Alcalde Bessia, F., M. Pérez, M. Gomez Berisso, N. Piunno, H. Mateos, F. J. Pomiro, I. Sidelnik, J. J. Blostein, M. Sofo Haro, and J. Lipovetzky. "X-ray micrographic imaging system based on COTS CMOS sensors." In 2017 Argentine Conference of Micro-Nanoelectronics, Technology and Applications (CAMTA), 1–4. 2017 Argentine Conference of Micro-Nanoelectronics, Technology and Applications (CAMTA)., 2017.
Abstract: This paper presents the use of Commercial Off The Shelf CMOS image sensors for the acquisition of X-ray images with high spatial resolution. The X-ray images, with application in biology, electronic components inspection or paleontology research, are obtained with 8 keV photons from a Cu tube. The quantum efficiency of the detector is estimated using attenuation lengths of photons in the sensor, and compared to traditional scintillator conversion layers.
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Amigó, M. L., M. V. Ale Crivillero, D. G. Franco, A. Badía–Majós, J. Guimpel, J. Campo, F. Damay, F. Porcher, A. M. Condó, and G. Nieva. "Intrinsic pinning by naturally occurring correlated defects in FeSe 1− x Te x superconductors." Superconductor Science and Technology 30, no. 8 (2017): 085010.
Abstract: We study the angular dependence of dissipation in the superconducting state of FeSe and Fe(Se 1− x Te x ) through electrical transport measurements, using crystalline intergrown materials. We reveal the key role of the inclusions of the non superconducting magnetic phase Fe 1− y (Se 1− x Te x ), growing into the Fe(Se 1− x Te x ) pure β -phase, in the development of a correlated defect structure. The matching of both atomic structures defines the growth habit of the crystalline material as well as the correlated planar defects orientation.
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Amigó, M. L., N. Haberkorn, P. Pérez, S. Suárez, and G. Nieva. "Vortex dynamics in β -FeSe single crystals: effects of proton irradiation and small inhomogeneous stress." Superconductor Science and Technology 30, no. 12 (2017): 125017.
Abstract: We report on the critical current density J c and the vortex dynamics of pristine and 3 MeV proton irradiated (cumulative dose equal to ##IMG## [http://ej.iop.org/images/0953-2048/30/12/125017/sustaa9518ieqn1.gif] {$2\times {10}^{16}$} cm −2 ) β -FeSe single crystals. We also analyze a remarkable dependence of the superconducting critical temperature T c , J c and the flux creep rate S on the sample mounting method. Free-standing crystals present T c = 8.4(1) K, which increases to 10.5(1) K when they are fixed to the sample holder by embedding them with GE-7031 varnish. On the other hand, the irradiation has a marginal effect on T c . The pinning scenario can be ascribed to twin boundaries and random point defects. We find that the main effect of irradiation is to increase the density of random point defects, while the embedding mainly reduces the density of twin boundaries. Pristine and irradiated crystals present two outstanding features in the temperature dependence of the flux creep rate: ##IMG## [http://ej.iop.org/images/0953-2048/30/12/125017/sustaa9518ieqn2.gif] {$S(T)$} presents large values at low temperatures, which can be attributed to small pinning energies, and a plateau at intermediate temperatures, which can be associated with glassy relaxation. From Maley analysis, we observe that the characteristic glassy exponent μ changes from ∼1.7 to 1.35–1.4 after proton irradiation.
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Amigó, M. L., T. Herrera, L. Neñer, L. Peralta Gavensky, F. Turco, and J. Luzuriaga. "A quantitative experiment on the fountain effect in superfluid helium." European Journal of Physics 38, no. 5 (2017): 055103.
Abstract: Superfluid helium, a state of matter existing at low temperatures, shows many remarkable properties. One example is the so called fountain effect, where a heater can produce a jet of helium. This converts heat into mechanical motion; a machine with no moving parts, but working only below 2 K. Allen and Jones first demonstrated the effect in 1938, but their work was basically qualitative. We now present data of a quantitative version of the experiment. We have measured the heat supplied, the temperature and the height of the jet produced. We also develop equations, based on the two-fluid model of superfluid helium, that give a satisfactory fit to the data. The experiment has been performed by advanced undergraduate students in our home institution, and illustrates in a vivid way some of the striking properties of the superfluid state.
