arises due to a vertical temperature
gradient across the drop and is driven by the surface tension variations at the
free drop interface. Using the analytical basis for the solutions of Stokes
equation in coordinates of oblate spheroid we have derived the linearly
independent stationary solutions for Marangoni convection in terms of Stokes
stream functions. The numerical simulations of the thermocapillary motion in
the drops are used to study the onset of the stationary regime. Both analytical
and numerical calculations predict the axially-symmetric circulatory convection
motion in the drop, the dynamics of which is determined by the magnitude of the
temperature gradient across the drop. The analytical solutions for the critical
temperature distribution and velocity fields are obtained for the large
temperature gradients across the oblate drop. These solutions reveal the
lateral separation of the critical and stationary motions within the drops. The
critical vortices are localized near the central part of a drop, while the
intensive stationary flow is located closer to its butt end. A crossover to the
limit of the plane film is studied within the formalism of the stream functions
by reducing the droplet ellipticity ratio to zero value. The initial stationary
regime for the strongly oblate drops becomes unstable relative to the
many-vortex perturbations in analogy with the plane fluid films with free
boundaries
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