2,355 research outputs found
Dynamic coexistence of various configurations: clusters vs.nuclei
The presence of energy shells in metallic clusters and atomic nuclei leads to
a peculiar relation between the number of particles N and the structure, and
this leads to a strong correlation between the energy spectrum and N. An
analysis of experimental data leads to the conclusion that, in addition to the
static Jahn-Teller effect, the dynamic effect leading to the quantum
coexistence of different configurations (quantum oscillations) plays an
important role. Such suggested coexistence is an essential feature of clusters
as well as nuclei, both finite Fermi systems.Comment: 6 pages, 2 figure
A comparative study of high-field diamagnetic fluctuations in deoxygenated YBa2Cu3O(7-x) and polycrystalline (Bi-Pb)2Sr2Ca3O(10)
We studied three single crystals of YBa2Cu3O{7-x} with Tc= 62.5, 52, and 41
K, and a textured specimen of (Bi-Pb)2Sr2Ca2Cu3O10 with Tc=108 K, for H//c
axis. The reversible data were interpreted in terms of 2D lowest-Landau-level
fluctuation theory. The data were fit well by the 2D LLL expression for
magnetization obtained by Tesanovic etal., producing reasonable values of kappa
but larger values of dHc2/dT. Universality was studied by obtaining a
simultaneous scaling of Y123 data and Bi2223. An expression for the 2D x-axis
LLL scaling factor used to obtain the simultaneous scaling was extracted from
theory, and compared with the experimental values. The comparison between the
values of the x-axis produced a deviation of 40% which suggests that the
hypothesis of universality of the 2D-LLL fluctuations is not supported by the
studied samples. We finaly observe that Y123 magnetization data for
temperatures above obbey a universal scaling obtained for the diamagnetic
fluctuation magnetization from a theory considering non-local field effects.
The same scaling was not obbeyed by the corresponding magnetization calculated
from the two-dimensional lowest-Landau-level theory.Comment: 7 pages 5 figures, accept in Journ. Low Temp. Phy
Why are women still leaving academic medicine? A qualitative study within a London Medical School
Objectives: To identify factors that influenced women who chose to leave academic medicine. Design and main outcome measures: Independent consultants led a focus group of women in medicine who had left academia after completion of their postgraduate research degree at Imperial College London Faculty of Medicine. Thematic analysis was performed on the transcribed conversations. Participants and setting: Nine women physicians who completed a postgraduate degree (MD or PhD) at a large London Medical School and Academic Health Sciences Centre, Imperial College London, but did not go on to pursue a career in academic medicine. Results: Influences to leave clinical academia were summarised under eight themes—career intentions, supervisor support, institutional human resources support, inclusivity, work–life balance, expectations, mentors and role models, and pregnancy and maternity leave. Conclusion: The women in our focus group reported several factors contributing to their decision to leave clinical academia, which included lack of mentoring tailored to specific needs, low levels of acceptance for flexible working to help meet parental responsibilities and perceived explicit gender biases. We summarise the multiple targeted strategies that Imperial College London has implemented to promote retention of women in academic medicine, although more research needs to be done to ascertain the most effective interventions
On the Dynamics of solitons in the nonlinear Schroedinger equation
We study the behavior of the soliton solutions of the equation
i((\partial{\psi})/(\partialt))=-(1/(2m)){\Delta}{\psi}+(1/2)W_{{\epsilon}}'({\psi})+V(x){\psi}
where W_{{\epsilon}}' is a suitable nonlinear term which is singular for
{\epsilon}=0. We use the "strong" nonlinearity to obtain results on existence,
shape, stability and dynamics of the soliton. The main result of this paper
(Theorem 1) shows that for {\epsilon}\to0 the orbit of our soliton approaches
the orbit of a classical particle in a potential V(x).Comment: 29 page
Boltzmann brains and the scale-factor cutoff measure of the multiverse
To make predictions for an eternally inflating "multiverse", one must adopt a
procedure for regulating its divergent spacetime volume. Recently, a new test
of such spacetime measures has emerged: normal observers - who evolve in pocket
universes cooling from hot big bang conditions - must not be vastly outnumbered
by "Boltzmann brains" - freak observers that pop in and out of existence as a
result of rare quantum fluctuations. If the Boltzmann brains prevail, then a
randomly chosen observer would be overwhelmingly likely to be surrounded by an
empty world, where all but vacuum energy has redshifted away, rather than the
rich structure that we observe. Using the scale-factor cutoff measure, we
calculate the ratio of Boltzmann brains to normal observers. We find the ratio
to be finite, and give an expression for it in terms of Boltzmann brain
nucleation rates and vacuum decay rates. We discuss the conditions that these
rates must obey for the ratio to be acceptable, and we discuss estimates of the
rates under a variety of assumptions.Comment: 32 pp, 2 figs. Modified to conform to the version accepted by Phys.
