Papers by Francisco Rivadulla
arXiv (Cornell University), Mar 29, 2004
We report the divergence of the nonlinear component of the bulk susceptibility in Na x CoO 2 (0.3... more We report the divergence of the nonlinear component of the bulk susceptibility in Na x CoO 2 (0.3<x<0.62) as T→0K. These experiments provide an striking evidence of the existence of a ferromagnetic phase transition at zero Kelvin (quantum phase transition). The possible role of magnetic fluctuations in the superconductivity is discussed to the light of the observed (H,T) scaling of the magnetization, which implies a local character of the fluctuations. The breakdown of Fermi-Liquid theory in the paramagnetic phase of itinerant ferromagnets 1,2 and elemental paramagnets 3 , as well as unconventional forms of magnetically mediated superconductivity in heavy fermions 4,5 , have been related to the
arXiv (Cornell University), Mar 1, 2005
The technological impact of combining substrate technologies with the properties of functional ad... more The technological impact of combining substrate technologies with the properties of functional advanced oxide ceramics is colossal given its relevant role in the development of novel and more efficient devices. However the precise control of interfaces and crystallization mechanisms of dissimilar materials at the nanoscale needs to be further developed. As an example, the integration of hybrid structures of high quality epitaxial oxide films and nanostructures on silicon as remains extremely challenging because these materials present major chemical, structural and thermal differences. This book chapter describes the main promising strategies that are being used to accommodate advanced oxide nanostructured ceramics on different technological substrates via chemical solution deposition approaches. We will focus on novel examples separated in two main sections: (i) epitaxial ceramic nanomaterials entirely performed by soft chemistry, such as nanostructured piezoelectric quartz thin fi...
ACS applied materials & interfaces, Jan 3, 2018
Understanding diffusion of oxygen vacancies in oxides under different external stimuli is crucial... more Understanding diffusion of oxygen vacancies in oxides under different external stimuli is crucial for the design of ion-based electronic devices, improvement of catalytic performance, and so forth. In this manuscript, using an external electric field produced by an atomic force microscopy tip, we obtain the room-temperature diffusion coefficient of oxygen-vacancies in thin films of SrTiO under compressive/tensile epitaxial strain. Tensile strain produces a substantial increase of the diffusion coefficient, facilitating the mobility of vacancies through the film. Additionally, the effect of tip bias, pulse time, and temperature on the local concentration of vacancies is investigated. These are important parameters of control in the production and stabilization of nonvolatile states in ion-based devices. Our findings show the key role played by strain for the control of oxygen vacancy migration in thin-film oxides.
Nano Letters, 2016
We report the effect of interface symmetry-mismatch on the magnetic properties of LaCoO 3 (LCO) t... more We report the effect of interface symmetry-mismatch on the magnetic properties of LaCoO 3 (LCO) thin films. Growing epitaxial LCO under tensile strain on top of cubic SrTiO 3 (STO) produces a contraction along the c-axis, and a characteristic ferromagnetic response. However, we report here that ferromagnetism in LCO is completely suppressed when grown on top of a buffer layer of rhombohedral La 2/3 Sr 1/3 MnO 3 (LSMO), in spite of identical in-plane and out-of-plane lattice deformation. This confirms that it is the lattice symmetry mismatch and not just the total strain, which determines the magnetism of
Frontiers in Physics, 2015
The combination of standard wafer-scale semiconductor processing with the properties of functiona... more The combination of standard wafer-scale semiconductor processing with the properties of functional oxides opens up to innovative and more efficient devices with high value applications which can be produced at large scale. This review uncovers the main strategies that are successfully used to monolithically integrate functional complex oxide thin films and nanostructures on silicon: the chemical solution deposition approach (CSD) and the advanced physical vapor deposition techniques such as oxide molecular beam epitaxy (MBE). Special emphasis will be placed on complex oxide nanostructures epitaxially grown on silicon using the combination of CSD and MBE. Several examples will be presented, with a particular stress on the control of interfaces and crystallization mechanisms on epitaxial perovskite oxide thin films, nanostructured quartz thin films, and octahedral molecular sieve nanowires. This review enlightens on the potential of complex oxide nanostructures and the combination of both chemical and physical elaboration techniques for novel oxide-based integrated devices.
