arXiv subjects

E. J. Calegari

Publications and source records attributed to E. J. Calegari.

15 recordsLinked to original sources

arXiv2026

Coexistence of superconductivity and charge density wave in a correlated regime

To investigate the coexistence of superconductivity and charge density wave (CDW) in a correlated regime, we employ the Green's functions formalism, as well as the Hubbard-I approximation, as a way to introduce the correlations into the problem, in the form of a repulsive Coulomb interaction $U$. In addition, we investigate the effects of second-nearest neighbor hopping $t_1$ on a pure CDW state. The analysis of the results show that, for small values of $t_1$, both CDW and superconducting gaps compete for the same region on the Fermi surface. The increase of $t_1$ decreases the competition and may lead the system to a coexistence regime. Effects of temperature in the coexistence regime, are also investigated.

cond-mat.supr-con↗

arXiv2024

The interplay between a pseudogap and superconductivity in a two-dimensional Hubbard model

Strongly correlated electrons systems may exhibit a variety of interesting phenomena, for instance, superconductivity and pseudogap, as is the case of cuprates and pnictides. In strongly correlated electron systems, it is considered essential to understand, not only the nature of the pseudogap, but also the relationship between superconductivity and the pseudogap. In order to address this question, in the present work, we investigated a one-band Hubbard model treated by the Green's function method within an n-pole approximation. In the strongly correlated regime, antiferromagnetic correlations give rise to nearly flat band regions in the nodal points of the quasiparticle bands. As a consequence, a pseudogap emerges at the antinodal points of the Fermi surface. The obtained results indicate that the same antiferromagnetic correlations responsible for a pseudogap, can also favor superconductivity, providing an increase in the superconducting critical temperature Tc.

cond-mat.str-el↗

arXiv2021

Magnetic transitions induced by pressure and magnetic field in a two-orbital $5f$-electron model in cubic and tetragonal lattices

We investigate the onset and evolution of under the simultaneous application of pressure and magnetic field of distinct itinerant Néel states using the underscreened Anderson Lattice Model (UALM) which has been proposed to describe $5f$-electron systems. The model is composed by two narrow $f$-bands (of either $α$ or $β$ character) that hybridize with a wide $d$-band and local $5f$-electron interactions. We consider both cubic and tetragonal lattices. The Néel order parameters $ϕ^β$ and $ϕ^α$ are assumed to be fixed by an Ising anisotropy. The applied magnetic field $h_z$ is parallel to the anisotropy axis. It has been assumed that the variation of the band width $W$ is sensitive to pressure. In the absence of a magnetic field, the increase of $W$ takes the system from the phase AF$_1$ to another phase AF$_2$. The phase AF$_1$ occurs when $ϕ^β>ϕ^α>0$ while in the AF$_2$ phase the gaps satisfy $ϕ^α>ϕ^β>0$. In the presence of a magnetic field $h_z$, the phase AF$_2$ is quickly suppressed and reappears again at intermediate values of the magnetic field while it is predominant at higher magnetic fields. The analysis of the partial density of states close to the phase transition between the phases AF$_1$ and AF$_2$, allows a better understanding the mechanism responsible whereby the transition is induced by an increase in the magnetic field. As a important general result, we found that the magnetic field $h_z$ favours the phase AF$_2$ while the phase AF$_1$ is suppressed. For the tetragonal lattice, the phase AF$_2$ is even more favored when $h_z$ and $c/a$ increases concomitantly, where $c$ and $a$ are the lattice parameters.

cond-mat.str-el↗

arXiv2020

Unfolding of antiferromagnetic phases and multicritical points in a two-orbital model for Uranium compounds under pressure and magnetic field