Rev. D. The last paragraph of Sec. V-A, about Boltzmann brains in Minkowski
space, has been significantly enlarged. Two sentences were added to the
introduction concerning the classical approximation and the hope of finding a
motivating principle for the measure. Several references were adde
Effective dynamics for particles coupled to a quantized scalar field
We consider a system of N non-relativistic spinless quantum particles
(``electrons'') interacting with a quantized scalar Bose field (whose
excitations we call ``photons''). We examine the case when the velocity v of
the electrons is small with respect to the one of the photons, denoted by c
(v/c= epsilon << 1). We show that dressed particle states exist (particles
surrounded by ``virtual photons''), which, up to terms of order (v/c)^3, follow
Hamiltonian dynamics. The effective N-particle Hamiltonian contains the kinetic
energies of the particles and Coulomb-like pair potentials at order (v/c)^0 and
the velocity dependent Darwin interaction and a mass renormalization at order
(v/c)^{2}. Beyond that order the effective dynamics are expected to be
dissipative.
The main mathematical tool we use is adiabatic perturbation theory. However,
in the present case there is no eigenvalue which is separated by a gap from the
rest of the spectrum, but its role is taken by the bottom of the absolutely
continuous spectrum, which is not an eigenvalue.
Nevertheless we construct approximate dressed electrons subspaces, which are
adiabatically invariant for the dynamics up to order (v/c)\sqrt{\ln
(v/c)^{-1}}. We also give an explicit expression for the non adiabatic
transitions corresponding to emission of free photons. For the radiated energy
we obtain the quantum analogue of the Larmor formula of classical
electrodynamics.Comment: 67 pages, 2 figures, version accepted for publication in
Communications in Mathematical Physic
Group Theory Approach to Band Structure: Scarf and Lame Hamiltonians
The group theoretical treatment of bound and scattering state problems is
extended to include band structure. We show that one can realize Hamiltonians
with periodic potentials as dynamical symmetries, where representation theory
provides analytic solutions, or which can be treated with more general spectrum
generating algebraic methods. We find dynamical symmetries for which we derive
the transfer matrices and dispersion relations. Both compact and non-compact
groups are found to play a role.Comment: 4 pages + 2 figs. Revtex/epsf. To appear: Phys Rev Lett, v.83 199
Potential gene conversion and source genes for recently integrated Alu elements
Alu elements comprise \u3e10% of the human genome. We have used a computational biology approach to analyze the human genomic DNA sequence databases to determine the impact of gene conversion on the sequence diversity of recently integrated Alu elements and to identify Alu elements that were potentially retroposition competent. We analyzed 269 Alu Ya5 elements and identified 23 members of a new Alu subfamily termed Ya5a2 with an estimated copy number of 35 members, including the de novo Alu insertion in the NFI gene. Our analysis of Alu elements containing one to four (Ya1-Ya4) of the Ya5 subfamily-specific mutations suggests that gene conversion contributed as much as 10%-20% of the variation between recently integrated Alu elements. In addition, analysis of the middle A-rich region of the different Alu Ya5 members indicates a tendency toward expansion of this region and subsequent generation of simple sequence repeats. Mining the databases for putative retroposition-competent elements that share 100% nucleotide identity to the previously reported de novo Alu insertions linked to human diseases resulted in the retrieval of 13 exact matches to the NF1 Alu repeat, three to the Alu element in BRCA2, and one to the Alu element in FGFR2 (Apert syndrome). Transient transfections of the potential source gene for the Apert\u27s Alu with its endogenous flanking genomic sequences demonstrated the transcriptional and presumptive transpositional competency of the element
Scattering and absorption of ultracold atoms by nanotubes
We investigate theoretically how cold atoms, including Bose-Einstein
condensates, are scattered from, or absorbed by nanotubes with a view to
analysing recent experiments. In particular we consider the role of potential
strength, quantum reflection, atomic interactions and tube vibrations on atom
loss rates. Lifshitz theory calculations deliver a significantly stronger
scattering potential than that found in experiment and we discuss possible
reasons for this. We find that the scattering potential for dielectric tubes
can be calculated to a good approximation using a modified pairwise summation
approach, which is efficient and easily extendable to arbitrary geometries.
Quantum reflection of atoms from a nanotube may become a significant factor at
low temperatures, especially for non-metallic tubes. Interatomic interactions
are shown to increase the rate at which atoms are lost to the nanotube and lead
to non-trivial dynamics. Thermal nanotube vibrations do not significantly
increase loss rates or reduce condensate fractions, but lower frequency
oscillations can dramatically heat the cloud.Comment: 7 pages, 4 figure
- …