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Ítem Acceso Abierto Characterization of the Nb-B Superlattice System(Elsevier, 2016-12-15) Franco, D.G.; Sarmiento-Chavez, A.; Schenone, N.; Allcca, A.E.L.; Berisso, M.G.; Fasano, Y.; Guimpel, J.; Gerencia Física. Departamento Materia Condensada. División Bajas TemperaturasWe study the growth, stacking and superconducting properties of Nb and B thin films and superlattices. The interest in these resides in their possible use in transition edge neutron sensors. The samples were grown by magnetron sputtering over Si (1 0 0) substrates. The X-ray diffraction patterns for all Nb containing samples show a Nb (1 1 0) preferential orientation. From the low-angle X-ray reflectivity we obtain information on the superlattice structure. The superconducting transition temperatures of the superlattices, obtained from the temperature dependence of the magnetization, are higher than those of single Nb films of similar thickness. The temperature dependence of the perpendicular and parallel upper critical fields indicate that the superlattices behave as an array of decoupled superconducting Nb layers.Ítem Acceso Abierto Synthesis, structure and magnetic properties of La3Co2SbO9: A double perovskite with competing antiferromagnetic and ferromagnetic interactions(Elsevier, 2012-10-00) Franco, D.G.; Fuertes, V.C.; Blanco, M.C.; Fernández-Díaz, M.T.; Sánchez, R.D.; Carbonio, R.E.; Gerencia Física. Departamento Materia Condensada. División Bajas TemperaturasThe synthesis, structural characterization, and magnetic properties of La3Co2SbO9 double perovskite are reported. The crystal structure has been refined by X-ray and neutron powder diffraction data in the monoclinic space group P21/n. Co2+ and Sb5+ have the maximum order allowed for the La3Co2SbO9 stoichiometry. Rietveld refinements of powder neutron diffraction data show that at room temperature the cell parameters are a=5.6274(2) Å, b=5.6842(2) Å, c=7.9748(2) Å and β=89.999(3)°. Magnetization measurements indicate the presence of ferromagnetic correlations with TC=55 K attributed to the exchange interactions for non-linear Co2+–O–Sb5+–O–Co2+ paths. The effective magnetic moment obtained experimentally is μexp=4.38 μB (per mol Co2+), between the theoretical one for spin only (3.87 μB) and spin-orbit value (6.63 μB), indicating partially unquenched contribution. The low magnetization value at high magnetic field and low temperature (1 μB/f.u., 5 T and 5 K) and the difference between ZFC and FC magnetization curves (at 5 kOe) indicate that the ferromagnetism do not reach a long range order and that the material has an important magnetic frustration.Ítem Acceso Abierto Synthesis and structural and magnetic characterization of the frustrated magnetic system La2Ni4/3-xCoxSb2/3O6(Elsevier, 2013-11-00) Franco, D.G.; Carbonio, R.E.; Nieva, G.; Gerencia Física. Departamento Materia Condensada. División Bajas TemperaturasWe report the synthesis of double perovskites La2Ni4/3−xCoxSb2/3O6 with x=0, 1/3, 2/3 and 1 by a solid state method. Rietveld refinements of X-ray and neutron powder diffraction data show that all samples crystallize in space group P21/n, with almost perfect occupation of the 2d octahedral site with the transition metals, while all Sb5+ are randomly distributed in a 2c octahedral site. The saturation magnetization in hysteresis loops indicates that the samples are ferrimagnetic throughout all the series. Virgin magnetization curves lie outside hysteresis loops at low temperatures and thermal evolution of Hm – defined as the inflection point of these curves – follows the de Almeida–Thouless dependence for . This spin glass like behavior below 30 K is also supported by thermal evolution of the coercivity, which follows an exponential law typical of magnetic clusters, not found in the pure Ni2+ perovskite, x=0 extreme.