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Accessing the regime of coherent phonon propagation in nanostructures opens enormous possibilities to control the thermal conductivity in energy harvesting devices, phononic circuits, etc. In this paper we show that coherent phonons contribute substantially to the thermal conductivity of LaCoO3/SrTiO3 oxide superlattices, up to room temperature. We show that their contribution can be tuned through small variations of the superlattice periodicity, without changing the total superlattice thickness. Using this strategy, we tuned the thermal conductivity by 20% at room temperature. We also discuss the role of interface mixing and epitaxial relaxation as an extrinsic, material dependent key parameter for understanding the thermal conductivity of oxide superlattices.
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We present the analysis of the thermal conductivity, κ, and heat capacity, Cp, of a wide variety of liquids, covering organic molecular solvents, ionic liquids and water-polymer mixtures. These data were obtained from ≈0.6 µL samples, using an experimental development based on the 3ω method, capable of the simultaneous measurement of κ and Cp. In spite of the different type and strength of interactions, expected in a priori so different systems, the ratio of κ to the sound velocity is approximately constant for all of them. This is the consequence of a similar atomic density for all these liquids, notwithstanding their different molecular structures. This was corroborated experimentally by the observation of a Cp/V ≈ 1.89 × 106 J K-1 m-3 (≈3R/2 per atom), for all liquids studied in this work. Finally, the very small volume of the sample required in this experimental method is an important advantage for the characterization of systems like nanofluids, in which having a large amount of the dispersed phase is sometimes extremely challenging.
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We report thermoelectric power experiments in e-doped thin films of SrTiO3 (STO) which demonstrate that the electronic band degeneracy can be lifted through defect management during growth. We show that even small amounts of cationic vacancies, combined with epitaxial stress, produce a homogeneous tetragonal distortion of the films, resulting in a Kondo-like resistance upturn at low temperature, large anisotropic magnetoresistance, and nonlinear Hall effect. Ab initio calculations confirm a different occupation of each band depending on the degree of tetragonal distortion. The phenomenology reported in this Letter for tetragonally distorted e-doped STO thin films, is similar to that observed in LaAlO3/STO interfaces and magnetic STO quantum wells.
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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.
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In this work we show that the electrical conductivity and thermoelectric power of random networks of carbon nanotube (CNT) films can be tuned by means of a layer-by-layer polymer coating technique of the individual nanotubes. Using this approach, nanotubes dispersed in water are coated before the formation of the film, achieving a control of the transport properties independent of the substrate. Below a certain temperature the conductivity departs from the ohmic behavior and enters a tunneling regime with an energy barrier for electron transport determined by the width of the polymer layer. The excellent control over the conductivity and the possibility of using the polymer coating as a gate dielectric material could be an important step for the development of an easy and scalable approach for the fabrication of CNT-based thin film transistors.
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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 Ti3+-Ti3+ dimerized phase existing already at room temperature. Our results demonstrate the formation of a metal-metal bond between Ti3+ 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.
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Electronic structure calculations for spinel vanadate ZnV2O4 show that partial electronic delocalization in this system leads to a structural instability, with the formation of V-V dimers along the [011] and [101] directions, and readily accounts for the intriguing magnetic structure of this material. The formation of V-V bonds is a consequence of the proximity to the itinerant-electron boundary and is not related to orbital ordering. We discuss how this mechanism naturally couples charge and lattice degrees of freedom in magnetic insulators close to such a crossover.
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Gamma-Fe2O3 nanoparticles have been synthesized and dispersed in a polymeric matrix, forming a series of composites with different concentrations of magnetic particles. The effect of volume polydispersity and dipolar interactions on the relaxation behavior is discussed. We have paid special attention to the dynamic approach to discuss a possible true superspin-glass transition in highly concentrated composites. To avoid the practical limitations that appear in highly concentrated systems of particles, like the formation of aggregates, etc., we have studied the glassy phase that appears spontaneously in certain strongly electronic correlated materials close to a metal-insulator transition. It must be emphasized that from a theoretical point of view these inhomogenous magnetic states could present important advantages over classical dispersions of particles, like field-control of the effective particle size. The results are compared with other recently obtained for classical systems of particles.
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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 Zn[V(2)]O(4) and Mg[V(2)]O(4) exhibiting mobile holes as large polarons. Partial electronic delocalization, cooperative ordering of V-V pairs in Zn[V(2)]O(4) below T(s) approximately T(N) and dT(N)/dP<0, signals that lattice instabilities associated with the electronic crossover are a universal phenomenon.
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From Raman spectroscopy, magnetization, and thermal expansion on the system La(2/3)(Ca(1-x)Sr(x))(1/3)MnO3, we have been able to provide a quantitative basis for the heterogeneous electronic model for manganites exhibiting colossal magnetoresistance (CMR). We construct a mean-field model that accounts quantitatively for the measured deviation of T(C)(x) from the T(C) predicted by de Gennes double-exchange in the adiabatic approximation and predicts the occurrence of a first-order transition for a strong coupling regime, in accordance with the experiments. The existence of a temperature interval T(C) < T < T*, where CMR may be found, is discussed in connection with the occurrence of an idealized Griffiths phase.
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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.
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MnAs exhibits a first-order phase transition from a ferromagnetic, high-spin metal hexagonal phase to a paramagnetic, lower-spin insulator orthorhombic phase at T(C)=313 K. Here, we report the results of neutron diffraction experiments showing that an external magnetic field, B, stabilizes the hexagonal phase above T(C). The phase transformation is reversible and constitutes the first demonstration of a bond-breaking transition induced by a magnetic field. The field-induced phase transition is accompanied by an enhanced magnetoresistance of about 17% at 310 K. The phenomenon appears to be similar to that of the colossal magnetoresistance response observed in the Mn [corrected] perovskite family.
