arXiv subjects

Cecilia I. Ventura

Publications and source records attributed to Cecilia I. Ventura.

3 recordsLinked to original sources

arXiv2018

Spin excitations of half-doped bilayer manganites: intermediate phase

The ground state of half-doped manganites involves intricate spin, charge and orbital orderings, which are difficult to discern experimentally. In this work, we resort to the theoretical analysis of the spin fluctuation spectrum of the half-doped bilayer Pr(Ca$_{0.9}$Sr$_{0.1}$)$_2$ Mn$_2$O$_7$ in order to get an insight of its electronic ground state. By means of the linear spin wave approximation, we compute the magnon dispersion for a family of localized spin models, which can describe several phases proposed for the ground state of half-doped manganites, like the intermediate one proposed by Efremov et al. [Nat.Mats. 3, 853, (2004)] along with its particular cases corresponding to Goodenough's CE phase [Phys. Rev. 100, 564, (1955)] and Zener polaron or dimer phases. We obtain an excellent agreement between theory and experiment when the ground state is assumed to be a generalized Goodenough's CE phase, with a Mn-charge disproportionation inside the experimentally expected range. As essential ingredients for our improved fit of the upper and lower magnon branches measured around the gap, we identified two next-nearest neighbour exchange interactions between the planar Mn zig-zag chains, one for each type of Mn ion present. In connection with this finding, we revisited the magnetic excitations of the laminar related compounds, focusing on the upper magnon branches. Here we prove that their measurement would provide the key to identify unambiguously the ground state present in the layered half-doped manganites.

cond-mat.str-el↗

arXiv2015

Magnetic excitations of perovskite rare-earth nickelates: RNiO$_3$

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$^{(3-δ)+}$ and Ni$^{(3+δ)+}$, though agreement has not been reached on the precise value of $δ$ (e.g. for NdNiO$_3$, $δ=0.0$ and $δ=0.29$ 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 ($θ=π/2$) phases, we predict differences in the magnon spectrum which would allow to distinguish between them in future inelastic neutron scattering experiments.

cond-mat.str-el↗

arXiv2013

Quantum magnons of the intermediate phase of half-doped manganite oxides

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 $Φ$ between them. Varying Mn-charge disproportionation and $Φ$, 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 $π/2-$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 $π/2$ phases, in the context of recent experimental results.

cond-mat.str-el↗