We investigate the occurrence of multicritical points under pressure and magnetic field in a model that describes two 5f bands (of either $α$ or $β$ characters) which hybridize with a single itinerant conduction band. The 5f-electrons interact through Coulomb and exchange terms. The AF order parameter is a Néel vector, which is assumed to be fixed by an Ising anisotropy. The applied magnetic field is transverse to the anisotropy axis. Without field, our results for the temperature - pressure phase diagram show that, at low temperatures, a first-order phase transition occurs between two distinct antiferromagnetic phases, AF$_1$ and AF$_2$, as the pressure is increased. The two phases are characterized by the gaps of bands $α$ and $β$ given by $Δ_α$ and $Δ_β$, respectively. The AF$_1$ phase occurs when $Δ_β>Δ_α>0$, while in the AF$_2$ phase, the gaps satisfy $Δ_α>Δ_β>0$. The application of a magnetic field produces a drastic change in the phase diagram. The AF1 and AF2 phases separate with the latter acquiring a dome shape which is eventually suppressed for large values of the applied field. The evolution of the phase diagram under pressure, without and with magnetic field, shows the presence of multicritical points. Our results show that the evolution of these multicritical points by the simultaneous application of pressure and field is also drastic with the suppression of some multicritical points and the emergence of others ones. We believe that these results may have relevance for the growing field of multicritical points (classical and quantum) in the physics of Uranium compounds.

cond-mat.str-el↗

arXiv2019

Effects of a k-dependent Hybridization on the Fermi Surface of an Extended $d-p$ Hubbard Model

The topology of the Fermi surface of an extended $d-p$ Hubbard model is investigated using the Green's function technique in a n-pole approximation. The effects of the $d-p$ hybridization on the Fermi surface are the main focus in the present work. Nevertheless, the effects of doping, Coulomb interaction and hopping to second-nearest-neighbors on the Fermi surface, are also studied. Particularly, it is shown that the crossover from hole-like to electron-like Fermi surface (Lifshitz transition) is deeply affected by the $d-p$ hybridization. Moreover, the pseudogap present in the low doping regime is also affected by the hybridization. The results show that both the doping and the hybridization act in the sense of suppresses the pseudogap. Therefore, the systematic investigation of the Fermi surface topology, shows that not only the doping but also the hybridization can be considered as a control parameter for both the pseudogap and the Lifshitz transition. Assuming that the hybridization is sensitive to external pressure, the present results agree qualitatively with recent experimental data for the cuprate Nd-LSCO.

cond-mat.str-el↗

arXiv2014

Pseudogap and the specific heat of high T$_c$ superconductors: a Hubbard model in a n-pole approximation

In this work the specific heat of a two-dimensional Hubbard model, suitable to discuss high-$T_c$ superconductors (HTSC), is studied taking into account hopping to first ($t$) and second ($t_2$) nearest neighbors. Experimental results for the specific heat of HTSC's, for instance, the YBCO and LSCO, indicate a close relation between the pseudogap and the specific heat. In the present work, we investigate the specific heat by the Green's function method within a $n$-pole approximation. The specific heat is calculated on the pseudogap and on the superconducting regions. In the present scenario, the pseudogap emerges when the antiferromagnetic (AF) fluctuations become sufficiently strong. The specific heat jump coefficient $Δγ$ decreases when the total occupation per site ($n_T$) reaches a given value. Such behavior of $Δγ$ indicates the presence of a pseudogap in the regime of high occupation.

cond-mat.str-el↗

arXiv2014

Interplay between condensation energy, pseudogap and the specific heat of a Hubbard model in a n-pole approximation

The condensation energy and the specific heat jump of a two-dimensional Hubbard model, suitable to discuss high-$T_c$ superconductors, is studied. In this work, the Hubbard model is investigated by the Green's function method within a $n$-pole approximation, which allows to consider superconductivity with $d_{x^2-y^2}$-wave pairing. In the present scenario, the pseudogap regime emerges when the antiferromagnetic (AF) correlations become sufficiently strong to move to lower energies the region around of the nodal point $(π,π)$ on the renormalized bands. It is observed that above a given total occupation $n_T$, the specific heat jump $ΔC$ and also the condensation energy $U(0)$ decrease signaling the presence of the pseudogap.

