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Search Results: 1 - 10 of 93496 matches for " Cecilia I. Ventura "
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Magnetic excitations of perovskite rare-earth nickelates: RNiO$_3$
Ivon R. Buitrago,Cecilia I. Ventura
Physics , 2015, DOI: 10.1016/j.jmmm.2015.06.056
Abstract: The perovskite nickelates RNiO$_3$ (R: rare-earth) have been studied as potential multiferroic compounds. A certain degree of charge disproportionation in the Ni ions has been confirmed by high resolution synchrotron power diffraction: instead of the nominal Ni$^{3+}$ valence, they can have the mixed-valence state Ni$^{(3-\delta)+}$ and Ni$^{(3+\delta)+}$, though agreement has not been reached on the precise value of $\delta$ (e.g. for NdNiO$_3$, $\delta=0.0$ and $\delta=0.29$ were reported). Also, the magnetic ground state is not yet clear: collinear and non-collinear Ni-O magnetic structures have been proposed to explain neutron diffraction and soft X-ray resonant sccattering results in these compounds, and more recently a canted antiferromagnetic spin arrangement was proposed on the basis of magnetic susceptibility measurements. This scenario is reminiscent of the situation in the half-doped manganites. In order to gain insight into the ground state of these compounds, we studied the magnetic excitations of some of the different phases proposed, using a localized spin model for a simplified spin chain which could describe these compounds. We first analize the stability of the collinear, orthogonal, and intermediate phases in the classical case. We then explore the quantum ground state indirectly, calculating the spin excitations obtained for each phase, using the Holstein-Primakoff transformation and the linear spin-wave approximation. For the collinear and orthogonal ($\theta=\pi/2$) phases, we predict differences in the magnon spectrum which would allow to distinguish between them in future inelastic neutron scattering experiments.
Quantum magnons of the intermediate phase of half-doped manganite oxides
Ivon R. Buitrago,Cecilia I. Ventura,Luis O. Manuel
Physics , 2013, DOI: 10.1109/TMAG.2013.2261417
Abstract: At half doping, the ground state of three-dimensional manganite perovskite oxides like R$_{1-x}$Ca$_x$MnO$_3$, where R is a trivalent ion such as La, Pr, etc, is still unclear. Many experimental findings agree better with the combined magnetic, charge, and orbital order characteristic of the "intermediate phase", introduced by Efremov et al. in 2004 [Nature Mats. 3, 853]. This phase consists of spin dimers (thus incorporating aspects of the Zener polaron phase (ZP) proposed in 2002 by Daoud-Aladine et al. [Phys. Rev. Lett. 89, 097205]), though formed by a pair of parallel Mn spins of different magnitude, in principle (thereby allowing for a degree of Mn charge disproportionation: not necessarily as large as that of Mn$^{3+}$-Mn$^{4+}$ in Goodenough's original CE phase [Phys. Rev. 100, 564 (1955)]). In the intermediate phase, consecutive spin dimers localed along the planar zig-zag chains are oriented at a constant relative angle $\Phi$ between them. Varying Mn-charge disproportionation and $\Phi$, the intermediate phase should allow to continuously interpolate between the two limiting cases of the CE phase and the dimer phase denoted as "orthogonal intermediate $\pi/2-$phase". It is not easy to find a microscopic model able to describe the phenomenological intermediate phase adequately for the spin, charge, and orbital degrees of freedom simultaneously. Here, we study the quantum spin excitations of a planar model of interacting localized spins, which we found can stabilize the intermediate phase classically. We compare the quantum magnons of the intermediate phase with those of the CE and orthogonal $\pi/2$ phases, in the context of recent experimental results.
Spin excitations of half-doped bilayer manganites: intermediate phase
Ivon R Buitrago,Cecilia I. Ventura,Luis O. Manuel
Physics , 2014,
Abstract: Few magnon measurements in half-doped manganites are available. Recently, the spin-wave spectrum of half-doped bilayer Pr(Ca0.9Sr0.1)2Mn2O7 was reported (Johnstone et al 2012 Phys.Rev.Lett.109 237202). The best fit included, using a localized spin model, corresponds to a ground state with a combined spin, charge and orbital ordering analogous to the CE phase proposed by Goodenough in 1955, though with a quite larger Mn-charge disproportionation. The latter contradicts previous experimental indications in half-doped manganites, pointing toward smaller Mn-disproportionations: in a range between the minimum value corresponding to the Zener-polaron or dimer phase (with all Mn-charges formally equal to 3.5+), and the maximum disproportionation found in Goodenough's CE phase (with Mn3+ and Mn4+). Such indications led to the intermediate phase proposal (Efremov et al 2004 Nat.Mats.3 853). Here, we analize the spin excitations of half-doped bilayer (and layered) manganites, calculating them in terms of a localized spin model suitable to describe the intermediate, CE and dimer phases. We discuss the effects of an extensive set of magnetic couplings, including all those introduced in previous fits. An improved description of the magnons reported for bilayer manganites is obtained assuming a generalized CE phase with Mn-disproportionation inside the expected range, by including next-nearest-neighbour magnetic coupling parameters between the planar Mn zig-zag chains.