Ítem Acceso Abierto Magnetic Properties of the Double Perovskites LaPbMSbO6 (M = Mn, Co, and Ni)(IEEE (Institute of Electrical and Electronics Engineers), 2013-07-23) Franco, D.G.; Carbonio, R.E.; Nieva, G.; Gerencia Física. Departamento Materia Condensada. División Bajas TemperaturasNew double perovskites LaPbMSbO 6 , where M 2+ = Mn 2+ , Co 2+ , and Ni 2+ , were synthesized as polycrystals by an aqueous synthetic route at temperatures below 1000 ° C. All samples are monoclinic, space group P2 1 /n, as it is observed from Rietveld analysis of X-ray powder diffraction patterns. The distribution of M 2+ and Sb 5+ among the two octahedral sites have 3% of disorder for M 2+ = Ni 2+ , whereas for M 2+ = Mn 2+ and Co 2+ less disorder is found. The three samples have an antiferromagnetic transition, due to the antiferromagnetic coupling between M 2 + through super-superexchange paths M 2+ -O 2- -Sb 5+ -O 2- -M 2+ . Transition temperatures are low: 8, 10 and 17 K for Mn 2+ , Co 2+ , and Ni 2 + respectively, as a consequence of the relatively long distances between the magnetic ions M 2+ . Besides, for LaPbMnSbO 6 a small transition at 45 K was found, with ferrimagnetic characteristics, possibly as a consequence of a small disorder between Mn 2+ and Sb 5+ . This disorder would give additional and shorter interaction paths: superexchange Mn 2+ -O 2- -Mn 2+ .Ítem Acceso Abierto Tailoring the ground state of the ferrimagnet La2Ni(Ni1/3Sb2/3)O6(Elsevier, 2013-11-00) Franco, D.G.; Carbonio, R.E.; Kaul, E.E.; Nieva, G.; Gerencia Física. Departamento Materia Condensada. División Bajas TemperaturasWe report on the magnetic and structural properties of La2Ni(Ni1/3Sb2/3)O6 in polycrystal, single crystal and thin film samples. We found that this material is a ferrimagnet (T c≈100 K) which possesses a very distinctive and uncommon feature in its virgin curve of the hysteresis loops. We observe that bellow 20 K it lies outside the hysteresis cycle, and this feature was found to be an indication of a microscopically irreversible process possibly involving the interplay of competing antiferromagnetic interactions that hinder the initial movement of domain walls. This initial magnetic state is overcome by applying a temperature dependent characteristic field. Above this field, an isothermal magnetic demagnetization of the samples yield a ground state different from the initial thermally demagnetized one.Ítem Acceso Abierto Change in the Magnetic Domain Alignment Process at the Onset of a Frustrated Magnetic State in Ferrimagnetic La2Ni(Ni1/3Sb2/3)O6 Double Perovskite(IEEE (Institute of Electrical and Electronics Engineers), 2013-07-23) Franco, D.G.; Carbonio, R.E.; Nieva, G.; Gerencia Física. Departamento Materia Condensada. División Bajas TemperaturasWe have performed a combined study of magnetization hysteresis loops and time dependence of the magnetization in a broad temperature range for the ferrimagnetic La 2 Ni(Ni 1/3 Sb 2/3 )O 6 double perovskite. This material has a ferrimagnetic order transition at ~100 K and at lower temperatures (~20 K) shows the signature of a frustrated state due to the presence of two competing magnetic exchange interactions. The temperature dependence of the coercive field shows an important upturn below the point where the frustrated state sets in. The use of hysteresis data, magnetization versus applied magnetic field, together with the magnetization versus time data provides a unique opportunity to distinguish between different scenarios for the low temperature regime. From our analysis, a strong domain wall pinning results in the best scenario for the low temperature regime. For temperatures larger than 20 K, the adequate scenario seems to correspond to a weak domain wall pinning.Ítem Acceso Abierto Phase diagram and superconductive properties of splat-cooled CuxLa1-x(American Physical Society, 1980-04-01) Favaron, J.; de la Cruz, M.E.; Esquinazi, P.; de la Cruz, F.; Gerencia Física. Departamento Materia Condensada. División Bajas TemperaturasThe phase diagram of nonequilibrium, splat-cooled CuxLa1−x is interpreted by means of superconductive-transition and x-ray measurements. At the eutectic composition the material is shown to be in a highly disordered state. Measurements of dc resistance and ac susceptibility show that there are two different critical fields at the disordered concentration although there is a unique critical temperature.