Fermi-Fermi Mixtures in the Strong Attraction Limit

The phase diagrams of low density Fermi-Fermi mixtures with equal or unequal masses and equal or unequal populations are described at zero and finite temperatures in the strong attraction limit. In this limit, the Fermi-Fermi mixture can be described by a weakly interacting Bose-Fermi mixture, where the bosons correspond to Feshbach molecules and the fermions correspond to excess atoms. First, we discuss the three and four fermion scattering processes, and use the exact boson-fermion and boson-boson scattering lengths to generate the phase diagrams in terms of the underlying fermion-fermion scattering length. In three dimensions, in addition to the normal and uniform superfluid phases, we find two stable non-uniform states corresponding to (1) phase separation between pure unpaired (excess) and pure paired fermions (molecular bosons); and (2) phase separation between pure excess fermions and a mixture of excess fermions and molecular bosons. Lastly, we also discuss the effects of the trapping potential in the density profiles of condensed and non-condensed molecular bosons, and excess fermions at zero and finite temperatures, and discuss possible implications of our findings to experiments involving mixtures of ultracold fermions.Comment: 12 Pages, 6 Figures and 1 Tabl

Two-species fermion mixtures with population imbalance

We analyze the phase diagram of uniform superfluidity for two-species fermion mixtures from the Bardeen-Cooper-Schrieffer (BCS) to Bose-Einstein condensation (BEC) limit as a function of the scattering parameter and population imbalance. We find at zero temperature that the phase diagram of population imbalance versus scattering parameter is asymmetric for unequal masses, having a larger stability region for uniform superfluidity when the lighter fermions are in excess. In addition, we find topological quantum phase transitions associated with the disappearance or appearance of momentum space regions of zero quasiparticle energies. Lastly, near the critical temperature, we derive the Ginzburg-Landau equation, and show that it describes a dilute mixture of composite bosons and unpaired fermions in the BEC limit.Comment: 4 pages with 3 figures, accepted version to PR

F-wave versus P-wave Superconductivity in Organic Conductors

Current experimental results suggest that some organic quasi-one-dimensional superconductors exhibit triplet pairing symmetry. Thus, we discuss several potential triplet order parameters for the superconducting state of these systems within the functional integral formulation. We compare weak spin-orbit coupling $f_{xyz}$, $p_x$, $p_y$ and $p_z$ symmetries via several thermodynamic quantities. For each symmetry, we analyse the temperature dependences of the order parameter, condensation energy, specific heat, and superfluid density tensor.Comment: 5 pages, 4 figure

Nambu monopoles interacting with lattice defects in two-dimensional artificial square spin ice

The interactions between an excitation (similar to a pair of Nambu monopoles) and a lattice defect are studied in an artificial two-dimensional square spin ice. This is done by considering a square array of islands containing only one island different from all others. This difference is incorporated in the magnetic moment (spin) of the "imperfect" island and several cases are studied, including the special situation in which this distinct spin is zero (vacancy). We have shown that the two extreme points of a malformed island behave like two opposite magnetic charges. Then, the effective interaction between a pair of Nambu monopoles with the deformed island is a problem involving four magnetic charges (two pairs of opposite poles) and a string. We also sketch the configuration of the field lines of these four charges to confirm this picture. The influence of the string on this interaction decays rapidly with the string distance from the defect.Comment: 7 pages, 13 figure

Bopp-Podolsky black holes and the no-hair theorem

Bopp-Podolsky electrodynamics is generalized to curved space-times. The equations of motion are written for the case of static spherically symmetric black holes and their exterior solutions are analyzed using Bekenstein's method. It is shown the solutions split-up into two parts, namely a non-homogeneous (asymptotically massless) regime and a homogeneous (asymptotically massive) sector which is null outside the event horizon. In addition, in the simplest approach to Bopp-Podolsky black holes, the non-homogeneous solutions are found to be Maxwell's solutions leading to a Reissner-Nordstr\"om black hole. It is also demonstrated that the only exterior solution consistent with the weak and null energy conditions is the Maxwell's one. Thus, in light of energy conditions, it is concluded that only Maxwell modes propagate outside the horizon and, therefore, the no-hair theorem is satisfied in the case of Bopp-Podolsky fields in spherically symmetric space-times.Comment: 9 pages, updated to match published versio

Search for associations containing young stars (SACY). V. Is multiplicity universal? Tight multiple systems

Context: Dynamically undisrupted, young populations of stars are crucial to study the role of multiplicity in relation to star formation. Loose nearby associations provide us with a great sample of close ($<$150 pc) Pre-Main Sequence (PMS) stars across the very important age range ($\approx$5-70 Myr) to conduct such research. Aims: We characterize the short period multiplicity fraction of the SACY (Search for Associations Containing Young stars) accounting for any identifiable bias in our techniques and present the role of multiplicity fractions of the SACY sample in the context of star formation. Methods: Using the cross-correlation technique we identified double-lined spectroscopic systems (SB2), in addition to this we computed Radial Velocity (RV) values for our subsample of SACY targets using several epochs of FEROS and UVES data. These values were used to revise the membership of each association then combined with archival data to determine significant RV variations across different data epochs characteristic of multiplicity; single-lined multiple systems (SB1). Results: We identified 7 new multiple systems (SB1s: 5, SB2s: 2). We find no significant difference between the short period multiplicity fraction ($F_\mathrm{m}$) of the SACY sample and that of nearby star forming regions ($\approx$1-2 Myr) and the field ($F_\mathrm{m}\leq$10%) both as a function of age and as a function of primary mass, $M_1$, in the ranges $P$ [1:200 day] and $M_2$ [0.08 $M_{\odot}$-$M_1$]. Conclusions: Our results are consistent with the picture of universal star formation, when compared to the field and nearby star forming regions (SFRs). We comment on the implications of the relationship between increasing multiplicity fraction with primary mass, within the close companion range, in relation to star formation.Comment: 14 pages, 18 figures, published, A&A http://dx.doi.org/10.1051/0004-6361/20142385