Examinando por Autor "Luzuriaga, J."
Mostrando 1 - 20 de 33
Resultados por página
Opciones de ordenación
Ítem Acceso Abierto A quantitative experiment on the fountain effect in superfluid helium(IOP Publishing, 2017-08-17) Amigó, M.L.; Herrera, T.; Neñer, L.; Peralta Gavensky, L.; Turco, F.; Luzuriaga, J.; Gerencia Física. Departamento Materia Condensada. División Bajas TemperaturasSuperfluid 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.Ítem Acceso Abierto Analysis of motion of solid hydrogen tracer particles in oscillating superfluid flows(IOP Publishing, 2014-00-00) Zemma, E.; Luzuriaga, J.; Babuin, S.; Gerencia Física. Departamento Materia Condensada. División Bajas TemperaturasWe have developed a relatively simple cryostat which allows us to image turbulent flows in superfluid helium at temperatures below 2 K, using frozen H2 particles. We analyze the statistics of the velocities of these solid tracers, which follow the turbulent flow generated by oscillating bodies. We have also studied one of the oscillators working in air at room temperature, and traced the flow with solid talcum particles for comparison. Images were recorded by a digital camera at 240 frames per second, while frequencies of the oscillators are between 20 to 45 Hz. The flow is characterized by a modified Reynolds number Reδ based on the viscous penetration depth δ. Software in a dedicated particle tracking velocimetry code allows us to compute the trajectories and velocities of tens of thousands of particles. We have obtained the number of particles for equally spaced intervals of the velocity modulus. For the oscillators in the superfluid, the probability of finding particles at higher velocities has an exponential decay. Within our resolution the statistics in the superfluid for oscillating objects with sharp borders is largely independent of Reδ, while the logarithmic decay at low velocities seems faster than for high velocities for rounded objects. On the other hand, for data taken in air the result is closer to a classical Gaussian distribution of velocities.Ítem Acceso Abierto Angular variation of pinning near the irreversibility temperature in single crystal Y Ba2Cu3O7−δ with splayed columnar defects(Elsevier, 1998-08-01) Herbsommer, J.A.; Luzuriaga, J.; Civale, L.; Nieva, G.; Pasquini, G.; Lanza, H.; Levy, P.; Gerencia Física. Departamento Materia Condensada. División Bajas TemperaturasWe have studied vortex pinning through ac susceptibility measurements in single crystals of YBCO with columnar defects (CDs). The CDs have 0°, 10° and 20° splay angle and average direction 10° off the c-axis. By studying the angular variation we can compensate for the anisotropy and effects of twins, etc. Using a simple expression we can obtain the angular spread when the field direction is outside the splay angle. An increase of pinning when the field direction is inside the angle defined by the CDs can be attributed to vortex entanglement due to the splay and the suppression of the sliding of double kinks.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 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 Anomalous Trajectories of H2 Solid Particles Observed Near a Sphere Oscillating in Superfluid Turbulent 4He(Springer Nature, 2013-05-13) Zemma, E.; Luzuriaga, J.; Gerencia Física. Departamento Materia Condensada. División Bajas TemperaturasUsing a relatively low cost apparatus, consisting of a glass dewar and a digital camera capable of taking images at 240 frames per second we have observed trajectories of frozen H2 particles which follow the flow of liquid helium below 2 K, around a sphere oscillating at 38 Hz. In some of the images the motion is compatible with laminar flow, while at high amplitudes, where we can reach Reynolds numbers of a few thousand in the normal component, the flow is clearly turbulent. In some of the videos taken we find particles being suddenly accelerated to several times the velocity of the oscillating sphere.Ítem Acceso Abierto Current-voltage characteristics in collective pinning(American Physical Society, 1987-11-01) de la Cruz, F.; Luzuriaga, J.; Martinez, E.N.; Osquiguil, E.J.; Gerencia Física. Departamento Materia Condensada. División Bajas TemperaturasThe critical-current curves of amorphous samples which exhibit collective flux pinning have been analyzed, and it was found that the nonlinear part can be fitted by an expression of the form V(I)∼[(I-IK)/IK]ζ. This form, which could be due to critical behavior, is consistent with an interpretation of the unpinning of vortices in the collective regime as a phase transition between the static flux-line lattice (FLL) and the FLL in steady-state movement as has been proposed by Fisher.Ítem Acceso Abierto Dynamic and static mechanical measurements of flux-lattice softening and associated hysteretic depinning dissipation peak in YBa2Cu3Ox ceramic(IOP Publishing, 1990-00-00) D'Anna, G.; Benoit, W.; Luzuriaga, J.; Berger, H.; Gerencia Física. Departamento Materia Condensada. División Bajas TemperaturasWe have studied flux-lattice dissipation in YBa2Cu3Ox ceramic using a forced inverted torsional pendulum. Reversible restoring force variations during temperature sweep suggest that flux lines experience a "hard-soft" transition between 50 K and Tc. Measurements of internal friction and modulus change as a function of the applied stress show a typical depinning behaviour. This dissipation peak during the temperature sweep is understood as a hysteretic depinning in which the critical force depends upon the penetration depth.