2,219 research outputs found
Magnetic trapping of neutral particles: Classical and Quantum-mechanical study of a Ioffe-Pritchard type trap
Recently, we developed a method for calculating the lifetime of a particle
inside a magnetic trap with respect to spin flips, as a first step in our
efforts to understand the quantum-mechanics of magnetic traps. The 1D toy model
that was used in this study was physically unrealistic because the magnetic
field was not curl-free. Here, we study, both classically and
quantum-mechanically, the problem of a neutral particle with spin S, mass m and
magnetic moment mu, moving in 3D in an inhomogeneous magnetic field
corresponding to traps of the Ioffe-Pritchard, `clover-leaf' and `baseball'
type. Defining by omega_p, omega_z and omega_r the precessional, the axial and
the lateral vibrational frequencies, respectively, of the particle in the
adiabatic potential, we find classically the region in the $(\omega_{r}%
(omega_r -- omega_z) plane where the particle is trapped.
Quantum-mechanically, we study the problem of a spin-one particle in the same
field. Treating omega_r / omega_p and omega_z / omega_p as small parameters for
the perturbation from the adiabatic Hamiltonian, we derive a closed-form
expression for the transition rate 1/T_{esc} of the particle from its trapped
ground-state. We find that in the extreme cases, the expression for 1/T_{esc}
is dominated by the largest of the two frequencies omega_r and omega_z.Comment: 25 pages + 1 EPS figur
Wigner-Crystal Formulation of Strong-Coupling Theory for Counter-ions Near Planar Charged Interfaces
We present a new analytical approach to the strong electrostatic coupling
regime (SC), that can be achieved equivalently at low temperatures, high
charges, low dielectric permittivity etc. Two geometries are analyzed in
detail: one charged wall first, and then, two parallel walls at small
distances, that can be likely or oppositely charged. In all cases, one type of
mobile counter-ions only is present, and ensures electroneutrality (salt free
case). The method is based on a systematic expansion around the ground state
formed by the two-dimensional Wigner crystal(s) of counter-ions at the
plate(s). The leading SC order stems from a single-particle theory, and
coincides with the virial SC approach that has been much studied in the last 10
years. The first correction has the functional form of the virial SC
prediction, but the prefactor is different. The present theory is free of
divergences and the obtained results, both for symmetrically and asymmetrically
charged plates, are in excellent agreement with available data of Monte-Carlo
simulations under strong and intermediate Coulombic couplings. All results
obtained represent relevant improvements over the virial SC estimates. The
present SC theory starting from the Wigner crystal and therefore coined Wigner
SC, sheds light on anomalous phenomena like the counter-ion mediated
like-charge attraction, and the opposite-charge repulsion
On the structure and spectrum of classical two-dimensional clusters with a logarithmic interaction potential
We present a numerical study of the effect of the repulsive logarithmic
inter-particle interaction on the ground state configuration and the frequency
spectrum of a confined classical two-dimensional cluster containing a finite
number of particles. In the case of a hard wall confinement all particles form
one ring situated at the boundary of the potential. For a general r^n
confinement potential, also inner rings can form and we find that all
frequencies lie below the frequency of a particular mode, namely the
breathing-like mode. An interesting situation arises for the parabolic confined
system(i.e. n=2). In this case the frequency of the breathing mode is
independent of the number of particles leading to an upper bound for all
frequencies. All results can be understood from Earnshaw's theorem in two
dimensions. In order to check the sensitivity of these results, the spectrum of
vortices in a type II superconductor which, in the limit of large penetration
depths, interact through a logarithmic potential, is investigated.Comment: 11 pages, 6 figure
Constraints on stable equilibria with fluctuation-induced forces
We examine whether fluctuation-induced forces can lead to stable levitation.
First, we analyze a collection of classical objects at finite temperature that
contain fixed and mobile charges, and show that any arrangement in space is
unstable to small perturbations in position. This extends Earnshaw's theorem
for electrostatics by including thermal fluctuations of internal charges.
Quantum fluctuations of the electromagnetic field are responsible for
Casimir/van der Waals interactions. Neglecting permeabilities, we find that any
equilibrium position of items subject to such forces is also unstable if the
permittivities of all objects are higher or lower than that of the enveloping
medium; the former being the generic case for ordinary materials in vacuum.Comment: 4 pages, 1 figur
Evaluating the REF2014 Results in Art and Design
In the UK, periodic assessment of research in universities has taken place since 1986. The most recent took place during 2014 with the results being published at the end of the year. This evaluation is concerned with art and design though all disciplines were included in the assessment. The method used for assessing research quality is outlined and the results summarised. The lessons drawn from the evaluation by REF2014 are detailed. A number of issues are identified and discussed. These include the staff selected for submission, the method of evaluation, and a cost-benefit analysis of the process
An AC electric trap for ground-state molecules
We here report on the realization of an electrodynamic trap, capable of
trapping neutral atoms and molecules in both low-field and high-field seeking
states. Confinement in three dimensions is achieved by switching between two
electric field configurations that have a saddle-point at the center of the
trap, i.e., by alternating a focusing and a defocusing force in each direction.
AC trapping of 15ND3 molecules is experimentally demonstrated, and the
stability of the trap is studied as a function of the switching frequency. A 1
mK sample of 15ND3 molecules in the high-field seeking component of the
|J,K>=|1,1> level, the ground-state of para-ammonia, is trapped in a volume of
about 1 mm^3
A Black Hole Levitron
We study the problem of spatially stabilising four dimensional extremal black
holes in background electric/magnetic fields. Whilst looking for stationary
stable solutions describing black holes kept in external fields we find that
taking a continuum limit of Denef et al's multi-center solutions provides a
supergravity description of such backgrounds within which a black hole can be
trapped in a given volume. This is realised by levitating a black hole over a
magnetic dipole base. We comment on how such a construction resembles a
mechanical Levitron.Comment: 5 pages, 1 figur
Collaborative Research in Art, Design and New Media - Challenges and Opportunities
Collaboration between the arts and the sciences has a long history of tension and antipathy, and also, paradoxically, interdependence. Artists such as Leonardo da Vinci were involved with the creation of artistic works and also novel inventions that were intended to serve a purpose in the natural world. More recently the rapid advances of technology underpinning new media have facilitated both the collaboration of artists in virtual spaces and the use of new media to create, view, and interact with artistic works. Technology and applications are changing the way ideas are formed and the way research is developed and advanced. In addition, the ubiquity of new media has facilitated its use to support and develop a number of application domains. This paper explores the relationship between creativity, collaboration, and artistic works, and their interaction and interplay with social media
Stiffness and energy losses in cylindrically symmetric superconductor levitating systems
Stiffness and hysteretic energy losses are calculated for a magnetically
levitating system composed of a type-II superconductor and a permanent magnet
when a small vibration is produced in the system. We consider a cylindrically
symmetric configuration with only vertical movements and calculate the current
profiles under the assumption of the critical state model. The calculations,
based on magnetic energy minimization, take into account the demagnetization
fields inside the superconductor and the actual shape of the applied field. The
dependence of stiffness and hysteretic energy losses upon the different
important parameters of the system such as the superconductor aspect ratio, the
relative size of the superconductor-permanent magnet, and the critical current
of the superconductor are all systematically studied. Finally, in view of the
results, we provide some trends on how a system such as the one studied here
could be designed in order to optimize both the stiffness and the hysteretic
losses.Comment: 8 pages; 8 figure
- …