We report specific heat, resistivity and susceptibility measurements at different temperatures, m... more We report specific heat, resistivity and susceptibility measurements at different temperatures, magnetic fields, and pressures to provide solid evidence of CoS2 being a marginal Fermi liquid. The presence of a tricritical point in the phase diagram of the system provides an opportunity to test the spin fluctuation theory with a high limit of accuracy. A magnetic field suppresses the amplitude
Apl Materials, 2013
We propose a novel route for optimizing the thermoelectric power of a polaronic conductor, indepe... more We propose a novel route for optimizing the thermoelectric power of a polaronic conductor, independent of its electronic conductivity. This mechanism is exemplified here in thin-films of La 2 NiO 4+δ. Tensile stress induced by epitaxial growth on SrTiO 3 doubles the thermoelectric power of ≈15 nm thick films relative to ≈90 nm films, while the electronic conductivity remains practically unchanged. Epitaxial strain influences the statistical contribution to the high temperature thermopower, but introduces a smaller correction to the electronic conductivity. This mechanism provides a new way for optimizing the high temperature thermoelectric performance of polaronic conductors.
Physical Review B, 1998
We have performed electron spin resonance ͑ESR͒ and dc susceptibility measurements in Mn perovski... more We have performed electron spin resonance ͑ESR͒ and dc susceptibility measurements in Mn perovskites up to 1000 K. Assuming an effective Heisenberg-like interaction for Mn 3ϩ ϪMn 4ϩ spin pairs, the dc susceptibility, dc (T), is well described in the paramagnetic regime by the constant coupling approximation. Absolute determination of the ESR intensity indicates that all Mn spins contribute to the ESR line in the temperature range studied. The ESR linewidth can be described by ⌬H pp (T)ϭ⌬H pp (ϱ) ͓C/T dc (T)͔, thus presenting a universal behavior in a temperature scale normalized to T c. A single relaxation mechanism, related to spinonly interactions, explains the T dependence of ⌬H pp
Physical Review Letters, 2009
We report x-ray diffraction and magnetization measurements under pressure combined with ab initio... more We report x-ray diffraction and magnetization measurements under pressure combined with ab initio calculations to show that high-pressure TiOCl corresponds to an enhanced Ti 3þ-Ti 3þ dimerized phase existing already at room temperature. Our results demonstrate the formation of a metal-metal bond between Ti 3þ ions along the b axis of TiOCl, accompanied by a strong reduction of the electronic gap. The evolution of the dimerization with pressure suggests a crossover from the spin-Peierls to a conventional Peierls situation at high pressures.
Physical Review Letters, 2004
In this Letter we demonstrate that the phase segregated state observed in many rare-earth perovsk... more In this Letter we demonstrate that the phase segregated state observed in many rare-earth perovskites constitutes a sort of self-generated assembly of magnetic clusters in which magnetic interaction introduces collectivity among them. We show that the observed glassy behavior (memory, aging, etc.) can be perfectly understood taking into account only the intercluster interactions. We address the fundamental question about whether this state constitutes classical spin glass or if, on the other hand, a new universality class must be defined.
Physical Review Letters, 2003
MnAs exhibits a first-order phase transition from a ferromagnetic, high-spin metal NiAs-type hexa... more MnAs exhibits a first-order phase transition from a ferromagnetic, high-spin metal NiAs-type hexagonal phase to a paramagnetic, lower-spin insulator MnP-type orthorhombic phase at TC = 313 K. Here, we report the results of neutron diffraction experiments showing that an external magnetic field, B, stabilizes the hexagonal metallic phase above TC. The phase transformation is reversible and constitutes the first demonstration of a bond-breaking transition induced by a magnetic field. At 322 K the hexagonal structure is restored for B > 4 tesla. The field-induced phase transition is accompanied by an enhanced magnetoresistance of about 17 % at 310 K. We discuss the origin of this phenomenon, which appears to be similar to that of the colossal magnetoresistance response observed in some members of the manganese perovskite family.
