arXiv subjects

C. M. Chaves

Publications and source records attributed to C. M. Chaves.

At least 19 recordsLinked to original sources

arXiv2015

Temperature dependence of the magnetic hyperfine field at an s-p impurity diluted in $R$Ni$_{2}$

We study the formation of local magnetic moments and magnetic hyperfine fields at an \textit{s-p} impurity diluted in intermetallic Laves phases compounds $R$Ni$_{2}$ ($R$ = Nd, Sm, Gd, Tb, Dy) at finite temperatures. We start with a clean host and later the impurity is introduced. The host has two-coupled ($R$ and Ni) sublattice Hubbard Hamiltonians but the Ni sublattice can be disregarded because its $\textit{d}$ band, being full, is magnetically ineffective. Also, the effect of the $\textit{4f}$ electrons of $R$ is represented by a polarization of the \textit{d} band that would be produced by the $\textit{4f}$ magnetic field.This leaves us with a lattice of \textit{effective} rare earth \textit{R}-ions with only \textit{d} electrons. For the \textit{dd} electronic interaction we use the Hubbard-Stratonovich identity in a functional integral approach in the static saddle point approximation.

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

Quantum and classical correlations and Werner states in finite spin linear arrays

Pairwise quantum correlations in the ground state of a N-spins antiferromagnetic chain described by the Heisenberg model with nearest neighbor exchange coupling are investigated. By varying a single coupling between two neighboring sites it is possible to drive spins from entangled to disentangled states, reversibly. For even N the two-spin density matrix is written in the form of a Werner state, allowing identification of the weight parameter with the usual spin-spin correlation function $\langle S_i^z \, S_j^z \rangle = Γ_{ij}$. The correlation functions show universal behavior in the $Γ$-dependence. This study presents a concrete possibility for the practical demonstration of entanglement control, opening alternatives for probing non-classical correlations and the realization of Werner states in familiar condensed matter systems. All required fabrication and measurement ingredients are currently available.

cond-mat.mes-hall↗

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↗

arXiv2012

Theoretical study of the hyperfine field at Cu impurities diluted in an iron host

Magnetic hyperfine field at Cu isotopes as impurities in Fe were recently measured at low temperature. A model to explain these experimental results is proposed. The diluted Cu impurities in the ferromagnetic Fe host are described by an extension of the Daniel-Friedel model, including the next neighbor perturbation. In order to account for the available experimental data in Cu isotopes with atomic masses $A$ = 59, 67, 69 and 71 as impurity, we needed to incorporate the Cu anomaly volume in the effective charge to be screened and in the self-consistent procedures.

nucl-ex↗

arXiv2011

Magnetic moment formation at a dilute Ce(140) impurity in RCo_2 compounds

A great deal of experimental work using perturbed angular correlation (PAC) has succeed in measuring hyperfine fields in Ce diluted in metallic systems, thus allowing the determination of the local impurity moment at low temperatures. Motivated by such experimental work on Ce(140) placed on a R site of the rare earth (R = Gd, Tb, Dy, Ho, Er) in RCo_2, we theoretically discuss, within a simple model, the local magnetic moments and thereby calculate the magnetic hyperfine fields. The results are in good agreement with the experimental data. For the sake of comparison we recall our previous results on Ta d-impurity embedded in the same hosts.

cond-mat.str-el↗

arXiv2011

Magnetic hyperfine field at \textit{s-p} impurities on Laves phase compounds

Recent experimental results for the magnetic hyperfine field B_{\rm hf} at the nuclei of \textit{s-p} impurities such as ${}^{119}$Sn in intermetallic Laves phases RM_2 (R = Gd, Tb, Dy, Ho, Er; M = Fe, Co) and ${}^{111}$Cd in RCo_{2}, the impurity occupying a $R$ site indicate that the ratio B_{\rm hf}/μ_{3d} exhibits different behavior when one goes from RFe_2 to RCo_2. In this work, we calculate these local moments and the magnetic hyperfine fields. In our model, B_{\rm hf} has two contributions: one arising from the $R$ ions, and the other arising from magnetic 3d-elements; these separate contributions allow the identification of the origin of different behavior of the ratio mentioned above. For {}^{111}Cd in RCo_2 we present also the contributions for B_{\rm hf} in the light rare earth Pr, Nd, Pm, Sm compounds. For the sake of comparison we apply also the model to {}^{111}Cd diluted in RNi_2. Our self-consistent magnetic hyperfine field results are in good agreement with those recent experimental data.

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↗

arXiv2009

Effect of hybridization on the magnetic properties of correlated two-band metals

The magnetic properties of transition-like metals are discussed within the single site approximation, which is a picture to take into account electron correlations. The metal is described by two hybridized bands one of which includes Coulomb correlation. The presented results indicate that ferromagnetism arises for adequate values of hybridization (V), correlation (U) and occupation number($n_σ$). Some similarities with Dynamical Mean-Field Theory (DMFT) are indicated.

cond-mat.str-el↗

arXiv2009

Model for spin coupling disorder effects on the susceptibility of antiferromagnetic nanochains

The temperature dependence of the static magnetic susceptibility of exchange-disordered antiferromagnetic Heisenberg spin-1/2 finite chains with an odd number of spins is investigated as a function of size and type of disorder in the exchange coupling. Two models for the exchange disorder distribution are considered. At sufficiently low temperatures each chain behaves like an isolated spin-1/2 particle. As the size of the chains increases, this analogy is lost and the chains evolve into the thermodynamic limit behavior. The present study provides a simple criterion, based on susceptibility measurements, to establish when odd-sized chains effectively simulate a single spin-1/2 particle.