Ítem Acceso Abierto Even in translation, Richter's 'science' unimpressive(AIP Publishing, 2005-01-01) Luzuriaga, J.; Gerencia Física. Departamento Materia Condensada. División Bajas TemperaturasHaving read the exchange of letters about the value of Ronald Richter´s nuclear fusion work in Bariloche, Argentina, in the early 1950s (PHYSICS TODAY, August 2003, page 12, and March 2004, page 14), I thought it unfortunate that most challenges to Richter’s results are in Spanish. Analyses appear in two reports from the scientific panel that Juan Perón’s government appointed to evaluate the project after Richter’s erratic behavior became apparent.Ítem Acceso Abierto Experimental and numerical study of transverse flux shaking in MgB2 superconductors(IOP Publishing, 2009-00-00) Luzuriaga, J.; Badía-Majós, A.; Nieva, G.; Giordano, J.L.; Lopez, C.; Serquis, A.; Serrano, G.; Gerencia Física. Departamento Materia Condensada. División Bajas TemperaturasMagnetization measurements in the mixed state of thick strips of carbon nanotube doped MgB2 in crossed fields configurations are reported, together with numerical simulations performed with a geometry equivalent to the sample shape. The samples were subjected to magnetic field components along mutually perpendicular directions, an oscillatory field in one direction and a remanent magnetization in the perpendicular direction. The magnetic response along the oscillatory field and the magnetic relaxation perpendicular to it are observed and simulated using the critical state theory. A remarkable quantitative agreement between the experiment and the theory was obtained.Ítem Acceso Abierto Interplay between pinning energy and vortex interaction in YBa2Cu3O7-delta with oriented twin boundaries in tilted magnetic fields: Bitter decoration and tilt-modulus measurements(American Physical Society, 2000-08-01) Herbsommer, J.A.; Nieva, G.; Luzuriaga, J.; Gerencia Física. Departamento Materia Condensada. División Bajas TemperaturasWe have performed Bitter decoration and ac susceptibility measurements in single-crystal YBa2Cu3O7−δ with oriented twin boundaries. The twin boundaries (TB’s) pin vortices over approximately 65% of the sample. The pinned areas are unevenly distributed and some relatively large TB-free regions are present. The Bitter decorations were performed in a 52 Oe dc magnetic field rotated off the c axis so that the plane defined by the field direction and the c axis is perpendicular to the TB’s. Several decorations were performed in the same sample. Additional dynamical information was obtained from ac susceptibility measurements. Results show that for small tilts the vortices remain locked to the c-axis direction, for angles greater than 12° they form a staircase pattern, and in this case pinning by the twin boundaries remains effective up to 75°. We observe vortex chains in twin-free zones of the sample for tilted fields at 65° and 40°. Due to our particular experimental arrangement, in the twinned regions the interplay of the potentials giving rise to the chains and the pinning potential produces a structure with a disordered square symmetry. The data allow us to estimate the dominant energy of the vortex system for some inclinations of the applied magnetic field.Ítem Acceso Abierto Logarithmic to nonlogarithmic flux creep transition and magnetic flux hardening in BiSrCaCuO superconducting ceramics(Elsevier, 1989-12-00) Safar, H.; Durán, C.; Guimpel, J.; Civale, L.; Luzuriaga, J.; Rodríguez, E.; de la Cruz, F.; Fainstein, C.; Schneemeyer, L.F.; Waszczak, J.V.; Gerencia Física. Departamento Materia Condensada. División Bajas TemperaturasA sharp temperature and field dependent transition from a logarithmic to a non-logarithmic flux creep behavior has been found in BiSrCaCuO superconductors. The transition is not followed by any noticeable change in the magnetization. However, vibrating reed measurements show that it coincides with a strong hardening of the magnetic structure. The transition occurs at fields and temperatures well below the reversibility line. The possible relation to the recently proposed flux lattice melting is discussed.Ítem Acceso Abierto Magnetic relaxation induced by transverse flux shaking in MgB2 superconductors(IOP Publishing, 2009-00-00) Luzuriaga, J.; Badía-Majós, A.; Nieva, G.; Giordano, J.L.; Lopez, C.; Serquis, A.; Serrano, G.; Gerencia Física. Departamento Materia Condensada. División Bajas TemperaturasWe report on measurements and numerical simulations of the behavior of MgB2 superconductors when magnetic field components are applied along mutually perpendicular directions. By closely matching the geometry in simulations and measurements, full quantitative agreement is found. The critical state theory and a single phenomenological law, i.e. the field dependence of the critical current density Jc(B), are sufficient for a full quantitative description of the measurements. These were performed in thick strips of carbon nanotube doped MgB2 samples. Magnetization was measured in two orthogonal directions using a SQUID magnetometer. Magnetic relaxation effects induced by the application of an oscillatory perpendicular field were observed and simulated numerically. The measurements confirm the numerical predictions, that two relaxation regimes appear, depending on the amplitude of the applied magnetic field. The overall agreement constitutes a convincing validation of the critical state model and the numerical procedures used.