Polymer-assisted deposition of epitaxial oxide thin films

Traballo de Fin de Grao en Química. Curso 2014-2015The possibility of growing epitaxial thin-films has been intensely studied during the last decades. The development of the physical deposition techniques, such as Pulsed Laser Deposition (PLD), Sputtering or Molecular Beam Epitaxy (MBE) made possible the production films and multilayers of very high quality, although they have the serious drawback of the huge price of the equipment, particularly for small-laboratory basic research. For this reason, there is a great interest in developing more cost-effective chemical methods of deposition, which however must produce films of a quality similar to the physical techniques to meet the requirements of demanding applications. In this work we use a chemical method to grow metal-oxide thin films of a structural and morphological quality which is similar to that obtained by traditional physical high-vacuum methods. The process basis is the preparation of solutions containing the metal-cations precursors of the thin film; a soluble metal salt, a complexing agent and a polymer for retaining the cations are the basic components of the precursor solutions used in this study. By spin-coating the solutions over commercial monocrystalline substrates (like (001) oriented SrTiO3) and after a thermal treatment, thin-films of the materials of interest are obtained. A good control over the thickness and stoichiometry can be achieved through the control of the initial concentration of the solutions and the gaseous atmosphere during the annealing. In this study we obtained thin films epitaxially grown of BiFeO3, SrRuO3, La0.7Sr0.3MnO3 and a bilayer of BiFeO3/La0.7Sr0.3MnO3. These compositions were selected due to their scientific and technologic relevance. The structure and morphological characterization was carried out by X-ray and microscopy techniques. The electric and magnetic properties are also investigated to study the effect of the low dimensionality and epitaxial strain in these materials

Anomalous and planar Nernst effects in thin films of the half-metallic ferromagnet La2/3Sr1/3 MnO3

We report the planar and anomalous Nernst effects in epitaxial thin films of spin polarized La2/3Sr1/3MnO3. The thermal counterpart of the anomalous Hall effect in this material (i.e., the anomalous Nernst effect) shows an extreme sensitivity to any parasitic thermal gradient, resulting in large asymmetric voltages under small temperature differences. This should be considered when interpreting the magnitude of the electrical response in nanostructures and devices that operate under high current densities. Finally, none of the observed magnetothermoelectric signals is related to the spin Seebeck effect in this materialThis research was supported by the European Research Council (ERC StrG-259082, 2DTHERMS) and Xunta de Galicia (2012-CP071)S

Suppression of V1 feedback produces a shift in the topographic representation of receptive fields of LGN cells by unmasking latent retinal drives

[Abstract] In awake monkeys, we used repetitive transcranial magnetic stimulation (rTMS) to focally inactivate visual cortex while measuring the responsiveness of parvocellular lateral geniculate nucleus (LGN) neurons. Effects were noted in 64/75 neurons, and could be divided into 2 main groups: (1) for 39 neurons, visual responsiveness decreased and visual latency increased without apparent shift in receptive field (RF) position and (2) a second group (n = 25, 33% of the recorded cells) whose excitability was not compromised, but whose RF position shifted an average of 4.5°. This change is related to the retinotopic correspondence observed between the recorded thalamic area and the affected cortical zone. The effect of inactivation for this group of neurons was compatible with silencing the original retinal drive and unmasking a second latent retinal drive onto the studied neuron. These results indicate novel and remarkable dynamics in thalamocortical circuitry that force us to reassess constraints on retinogeniculate transmission

Thermoelectric properties of heavy-element doped CrN

CrN was doped with Mo and W to study the effect of heavy elements alloying on its thermoelectric properties. An spontaneous phase segregation into Mo- and W-rich regions was observed even at the lowest concentrations probed at this work (≃1%). In the particular case of W, this segregation creates nanoinclusions into the Cr1–xWxN matrix, which results in a substantial reduction of the thermal conductivity in the whole temperature range compared to undoped CrN. In addition, an increased hybridization of N:2p and 4d/5d orbitals with respect to Cr:3d decreases the electrical resistivity in lightly doped samples. This improves substantially the thermoelectric figure of merit with respect to the undoped compound, providing a pathway for further improvement of the thermoelectric performance of CrNS

Tunable resistivity exponents in the metallic phase of epitaxial nickelates

We report a detailed analysis of the electrical resistivity exponent of thin films of NdNiO3 as a function of epitaxial strain. Thin films under low strain conditions show a linear dependence of the resistivity versus temperature, consistent with a classical Fermi gas ruled by electron-phonon interactions. In addition, the apparent temperature exponent, n, can be tuned with the epitaxial strain between n = 1 and n = 3. We discuss the critical role played by quenched random disorder in the value of n. Our work shows that the assignment of Fermi/Non-Fermi liquid behaviour based on experimentally obtained resistivity exponents requires an in-depth analysis of the degree of disorder in the material

