2,888 research outputs found
Cavity-induced temperature control of a two-level system
We consider a two-level atom interacting with a single mode of the
electromagnetic field in a cavity within the Jaynes-Cummings model. Initially,
the atom is thermal while the cavity is in a coherent state. The atom interacts
with the cavity field for a fixed time. After removing the atom from the cavity
and applying a laser pulse the atom will be in a thermal state again. Depending
on the interaction time with the cavity field the final temperature can be
varied over a large range. We discuss how this method can be used to cool the
internal degrees of freedom of atoms and create heat baths suitable for
studying thermodynamics at the nanoscale
On the maximum size of an anti-chain of linearly separable sets and convex pseudo-discs
We show that the maximum cardinality of an anti-chain composed of
intersections of a given set of n points in the plane with half-planes is close
to quadratic in n. We approach this problem by establishing the equivalence
with the problem of the maximum monotone path in an arrangement of n lines. For
a related problem on antichains in families of convex pseudo-discs we can
establish the precise asymptotic bound: it is quadratic in n. The sets in such
a family are characterized as intersections of a given set of n points with
convex sets, such that the difference between the convex hulls of any two sets
is nonempty and connected.Comment: 10 pages, 3 figures. revised version correctly attributes the idea of
Section 3 to Tverberg; and replaced k-sets by "linearly separable sets" in
the paper and the title. Accepted for publication in Israel Journal of
Mathematic
INTERACTION OF IMPLICIT THEORIES AND ORIENTATION STYLE IN TEACHER PRACTICUM
The classroom practicum for student teachers is evaluated controversely both by
educational researches and by student teachers. We suggested that student teachers'
differing evaluations are due to interpersonal differences: according to their own
experiences as pupils and to their uncertainty vs. certainty orientation student teachers
developed implicit theories of teaching, which determine their actual practicum
experiences. Interviews with 18 student teachers selected on the basis oftheir orientation
style were analyzed using a combination of qualitative and quantitative methods. The
results show a clear interaction of central components of their implicit theories of
teaching and their orientation style. Consequences for the organization of teacher
practica are outlined.La práctica en el aula es evaluada de un modo controvertido tanto por parte de los investigadores en educación como de los futuros profesores. Nuestra hipótesis es que las diferentes evaluaciones del profesor en formación son debidas a diferencias
interpersonales: según sus propias experiencias como alumno; su orientación hacia la certidumbre o la incertidumbre, los futuros profesores desarrollan teorías implícitas de enseñanza que determinan el tipo de experiencia que sacarán de la propia práctica. Con este fin se analizaron -usando una combinación de métodos cuantitativos y cualitativos- 18 entrevistas con estudiantes de Pedagogía seleccionados sobre la base de su estilo de orientación. Los resultados muestran una clara interacción entre los componentes centrales de sus teorías implícitas acerca de la enseñanza y sus estilos de orientación. Se deducen, como conclusión, algunas consecuencias para la organización de la práctica
An experimental approach for investigating many-body phenomena in Rydberg-interacting quantum systems
Recent developments in the study of ultracold Rydberg gases demand an
advanced level of experimental sophistication, in which high atomic and optical
densities must be combined with excellent control of external fields and
sensitive Rydberg atom detection. We describe a tailored experimental system
used to produce and study Rydberg-interacting atoms excited from dense
ultracold atomic gases. The experiment has been optimized for fast duty cycles
using a high flux cold atom source and a three beam optical dipole trap. The
latter enables tuning of the atomic density and temperature over several orders
of magnitude, all the way to the Bose-Einstein condensation transition. An
electrode structure surrounding the atoms allows for precise control over
electric fields and single-particle sensitive field ionization detection of
Rydberg atoms. We review two experiments which highlight the influence of
strong Rydberg--Rydberg interactions on different many-body systems. First, the
Rydberg blockade effect is used to pre-structure an atomic gas prior to its
spontaneous evolution into an ultracold plasma. Second, hybrid states of
photons and atoms called dark-state polaritons are studied. By looking at the
statistical distribution of Rydberg excited atoms we reveal correlations
between dark-state polaritons. These experiments will ultimately provide a
deeper understanding of many-body phenomena in strongly-interacting regimes,
