arXiv subjects

Heron Caldas

Publications and source records attributed to Heron Caldas.

35 records · Page 2Linked to original sources

arXiv2012

Superfluidity in Two-Dimensional Imbalanced Fermi Gases

We study the zero temperature ground state of a two-dimensional atomic Fermi gas with chemical potential and population imbalance in the mean-field approximation. All calculations are performed in terms of the two-body binding energy $ε_B$, whose variation allows to investigate the evolution from the BEC to the BCS regimes. By means of analytical and exact expressions we show that, similarly to what is found in three dimensions, at fixed chemical potentials, BCS is the ground state until the critical imbalance $h_c$ after which there is a first-order phase transition to the normal state. We find that $h_c$, the Chandrasekhar-Clogston limit of superfluidity, has the same value as in three dimensional systems. We show that for a fixed ratio $ε_B/ε_F$, where $ε_F$ is the two-dimensional Fermi energy, as the density imbalance $m$ is increased from zero, the ground state evolves from BCS to phase separation to the normal state. At the critical imbalance $m_c$ phase separation is not supported and the normal phase is energetically preferable. The BCS-BEC crossover is discussed in balanced and imbalanced configurations. Possible pictures of what may be found experimentally in these systems are also shown. We also investigate the necessary conditions for the existence of bound states in the balanced and imbalanced normal phase.

cond-mat.other↗

arXiv2012

Quantum normal-to-inhomogeneous superconductor phase transition in nearly two-dimensional metals

In multi-band systems, electrons from different orbitals coexist at the Fermi surface. An attractive interaction among these quasi-particles gives rise to inter-band or hybrid pairs which eventually condense in a superconducting state. These quasi-particles have a natural mismatch of their Fermi wave-vectors, $δk_F$, which depends on the strength of the hybridization between their orbitals. The existence of this natural scale suggests the possibility of inhomogeneous superconducting ground states in these systems, even in the absence of an applied magnetic field. Furthermore, since hybridization $V$ depends on pressure, this provides an external parameter to control the wave-vectors mismatch at the Fermi surface. In this work, we study the phase diagram of a two-dimensional, two-band metal with inter-band pairing. We show that as the mismatch between the Fermi wave-vectors of the two hybrid bands is reduced, the system presents a normal-to-inhomogeneous superconductor quantum phase transition at a critical value of the hybridization $V_c=Δ_0$. The superconducting ground state for $V<V_c$ is characterized by a wave-vector with magnitude $|\mathbf{q}_c|=q_c=2 Δ_0/\bar{v}_f$. Here $Δ_0$ is the superconducting gap in the homogeneous state and $\bar{v}_f$ the average Fermi velocity. We discuss the nature of the quantum critical point (QCP) at $V_c$ and obtain the associated quantum critical exponents.

cond-mat.str-el↗

arXiv2012

Finite Temperature Phase Diagram of Quasi-Two-Dimensional Imbalanced Fermi Gases Beyond Mean-Field

We investigate the superfluid transition temperature of quasi-two-dimensional imbalanced Fermi gases beyond the mean-field approximation, through the second-order (or induced) interaction effects. For a balanced Fermi system the transition temperature is suppressed by a factor $\approx 2.72$. For imbalanced Fermi systems, the polarization and transition temperature of the tricritical point are significantly reduced as the two-body binding energy $|ε_B|$ increases.

cond-mat.quant-gas↗

arXiv2011

An Effective Field Theory Model for One-Dimensional ${\rm CH}$ Chains: Effects at Finite Chemical Potential, Temperature and External Zeeman Magnetic Field

In this work we use an effective field theory model to investigate doped $\rm (CH)_x$ chains under the influence of an external constant Zeeman magnetic field $B_0$, at zero and finite temperature, in the mean-field approximation and beyond. We consider both homogeneous and inhomogeneous $Δ(x)$ condensates and briefly discuss the possibility of using these materials as partially polarized 1D organic conductors.

cond-mat.str-el↗

arXiv2010

Temperature effects on the magnetization of quasi-one-dimensional Peierls distorted materials

It is shown that temperature acts to disrupt the magnetization of Peierls distorted quasi-one-dimensional materials (Q1DM). The mean-field finite temperature phase diagram for the field theory model employed is obtained by considering both homogeneous and inhomogeneous condensates. The tricritical points of the second order transition lines of the gap parameter and magnetization are explicitly calculated. It is also shown that in the absence of an external static magnetic field the magnetization is always zero, at any temperature. As expected, temperature does not induce any magnetization effect on Peierls distorted Q1DM.

cond-mat.str-el↗

arXiv2009

Magnetization of Planar Four-Fermion Systems

We consider a planar system of fermions, at finite temperature and density, under a static magnetic field parallel to the two-dimensional plane. This magnetic field generates a Zeeman effect and, then, a spin polarization of the system. The critical properties are studied from the Landau's free energy. The possible observable consequences of the magnetization of planar systems such as polymer films and graphene are discussed.

cond-mat.soft↗

arXiv2008

Asymmetrically Doped Polyacetylene

Doped one-dimensional (1D) conjugated polymers, such as polyacetylene, have a conductivity of some metals, like copper. We show that when this polymer is asymmetrically doped, unexpected properties are revealed, when compared to the behavior of the symmetrically standard doped systems (SDS). Depending on the level of imbalance between the chemical potentials of the two involved fermionic species, the polymer can be converted into a 1D partially or fully spin polarized organic conductor.

