7,016 research outputs found
Trapped fermions with density imbalance in the BEC limit
We analyze the effects of imbalancing the populations of two-component
trapped fermions, in the BEC limit of the attractive interaction between
different fermions. Starting from the gap equation with two fermionic chemical
potentials, we derive a set of coupled equations that describe composite bosons
and excess fermions. We include in these equations the processes leading to the
correct dimer-dimer and dimer-fermion scattering lengths. The coupled equations
are then solved in the Thomas-Fermi approximation to obtain the density
profiles for composite bosons and excess fermions, which are relevant to the
recent experiments with trapped fermionic atomsComment: 5 pages, 4 figure
Mass imbalance effect in resonant Bose-Fermi mixtures
We consider a homogeneous Bose-Fermi mixture, with the boson-fermion
interaction tuned by a Fano-Feshbach resonance, in the presence of mass and
density imbalance between the two species. By using many-body diagrammatic
methods, we first study the finite-temperature phase diagram for the specific
case of the mass-imbalanced mixture Rb K for different values of
the density imbalance. We then analyse the quantum phase transition associated
with the disappearance at zero temperature of the boson condensate above a
critical boson-fermion coupling. We find a pronounced dependence of the
critical coupling on the mass ratio and a weak dependence on the density
imbalance. For a vanishingly small boson density, we derive, within our
approximation, the asymptotic expressions for the critical coupling in the
limits of small and large mass ratios. These expressions are relevant also for
the polaron-molecule transition in a Fermi mixture at small and large mass
ratios. The analysis of the momentum distribution functions at sufficiently
large density imbalances shows an interesting effect in the bosonic momentum
distribution due to the simultaneous presence of composite fermions and
unpaired fermions.Comment: Final versio
Temperature and coupling dependence of the universal contact intensity for an ultracold Fermi gas
Physical properties of an ultracold Fermi gas in the temperature-coupling
phase diagram can be characterized by the contact intensity C, which enters the
pair-correlation function at short distances and describes how the two-body
problem merges into its surrounding. We show that the local order established
by pairing fluctuations about the critical temperature Tc of the superfluid
transition considerably enhances the contact C in a temperature range where
pseudogap phenomena are maximal. Our ab initio results for C in a trap compare
well with recently available experimental data over a wide coupling range. An
analysis is also provided for the effects of trap averaging on C.Comment: 5 pages, 5 figure
Ferromagnetism of the Hubbard Model at Strong Coupling in the Hartree-Fock Approximation
As a contribution to the study of Hartree-Fock theory we prove rigorously
that the Hartree-Fock approximation to the ground state of the d-dimensional
Hubbard model leads to saturated ferromagnetism when the particle density (more
precisely, the chemical potential mu) is small and the coupling constant U is
large, but finite. This ferromagnetism contradicts the known fact that there is
no magnetization at low density, for any U, and thus shows that HF theory is
wrong in this case. As in the usual Hartree-Fock theory we restrict attention
to Slater determinants that are eigenvectors of the z-component of the total
spin, {S}_z = sum_x n_{x,\uparrow} - n_{x,\downarrow}, and we find that the
choice 2{S}_z = N = particle number gives the lowest energy at fixed 0 < mu <
4d.Comment: v2: Published version. 30 pages latex. Changes in title, abstract,
introductio
Anisotropic Galactic Outflows and Enrichment of the Intergalactic Medium. I: Monte Carlo Simulations
We have developed an analytical model to describe the evolution of
anisotropic galactic outflows. With it, we investigate the impact of varying
opening angle on galaxy formation and the evolution of the IGM. We have
implemented this model in a Monte Carlo algorithm to simulate galaxy formation
and outflows in a cosmological context. Using this algorithm, we have simulated
the evolution of a comoving volume of size [12h^(-1)Mpc]^3 in the LCDM
universe. Starting from a Gaussian density field at redshift z=24, we follow
the formation of ~20,000 galaxies, and simulate the galactic outflows produced
by these galaxies. When these outflows collide with density peaks, ram pressure
stripping of the gas inside the peak may result. This occurs in around half the
cases and prevents the formation of galaxies. Anisotropic outflows follow the
path of least resistance, and thus travel preferentially into low-density
regions, away from cosmological structures (filaments and pancakes) where
galaxies form. As a result, the number of collisions is reduced, leading to the
formation of a larger number of galaxies. Anisotropic outflows can
significantly enrich low-density systems with metals. Conversely, the
cross-pollution in metals of objects located in a common cosmological
structure, like a filament, is significantly reduced. Highly anisotropic
outflows can travel across cosmological voids and deposit metals in other,
unrelated cosmological structures.Comment: 32 pages, 9 figures (2 color). Revised version accepted in Ap
Comparison between a diagrammatic theory for the BCS-BEC crossover and Quantum Monte Carlo results
Predictions for the chemical potential and the excitation gap recently
obtained by our diagrammatic theory for the BCS-BEC crossover in the superfluid
phase are compared with novel Quantum Monte Carlo results at zero temperature
now available in the literature. A remarkable agreement is found between the
results obtained by the two approachesComment: 3 pages, 2 figure