Ítem Acceso Abierto Effect of physical and chemical pressure on the superconductivity of high-temperature oxide superconductors(American Physical Society, 1991-10-01) Fernandes, A.A.R.; Santamaria, J.; Bud’ko, S.L.; Nakamura, O.; Guimpel, J.; Schuller, I.K.; Gerencia Física. Departamento Materia Condensada. División Bajas TemperaturasWe have compared the changes in the structure and superconductivity of the Gd1−xYxBa2Cu3O7−δ system produced by rare-earth-ion substitution (chemical pressure) and by hydrostatic pressure. Although the application of both chemical and physical pressure result in an overall compression of the unit cell, the effect on Tc is of opposite sign. A detailed comparison of the evolution of the structure under both kinds of pressure shows that the origin of this qualitatively different behavior in Tc is correlated with the apical oxygen to CuO2-plane distance which also presents opposite trends. The origin of this difference is found in the extremely inhomogeneous character of the chemical pressure which, in spite of inducing an overall compression of the structure, produces an expansion of certain regions of the unit cell.Ítem Acceso Abierto Unconventional superconductivity in the strong-coupling limit for the heavy fermion system CeCoIn5(Elsevier, 2018-05-01) Fasano, Y.; Szabó, P.; KaÄ_x008d_marÄ_x008d_ík, J.; Pribulová, Z.; Pedrazzini, P.; Samuely, P.; Correa, V.F.; Gerencia Física. Departamento Materia Condensada. División Bajas TemperaturasWe present scanning tunneling spectroscopy measurements of the local quasiparticles' excitation spectra of the heavy fermion CeCoIn5 between 440 mK and 3 K in samples with a bulk T = 2.25 Kc. The spectral shape of our low-temperature tunneling data, quite textbook nodal- conductance, allow us to confidently fit the spectra with a d-wave density of states considering also a shortening of quasiparticles' lifetime term Γ . The Δ(0). The value obtained from the fits yields a BCS ratio 2 Δ /kTc = 7.73 suggesting that CeCoIn5 is an unconventional superconductor in the strong coupling limit. The fits also reveal that the height of coherence peaks in CeCoIn5 is reduced with respect to a pure BCS spectra and therefore the coupling of quasiparticles with spin excitations should play a relevant role. The tunneling conductance shows a depletion at energies smaller than Δ for temperatures larger than the bulk Tc, giving further support to the existence of a pseudogap phase that in our samples span up to T* ∼ 1.2 T c. The phenomenological scaling of the pseudogap temperature observed in various families of cuprates 2 Δ / kT * ∼ 4.3, is not fulfilled in our measurements. This suggests that in CeCoIn5 the strong magnetic fluctuations might conspire to close the local superconducting gap at a smaller pesudogap temperature-scale than in cuprates.Ítem Acceso Abierto Commensurability and stability in nonperiodic systems(National Academy of Science, 2005-01-24) Fasano, Y.; Seta, M.D.; Menghini, M.; Pastoriza, H.; Cruz, F. de la; Gerencia Física. Departamento Materia Condensada. División Bajas TemperaturasWe have investigated the response of 3D Bi2Sr2CaCu2O8 vortex structures to a weak perturbation induced by 2D Fe pinning structures acting on one extremity of vortex lines. The pinning patterns were nano-engineered at the sample surface by means of either a Bitter decoration of the vortex lattice or electron-beam lithography. The commensurability conditions between 2D rigid pinning potentials and 3D elastic structures with short-range positional and long-range orientational correlation have been experimentally determined. When the 2D potential is a replica of the nonperiodic vortex structure an amplification of its interaction with the vortex structure takes place. This effect is detected only for the first matching field, becoming negligible for other matching fields. On the other hand, a periodic 2D perturbation is shown to transform the nonperiodic Bragg glass-like structure into an Abrikosov crystal with an effective Debye–Waller factor.