7,152 research outputs found
Fermi-Bose quantum degenerate ^40 K - ^87 Rb mixture with attractive interaction
We report on the achievement of simultaneous quantum degeneracy in a mixed
gas of fermionic ^40 K and bosonic ^87 Rb. Potassium is cooled to 0.3 times the
Fermi temperature by means of an efficient thermalization with evaporatively
cooled rubidium. Direct measurement of the collisional cross-section confirms a
large interspecies attraction. This interaction is shown to affect the
expansion of the Bose-Einstein condensate released form the magnetic trap,
where it is immersed in the Fermi sea.Comment: 5 pages, 4 figures, replaced one figure plus some change
Mean-field analysis of the stability of a K-Rb Fermi-Bose mixture
We compare the experimental stability diagram of a Fermi-Bose mixture of K-40
and Rb-87 atoms with attractive interaction to the predictions of a mean-field
theoretical model. We discuss how this comparison can be used to give a better
estimate of the interspecies scattering length, which is currently known from
collisional measurements with larger uncertainty.Comment: 5 pages, 4 figure
Non-Fermi-liquid scattering rates and anomalous band dispersion in ferropnictides
Angle-resolved photoemission spectroscopy (ARPES) is used to study the band
dispersion and the quasiparticle scattering rates in two ferropnictides
systems. Our ARPES results show linear-in-energy dependent scattering rates
which are constant in a wide range of control parameter and which depend on the
orbital character of the bands. We demonstrate that the linear energy
dependence gives rise to weakly dispersing band with a strong mass enhancement
when the band maximum crosses the chemical potential. In the superconducting
phase the related small effective Fermi energy favors a
Bardeen-Cooper-Schrieffer (BCS)\,\cite{Bardeen1957}-Bose-Einstein
(BE)\,\cite{Bose1924} crossover state.Comment: 5 pages, 4 figures Supplement 4 pages, 6 figure
Collective excitations in trapped boson-fermion mixtures: from demixing to collapse
We calculate the spectrum of low-lying collective excitations in a gaseous
cloud formed by a Bose-Einstein condensate and a spin-polarized Fermi gas over
a range of the boson-fermion coupling strength extending from strongly
repulsive to strongly attractive. Increasing boson-fermion repulsions drive the
system towards spatial separation of its components (``demixing''), whereas
boson-fermion attractions drive it towards implosion (``collapse''). The
dynamics of the system is treated in the experimentally relevant collisionless
regime by means of a Random-Phase approximation and the behavior of a
mesoscopic cloud under isotropic harmonic confinement is contrasted with that
of a macroscopic mixture at given average particle densities. In the latter
case the locations of both the demixing and the collapse phase transitions are
sharply defined by the same stability condition, which is determined by the
softening of an eigenmode of either fermionic or bosonic origin. In contrast,
the transitions to either demixing or collapse in a mesoscopic cloud at fixed
confinement and particle numbers are spread out over a range of boson-fermion
coupling strength, and some initial decrease of the frequencies of a set of
collective modes is followed by hardening as evidenced by blue shifts of most
eigenmodes. The spectral hardening can serve as a signal of the impending
transition and is most evident when the number of bosons in the cloud is
relatively large. We propose physical interpretations for these dynamical
behaviors with the help of suitably defined partial compressibilities for the
gaseous cloud under confinement.Comment: 16 pages, 7 figures, revtex
Finite temperature effects on the collapse of trapped Bose-Fermi mixtures
By using the self-consistent Hartree-Fock-Bogoliubov-Popov theory, we present
a detailed study of the mean-field stability of spherically trapped Bose-Fermi
mixtures at finite temperature. We find that, by increasing the temperature,
the critical particle number of bosons (or fermions) and the critical
attractive Bose-Fermi scattering length increase, leading to a significant
stabilization of the mixture.Comment: 5 pages, 4 figures; minor changes, proof version, to appear in Phys.
