212 research outputs found
Phase separation in a boson-fermion mixture of Lithium atoms
We use a semiclassical three-fluid model to analyze the conditions for
spatial phase separation in a mixture of fermionic Li-6 and a (stable)
Bose-Einstein condensate of Li-7 atoms under cylindrical harmonic confinement,
both at zero and finite temperature. We show that with the parameters of the
Paris experiment [F. Schrek et al., Phys. Rev. Lett. 87 080403 (2001)] an
increase of the boson-fermion scattering length by a factor five would be
sufficient to enter the phase-separated regime. We give examples of
configurations for the density profiles in phase separation and estimate that
the transition should persist at temperatures typical of current experiments.
For higher values of the boson-fermion coupling we also find a new phase
separation between the fermions and the bosonic thermal cloud at finite
temperature.Comment: 8 pages, 4 figures, new version of Fig. 4 and typos correcte
Collective excitations of a trapped boson-fermion mixture across demixing
We calculate the spectrum of low-lying collective excitations in a mesoscopic
cloud formed by a Bose-Einstein condensate and a spin-polarized Fermi gas as a
function of the boson-fermion repulsions. The cloud is under isotropic harmonic
confinement and its dynamics is treated in the collisional regime by using the
equations of generalized hydrodynamics with inclusion of surface effects. For
large numbers of bosons we find that, as the cloud moves towards spatial
separation (demixing) with increasing boson-fermion coupling, the frequencies
of a set of collective modes show a softening followed by a sharp upturn. This
behavior permits a clear identification of the quantum phase transition. We
propose a physical interpretation for the dynamical transition point in a
confined mixture, leading to a simple analytical expression for its location.Comment: revtex4, 9 pages, 8 postscript file
The Lorentzian distance formula in noncommutative geometry
For almost twenty years, a search for a Lorentzian version of the well-known
Connes' distance formula has been undertaken. Several authors have contributed
to this search, providing important milestones, and the time has now come to
put those elements together in order to get a valid and functional formula.
This paper presents a historical review of the construction and the proof of a
Lorentzian distance formula suitable for noncommutative geometry.Comment: 16 pages, final form, few references adde
Time-Dependent Multi-Centre Solutions from New Metrics with Holonomy Sim(n-2)
The classifications of holonomy groups in Lorentzian and in Euclidean
signature are quite different. A group of interest in Lorentzian signature in n
dimensions is the maximal proper subgroup of the Lorentz group, SIM(n-2).
Ricci-flat metrics with SIM(2) holonomy were constructed by Kerr and Goldberg,
and a single four-dimensional example with a non-zero cosmological constant was
exhibited by Ghanam and Thompson. Here we reduce the problem of finding the
general -dimensional Einstein metric of SIM(n-2) holonomy, with and without
a cosmological constant, to solving a set linear generalised Laplace and
Poisson equations on an (n-2)-dimensional Einstein base manifold. Explicit
examples may be constructed in terms of generalised harmonic functions. A
dimensional reduction of these multi-centre solutions gives new time-dependent
Kaluza-Klein black holes and monopoles, including time-dependent black holes in
a cosmological background whose spatial sections have non-vanishing curvature.Comment: Typos corrected; 29 page
Free expansion of impenetrable bosons on one-dimensional optical lattices
We review recent exact results for the free expansion of impenetrable bosons
on one-dimensional lattices, after switching off a confining potential. When
the system is initially in a superfluid state, far from the regime in which the
Mott-insulator appears in the middle of the trap, the momentum distribution of
the expanding bosons rapidly approaches the momentum distribution of
noninteracting fermions. Remarkably, no loss in coherence is observed in the
system as reflected by a large occupation of the lowest eigenstate of the
one-particle density matrix. In the opposite limit, when the initial system is
a pure Mott insulator with one particle per lattice site, the expansion leads
to the emergence of quasicondensates at finite momentum. In this case,
one-particle correlations like the ones shown to be universal in the
equilibrium case develop in the system. We show that the out-of-equilibrium
behavior of the Shannon information entropy in momentum space, and its contrast
with the one of noninteracting fermions, allows to differentiate the two
different regimes of interest. It also helps in understanding the crossover
between them.Comment: 21 pages, 14 figures, invited brief revie
Simultaneity and generalized connections in general relativity
Stationary extended frames in general relativity are considered. The
requirement of stationarity allows to treat the spacetime as a principal fiber
bundle over the one-dimensional group of time translations. Over this bundle a
connection form establishes the simultaneity between neighboring events
accordingly with the Einstein synchronization convention. The mathematics
involved is that of gauge theories where a gauge choice is interpreted as a
global simultaneity convention. Then simultaneity in non-stationary frames is
investigated: it turns to be described by a gauge theory in a fiber bundle
without structure group, the curvature being given by the Fr\"olicher-Nijenhuis
bracket of the connection. The Bianchi identity of this gauge theory is a
differential relation between the vorticity field and the acceleration field.