Ítem Acceso Abierto Vortex pinning by intrinsic correlated defects in Fe1-ySe(IOP Publishing, 2014-00-00) Amigó, M.L.; Crivillero, V.A.; Franco, D.G.; Badía-Majós, A.; Guimpel, J.; Nieva, G.; Gerencia Física. Departamento Materia Condensada. División Bajas TemperaturasWe present a study on the transport and magnetic properties of superconducting Fe1−ySe single crystals. In the superconducting state, the in-plane electrical resistivity of the crystal is measured for fields up to 16 T and as a function of field direction, in order to understand how the vortex dynamics is affected by the presence of defects. A strong deviation from the slightly anisotropic crystal (electronic anisotropy constant 7 ∼ 1.08) is observed as a steep angular dependence, which is interpreted as a signature of the presence of correlated defects. The influence of the correlated defects on the critical current is studied through the angular dependence of the magnetization, and compared to numerical simulations.Ítem Acceso Abierto Annealing disorder and photoinduced order of oxygen chains in detwinned YBa2Cu3O6.65 single crystals probed by Raman scattering(American Physical Society, 2000-02-01) Fainstein, A.; Maiorov, B.; Guimpel, J.; Nieva, G.; Osquiguil, E.; Gerencia Física. Departamento Materia Condensada. División Bajas TemperaturasRaman scattering in detwinned YBa2Cu3O6.65 single crystals is studied as a function of photoexcitation and annealing. Copper-oxygen chain-related forbidden Raman bands that are known to strongly bleach with illumination at low temperatures, increase their intensity with chain fragmentation induced by annealing at high temperature. This contrasting behavior proves the conjunction of short Cu-O fragments into longer chains on photoexcitation. We interpret the Raman modes as due to vibrations at the end of CuO chain fragments and Cu-O-Cu monomers, and use their evolution with illumination and annealing as anisotropic sensitive markers of oxygen reordering processes. The identification of the “forbidden” Raman bands is discussed in the context of our results and recent literature in the subject. We also present absorption measurements performed on GdBa2Cu3Ox thin films with varying oxygen content. These experiments show that the 2.2-eV absorption and the chain-related Raman peaks have different dependencies with oxygen content and illumination, ruling out an explanation that suggests that the Raman intensity reduction of these modes is due to a photobleaching of intermediate defect states. These results highlight the potentialities of Raman scattering for oxygen dynamics studies and demonstrate the presence of photoinduced oxygen ordering in these high-Tc superconductor compounds.Ítem Acceso Abierto Imaging the structure of the interface between symmetries interconnected by a discontinuous transition(Elsevier, 2003-10-00) Fasano, Y.; de Seta, M.; Menghini, M.; Pastoriza, H.; de la Cruz, F.; Gerencia Física. Departamento Materia Condensada. División Bajas TemperaturasWe have been able to observe with single particle resolution the interface between two structural symmetries that cannot be interconnected by a continuous transition. By means of an engineered 2D potential that pins the extremity of vortex strings a square symmetry was imposed at the surface of a 3D vortex solid. Using the Bitter decoration technique and on account of the continuous vortex symmetry, we visualize how the induced structure transforms along the vortex direction before changing into the expected hexagonal structure at a finite distance from the surface.Ítem Acceso Abierto Why the Co-based 115 compounds are different: The case study of GdMIn5 (M = Co,Rh,Ir)(American Physical Society, 2015-01-00) Facio, J.I.; Betancourth, D.; Pedrazzini, P.; Correa, V. F; Vildosola, V.; García, D. J.; Cornaglia, P.S.; Gerencia Física. Departamento Materia Condensada. División Bajas TemperaturasThe discovery in 2001 of superconductivity in some heavy fermion compounds of the RMIn5 (R=4f or 5f elements; M=Co, Rh, Ir) family, has triggered an enormous amount of research into understanding the physical origin of superconductivity and its relation with magnetism. Although many properties have been clarified, there are still crucial questions that remain unanswered. One of these questions is the particular role of the transition metal in determining the value of critical superconducting temperature (TC). In this work, we analyze an interesting regularity that is experimentally observed in this family of compounds, where the lowest Néel temperatures are obtained in the Co-based materials. We focus our analysis on the GdMIn5 compounds and perform density-functional-theory-based