2,097 research outputs found
Energies of knot diagrams
We introduce and begin the study of new knot energies defined on knot
diagrams. Physically, they model the internal energy of thin metallic solid
tori squeezed between two parallel planes. Thus the knots considered can
perform the second and third Reidemeister moves, but not the first one. The
energy functionals considered are the sum of two terms, the uniformization term
(which tends to make the curvature of the knot uniform) and the resistance term
(which, in particular, forbids crossing changes). We define an infinite family
of uniformization functionals, depending on an arbitrary smooth function
and study the simplest nontrivial case , obtaining neat normal forms
(corresponding to minima of the functional) by making use of the Gauss
representation of immersed curves, of the phase space of the pendulum, and of
elliptic functions
Auger decay, Spin-exchange, and their connection to Bose-Einstein condensation of excitons in Cu_2O
In view of the recent experiments of O'Hara, et al. on excitons in Cu_2O, we
examine the interconversion between the angular-momentum triplet-state excitons
and the angular-momentum singlet-state excitons by a spin-exchange process
which has been overlooked in the past. We estimate the rate of this
particle-conserving mechanism and find a substantially higher value than the
Auger process considered so far. Based on this idea, we give a possible
explanation of the recent experimental observations, and make certain
predictions, with the most important being that the singlet-state excitons in
Cu_2O is a very serious candidate for exhibiting the phenomenon of
Bose-Einstein condensation.Comment: 4 pages, RevTex, 1 ps figur
Efficient and robust initialization of a qubit register with fermionic atoms
We show that fermionic atoms have crucial advantages over bosonic atoms in
terms of loading in optical lattices for use as a possible quantum computation
device. After analyzing the change in the level structure of a non-uniform
confining potential as a periodic potential is superimposed to it, we show how
this structure combined with the Pauli principle and fermion degeneracy can be
exploited to create unit occupancy of the lattice sites with very high
efficiency.Comment: 4 pages, 3 figure
Pure Gas of Optically Trapped Molecules Created from Fermionic Atoms
We report on the production of a pure sample of up to 3x10^5 optically
trapped molecules from a Fermi gas of 6Li atoms. The dimers are formed by
three-body recombination near a Feshbach resonance. For purification a
Stern-Gerlach selection technique is used that efficiently removes all trapped
atoms from the atom-molecule mixture. The behavior of the purified molecular
sample shows a striking dependence on the applied magnetic field. For very
weakly bound molecules near the Feshbach resonance, the gas exhibits a
remarkable stability with respect to collisional decay.Comment: 4 pages, 5 figure
Resonator-Enhanced Optical Dipole Trap for Fermionic Lithium Atoms
We demonstrate a novel optical dipole trap which is based on the enhancement
of the optical power density of a Nd:YAG laser beam in a resonator. The trap is
particularly suited for experiments with ultracold gases, as it combines a
potential depth of order 1 mK with storage times of several tens of seconds. We
study the interactions in a gas of fermionic lithium atoms in our trap and
observe the influence of spin-changing collisions and off-resonant photon
scattering. A key element in reaching long storage times is an ultra-low noise
laser. The dependence of the storage time on laser noise is investigated.Comment: 4 pages 3 figures Revised 17.07.2001; Corrected calibration of noise
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Fictitious Magnetic Resonance by Quasi-Electrostatic Field
We propose a new kind of spin manipulation method using a {\it fictitious}
magnetic field generated by a quasi-electrostatic field. The method can be
applicable to every atom with electron spins and has distinct advantages of
small photon scattering rate and local addressability. By using a
laser as a quasi-electrostatic field, we have experimentally demonstrated the
proposed method by observing the Rabi-oscillation of the ground state hyperfine
spin F=1 of the cold atoms and the Bose-Einstein condensate.Comment: 5 pages, 5 figure
Very long storage times and evaporative cooling of cesium atoms in a quasi-electrostatic dipole trap
We have trapped cesium atoms over many minutes in the focus of a CO-laser
beam employing an extremely simple laser system. Collisional properties of the
unpolarized atoms in their electronic ground state are investigated. Inelastic
binary collisions changing the hyperfine state lead to trap loss which is
quantitatively analyzed. Elastic collisions result in evaporative cooling of
the trapped gas from 25 K to 10 K over a time scale of about 150 s.Comment: 5 pages, 3 figure
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Morphology controls the thermoelectric power factor of a doped semiconducting polymer
The electrical performance of doped semiconducting polymers is strongly governed by processing methods and underlying thin-film microstructure. We report on the influence of different doping methods (solution versus vapor) on the thermoelectric power factor (PF) of PBTTT molecularly p-doped with FnTCNQ (n = 2 or 4). The vapor-doped films have more than two orders of magnitude higher electronic conductivity (s) relative to solution-doped films. On the basis of resonant soft x-ray scattering, vapor-doped samples are shown to have a large orientational correlation length (OCL) (that is, length scale of aligned backbones) that correlates to a high apparent charge carrier mobility (m). The Seebeck coefficient (a) is largely independent of OCL. This reveals that, unlike s, leveraging strategies to improve m have a smaller impact on a. Our best-performing sample with the largest OCL, vapor-doped PBTTT: F4TCNQ thin film, has a s of 670 S/cm and an a of 42 μV/K, which translates to a large PF of 120 mW m-1 K-2. In addition, despite the unfavorable offset for charge transfer, doping by F2TCNQ also leads to a large PF of 70 μW m-1 K-2, which reveals the potential utility of weak molecular dopants. Overall, our work introduces important general processing guidelines for the continued development of doped semiconducting polymers for thermoelectrics
Development of an apparatus for cooling 6Li-87Rb Fermi-Bose mixtures in a light-assisted magnetic trap
We describe an experimental setup designed to produce ultracold trapped gas
clouds of fermionic 6Li and bosonic 87Rb. This combination of alkali metals has
the potential to reach deeper Fermi degeneracy with respect to other mixtures
since it allows for improved heat capacity matching which optimizes sympathetic
cooling efficiency. Atomic beams of the two species are independently produced
and then decelerated by Zeeman slowers. The slowed atoms are collected into a
magneto-optical trap and then transferred into a quadrupole magnetic trap. An
ultracold Fermi gas with temperature in the 10^-3 T_F range should be
attainable through selective confinement of the two species via a properly
detuned laser beam focused in the center of the magnetic trap.Comment: Presented at LPHYS'06, 8 figure
Effective s- and p-Wave Contact Interactions in Trapped Degenerate Fermi Gases
The structure and stability of dilute degenerate Fermi gases trapped in an
external potential is discussed with special emphasis on the influence of s-
and p-wave interactions. In a first step an Effective Contact Interaction for
all partial waves is derived, which reproduces the energy spectrum of the full
potential within a mean-field model space. Using the s- and p-wave part the
energy density of the multi-component Fermi gas is calculated in Thomas-Fermi
approximation. On this basis the stability of the one- and two-component Fermi
gas against mean-field induced collapse is investigated. Explicit stability
conditions in terms of density and total particle number are given. For the
single-component system attractive p-wave interactions limit the density of the
gas. In the two-component case a subtle competition of s- and p-wave
interactions occurs and gives rise to a rich variety of phenomena. A repulsive
p-wave part, for example, can stabilize a two-component system that would
otherwise collapse due to an attractive s-wave interaction. It is concluded
that the p-wave interaction may have important influence on the structure of
degenerate Fermi gases and should not be discarded from the outset.Comment: 18 pages, 11 figures (using RevTEX4
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