1,885 research outputs found
Observation of Interactions between Trapped Ions and Ultracold Rydberg Atoms
We report on the observation of interactions between ultracold Rydberg atoms
and ions in a Paul trap. The rate of observed inelastic collisions, which
manifest themselves as charge transfer between the Rydberg atoms and ions,
exceeds that of Langevin collisions for ground state atoms by about three
orders of magnitude. This indicates a huge increase in interaction strength. We
study the effect of the vacant Paul trap's electric fields on the Rydberg
excitation spectra. To quantitatively describe the exhibited shape of the ion
loss spectra, we need to include the ion-induced Stark shift on the Rydberg
atoms. Furthermore, we demonstrate Rydberg excitation on a dipole-forbidden
transition with the aid of the electric field of a single trapped ion. Our
results confirm that interactions between ultracold atoms and trapped ions can
be controlled by laser coupling to Rydberg states. Adding dynamic Rydberg
dressing may allow for the creation of spin-spin interactions between atoms and
ions, and the elimination of collisional heating due to ionic micromotion in
atom-ion mixtures.Comment: 7 pages, 5 figures, including appendices. Note that the title has
been changed in version
Computer simulations of two-dimensional melting with dipole-dipole interactions
We perform molecular dynamics and Monte Carlo simulations of two-dimensional
melting with dipole-dipole interactions. Both static and dynamic behaviors are
examined. In the isotropic liquid phase, the bond orientational correlation
length 6 and susceptibility 6 are measured, and the data are fitted to the
theoretical ansatz. An algebraic decay is detected for both spatial and
temporal bond orientational correlation functions in an intermediate
temperature regime, and it provides an explicit evidence for the existence of
the hexatic phase. From the finite-size scaling analysis of the global bond
orientational order parameter, the disclination unbinding temperature Ti is
estimated. In addition, from dynamic Monte Carlo simulations of the positional
order parameter, we extract the critical exponents at the dislocation unbinding
temperature Tm. All the results are in agreement with those from experiments
and support the Kosterlitz-Thouless-Halperin-Nelson-Young (KTHNY) theory.Comment: 23 pages, 12figure
Dynamical Diffraction Theory for Wave Packet Propagation in Deformed Crystals
We develop a theory for the trajectory of an x ray in the presence of a
crystal deformation. A set of equations of motion for an x-ray wave packet
including the dynamical diffraction is derived, taking into account the Berry
phase as a correction to geometrical optics. The trajectory of the wave packet
has a shift of the center position due to a crystal deformation. Remarkably, in
the vicinity of the Bragg condition, the shift is enhanced by a factor (: frequency of an x ray, : gap frequency
induced by the Bragg reflection). Comparison with the conventional dynamical
diffraction theory is also made.Comment: 4 pages, 2 figures. Title change
Two-Dimensional Wigner Crystal in Anisotropic Semiconductor
We investigate the effect of mass anisotropy on the Wigner crystallization
transition in a two-dimensional (2D) electron gas. The static and dynamical
properties of a 2D Wigner crystal have been calculated for arbitrary 2D Bravais
lattices in the presence of anisotropic mass, as may be obtainable in Si
MOSFETs with (110) surface. By studying the stability of all possible lattices,
we find significant change in the crystal structure and melting density of the
electron lattice with the lowest ground state energy.Comment: 4 pages, revtex, 4 figure
Boundary conditions for interfaces of electromagnetic (photonic) crystals and generalized Ewald-Oseen extinction principle
The problem of plane-wave diffraction on semi-infinite orthorhombic
electromagnetic (photonic) crystals of general kind is considered. Boundary
conditions are obtained in the form of infinite system of equations relating
amplitudes of incident wave, eigenmodes excited in the crystal and scattered
spatial harmonics. Generalized Ewald-Oseen extinction principle is formulated
on the base of deduced boundary conditions. The knowledge of properties of
infinite crystal's eigenmodes provides option to solve the diffraction problem
for the corresponding semi-infinite crystal numerically. In the case when the
crystal is formed by small inclusions which can be treated as point dipolar
scatterers with fixed direction the problem admits complete rigorous analytical
solution. The amplitudes of excited modes and scattered spatial harmonics are
expressed in terms of the wave vectors of the infinite crystal by closed-form
analytical formulae. The result is applied for study of reflection properties
of metamaterial formed by cubic lattice of split-ring resonators.Comment: 15 pages, 8 figures, submitted to PR
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Evolutionary bi-stability in pathogen transmission mode
Many pathogens transmit to new hosts by both infection (horizontal transmission) and transfer to the
infected host's offspring (vertical transmission). These two transmission modes require speci®c adap-
