139,202 research outputs found
Fast and dense magneto-optical traps for Strontium
We improve the efficiency of sawtooth-wave-adiabatic-passage (SWAP) cooling
for strontium atoms in three dimensions and combine it with standard
narrow-line laser cooling. With this technique, we create strontium
magneto-optical traps with bosonic Sr (
fermionic Sr) atoms at phase-space densities of
(). Our method is simple to implement and is faster and more
robust than traditional cooling methods.Comment: 9 pages, 6 figure
In Defense of the Epistemic Imperative
Sample (2015) argues that scientists ought not to believe that their theories are true because they cannot fulfill the epistemic obligation to take the diachronic perspective on their theories. I reply that Sample’s argument imposes an inordinately heavy epistemic obligation on scientists, and that it spells doom not only for scientific theories but also for observational beliefs and philosophical ideas that Samples endorses. I also delineate what I take to be a reasonable epistemic obligation for scientists. In sum, philosophers ought to impose on scientists only an epistemic standard that they are willing to impose on themselves
Electron Paramagnetic Resonance Linewidths and Lineshapes for the Molecular Magnets Fe8 and Mn12
We study theoretically Electron Paramagentic Resonance (EPR) linewidths for
single crystals of the molecular magnets Fe and Mn as functions of
energy eigenstates , frequency, and temperature when a magnetic field
along the easy axis is swept at fixed excitation frequency. This work was
motivated by recent EPR experiments. To calculate the linewidths, we use
density-matrix equations, including dipolar interactions and distributions of
the uniaxial anisotropy parameter and the Land\'{e} factor. Our
calculated linewidths agree well with the experimental data. We also examine
the lineshapes of the EPR spectra due to local rotations of the magnetic
anisotropy axes caused by defects in samples. Our preliminary results predict
that this effect leads to asymmetry in the EPR spectra.Comment: 2001 MMM conferenc
Response of flexible space vehicles to docking impact. Volume 2 - Numerical investigation
Mathematical model and computerized simulation of flexible space vehicle response to docking impac
Holographic End-Point of Spatially Modulated Phase Transition
In the previous paper [arXiv:0911.0679], we showed that the
Reissner-Nordstrom black hole in the 5-dimensional anti-de Sitter space coupled
to the Maxwell theory with the Chern-Simons term is unstable when the
Chern-Simons coupling is sufficiently large. In the dual conformal field
theory, the instability suggests a spatially modulated phase transition. In
this paper, we construct and analyze non-linear solutions which describe the
end-point of this phase transition. In the limit where the Chern-Simons
coupling is large, we find that the phase transition is of the second order
with the mean field critical exponent. However, the dispersion relation with
the Van Hove singularity enhances quantum corrections in the bulk, and we argue
that this changes the order of the phase transition from the second to the
first. We compute linear response functions in the non-linear solution and find
an infinite off-diagonal DC conductivity in the new phase.Comment: 21 pages, 22 figures. v2: a note and a reference adde
Effect of Defects on the Line shape of Electron Paramagnetic Resonance Signals from the Single-Molecule Magnet Mn12: A Theoretical Study
We herein estimate the effect of lattice defects on the line shape of
electron paramagnetic resonance (EPR) signals from a single crystal of the S=10
single-molecule magnet Mn with the external magnetic field along the
crystal c axis. A second-order perturbation treatment of an effective
single-spin Hamiltonian indicates that a small, random, static misorientation
of the magnetic symmetry axes in a crystalline lattice can lead to asymmetric
EPR peaks. Full spectra are simulated by calculating probability-distribution
functions for the resonant fields, employing distributions in the tilt angle of
the easy axis from the c axis, in the uniaxial anisotropy parameter, and in the
-factor. We discuss conditions under which the asymmetry in the EPR spectra
becomes prominent. The direction and magnitude of the asymmetry provide
information on the specific energy levels involved with the EPR transition, the
EPR frequency, and the distribution in the tilt angle.Comment: published versio
Spectroscopy of Rb dimers in solid He
We present experimental and theoretical studies of the absorption, emission
and photodissociation spectra of Rb molecules in solid helium. We have
identified 11 absorption bands of Rb. All laser-excited molecular states
are quenched by the interaction with the He matrix. The quenching results in
efficient population of a metastable (1) state, which emits
fluorescence at 1042 nm. In order to explain the fluorescence at the forbidden
transition and its time dependence we propose a new molecular exciplex
RbHe. We have also found evidence for the formation of
diatomic bubble states following photodissociation of Rb
Scaling in the crossover from random to correlated growth
In systems where deposition rates are high compared to diffusion, desorption
and other mechanisms that generate correlations, a crossover from random to
correlated growth of surface roughness is expected at a characteristic time
t_0. This crossover is analyzed in lattice models via scaling arguments, with
support from simulation results presented here and in other authors works. We
argue that the amplitudes of the saturation roughness and of the saturation
time scale as {t_0}^{1/2} and t_0, respectively. For models with lateral
aggregation, which typically are in the Kardar-Parisi-Zhang (KPZ) class, we
show that t_0 ~ 1/p, where p is the probability of the correlated aggregation
mechanism to take place. However, t_0 ~ 1/p^2 is obtained in solid-on-solid
models with single particle deposition attempts. This group includes models in
various universality classes, with numerical examples being provided in the
Edwards-Wilkinson (EW), KPZ and Villain-Lai-Das Sarma (nonlinear molecular-beam
epitaxy) classes. Most applications are for two-component models in which
random deposition, with probability 1-p, competes with a correlated aggregation
process with probability p. However, our approach can be extended to other
systems with the same crossover, such as the generalized restricted
solid-on-solid model with maximum height difference S, for large S. Moreover,
the scaling approach applies to all dimensions. In the particular case of
one-dimensional KPZ processes with this crossover, we show that t_0 ~ nu^{-1}
and nu ~ lambda^{2/3}, where nu and lambda are the coefficients of the linear
and nonlinear terms of the associated KPZ equations. The applicability of
previous results on models in the EW and KPZ classes is discussed.Comment: 14 pages + 5 figures, minor changes, version accepted in Phys. Rev.
Does Scientific Progress Consist in Increasing Knowledge or Understanding?
Bird argues that scientific progress consists in increasing knowledge. Dellsén objects that increasing knowledge is neither necessary nor sufficient for scientific progress, and argues that scientific progress rather consists in increasing understanding. Dellsén also contends that unlike Bird’s view, his view can account for the scientific practices of using idealizations and of choosing simple theories over complex ones. I argue that Dellsén’s criticisms against Bird’s view fail, and that increasing understanding cannot account for scientific progress, if acceptance, as opposed to belief, is required for scientific understanding
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