Popov approximation for composite bosons in the BCS-BEC crossover
Theoretical treatments of the BCS-BEC crossover need to provide as accurate
as possible descriptions of the two regimes where the diluteness condition
applies, either in terms of the constituent fermions (BCS limit) or of the
composite bosons which form as bound-fermion pairs (BEC limit). This has to
occur via a single fermionic theory that bridges across these two limiting
representations. In this paper, we set up successive improvements of the
fermionic theory, that result into composite bosons described at the level of
either the Bogoliubov or the Popov approximations for point-like bosons. This
work bears on the recent experimental advances on the BCS-BEC crossover with
trapped Fermi atoms, which show the need for accurate theoretical descriptions
of BEC side of the crossover.Comment: 13 pages, 4 figure
Magnetic Field Effect on the Pseudogap Temperature within Precursor Superconductivity
We determine the magnetic field dependence of the pseudogap closing
temperature T* within a precursor superconductivity scenario. Detailed
calculations with an anisotropic attractive Hubbard model account for a
recently determined experimental relation in BSCCO between the pseudogap
closing field and the pseudogap temperature at zero field, as well as for the
weak initial dependence of T* at low fields. Our results indicate that the
available experimental data are fully compatible with a superconducting origin
of the pseudogap in cuprate superconductors.Comment: 4 pages, 3 figure
The recent non-marine ostracods of tunisia: An updated checklist with remarks on their regional distribution patterns and ecological preferences
Different lines of investigation have recently contributed to increasing the available knowledge about the invertebrates inhabiting inland waters of north Africa, but a comprehensive synopsis on Tunisian Ostracoda is missing to date. An updated checklist of Recent non-marine ostracods from Tunisia and data on their distribution is thus offered here, representing the most extensive survey on this crustacean group ever carried out in inland waters throughout the country. One-hundred-five sites covering various climate zones, from Mediterranean to desert areas, were sampled between 2002 and 2012. Most of the considered water bodies were temporary or ephemeral habitats, but a few permanent sites were sampled as well. Overall, 18 genera and 32 taxa of putative species rank were collected in the frame of this survey, among which nine species and five genera were new to Tunisian fauna. As a result of this study and based on pre-vious investigations, nine families (Candonidae, Cyprididae, Cytherideidae, Darwinulidae, Ilyocyprididae, Leptocytheridae, Limno-cytheridae, Loxoconchidae, Paradoxostomatidae), 29 genera and at least 45 species of non-marine ostracods are currently known for Tunisia, which thus prove to host the most diverse ostracod fauna among north African countries. The number of species occurring in a single sample varied from 1 to 4. The Eucypris virens complex was the most widespread taxon (58 records), followed by Heterocypris barbara (30 records), Heterocypris incongruens (22 records), and Sarscypridopsis aculeata (16 records). For some ostracod species, clear distributional gradients associated with different climatic conditions were observed. The affinities with adjacent Maghrebian os-tracod faunas are discussed. This study confirms the crucial role played by marginal aquatic habitats for the conservation of biodiversity, in particular in arid and semi-arid regions
An updated checklist of Recent ostracods (Crustacea: Ostracoda) from inland waters of Sicily and adjacent small islands with notes on their distribution and ecology
Because of its position in the middle of the Mediterranea Sea and a complex geological history that has promoted repeated waves of biological colonization, Sicily (Southern Italy) is of particular interest from a biogeographical perspective. A number of previous investigations, dating back as far as the end of the 19th century, have contributed to gather information about the occurrence of Recent non-marine ostracods in Sicily, making this region one of the most intensively studied areas of the Central Mediterranean. Published data on ostracod distributions have been integrated through an extensive field survey on mainland Sicily and surrounding small islands and archipelagos. Altogether, 271 ostracod samples and 11 sediment samples from dry water bodies were analysed from 218 sites visited between 2002 and 2017. Sampling sites were selected to encompass all the most common types of freshwater aquatic habitats, both natural and artificial, present in the area. Thirty-nine ostracods were identified at species level and 12 at supraspecific level. The present study reports four species (Cypria subsalsa, Eucypris mareotica, Physocypria kerkyrensis, Vestalenula boteai) and one genus (Vestalenula) as new for both peninsular Italy and adjacent islands, and three species (Candonopsis novaezelandiae, Ilyocypris inermis, Neglecandona neglecta) and two genera (Candonopsis and Physocypria) as new for Sicily. The updated checklist of the study area now includes at least 46 nominal species and other taxa identified at supraspecific level, belonging to 28 genera in 8 families (Candonidae, Cyprididae, Cytherideidae, Darwinulidae, Hemicytheridae, Ilyocyprididae, Limnocytheridae and Notodromadidae). The present investigation represents a significant addition to the knowledge of the ostracod diversity and distribution in the Sicilian area and in Italy as a whole. It also provides a sound baseline data for further comparative faunal studies aimed at investigating the affinities and origins of the central Mediterranean inland-water ostracod faunas, and to analyse their biogeographic patterns
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