Endocannabinoid CB1 receptors modulate visual output from the thalamus

[Abstract] Rationale Endocannabinoids have emerged as a modulatory brain system affecting different types of synapses, broadly distributed throughout the CNS, which explain the diverse psychophysical effects observed following activation of the endocannabinoid system. Objectives and methods The present study aimed to characterize the effect of CB1-mediated activity in the visual thalamus. In vivo single-unit extracellular recordings were performed in anaesthetized adult pigmented rats, measuring visual and spontaneous activity, combined with application of CB1 receptor agonists (anandamide, 2-AG, and O2545) and one antagonist, AM251. Results CB1 receptors activation revealed two cellular populations, with excitatory effects on ∼28% of cells and inhibitory in ∼72%, actions which were blocked by the antagonist AM251. The agonist action significantly altered both spontaneous and visual activity, shifting the signal-tonoise ratio (S/N), with accompanying changes in the variability within the visual response. Increased responses by agonist application were accompanied by a decrease in S/N and an increase in variability, while those cells inhibited by the agonist showed an increase in S/N and a decrease in variability. There was no obvious correlation between the two effects and any other response property suggesting a more general role in modulating all information passing from LGN to cortex. Conclusions Our data support a role for CB1 at the level of the thalamus acting as a dynamic modulator of visual information being sent to the cortex, apparently maintaining the salience of the signal within upper and lower boundaries. This may account for some of the behavioral effects of cannabis.Ministerio de Ciencia e Innovación; BFU2009-08169Xunta de Galicia; 2007/000140-

High quality thin films of thermoelectric misfit cobalt oxides prepared by a chemical solution method

Misfit cobaltates ([Bi/Ba/Sr/Ca/CoO]nRS[CoO2]q) constitute the most promising family of thermoelectric oxides for high temperature energy harvesting. However, their complex structure and chemical composition makes extremely challenging their deposition by high-vacuum physical techniques. Therefore, many of them have not been prepared as thin films until now. Here we report the synthesis of high-quality epitaxial thin films of the most representative members of this family of compounds by a water-based chemical solution deposition method. The films show an exceptional crystalline quality, with an electrical conductivity and thermopower comparable to single crystals. These properties are linked to the epitaxial matching of the rock-salt layers of the structure to the substrate, producing clean interfaces free of amorphous phases. This is an important step forward for the integration of these materials with complementary n-type thermoelectric oxides in multilayer nanostructuresThis research was supported by the European Research Council (ERC StG-2DTHERMS), Ministerio de Economía y Competitividad of Spain (MAT2010-16157 & MAT2013-44673-R) and Xunta de Galicia (2012-CP071). J.M.V-F acknowledges the MINECO for support with a PhD grant of the FPI programS

Intracuneate mechanisms underlying primary afferent cutaneous processing in anaesthetized cats

[Abstract] The cutaneous primary afferents from the upper trunk and forelimbs reach the medial cuneate nucleus in their way towards the cerebral cortex. The aim of this work was twofold: (i) to study the mechanisms used by the primary afferents to relay cutaneous information to cuneate cuneolemniscal (CL) and noncuneolemniscal (nCL) cells, and (ii) to determine the intracuneate mechanisms leading to the elaboration of the output signal by CL cells. Extracellular recordings combined with microiontophoresis demonstrated that the primary afferent cutaneous information is communicated to CL and nCL cells through AMPA, NMDA and kainate receptors. These receptors were sequentially activated: AMPA receptors participated mainly during the initial phase of the response, whereas kainate- and NMDA-mediated activity predominated during a later phase. The involvement of NMDA receptors was confirmed by in vivo intracellular recordings. The cutaneous-evoked activation of CL cells was decreased by GABA and increased by glycine acting at a strychnine-sensitive site, indicating that glycine indirectly affects CL cells. Two subgroups of nCL cells were distinguished based on their sensitivity to iontophoretic ejection of glycine and strychnine. Overall, the results support a model whereby the primary afferent cutaneous input induces a centre-surround antagonism in the cuneate nucleus by activating (via AMPA, NMDA and kainate receptors) and disinhibiting (via serial glycinergic–GABAergic interactions) a population of CL cells with overlapped receptive fields that at the same time inhibit (via GABAergic cells) other neighbouring CL cells with different receptive fields.Ministerio de Ciencia y Tecnología; BFI 2003-0194