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Bolecek, N. R. C., M. I. Dolz, H. Pastoriza, M. Konczykowski, C. J. van der Beek, A. B. Kolton, and Y. Fasano. "Excess of topological defects induced by confinement in vortex nanocrystals." Physical Review B 96, no. 2 (2017): 024507.
Abstract: We directly image individual vortex positions in nanocrystals in order to unveil the structural property that contributes to the depletion of the entropy jump entailed at the first-order transition. On reducing the nanocrystal size, the density of topological defects increases near the edges over a characteristic length. Within this “healing-length†distance from the sample edge, vortex rows tend to bend, while towards the center of the sample, the positional order of the vortex structure is what is expected for the Bragg-glass phase. This suggests that the healing length may be a key quantity to model confinement effects in the first-order transition of extremely layered vortex nanocrystals.
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Giriat, G., G. W. Scheerer, Z. Ren, P. Pedrazzini, G. Lapertot, and D. Jaccard. "A watchmaker’s precision at high-pressure reveals CeCu 2 Si 2 and CeAu 2 Si 2 p-T phase diagrams." In Journal of Physics: Conference Series, 032008. Vol. 950., 2017.
Abstract: Gaétan Giriat and Gernot Werner Scheerer and Zhi Ren and Pablo Pedrazzini and Gerard Lapertot and Didier Jaccard
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Gonzalez Sutter, J., L. Neñer, H. Navarro, G. Leyva, S. Fusil, K. Bouzehouane, N. Haberkorn, and M. Sirena. "Oxygen influence in the magnetic and the transport properties of ferroelectric/ferromagnetic heterostructures." Thin Solid Films 639 (2017): 42–46.
Abstract: Oxygen vacancies in oxides nanostructures are the origin of many intriguing phenomena. We have studied the influence of the oxygen pressure in the tunneling properties of a ferroelectric barrier, Ba0.25Sr0.75TiO3 (BSTO), grown over a ferromagnetic electrode. A phenomenological model description was used to obtain critical information about the structure and electrical properties of ultra-thin BSTO layers using conductive atomic force microscopy. The BSTO layers present good insulation properties. Reducing the oxygen content increases the conductivity of the samples. The tunneling of the current carriers is probably the main conduction mechanism for samples with higher barrier thicknesses.
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Haberkorn, N., J. Guimpel, S. Suárez, H. E. Troiani, P. Granell, F. Golmar, J. - H. Lee, S. H. Moon, and H. Lee. "Strong influence of the oxygen stoichiometry on the vortex bundle size and critical current densities J c of GdBa 2 Cu 3 O x -coated conductors grown by co-evaporation." Superconductor Science and Technology 30, no. 9 (2017): 095009.
Abstract: We report on the influence of oxygen stoichiometry on the vortex creep mechanism of GdBa 2 Cu 3 O x -coated conductors produced by co-evaporation. The oxygen stoichiometry of the films, x , was modified in a controlled way between 6.85 and 7, which resulted in systematic and reversible control of the superconducting critical temperature between about 78 and 93 K. The change in the oxygen stoichiometry produces a strong reduction in the self-field critical current densities J c without significantly modifying the power-law dependence at intermediate magnetic fields, which indicates a negligible contribution of oxygen vacancies to the pinning. In addition, the characteristic glassy exponent μ shows a systematic diminution from about 1.63 at x = 7 to about 1.12 at x = 6.85. The results are compared with those obtained for proton- and oxygen-irradiated films, in which the vortex dynamics is determined by a balance between the improved pinning, originating from nanocluster inclusion, and the suppressed superconducting properties due to disorder in the nanoscale.
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Haberkorn, N., S. Suárez, P. D. Pérez, H. Troiani, P. Granell, F. Golmar, J. - H. Lee, and S. H. Moon. "Effect of mixed pinning landscapes produced by 6 MeV oxygen irradiation on the resulting critical current densities Jc in 1.3 µm thick GdBa2Cu3O7-d coated conductors grown by co-evaporation." Physica C: Superconductivity and its Applications 542 (2017): 6–11.