Rev. A (Nov. 1, 2003
Mathematical Aspects of Vacuum Energy on Quantum Graphs
We use quantum graphs as a model to study various mathematical aspects of the
vacuum energy, such as convergence of periodic path expansions, consistency
among different methods (trace formulae versus method of images) and the
possible connection with the underlying classical dynamics.
We derive an expansion for the vacuum energy in terms of periodic paths on
the graph and prove its convergence and smooth dependence on the bond lengths
of the graph. For an important special case of graphs with equal bond lengths,
we derive a simpler explicit formula.
The main results are derived using the trace formula. We also discuss an
alternative approach using the method of images and prove that the results are
consistent. This may have important consequences for other systems, since the
method of images, unlike the trace formula, includes a sum over special
``bounce paths''. We succeed in showing that in our model bounce paths do not
contribute to the vacuum energy. Finally, we discuss the proposed possible link
between the magnitude of the vacuum energy and the type (chaotic vs.
integrable) of the underlying classical dynamics. Within a random matrix model
we calculate the variance of the vacuum energy over several ensembles and find
evidence that the level repulsion leads to suppression of the vacuum energy.Comment: Fixed several typos, explain the use of random matrices in Section
Interleukin-10 inhibits tumor metastasis through an NK-cell dependent mechanism
Interleukin-10 (IL-10) is a recently described pleiotropic cytokine secreted mainly by type 2 helper T cells. Previous studies have shown that IL-10 suppresses cytokine expression by natural killer (NK) and type 1 T cells, thus down-regulating cell-mediated immunity and stimulating humoral responses. We here report that injected IL-10 protein is an efficient inhibitor of tumor metastasis in experimental (B16-F10) and spontaneous (M27 and Lox human melanoma) metastasis models in vivo at doses that do not have toxic effects on normal or cancer cells. Histological characterization after IL-10 treatment confirmed the absence of CD8+ and CD4+ T cells and macrophages at the sites of tumor growth, but abundant NK cells were localized at these sites. This unexpected finding was confirmed by showing that IL-10 inhibits most B16-F10 and Lox metastases in mice deficient in T or B cells (SCID and nu/nu mice), but not in those deficient in NK cells (beige mice or NK cell-depleted mice). However, IL-10 downregulation of pro-inflammatory cytokine production and/or recruitment of additional effector cells may also be involved in the anti-tumor effect at higher local concentrations of IL-10, since transfected B16 tumor cells expressing high amounts of IL-10 were rejected by normal, nu/nu, or SCID mice at the primary tumor stage, and there was still a 33% inhibition of tumor metastasis in beige mice
Collective ferromagnetism in two-component Fermi-degenerate gas trapped in finite potential
Spin asymmetry of the ground states is studied for the trapped
spin-degenerate (two-component) gases of the fermionic atoms with the repulsive
interaction between different components, and, for large particle number, the
asymmetric (collective ferromagnetic) states are shown to be stable because it
can be energetically favorable to increase the fermi energy of one component
rather than the increase of the interaction energy between up-down components.
We formulate the Thomas-Fermi equations and show the algebraic methods to solve
them. From the Thomas-Fermi solutions, we find three kinds of ground states in
finite system: 1) paramagnetic (spin-symmetric), 2) ferromagnetic (equilibrium)
and 3) ferromagnetic (nonequilibrium) states. We show the density profiles and
the critical atom numbers for these states obtained analytically, and, in
ferromagnetic states, the spin-asymmetries are shown to occur in the central
regions of the trapped gas, and grows up with increasing particle number. Based
on the obtained results, we discuss the experimental conditions and current
difficulties to realize the ferromagnetic states of the trapped atom gas, which
should be overcome.Comment: submit to PR
Polytetrahedral Clusters
By studying the structures of clusters bound by a model potential that
favours polytetrahedral order, we find a previously unknown series of `magic
numbers' (i.e. sizes of special stability) whose polytetrahedral structures are
characterized by disclination networks that are analogous to hydrocarbons.Comment: 4 pages, 4 figure
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