cond-mat.str-el↗

arXiv2013

Specific heat of a non-local attractive Hubbard model

The specific heat of an attractive (interaction $G<0$) non-local Hubbard model is investigated. We use a two-pole approximation which leads to a set of correlation functions. In particular, the correlation function $\ <\vec{S}_i\cdot\vec{S}_j\ >$ plays an important role as a source of anomalies in the normal state of the model. Our results show that for a giving range of $G$ and $δ$ where $δ=1-n_T$ ($n_T=n_{\uparrow}+n_{\downarrow}$), the specific heat as a function of the temperature presents a two peak structure. Nevertehelesss, the presence of a pseudogap on the anti-nodal points $(0,\pmπ)$ and $(\pmπ,0)$ eliminates the two peak structure, the low temperature peak remaining. The effects of the second nearest neighbor hopping on the specific heat are also investigated.

cond-mat.str-el↗

arXiv2013

Pseudogap and the specific heat of high $T_c$ superconductors

The specific heat of a two dimensional repulsive Hubbard model with local interaction is investigated. We use the two-pole approximation which exhibits explicitly important correlations that are sources of the pseudogap anomaly. The interplay between the specific heat and the pseudogap is the main focus of the present work. Our self consistent numerical results show that above the occupation $n_T\approx 0.85$, the specific heat starts to decrease due to the presence of a pseudogap in the density of states. We have also observed a two peak structure in the specific heat. Such structure is robust with respect to the Coulomb interaction $U$ but it is significantly affected by the occupation $n_T$. A detailed study of the two peak structure is carried out in terms of the renormalized quasi-particle bands. The role of the second nearest neighbor hopping on the specific heat behavior and on the pseudogap, is extensively discussed.

cond-mat.str-el↗

arXiv2011

Superconductivity in an extended Hubbard model with attractive interaction

In this work, a two-dimensional one-band Hubbard model is investigated within a two-pole approximation. The model presents a non-local attractive potential $U (U<0)$ that allows the study of d-wave superconductivity and also includes hopping up to second-nearest-neighbors. The two-pole scheme has been proposed to improve the Hubbard-I approximation. The analytical results show a more complex form for the gap $Δ(T)$, when compared to the one obtained in the latter approximation. Indeed, new anomalous correlation functions associated with the superconductivity are involved in the calculation of $Δ(T)$. Numerical results in a range of temperatures are presented. Moreover, the structure of the quasiparticle bands and the topology of the Fermi surface are studied in detail in the normal state. Connections with some experimental results are also included.

cond-mat.str-el↗

arXiv2010

Spectral Function of a $d-p$ Hubbard Model

This work investigates a d-p Hubbard model by the n-pole approximation in the hole-doped regime. In particular, the spectral function $A(ω,\vec{k})$ is analyzed varying the filling, the local Coulomb interaction and the $d-p$ hybridization. It should be remarked that the original n-pole approximation (Phys. Rev. 184 (1969) 451) has been improved in order to include adequately the $\vec{k}$-dependence of the important correlation function $< \vec{S}_j\cdot\vec{S}_i>$ present in the poles of the Green's functions. It has been verified that the topology of the Fermi surface (defined by $A(ω=0,\vec{k})$) is deeply affected by the doping, the strength of the Coulomb interaction and also by the hybridization. Particularly, in the underdoped regime, the spectral function $A(ω=0,\vec{k})$ presents very low intensity close to the anti-nodal points $(0,\pm π)$ and $(\pm π,0)$. Such a behavior produces an anomalous Fermi surface (pockets) with pseudogaps in the region of the anti-nodal points. On the other hand, if the $d-p$ hybridization is enhanced sufficiently, such pseudogaps vanish. It is precisely the correlation function $< \vec{S}_j\cdot\vec{S}_i>$ present in the poles of the Green's functions which plays the important role in the underdoped situation. In fact, antiferromagnetic correlations coming from $< \vec{S}_j\cdot\vec{S}_i>$ strongly modify the quasi-particle band structure. This is the ultimate source of anomalies in the Fermi surface in the present approach.