In order for the simultaneity connection to be principal, a necessary and
sufficient condition on the 4-velocity of the observers is given.Comment: RevTeX, 9 pages, 2 figures, 1 table. Previous title "The gauge nature
of simultaneity". Classical and Quantum Gravity
http://www.iop.org/EJ/journal/CQ
Demixing in mesoscopic boson-fermion clouds inside cylindrical harmonic traps: quantum phase diagram and role of temperature
We use a semiclassical three-fluid thermodynamic model to evaluate the
phenomena of spatial demixing in mesoscopic clouds of fermionic and bosonic
atoms at high dilution under harmonic confinement, assuming repulsive
boson-boson and boson-fermion interactions and including account of a bosonic
thermal cloud at finite temperature T. The finite system size allows three
different regimes for the equilibrium density profiles at T=0: a fully mixed
state, a partially mixed state in which the overlap between the boson and
fermion clouds is decreasing, and a fully demixed state where the two clouds
have zero overlap. We propose simple analytical rules for the two cross-overs
between the three regimes as functions of the physical system parameters and
support these rules by extensive numerical calculations. A universal ``phase
diagram'' expressed in terms of simple scaling parameters is shown to be valid
for the transition to the regime of full demixing, inside which we identify
several exotic configurations for the two phase-separated clouds in addition to
simple ones consisting of a core of bosons enveloped by fermions and "vice
versa". With increasing temperature the main role of the growing thermal cloud
of bosons is to transform some exotic configurations into more symmetric ones,
until demixing is ultimately lost. For very high values of boson-fermion
repulsive coupling we also report demixing between the fermions and the
thermally excited bosons.Comment: 11 pages, 8 figure
Diffraction of a superfluid Fermi gas by an atomic grating
An atomic grating generated by a pulsed standing wave laser field is proposed
to manipulate the superfluid state in a quantum degenerate gas of fermionic
atoms. We show that in the presence of atomic Cooper pairs, the density
oscillations of the gas caused by the atomic grating exhibit a much longer
coherence time than that in the normal Fermi gas. Our result indicates that the
technique of a pulsed atomic grating can be a potential candidate to detect the
atomic superfluid state in a quantum degenerate Fermi gas.Comment: 4 pages, 2 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
Electrodeposited cu thin layers as low cost and effective underlayers for Cu2O photocathodes in photoelectrochemical water electrolysis
Cu2O is one of the most studied semiconductors for photocathodes in photoelectrochemical water splitting (PEC-WS). Its low stability is counterbalanced by good activity, provided that a suitable underlayer/support is used. While Cu2O is mostly studied on Au underlayers, this paper proposes Cu(0) as a low-cost, easy to prepare and highly efficient alternative. Cu and Cu2O can be electrodeposited from the same bath, thus allowing in principle to tune the final material\u2019s physico-chemical properties with high precision with a scalable method. Electrodes and photoelectrodes are studied by means of electrochemical methods (cyclic voltammetry, Pb underpotential deposition) and by ex-situ X-ray absorption spectroscopy (XAS). While the potential applied for the deposition of Cu has no influence on the bulk structure and on the photocurrent displayed by the semiconductor, it plays a role on the dark currents, making this strategy promising for improving the material\u2019s stability. Au/Cu2O and Cu/Cu2O show similar performances, the latter having clear advantages in view of future use in practical applications. The influence of Cu underlayer thickness was also evaluated in terms of obtained photocurrent
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