Abstract: We report the influence of crystalline defects introduced by 6 MeV 16O3+ irradiation on the critical current densities Jc and flux creep rates in 1.3 µm thick GdBa2Cu3O7-δ coated conductor produced by co-evaporation. Pristine films with pinning produced mainly by random nanoparticles with diameter close to 50 nm were irradiated with doses between 2 × 1013 cm−2 and 4 × 1014 cm−2. The irradiations were performed with the ion beam perpendicular to the surface of the samples. The Jc and the flux creep rates were analyzed for two magnetic field configurations: magnetic field applied parallel (H║c) and at 45° (H║45°) to the c-axis. The results show that at temperatures below 40 K the in-field Jc dependences can be significantly improved by irradiation. For doses of 1 × 1014 cm−2 the Jc values at μ0H = 5 T are doubled without affecting significantly the Jc at small fields. Analyzing the flux creep rates as function of the temperature in both magnetic field configurations, it can be observed that the irradiation suppresses the peak associated with double-kink relaxation and increases the flux creep rates at intermediate and high temperatures. Under 0.5 T, the flux relaxation for H‖c and H||45° in pristine films presents characteristic glassy exponents μ = 1.63 and μ = 1.45, respectively. For samples irradiated with 1 × 1014 cm−2, these values drop to μ = 1.45 and μ = 1.24, respectively
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Martínez, E. D., A. F. García Flores, H. Pastoriza, R. R. Urbano, and C. Rettori. "Electrothermal Silver Nanowire Thin Films for In-Situ Observation of Thermally-Driven Chemical Processes." Sensors and Actuators B: Chemical 259 (2017): 475–483.
Abstract: We develop a novel device comprised of high optical transmittance thin films containing silver nanowires (AgNWs) in poly(methyl methacrylate) (PMMA) acting as heating elements. The electrothermal control of the AgNWs network allows us to externally trigger and tune the temperature conditions required to run chemical reactions and physicochemical processes. The device was successfully applied for the spectroscopic in-situ observation of three different model reactions: i) the thermal equilibrium of a CoCl2/HCl/H2O complex, ii) the reversible macromolecular phase transition of a pNIPAM solution, and iii) the nucleation and growth of gold nanoparticles (AuNPs). In the first case, the color of the Co2+ complex was reversibly switched from pink to blue when changing the thermal equilibrium condition. In the second one, the optical transmittance of an aqueous solution of carboxylic-terminated pNIPAM polymer was cycled from high to low as the temperature of the solution was below or above the lower critical solubility temperature (LCST) respectively. Finally, the electrothermal control on the device was applied to the study of the nucleation and growth of AuNPs in an organic solution of AuCl3 containing oleylamine acting as both the reducer and the stabilizing agent. The versatility of the electrothermal device provides an easy way to undertake thermally controlled processes and develop optical elements such as smart windows and lab-on-a-chip devices. The AgNWs-PMMA nanocomposite was also applied successfully as an electrothermal ink on the external side walls of a test tube.
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Mateos, H., J. Lipovetzky, F. Alcalde Bessia, M. Perez, P. Cappagli, and M. Gomez Berisso. "Characterization of sensors and design of an embedded photodetectors array for beam profile measurements in radiotherapy." In 2017 Eight Argentine Symposium and Conference on Embedded Systems (CASE), 1–6. 2017 Eight Argentine Symposium and Conference on Embedded Systems (CASE)., 2017.
Abstract: In this work, the dosimetric performance of several Commercial Of The Shelf photodiodes was studied. They were tested against gamma rays from a teletherapy source and were characterized with the aim of building a one-dimensional array of dosimeters able to measure the the spatial distribution of dose in a radiotherapy irradiation field. An embedded system comprising an array of 40 photodiodes and its acquisition electronics was built and tested. The dose resolution of each detector was 0.06 cGy and the spatial resolution of the system 0.5 cm.