Objetos rituales e identidad
Monserrat Ventura i Oller
Anthropologica , 2000,
Abstract: El chamanismo especialmente en las sociedades indígenas de América del Sur, se ha caracterizado por un complejo sistema de intercambio interétnico material y simbólico. Como parte de este sistema, los objetos utilizados en los rituales chamánicos no pueden ser interpretados a partir del significado que les otorga cada cultura particular de la que son originarios. Sino que deben ser analizados a partir del significado que cada cultura receptora da a su mundo sobrenatural, que es usualmente representado por formas materiales variables y diversas. Este tema será ilustrado con la etnografía del grupo indígena Tsachila del Ecuador, que despliega una extensa y compleja red de intercambios chamánicos.
Analyzing the Division of Household Labor within Spanish Families
Balcell s i Ventura, Laia
Revista Internacional de Sociologia , 2009,
Abstract: Do all women experience a labor “double burden”? Are women responsible for domestic tasks in all households? What explains variation in patterns of distribution of household labor across families? In this article, I approach these questions from the perspective of two opposing theoretical approaches: the New Home Economics framework, and the Social Capital or sociological framework. I test the validity of these two frameworks with data from the Spanish Fertility and Family Survey of 1995. The results point towards the validity of the sociological perspective: those families in which the husbands have medium labor status present more egalitarian patterns of household labor distribution. Additionally, the results confirm the importance of attitudinal variables such as religiosity to explain domestic labor division patterns: women with more traditional values experience a more severe double burden. Experimentan las mujeres una doble carga laboral? Son las mujeres responsables de las tareas domésticas en los hogares? Qué explica la variación en patrones de distribución de trabajo doméstico que tiene lugar a través de las familias? En este artículo se abordan estas cuestiones partiendo de dos marcos teóricos opuestos: el marco de la New Home Economics, y el del Capital Social, que se ponen a prueba a partir de datos de la Encuesta de Fecundidad y Familia de 1995. Los resultados apuntan hacia la validez de la perspectiva sociológica: las familias donde el marido tiene un estatus laboral medio presentan patrones más equitativos de distribución del trabajo doméstico. Adicionalmente, los resultados confirman la importancia de variables actitudinales como la religiosidad para explicar patrones de división del trabajo doméstico: las mujeres más tradicionales son las que experimentan una mayor doble carga.
Renormalized spin wave excitations in the antiferromagnetic Heisenberg-Kondo model for heavy fermions
M. Acquarone,C. I. Ventura,.
Physics , 2007,
Abstract: Recent inelastic neutron scattering experiments in CeIn3 and CePd2Si2 single crystals, measured spin wave excitations at low temperatures. These two heavy fermion compounds exhibit antiferromagnetic long-range order, but a strong competition between the Ruderman-Kittel-Kasuya-Yosida(RKKY) interaction and Kondo effect is evidenced by their nearly equal Neel and Kondo temperatures. Our aim is to show how magnons such as measured in the antiferromagnetic phase of these Ce compounds, can be described with a microscopic Heisenberg-Kondo model as introduced by J.R.Iglesias, C.Lacroix and B.Coqblin, used before for studies of the non-magnetic phase. The model includes the correlated Ce-4f electrons hybridized with the conduction band, and we consider competing RKKY (Heisenberg-like J_{H}) and Kondo (J_{K}) antiferromagnetic couplings. Carrying on a series of unitary transformations, we perturbatively derive a second-order effective Hamiltonian which, projected onto the antiferromagnetic electron ground state, describes the spin wave excitations, renormalized by their interaction with correlated itinerant electrons. We numerically study how the different parameters of the model influence the renormalization of the magnons, yielding useful information for the analysis of inelastic neutron scattering experiments in antiferromagnetic heavy fermion compounds. We also compare our results with available experimental data, finding good agreement with the spin wave measurements in cubic CeIn3.
Spin wave excitations in the antiferromagnetic Heisenberg-Kondo model for heavy fermions
M. Acquarone,C. I. Ventura
Physics , 2008,
Abstract: Recent inelastic neutron scattering experiments in CeIn$_{3}$ and CePd$_{2}$Si$_{2}$ single crystals measured spin wave excitations at low temperatures. These two heavy fermion compounds exhibit antiferromagnetic long-range order, but a strong competition between the Ruderman-Kittel-Kasuya-Yosida(RKKY) interaction and Kondo effect is evidenced by their nearly equal N\'eel and Kondo temperatures. Our aim is to show how magnons such as measured in the antiferromagnetic phase of these Ce compounds, can be described with a microscopic Heisenberg-Kondo model introduced by J.R.Iglesias, C.Lacroix and B.Coqblin, used before for studies of the non-magnetic phase. The model includes the correlated Ce-$4 f$ electrons hybridized with the conduction band, where we also allow for correlations, and we consider competing RKKY (Heisenberg-like $J_{H} $) and Kondo ($J_{K}$) antiferromagnetic couplings. Carrying on a series of unitary transformations, we perturbatively derive a second-order effective Hamiltonian which, projected onto the antiferromagnetic electron ground state, describes the spin wave excitations, renormalized by their interaction with correlated itinerant electrons. We numerically study how the different parameters of the model influence the renormalization of the magnons, yielding useful information for the analysis of inelastic neutron scattering experiments in antiferromagnetic heavy fermion compounds. We also compare our results with the available experimental data, finding good agreement with the spin wave measurements in cubic CeIn$_3$.