including the study of strongly-coupled plasmas and interfaces between atoms
and light at the quantum level.Comment: 14 pages, 11 figures; submitted to a special issue of 'Frontiers of
Physics' dedicated to 'Quantum Foundation and Technology: Frontiers and
Future
Nonlinear Schr\"odinger equation for a PT symmetric delta-functions double well
The time-independent nonlinear Schr\"odinger equation is solved for two
attractive delta-function shaped potential wells where an imaginary loss term
is added in one well, and a gain term of the same size but with opposite sign
in the other. We show that for vanishing nonlinearity the model captures all
the features known from studies of PT symmetric optical wave guides, e.g., the
coalescence of modes in an exceptional point at a critical value of the
loss/gain parameter, and the breaking of PT symmetry beyond. With the
nonlinearity present, the equation is a model for a Bose-Einstein condensate
with loss and gain in a double well potential. We find that the nonlinear
Hamiltonian picks as stationary eigenstates exactly such solutions which render
the nonlinear Hamiltonian itself PT symmetric, but observe coalescence and
bifurcation scenarios different from those known from linear PT symmetric
Hamiltonians.Comment: 16 pages, 9 figures, to be published in Journal of Physics
Electronic structure of fully epitaxial Co2TiSn thin films
In this article we report on the properties of thin films of the full Heusler
compound Co2TiSn prepared by DC magnetron co-sputtering. Fully epitaxial,
stoichiometric films were obtained by deposition on MgO (001) substrates at
substrate temperatures above 600{\deg}C. The films are well ordered in the L21
structure, and the Curie temperature exceeds slightly the bulk value. They show
a significant, isotropic magnetoresistance and the resistivity becomes strongly
anomalous in the paramagnetic state. The films are weakly ferrimagnetic, with
nearly 1 \mu_B on the Co atoms, and a small antiparallel Ti moment, in
agreement with theoretical expectations. From comparison of x-ray absorption
spectra on the Co L3/L2 edges, including circular and linear magnetic
dichroism, with ab initio calculations of the x-ray absorption and circular
dichroism spectra we infer that the electronic structure of Co2TiSn has
essentially non-localized character. Spectral features that have not been
explained in detail before, are explained here in terms of the final state band
structure.Comment: 11 pages, 8 figure
Diffusion Monte Carlo calculations for the ground states of atoms and ions in neutron star magnetic fields
The diffusion quantum Monte Carlo method is extended to solve the old
theoretical physics problem of many-electron atoms and ions in intense magnetic
fields. The feature of our approach is the use of adiabatic approximation wave
functions augmented by a Jastrow factor as guiding functions to initialize the
quantum Monte Carlo prodecure. We calcula te the ground state energies of atoms
and ions with nuclear charges from Z= 2, 3, 4, ..., 26 for magnetic field
strengths relevant for neutron stars.Comment: 6 pages, 1 figure, proceedings of the "9th International Conference
on Path Integrals - New Trends and Perspectives", Max-Planck-Institut fur
Physik komplexer Systeme, Dresden, Germany, September 23 - 28, 2007, to be
published as a book by World Scientific, Singapore (2008
Global and local relaxation of a spin-chain under exact Schroedinger and master-equation dynamics
We solve the Schroedinger equation for an interacting spin-chain locally
coupled to a quantum environment with a specific degeneracy structure. The
reduced dynamics of the whole spin-chain as well as of single spins is
analyzed. We show, that the total spin-chain relaxes to a thermal equilibrium
state independently of the internal interaction strength. In contrast, the
asymptotic states of each individual spin are thermal for weak but non-thermal
for stronger spin-spin coupling. The transition between both scenarios is found
for couplings of the order of , with denoting
the Zeeman-splitting. We compare these results with a master equation
treatment; when time averaged, both approaches lead to the same asymptotic
state and finally with analytical results.Comment: RevTeX, 8 pages, 14 figures, added DOI and forgotten reference
Lines pinning lines
A line g is a transversal to a family F of convex polytopes in 3-dimensional
space if it intersects every member of F. If, in addition, g is an isolated
point of the space of line transversals to F, we say that F is a pinning of g.
We show that any minimal pinning of a line by convex polytopes such that no
face of a polytope is coplanar with the line has size at most eight. If, in
addition, the polytopes are disjoint, then it has size at most six. We
completely characterize configurations of disjoint polytopes that form minimal
pinnings of a line.Comment: 27 pages, 10 figure
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