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Michor, H., J. G. Sereni, M. Giovannini, E. Kampert, L. Salamakha, G. Hilscher, and E. Bauer. "Elucidating the lack of magnetic order in the heavy-fermion CeCu2Mg." Physical Review B 95, no. 11 (2017): 115146.
Abstract: Magnetic, transport, and thermal properties of CeCu2Mg are investigated to elucidate the lack of magnetic order in this heavy-fermion compound with a specific heat value, Cmag/T|T→0≈1.2 J/mol K2 and robust effective magnetic moments (μeff≈2.46μB). The lack of magnetic order is attributed to magnetic frustration favored by the hexagonal configuration of the Ce sublattice. In fact, the effect of magnetic field on Cmag/T and residual resistivity Ï0 does not correspond to that of a Fermi liquid (FL) because a broad anomaly appears at Tmax≈1.2 K in Cmag(T)/T, without changing its position up to μ0H=7.5 T. However, the flattening of Cmag/T|T→0 and its magnetic susceptibility χT→0, together with the T2 dependence of Ï(T), reveal a FL behavior for T≤2 K which is also supported by Wilson and Kadowaki-Woods ratios. The unusual coexistence of FL and frustration phenomena can be understood by placing paramagnetic CeCu2Mg in an intermediate section of a frustration-Kondo model. The entropy, Smag, reaches 0.87Rln6 at T≃100 K, with a tendency to approach the expected value Smag=Rln6 of the J=5/2 ground state of Ce3+.
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Perez, M., F. Alcalde, M. S. Haro, I. Sidelnik, J. J. Blostein, M. Gomez Berisso, and J. Lipovetzky. "Implementation of an ionizing radiation detector based on a FPGA-controlled COTS CMOS image sensor." In 2017 XVII Workshop on Information Processing and Control (RPIC), 1–6. 2017 XVII Workshop on Information Processing and Control (RPIC)., 2017.
Abstract: This work presents the development and implementation of an ionizing radiation detector based on a commercial off the shelf CMOS image sensor and a FPGA. The response of the system was tested in irradiations with gamma photons, beta and alpha particles using different configurations of the image sensor. Finally, we analyze the possible uses of such configurations in the discrimination of events produced by alpha particles in mixed radiation fields.
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Pomiro, F., D. M. Gil, V. Nassif, A. Paesano, M. I. Gómez, J. Guimpel, R. D. Sánchez, and R. E. Carbonio. "Weak ferromagnetism and superparamagnetic clusters coexistence in YFe1−xCoxO_3 (0 ≤ x ≤ 1) perovskites." Materials Letters 94 (2017): 472–482.
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Pozo-López, G., A. M. Condó, L. M. Fabietti, E. Winkler, N. Haberkorn, and S. E. Urreta. "Microstructure of as-cast single and twin roller melt-spun Ni2MnGa ribbons." Materials Characterization 124 (2017): 171–181.
Abstract: Stoichiometric Ni2MnGa alloys are processed by two rapid solidification techniques – single-roller (SR) and twin-roller (TR) melt spinning – and the resulting microstructures and magnetic properties determined. Samples processed at tangential wheel speeds of 10m/s (V10) and 15m/s (V15) are studied in the as-cast condition to analyze the influence of the production methods on the microstructure. Important aspects like the resulting phases, their crystallographic texture, magnetic properties, martensitic transformation temperatures and Curie temperatures are compared. In addition, the magnetization mechanism involving twin boundary motion is explored. Our results indicate that the TR method provides lower cooling rates, thicker samples, higher internal stresses and larger MnS precipitates. However, the quenching rate is mainly determined by the tangential wheel velocity. TR samples also exhibit [100] texture normal to the ribbon plane but in a lesser extent than SR ribbons. Martensitic transformation temperatures are higher in samples V15 (~150K) than in V10 (~100K), with no clear difference between the SR and TR modes. This behavior is explained by considering distinct degrees of disorder in the L21 austenite phase resulting from quenching. The hysteresis of the transformation, defined as the difference Af−MS, takes similar values in the four samples analyzed. Pre-martensitic transformation temperatures are also slightly higher in samples V15, (230±3) K, than in samples V10, (222±3) K, as the magnitude of the Hopkinson effect, in good agreement with a higher residual stress level in TR ribbons. In the martensitic state, all ribbons exhibit hysteresis loops characteristic of a magnetization mechanism involving twin boundary motion. The switching magnetic fields for the onset of Type I twin boundary motion result between 220mT and 365mT, values equivalent to twinning stresses of about 1MPa. It is concluded that both procedures, SR and TR melt spinning, provide microstructures favoring magnetic field induced twin variant reorientation.