Magnons in CMR pyrochlore Tl2Mn2O7
C. I. Ventura,M. Acquarone
Physics , 2003,
Abstract: Well defined spin waves were observed when the spin dynamics of Tl2Mn2O7, the first pyrochlore compound found to exhibit colossal magnetoresistance, was measured [J.W.Lynn et al., Phys.Rev.Lett. 80,4582(1998)], in stark contrast with the experimental results on the larger family of magnetoresistive manganites with perovskite structure. In this work, we present our calculation for the spin waves in Tl2Mn2O7, which we described using the microscopic generic model proposed recently for this compound [C.I.Ventura and M.A.Gusmao, Phys.Rev.B 65, 14422(2002)]. We have employed a canonical transformation to determine perturbatively the effective spin-wave Hamiltonian, obtaining therefrom the renormalization of the ferromagnetic spin waves related to the localized Mn$^{4+}$ spins, due to their coupling with the conduction electrons present. We have calculated the magnon dispersion relations along different paths in the first Brillouin zone, comparing them with those which are obtained for an ideal isotropic ferromagnet. This comparison evidences an agreement between the ferromagnetic magnons obtained from the generic model and the bare spin waves, such as had been found in neutron scattering experiments.
Intermediate Valence Model for the Colossal Magnetoresistance in Tl_{2}Mn_{2}O_{7}
C. I. Ventura,B. Alascio
Physics , 1997, DOI: 10.1103/PhysRevB.56.14533
Abstract: The colossal magnetoresistance exhibited by Tl_{2}Mn_{2}O_{7} is an interesting phenomenon, as it is very similar to that found in perovskite manganese oxides although the compound differs both in its crystalline structure and electronic properties from the manganites. At the same time, other pyrochlore compounds, though sharing the same structure with Tl_{2}Mn_{2}O_{7}, do not exhibit the strong coupling between magnetism and transport properties found in this material. Mostly due to the absence of evidence for significant doping into the Mn-O sublattice, and the tendency of Tl to form conduction bands, the traditional double exchange mechanism mentioned in connection with manganites does not seem suitable to explain the experimental results in this case. We propose a model for Tl_{2}Mn_{2}O_{7} consisting of a lattice of intermediate valence ions fluctuating between two magnetic configurations, representing Mn-3d orbitals, hybridized with a conduction band, which we associate with Tl. This model had been proposed originally for the analysis of intermediate valence Tm compounds. With a simplified treatment of the model we obtain the electronic structure and transport properties of Tl_{2}Mn_{2}O_{7}, with good qualitative agreement to experiments. The presence of a hybridization gap in the density of states seems important to understand the reported Hall data.
Magnons in the half-doped manganites
C. I. Ventura,B. Alascio
Physics , 2003, DOI: 10.1103/PhysRevB.68.020404
Abstract: Recently, based on the refined crystal structure of Pr0.6Ca0.4MnO3 from neutron diffraction, Daoud-Aladine et al.[PRL89,97205(2002)] have proposed a new ground state structure for the half-doped manganites R0.5Ca0.5MnO3, where R is a trivalent ion like Bi,La,Pr,Sm or Y. Their proposal describes the CE magnetic structure attributed to these materials as an arrangement of dimers along the ferromagnetic Mn zig-zag chains that form it. However, the dimers' proposal is in conflict with the Goodenough-Kanamori-Anderson rules, which give a coherent description of many transition metal insulating compounds and predict the coexistence of Mn3+ and Mn4+ ions in equal parts in the half-doped manganites. On the other hand, Rivadulla et al.[PRB 66, 174432 (2002)] have studied several single crystal samples of half-doped manganites and propose a phase diagram in terms of the tolerance factor which contains both types of structures. In the present work we have calculated the magnon dispersion relations for the CE magnetic structure, arising for each type of proposal: the charge ordered and the dimer phases, respectively. We consider a three-dimensional unit cell containing 16 spins, and compare the magnetic excitations along different paths in the first Brillouin zone. We conclude that measurement of the magnon dispersion relations should allow a clear distinction between the two proposals, predicting qualitative differences arising along specific directions of propagation in the first Brillouin zone.
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