cond-mat.str-el↗

arXiv2008

Temperature Effects in a Fermi Gas with Population Imbalance

We investigate temperature effects in a Fermi gas with imbalanced spin populations. From the general expression of the thermal gap equation we find, in {\it weak coupling limit}, an analytical expression for the transition temperature $T_c$ as a function of various possibilities of chemical potential and mass asymmetries between the two particle species. For a range of asymmetry between certain specific values, this equation always has two solutions for $T_c$ which has been interpreted as a reentrant phenomena or a pairing induced by temperature effect. We show that the lower $T_c$ is never related to a stable solution. The same results are obtained in {\it strong coupling limit}. The thermodynamical potential is carefully analyzed to avoid the consideration of the unstable solutions. We also obtain the tricritical points for the chemical potential and mass imbalanced cases, and beyond these points we properly minimize the thermodynamic potential to find the stable and metastable first order transition lines.

cond-mat.str-el↗

arXiv2008

Critical Dopant Concentration in Polyacetylene and Phase Diagram from a Continuous Four-Fermi Model

The Optimized Perturbation Theory (OPT) method, at finite temperature and finite chemical potential, is applied to the field theory model for polyacetylene. The critical dopant concentration in trans-polyacetylene is evaluated and compared with the available experimental data and with previous calculations. The results obtained within the OPT go beyond the standard mean field (or large-N) approximation (MFA) by explicitly including finite N effects. A critical analysis of the possible theoretical prescriptions to implement and interpret these corrections to the mean field results, given the available data, is given. For typical temperatures probed in the laboratory, our results show that the critical dopant concentration is only weakly affected by thermal effects.

cond-mat.soft↗

arXiv2007

On the Chemical Potentials in Cold Heterogeneous Mixed Phases

We study the relations among the chemical potentials of paired and unpaired particles in a cold asymmetrical fermionic mixed phase in thermodynamical equilibrium. We show how the chemical potentials are expressed in terms of the parameters of the mixed phase energy. We propose a way of estimating the value of the BCS gap in optical atomic traps experiments.

cond-mat.supr-con↗

arXiv2007

Surface Energy in Cold Asymmetrical Fermion Superfluids

We derive the energy of the surface between the normal and superfluid components of a mixed phase of a system composed of two particle species with different densities. The surface energy is obtained by the integration of the free energy density in the interface between the two phases. We show that the mixed phase remains as the favored ground state over the gapless phase in weak coupling. We find that the surface energy effects emerge only at strong coupling.

cond-mat.supr-con↗

arXiv2006

Phase Transition in Imbalanced Fermion Superfluids

In this chapter the recent theoretical work on phase transition in imbalanced fermion superfluids is reviewed. The imbalanced systems are those in which the two fermionic species candidate to form pairing have different Fermi surfaces or densities. We consider systems subjected to weak interactions. In this scenario two distinct phase transitions are predicted to occur. A thermodynamical phase transition, induced by the temperature (T), and a quantum phase transition as a function of the increasing chemical potentials asymmetry, that takes place at zero temperature. We also briefly discuss some recent experimental work at non-zero T with imbalanced Fermi gases in cold atomic traps.

cond-mat.supr-con↗

arXiv2005

Phase Transition in Asymmetrical Superfluids I: Equal Fermi Surfaces

In this paper, we study phase transitions in asymmetrical fermion superfluids. In this scenario, the candidates to form pair are particles with mismatched masses and chemical potentials. We derive an expression for the critical temperature in terms of the gap and masses (or chemical potentials) when the constraint of equal Fermi surfaces $m_aμ_a = m_bμ_b$ is imposed.

hep-ph↗

arXiv2004

Infrared Behavior of the Pressure in $g ϕ^3$ Theory Reexamined

We reinvestigate the infrared behavior of the pressure in the $g ϕ^3$ scalar theory in six dimensions. This problem was first studied by Almeida and Frenkel and more recently by Carrington et al., that certified their results under certain approximations. We employ an alternative technique, instead of the approximation methods necessary to truncate the Schwinger-Dyson equations, often considered to calculate the pressure nonperturbatively. A daisy-type sum, implemented through the modified self-consistent resummation (MSCR), is enough to take care of the infrared divergences ensuring the finiteness of the pressure.

hep-ph↗

arXiv2004

Cold asymmetrical fermion superfluids

In this work we investigate the general properties and the ground state of an asymmetrical dilute gas of cold fermionic atoms, formed by two particle species having different densities. We have show in a recent paper, that a mixed phase composed of normal and superfluid components is the energetically favored ground state of such a cold fermionic system. Here we extend the analysis and verify that in fact, the mixed phase is the preferred ground state of an asymmetrical superfluid in various situatiations. We predict that the mixed phase can serve as a way of detecting superfluidity and estimating the magnitude of the gap parameter in asymmetrical fermionic systems.

hep-ph↗

arXiv2004

Neutral Pion Decay Width in a Hot and Dense Medium

We study the behavior of the $π^0$ width as a function of temperature and density. We provide simple expressions for the neutral pion width in a hot and dense medium based on a renormalized version of the microscopic Nambu Jona Lasinio model. Then we consider the two cases where the pion mass is finite or vanishes at the critical temperature and discuss the phenomenological consequences of both cases.

hep-ph↗

arXiv2003

Phase separation in asymmetrical fermion superfluids

Motivated by recent developments on cold atom traps and high density QCD we consider fermionic systems composed of two particle species with different densities. We argue that a mixed phase composed of normal and superfluid components is the energetically favored ground state. We suggest how this phase separation can be used as a probe of fermion superfluidity in atomic traps.

cond-mat↗