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Sereni, J. G. "Entropy Constraints in the Ground State Formation of Magnetically Frustrated Systems." Journal of Low Temperature Physics 190, no. 1-2 (2017): 1–19.
Abstract: A systematic modification of the entropy trajectory (Sm(T)) is observed at very low temperature in magnetically frustrated systems as a consequence of the constraint (Sm≥0) imposed by the Nernst postulate. The lack of magnetic order allows to explore and compare new thermodynamic properties by tracing the specific heat (Cm) behavior down to the sub-Kelvin range. Some of the most relevant findings are: (i) a common Cm/T|T→0≈7 J/mol K2 ‘plateau’ in at least five Yb-based very-heavy-fermions (VHF) compounds; (ii) quantitative and qualitative differences between VHF and standard non-Fermi-liquids; (iii) entropy bottlenecks governing the change of Sm(T) trajectories in a continuous transition into alternative ground states. A comparative analysis of Sm(T→0) dependencies is performed in compounds suitable for adiabatic demagnetization processes according to their ∂2Sm/∂T2 derivatives.
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Sereni, J. G. First observation of Ce volume collapse in CeN., 2017.
Abstract: On the occasion of the 80th anniversary of the first observation of Ce volume collapse in CeN a remembrance of the implications of that transcendent event is presented, along with a review of the knowledge of Ce physical properties available at that time. Coincident anniversary corresponds to the first proposal for Ce as a mix valence element, motivating to briefly review how the valence instability of Ce was investigated since that time.
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Sidelnik, I., H. Asorey, A. Mancilla, G. Anibal, N. Guarin, H. Arnaldi, M. Pérez, F. Alcalde Bessia, J. Lipovetzky, M. Sofo Haro et al. "Construccion de detectores Cherenkov en agua." Informe Tecnico CNEA, no. Diciembre (2017).
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Velarde, M. O., G. Mato, and D. Delavalle. "Mechanisms for pattern specificity of deep-brain stimulation in Parkinson's disease." PLoS One 12, no. 8 (2017): e0182884.
Abstract: Deep brain stimulation (DBS) has become a widely used technique for treating advanced stages of neurological and psychiatric illness. In the case of motor disorders related to basal ganglia (BG) dysfunction, several mechanisms of action for the DBS therapy have been identified which might be involved simultaneously or in sequence. However, the identification of a common key mechanism underlying the clinical relevant DBS configurations has remained elusive due to the inherent complexity related to the interaction between the electrical stimulation and the neural tissue, and the intricate circuital structure of the BG-thalamocortical network. In this work, it is shown that the clinically relevant range for both, the frequency and intensity of the electrical stimulation pattern, is an emergent property of the BG anatomy at the system-level that can be addressed using mean-field descriptive models of the BG network. Moreover, it is shown that the activity resetting mechanism elicited by electrical stimulation provides a natural explanation to the ineffectiveness of irregular (i.e., aperiodic) stimulation patterns, which has been commonly observed in previously reported pathophysiology models of Parkinson's disease. Using analytical and numerical techniques, these results have been reproduced in both cases: 1) a reduced mean-field model that can be thought as an elementary building block capable to capture the underlying fundamentals of the relevant loops constituting the BG-thalamocortical network, and 2) a detailed model constituted by the direct and hyperdirect loops including one-dimensional spatial structure of the BG nuclei. We found that the optimal ranges for the essential parameters of the stimulation patterns can be understood without taking into account biophysical details of the relevant structures.
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