Physical Review Letters, 2007
We report a systematic enhancement of the pressure dependence of T N in A 2 V 2 O 4 spinels as th... more We report a systematic enhancement of the pressure dependence of T N in A 2 V 2 O 4 spinels as the V-V separation approaches the critical separation for a transition to itinerant-electron behavior. An intermediate phase between localized and itinerant-electron behavior is identified in ZnV 2 O 4 and MgV 2 O 4 exhibiting mobile holes as large polarons. Partial electronic delocalization, cooperative ordering of V-V pairs in ZnV 2 O 4 below T s T N and dT N =dP < 0, signals that lattice instabilities associated with the electronic crossover are a universal phenomenon.
Physical Review Letters, 2008
Physical Review Letters, 2014
We study vanadium spinels AV2O4 (A = Cd, Mg) in pulsed magnetic fields up to 65 T. A jump in magn... more We study vanadium spinels AV2O4 (A = Cd, Mg) in pulsed magnetic fields up to 65 T. A jump in magnetization at µ0H ≈ 40 T is observed in the single-crystal MgV2O4, indicating a field induced quantum phase transition between two distinct magnetic orders. In the multiferroic CdV2O4, the field-induced transition is accompanied by a suppression of the electric polarization. By modeling the magnetic properties in the presence of strong spin-orbit coupling characteristic of vanadium spinels, we show that both features of the field-induced transition can be successfully explained by including the effects of the local trigonal crystal field.
Physical Review B, 2010
We use a Monte Carlo approach to simulate the influence of the dipolar interaction on assemblies ... more We use a Monte Carlo approach to simulate the influence of the dipolar interaction on assemblies of monodisperse superparamagnetic γ-Fe 2 O 3 nanoparticles. We have identified a critical concentration c*, that marks the transition between two different regimes in the evolution of the blocking temperature (T B) with interparticle interactions. At low concentrations (c < c*) magnetic particles behave as an ideal noninteracting system with a constant T B. At concentrations c > c* the dipolar energy enhances the anisotropic energy barrier and T B increases with increasing c, so that a larger temperature is required to reach the superparamagnetic state. The fitting of our results with classical particle models and experiments supports the existence of two differentiated regimes. Our data could help to understand apparently contradictory results from the literature.
Physical Review B, 2010
We report a high-energy inelastic neutron-scattering study on Sr 2 VO 4 that shows a transition f... more We report a high-energy inelastic neutron-scattering study on Sr 2 VO 4 that shows a transition from an orbital liquid phase at high temperature to an orbital-order phase at low temperature with an unquenched orbital angular momentum. Susceptibility under pressure shows a collapse of the magnetic phase transition due to enhanced orbital fluctuations. One possible explanation for our data is the recently proposed magnetically hidden order of Kramers doublets in d 1 electron systems.
Physical Review B, 2011
By studying the dielectric properties of the geometrically frustrated spinel CdV 2 O 4 , we obser... more By studying the dielectric properties of the geometrically frustrated spinel CdV 2 O 4 , we observe ferroelectricity developing at the transition into the collinear antiferromagnetic ground state. In this multiferroic spinel, ferroelectricity is driven by local exchange striction and not by the more common scenario of spiral magnetism. The experimental findings are corroborated by ab initio calculations of the electric polarization and the underlying spin and orbital order. The results point toward a charge rearrangement due to dimerization, where electronic correlations and the proximity to the insulator-metal transition play an important role.
Physical Review B, 2012
The magnetic, transport, and structural properties of cold-pressed Ba 3 CoRu 2 O 9 show (i) an an... more The magnetic, transport, and structural properties of cold-pressed Ba 3 CoRu 2 O 9 show (i) an antiferromagnetic transition, (ii) a semiconductor-semiconductor electronic transition where the resistivity is dominated by the electron hopping between the Ru 2 5+ O 9 dimers with itinerant Ru electrons at high temperatures, and (iii) a hexagonal to orthorhombic structural phase transition. All three transitions occur at 93 K and are related to the Ru 5+ ions in the Ru 2 O 9 dimers. The Ru-O bond distortion below 93 K further indicates a possible orbital ordering for Ru 5+ ions in isolated Ru 2 O 9 dimers, which accounts for the strong orbital, charge, and spin couplings.
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Papers by Francisco Rivadulla