total-energy calculations to obtain the parameters for the exchange coupling interactions between the magnetic moments located at the Gd3+ ions. Our calculations indicate that the ground state of the three compounds is a C-type antiferromagnet determined by the competition between the first- and second-neighbor exchange couplings inside GdIn3 planes and stabilized by the couplings across MIn2 planes. We then solve a model with these magnetic interactions using a mean-field approximation and quantum Monte Carlo simulations. The results obtained for the calculated Néel and Curie-Weiss temperatures, the specific heat, and the magnetic susceptibility are in very good agreement with the existent experimental data. Remarkably, we show that the first-neighbor interplane exchange coupling in the Co-based material is much smaller than in the Rh and Ir analogs which leads to a more two-dimensional magnetic behavior in the former. This result explains the observed lower Néel temperature in Co-115 systems and may shed light on the fact that the Co-based 115 superconductors present the highest TC.Ítem Acceso Abierto Vortex kinks in superconducting films with periodically modulated thickness(IOP Publishing, 2013-05-30) Facio, J.I.; Abate, A.; Guimpel, J.; Cornaglia, P.S.; Gerencia Física. Departamento Materia Condensada. División Bajas TemperaturasWe report magnetoresistance measurements in Nb films having a periodic thickness modulation. The cylinder shaped thicker regions of the sample, which form a square lattice, act as repulsive centers for the superconducting vortices. For low driving currents along one of the axes of the square lattice, the resistivity ρ increases monotonously with increasing magnetic field B and the ρ–B characteristics are approximately piecewise linear. The linear ρ versus B segments change their slope at matching fields where the number of vortices is an integer or a half integer times the number of protruding cylinders in the sample. Numerical simulations allow us to associate the different segments of linear magnetoresistance to different vortex-flow regimes, some of which are dominated by the propagation of discommensurations (kinks).Ítem Acceso Abierto Lattice specific heat for the RMIn5 (R=Gd, La,Y; M=Co, Rh) compounds: Non-magnetic contribution subtraction(Elsevier, 2016-06-01) Facio, J.I.; Betancourth, D.; Cejas Bolecek, N.R.; Jorge, G.A.; Pedrazzini, P.; Correa, V.F.; Cornaglia, P.S.; Vildosola, V.; García D.J.; Gerencia Física. Departamento Materia Condensada. División Bajas TemperaturasWe analyze theoretically a common experimental process used to obtain the magnetic contribution to the specific heat of a given magnetic material. In the procedure, the specific heat of a non-magnetic analog is measured and used to subtract the non-magnetic contributions, which are generally dominated by the lattice degrees of freedom in a wide range of temperatures. We calculate the lattice contribution to the specific heat for the magnetic compounds GdMIn5 (M=Co, Rh) and for the non-magnetic YMIn5 and LaMIn5 (M=Co, Rh), using density functional theory based methods. We find that the best non-magnetic analog for the subtraction depends on the magnetic material and on the range of temperatures. While the phonon specific heat contribution of YRhIn5 is an excellent approximation to the one of GdCoIn5 in the full temperature range, for GdRhIn5 we find a better agreement with LaCoIn5, in both cases, as a result of an optimum compensation effect between masses and volumes. We present measurements of the specific heat of the compounds GdMIn5 (M=Co, Rh) up to room temperature where it surpasses the value expected from the Dulong–Petit law. We obtain a good agreement between theory and experiment when we include anharmonic effects in the calculations.