cond-mat.str-el↗

arXiv2009

Ferromagnetism in two band metals: Combined effect of Coulomb correlation, hybridization and band widths

We study the possibility of ferromagnetism in metals. The metal is described by two hybridized bands one of which includes Hubbard correlation whereas the other is uncorrelated. We parametrize the ratio of the band widths and their centers as well. The original Hamiltonian is transformed in an effective and simpler one. Only one site retains the full correlation (U) while in the others acts as an internal field, the self-energy, in the framework of an alloy analogy approximation. This field, in turn, is self-consistently determined by imposing the translational invariance of the problem. For several total electronic occupation numbers (n_{total}) we compare the spin dependent free energies with the corresponding paramagnetic ones. We present several results pointing out the mechanism by which the self-consistency introduces a sort of constraints, for given values of band width and band shift .

cond-mat.str-el↗

arXiv2009

On the use of a single site approximation to describe correlation in pure metals

The magnetic properties of pure transition-like metals are discussed within the single site approximation, to take into account the electron correlation. The metal is described by two hybridized bands one of which includes the Coulomb correlation. Our results indicate that ferromagnetism follows from adequate values of the correlation and hybridization.

cond-mat.str-el↗

arXiv2009

Cross effect of Coulomb correlation and hybridization in the occurrence of ferromagnetism in two shifted band transition metals

In this work we discuss the occurrence of ferromagnetism in transition-like metals. The metal is represented by two hybridized($V$) and shifted $(ε_s$) bands one of which includes Hubbard correlation whereas the other is uncorrelated. The starting point is to transform the original Hamiltonian into an effective one. Only one site retains the full correlation (U) while in the others the correlations are represented by an effective field, the self-energy(single-site approximation). This field is self-consistently determined by imposing the translational invariance of the problem. Thereby one gets an exchange split quasi-particle density of states and then an electron-spin polarization for some values of the parameters $(U,V, α, ε_s)$, $α$ being the ratio of the effective masses of the two bands and of the occupation number $n$.

cond-mat.str-el↗

arXiv2008

Specific heat in different magnetic phases of RNi2B2C (R= Gd, Ho, Er): theory and experiment

The borocarbides RNi2B2C (R=Gd, Ho, Er) exhibit a large variety of magnetic states and as a consequence rich phase diagrams. We have analyzed the nature of these states by specific heat investigations. The data were measured down to 0.5 K and up to 80 kOe. The overall evolution of each Cm(T,H) curve is observed to reflect faithfully the features of the corresponding H-T phase diagram. Within the lower ranges of temperature and fields, the calculations based on linearized field-dependent spin-wave theory are found to reproduce satisfactorily the measured Cm(T,H) curves: accordingly, within these ranges, the thermodynamical properties of these compounds can be rationalized in terms of only two parameters: the spin-wave energy gap and the stiffness coefficient. For the intermediate fields ranges (H1<H<Hsat) wherein successive field-induced metamagnetic modes are stabilized, the evolution of Cm(T,H) is discussed in terms of the Maxwell relation (dCm/dH)T=T(d^2M/dT^2)H. For the particular case of GdNi2B2C wherein the anisotropy is dictated by the classical dipole interaction, Cm(T,H) across the whole ordered state is numerically evaluated within the model of Jensen and Rotter [PRB 77 (2008) 134408].

cond-mat.str-el↗

arXiv2008

On the ferromagnetic structure of the intermetallic borocarbide TbCo2B2C

Based on magnetization, specific heat, magnetostriction, and neutron diffraction studies on single-crystal TbCo2B2C, it is found out that the paramagnetic properties, down to liquid nitrogen temperatures, are well described by a Curie-Weiss behavior of the Tb+3 moments. Furthermore, below Tc= 6.3 K, the Tb-sublattice undergoes a ferromagnetic (FM) phase transition with the easy axis being along the (100) direction and, concomitantly, the unit cell undergoes a tetragonal-to-orthorhhombic distortion. For fields up to 90 kOe, no field-induced splitting of the Co 3d orbitals was observed; as such the internal field must be well below the critical value needed to polarize the Co 3d subsystem. The manifestation of a FM state in TbCo2B2C is unique among all other isomorphous borocarbides, in particular TbNi2B2C (Tn=15 K, incommensurate modulated magnetic state) even though the Tb-ions in both isomorphs have almost the same crystalline electric field properties. The difference in the magnetic modes of these Tb-based isomorphs is attributed to a difference in their exchange couplings caused by a variation in their lattice parameters and in the position of their Fermi levels.

cond-mat.str-el↗

arXiv2006

The magnetic susceptibility of exchange-disordered antiferromagnetic finite chains

The low-temperature behavior of the static magnetic susceptibility $χ(T)$ of exchange-disordered antiferromagnetic spin chains is investigated. It is shown that for a relatively small and even number of spins in the chain, two exchange distributions which are expected to occur in nanochains of P donors in silicon lead to qualitatively distinct behaviors of the low-temperature susceptibility. As a consequence, magnetic measurements might be useful to characterize whether a given sample meets the requirements compatible with Kane's original proposalfor the exchange gates in a silicon-based quantum computer hardware. We also explore the dependence of $χ(T)$ on the number of spins in the chain as it increases towards the thermodynamic limit, where any degree or distribution of disorder leads to the same low-temperature scaling behavior. We identify a crossover regime where the two distributions of disorder may not be clearly differentiated, but the characteristic scaling of the thermodynamic limit has not yet been reached.

cond-mat.stat-mech↗