cond-mat.str-el↗

arXiv2006

Effects of the Hybridization on the Fermi Surface of an Extended $d-p$ Hubbard Model

The Fermi surface (FS) of an extended $d-p$ Hubbard model is investigated by a two-pole approximation in both situations with hole and electron doping. Using the factorization procedure proposed by Beenen and Edwards, superconductivity with singlet $d_{x^2-y^2}$-wave pairing is considered. The effects of the $d-p$ hybridization on the FS are the main focus in the present work. Nevertheless, the asymmetries between the hole- and electron-doped regimes and the effects of doping and Coulomb interaction on FS are also investigated. Particularly, it is shown that the crossover from hole-like to electron-like FS is deeply affected by the $d-p$ hybridization in the hole-doped case. It has been verified that the effect of the hybridization is very pronounced around the saddle points $(0,\pmπ)$ and $(\pmπ,0)$, where the intensity of the superconducting order parameter is maximum in the particular case of $d_{x^2-y^2}$-wave symmetry. In the electron-doped case, the crossover in the FS is not verified. The doping dependence of the FS topology in the hole- and electron-doped regimes is in agreement with recent experimental ARPES results for La$_{2-x}$Sr$_{x}$CuO$_4$ (hole doping) and Nd$_{2-x}$Ce$_x$CuO$_4$ (electron doping).

cond-mat.supr-con↗

arXiv2005

Compressibility of a two-dimensional extended Hubbard model

The compressibility of an extended $d-p$ Hubbard model is investigated by the Roth's two-pole approximation. Using the factorization procedure proposed by Beenen and Edwards, superconductivity with singlet $d_{x^2-y^2}$-wave pairing is also considered. Within this framework, the effects of $d-p$ hybridization and Coulomb interaction $U$ on the compressibility are studied carefully. It has been found that the compressibility diverges and then it becomes negative near the half-filling. Within Roth's method, it has been verified that an important contribution for the negative compressibility comes from the spin-correlation term $ $ present in Roth's band shift. This correlation function plays an important role due to its high doping dependence. Also, its effects in the band shift and consequently in the compressibility are pronounced near the half-filling. The numerical results show that the hybridization acts in the sense of suppressing the negative compressibility near half-filling. Finally, the possibility of a connection between the negative compressibility and the phase separation is also discussed.

cond-mat.str-el↗

arXiv2005

Superconductivity in a two dimensional extended Hubbard model

The Roth's two-pole approximation has been used by the present authors to investigate the role of $d-p$ hybridization in the superconducting properties of an extended $d-p$ Hubbard model. Superconductivity with singlet $d_{x^2-y^2}$-wave pairing is treated by following Beenen and Edwards formalism. In this work, the Coulomb interaction, the temperature and the superconductivity have been considered in the calculation of some relevant correlation functions present in the Roth's band shift. The behavior of the order parameter associated with temperature, hybridization, Coulomb interaction and the Roth's band shift effects on superconductivity are studied.

cond-mat.str-el↗

arXiv2004

Role of Hybridization in the Superconducting Properties of an Extended $d-p$ Hubbard Model: a Detailed Numerical Study

The Roth's two-pole approximation has been used by the present authors to study the effects of the hybridization in the superconducting properties of a strongly correlated electron system. The model used is the extended Hubbard model which includes the $d-p$ hybridization, the $p$-band and a narrow $d$-band. The present work is an extension a previous reference [Intern. Journ. of Modern Phys. B, Vol. 18 No. 2 (2004) 241]. Nevertheless, some important correlation functions necessary to estimate the Roth's band shift, are included together with the temperature $T$ and the Coulomb interaction $U$ to describe the superconductivity. The superconducting order parameter of a cuprate system, is obtained following Beenen and Edwards formalism. Here, we investigate in detail the change of the order parameter associated to temperature, Coulomb interaction and Roth's band shift effects on superconductivity. The phase diagram with $T_c$ versus the total occupation numbers $n_T$, shows the difference respect to the previous work.

cond-mat.str-el↗