tations of the pathogen that can be mutually exclusive, resulting in a trade-off between horizontal and
vertical transmission. We show that in mathematical models such trade-offs can lead to the simultaneous
existence of two evolutionary stable states (evolutionary bi-stability) of allocation of resources to the two
modes of transmission. We also show that jumping between evolutionary stable states can be induced by
gradual environmental changes. Using quantitative PCR-based estimates of abundance in seed and vege-
tative parts, we show that the pathogen of wheat, Phaeosphaeria nodorum, has jumped between two
distinct states of transmission mode twice in the past 160 years, which, based on published evidence,
we interpret as adaptation to environmental change. The ®nding of evolutionary bi-stability has impli-
cations for human, animal and other plant diseases. An ill-judged change in a disease control
programme could cause the pathogen to evolve a new, and possibly more damaging, combination of
transmission modes. Similarly, environmental changes can shift the balance between transmission
modes, with adverse effects on human, animal and plant health
Laughlin-Jastrow-correlated Wigner crystal in a strong magnetic field
We propose a new ground state trial wavefunction for a two-dimensional Wigner
crystal in a strong perpendicular magnetic field. The wavefunction includes
Laughlin-Jastrow correlations between electron pairs, and may be interpreted as
a crystal state of composite fermions or composite bosons. Treating the power
of the Laughlin-Jastrow factor as a variational parameter, we use quantum
Monte Carlo simulations to compute the energy of these new states. We find that
our wavefunctions have lower energy than existing crystalline wavefunctions in
the lowest Landau level. Our results are consistent with experimental
observations of the filling factor at which the transition between the
fractional quantum Hall liquid and the Wigner crystal occurs for electron
systems. Exchange contributions to the wavefunctions are estimated
quantitatively and shown to be negligible for sufficiently small filling
factors
Improved analysis of neutral pion electroproduction off deuterium in chiral perturbation theory
Near threshold neutral pion electroproduction on the deuteron is studied in
the framework of heavy baryon chiral perturbation theory. We include the
next-to-leading order corrections to the three-body contributions. We find an
improved description of the total and differential cross section data measured
at MAMI. We also obtain more precise values for the threshold S-wave
multipoles. We discuss in detail the theoretical uncertainties of the
calculation.Comment: 14 pp, 11 figs, uses svjour.cl
Subtle competition between ferromagnetic and antiferromagnetic order in a Mn(II) - free radical ferrimagnetic chain
The macroscopic magnetic characterization of the Mn(II) - nitronyl nitroxide
free radical chain (Mn(hfac)2(R)-3MLNN) evidenced its transition from a
1-dimensional behavior of ferrimagnetic chains to a 3-dimensional ferromagnetic
long range order below 3 K. Neutron diffraction experiments, performed on a
single crystal around the transition temperature, led to a different conclusion
: the magnetic Bragg reflections detected below 3 K correspond to a canted
antiferromagnet where the magnetic moments are mainly oriented along the chain
axis. Surprisingly in the context of other compounds in this family of magnets,
the interchain coupling is antiferromagnetic. This state is shown to be very
fragile since a ferromagnetic interchain arrangement is recovered in a weak
magnetic field. This peculiar behavior might be explained by the competition
between dipolar interaction, shown to be responsible for the antiferromagnetic
long range order below 3 K, and exchange interaction, the balance between these
interactions being driven by the strong intrachain spin correlations. More
generally, this study underlines the need, in this kind of molecular compounds,
to go beyond macroscopic magnetization measurements.Comment: 12 pages, 10 figures, submitted to Phys. Rev.
Analysing the changing landscape of European financial centres: the role of financial products and the case of Amsterdam
The turn of the twenty-first century saw the re-emergence of debates about the
reconfiguration of European financial geographies and the role of stock
exchange mergers in this process. There has been, however, no systematic
attempt to date to analyse such changes. This paper proposes a specific
conceptual framework to explore these issues. It uses a product-based analysis
to examine, in the context of recent stock exchange mergers, the factors
affecting the competitiveness of a financial centre. It argues that it is important
to understand three intertwined influences – product complementarities, the
nature of local epistemic communities, and regulation – and their contingent
effects on change. This is exemplified by a tentative application of the
framework to the case of Amsterdam in order to better understand its recent
decline in competitiveness as a European financial centre
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