Ítem Acceso Abierto Anomalies in the internal friction and sound velocity in the high-temperature superconductor La1.8Sr0.2CuO4(American Physical Society, 1987-08-01) Esquinazi, P.; Luzuriaga, J.; Durán, C.; Esparza, D.A.; D'Ovidio, C.; Gerencia Física. Departamento Materia Condensada. División Bajas TemperaturasThe internal friction Q−1 and the Young's-modulus sound velocity vE have been measured in La1.8Sr0.2CuO4 using the vibrating-reed technique. Measurements were performed at a frequency of 700 Hz at temperatures between 0.2 and 100 K. A drastic change of slope in the internal friction versus temperature curve is observed at T=44 K just above the superconducting transition temperature Tc=40 K. A plateau in Q−1 is observed between 44 and 5 K. The sound velocity shows a decrease with decreasing temperature between 100 and 20 K and remains almost constant between 2 and 0.2 K.Ítem Acceso Abierto Annealing effects on the sound velocity and internal friction in the superconducting and normal states of the Cu30Zr70 amorphous alloy(American Physical Society, 1988-05-01) Esquinazi, P.; Luzuriaga, J.; Gerencia Física. Departamento Materia Condensada. División Bajas TemperaturasBy means of the vibrating-reed technique we have measured the evolution upon low-temperature annealing of the internal friction Q−1 and sound velocity v of the Cu30Zr70 amorphous alloy between 0.1 and 10 K. With an applied magnetic field of 5.6 T we measured also the normal-state behavior of both properties. Upon annealing, the internal friction and the slope of the logarithmic temperature dependence of v in the superconducting state decrease, in agreement with the standard tunneling model. Our results indicate that with thermal treatments, the coupling constant between tunneling systems (TS’s) and phonons decreases, and the density of states of the TS remains constant or decreases, at most 25%, in the fully relaxed state, which is in agreement with published specific-heat results in these materials. From our data we conclude that the coupling constant between the TS and phonons should be very sensitive to the relaxed state of the disordered structure. The sound-velocity behavior strongly indicates that relaxation processes up to 10 K involve the interaction with the TS. A well-defined change of slope in v at the superconducting critical temperature Tc is observed in the as-quenched and the first relaxed state, which cannot be explained with the standard tunneling model and the Korringa-like relaxation rate between the TS and electrons. Several features observed in the superconducting and normal states lead to the conclusion that the TS-electron interaction problem remains unsolved.Ítem Acceso Abierto Vibrating-reed studies of flux pinning in the superconducting metallic glass Zr70Cu30(American Physical Society, 1988-03-01) Esquinazi, P.; Luzuriaga, J.; Durán, C.; Gerencia Física. Departamento Materia Condensada. División Bajas TemperaturasWith the vibrating-reed technique we have measured the resonance frequency and damping of superconducting ribbons of a Zr70Cu30 amorphous alloy as a function of magnetic field and temperature for different heat treatments of the same sample. Through these measurements we obtain the Labusch parameter α in the reduced-field (b) range 5×10−4b=0.1, α=(3.8×1011)Bc2(T)2.15 N m−4 T−2.15, a result which is in agreement with the absolute value and temperature dependence of the pinning force obtained from critical-current measurements. The effect of annealing below the glass transition temperature is to produce changes in α which depend on field and temperature in a complicated fashion. There are indications that surface pinning could be important in these samples.Ítem Acceso Abierto Evidence of low-energy tunneling excitations in the high-Tc superconductor YBa2Cu3O7-x(American Physical Society, 1988-01-01) Esquinazi, P.; Durán, C.; Fainstein, C.; Nuñez Regueiro, M.; Gerencia Física. Departamento Materia Condensada. División Bajas TemperaturasWe have measured the internal friction Q−1 and the Young's-modulus sound velocity vE at 1 kHz for YBa2Cu3O7−x. Below 20 K the relative change in the sound velocity and the temperature dependence of the internal friction perfectly resemble the behavior obtained for Suprasil W. Below 1 K we observe a logarithmic temperature dependence for the sound velocity and between 1 and 5 K a well-defined plateau in the internal friction is obtained. We find quantitative agreement with the predictions of the standard tunneling model which is widely used to explain the low-temperature properties of amorphous materials. The implications of the existence of tunneling systems in these materials are discussed.