7,922 research outputs found
Dynamic structure factor of ultracold Bose and Fermi gases in optical lattices
We investigate the dynamic structure factor of atomic Bose and Fermi gases in
one-dimensional optical lattices at zero temperature. The focus is on the
generic behaviour of S(k,omega) as function of filling and interaction strength
with the aim of identifying possible experimental signatures for the different
quantum phase transitions. We employ the Hubbard or Bose-Hubbard model and
solve the eigenvalue problem of the Hamiltonian exactly for moderate lattice
sizes. This allows us to determine the dynamic structure factor and other
observables directly in the phase transition regime, where approximation
schemes are generally not applicable. We discuss the characteristic signatures
of the various quantum phases appearing in the dynamic structure factor and
illustrate that the centroid of the strength distribution can be used to
estimate the relevant excitation gaps. Employing sum rules, these quantities
can be evaluated using ground state expectation values only. Important
differences between bosonic and fermionic systems are observed, e.g., regarding
the origin of the excitation gap in the Mott-insulator phase.Comment: 15 pages, 7 figure
Long-range nature of Feshbach molecules in Bose-Einstein condensates
We discuss the long-range nature of the molecules produced in recent experiments on molecular Bose-Einstein condensation. The properties of these molecules depend on the full two-body Hamiltonian and not just on the states of the system in the absence of interchannel couplings. The very long-range nature of the state is crucial to the efficiency of production in the experiments. Our many-body treatment of the gas accounts for the full binary physics and describes properly how these molecular condensates can be directly probed
BCS Theory for Trapped Ultracold Fermions
We develop an extension of the well-known BCS-theory to systems with trapped
fermions. The theory fully includes the quantized energy levels in the trap.
The key ingredient is to model the attractive interaction between two atoms by
a pseudo-potential which leads to a well defined scattering problem and
consequently a BCS-theory free of divergences. We present numerical results for
the BCS critical temperature and the temperature dependence of the gap. They
are used as a test of existing semi-classical approximations.Comment: 4 pages, 3 figures, submitted to PR
EPA's Arsenic Rule: The Benefits of the Standard Do Not Justify the Costs
The U.S. Environmental Protection Agency recently finalized a rule that would reduce the maximum allowable level of arsenic in drinking water by 80 percent. While arsenic is thought to be essential for the human body at low levels, it can cause cancer when consumed at higher concentrations for extended periods of time. This regulatory analysis evaluates the benefits and costs of the EPA's rule. On the basis of currently available information, we find that the EPA's standard cannot be justified on economic grounds. We estimate that the costs of the final rule will exceed the benefits by about $190 million annually. We also find that the rule probably will result in a net loss of life. We find that the rule probably will result in a net loss of life. The direct effect of the rule will be to save about ten lives annually in the future. After taking into account the indirect impacts of the cost of the rule on items like health care expenditures, however, we find that the rule is likely to result in a net loss of about ten lives annually. A question that the rule does not examine carefully is whether other regulatory alternatives could result in positive net benefits. We explore the option of targeting specific water systems and find that this strategy is unlikely to be very helpful. Instead of regulating more stringently now, the agency should wait until more information becomes available over the next few years. Such a strategy would have the advantage of avoiding large capital expenditures until the time that evidence suggests that risks posed by arsenic in drinking water are significant.
Dynamics of the BCS-BEC crossover in a degenerate Fermi gas
We study the short-time dynamics of a degenerate Fermi gas positioned near a
Feshbach resonance following an abrupt jump in the atomic interaction resulting
from a change of external magnetic field. We investigate the dynamics of the
condensate order parameter and pair wavefunction for a range of field
strengths. When the abrupt jump is sufficient to span the BCS to BEC crossover,
we show that the rigidity of the momentum distribution precludes any
atom-molecule oscillations in the entrance channel dominated resonances
observed in the 40K and 6Li. Focusing on material parameters tailored to the
40K Feshbach resonance system at 202.1 gauss, we comment on the integrity of
the fast sweet projection technique as a vehicle to explore the condensed phase
in the crossover regionComment: 5 pages, 4 figure
Excitations of Bose-Einstein condensates in optical lattices
In this paper we examine the excitations observable in atoms confined in an
optical lattice around the superfluid-insulator transition. We use increases in
the number variance of atoms, subsequent to tilting the lattice as the primary
diagnostic of excitations in the lattice. We show that this locally determined
quantity should be a robust indicator of coherence changes in the atoms
observed in recent experiments. This was found to hold for commensurate or
non-commensurate fillings of the lattice, implying our results will hold for a
wide range of physical cases. Our results are in good agreement with the
quantitative factors of recent experiments. We do, howevers, find extra
features in the excitation spectra. The variation of the spectra with the
duration of the perturbation also turns out to be an interesting diagnostic of
atom dynamics.Comment: 6 pages, 7 figures, using Revtex4; changes to version 2: new data and
substantial revision of tex
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