Ítem Acceso Abierto Magnetization collapse in polycrystalline YBCO under transport current cycles(IOP Publishing, 2006-00-00) Giordano, J.L.; Luzuriaga, J.; Badía-Majós, A.; Nieva, G.; Ruiz-Tagle, I. A.; Gerencia Física. Departamento Materia Condensada. División Bajas TemperaturasWe report measurements of the hysteretic magnetization of YBCO under superimposed transport current cycles, together with numerical simulations of magnetization and current density profiles in the corresponding parallel configuration. Field cooled (FC) and zero-field-cooled (ZFC) experiments were carried out on polycrystalline YBa2Cu3O7−x cylinders, with both the applied magnetic field and transport current in the axial direction, and the current cycled several times, around and above the dissipative threshold. As in previously reported multicomponent field configuration experiments, the magnetization is seen to collapse to a more stable state both in FC and ZFC, because of the interplay between the shielding and transport currents. The results of our numerical simulations are in good qualitative agreement with the measurements, and the competition between shielding and transport due to vortex-pinning interactions and equilibrium magnetization effects are shown to play an important role in the range of our experiments.Ítem Acceso Abierto Magnetization measurements of hard superconductors under dissipative transport(IOP Publishing, 2006-00-00) Giordano, J.L.; Luzuriaga, J.; Ruíz-Tagle,; NIEVA, G.; Badía-Majós,; Gerencia Física. Departamento Materia Condensada. División Bajas TemperaturasThe hysteretic magnetic flux dynamics of ceramic type-II superconductors under superimposed transport current cycles has been investigated. Field cooled and zero field cooled experiments were carried-out on polycrystalline YBa2Cu3O7-x cylinders by using both temperature and applied field control, to drive the sample into the semi-reversible granular regime. The transport current magnitude was near and above the dissipative threshold. The interplay between shielding and transport due to vortex-pinning interactions, equilibrium magnetization effects, and diamagnetic/paramagnetic relaxation phenomena are shown to play an important role in the range of practical applications.Ítem Acceso Abierto Mean free path in soccer and gases(IOP Publishing, 2010-07-21) Luzuriaga, J.; Gerencia Física. Departamento Materia Condensada. División Bajas TemperaturasThe trajectories of the molecules in an ideal gas and of the ball in a soccer game are compared. The great difference between these motions and some similarities are discussed. This example could be suitable for discussing many concepts in kinetic theory in a way that can be pictured by students for getting a more intuitive understanding. It could be suitable for an introductory course in vacuum techniques or undergraduate courses in kinetic theory of gases. Without going into the slightly harder quantitative results, the analysis presented might be used for introducing some ideas of kinetic theory qualitatively to high school students.Ítem Acceso Abierto Mechanical response of the flux lines in ceramic YBa2Cu3O7-(American Physical Society, 1992-06-01) Luzuriaga, J.; Andre, M.-O.; Benoit, W.; Gerencia Física. Departamento Materia Condensada. División Bajas TemperaturasWe have studied the mechanical response of the flux-line lattice (FLL) in ceramic samples of YBa2Cu3O7 by means of a low-frequency forced pendulum. The internal friction and elastic modulus variation of the FLL have been measured as a function of temperature for different values of the applied stress. A somewhat different behavior was observed whether a zero-field-cooling or field-cooling procedure was followed. Measurements of the internal friction and elastic modulus as a function of the applied stress at constant temperature show amplitude-dependent dissipation, with a maximum dissipation at intermediate values of the stress. This dependence is well fitted by a rheological model of extended dry friction, if we restrict ourselves to the dissipation and modulus at fixed temperature. The agreement is not so good when attempting to extend the model to fit the temperature dependence.Ítem Acceso Abierto On the Non-linear Damping of Mechanical Oscillators in Flows of 4He(Springer Nature, 2014-00-00) Schmoranzer, D.; Jackson, M.J.; Luzuriaga, J.; Gerencia Física. Departamento Materia Condensada. División Bajas TemperaturasIn the studies of both classical and quantum turbulence, significant attention is devoted to the investigation of the behavior of various submerged resonators. Upon entering the turbulent regime, the oscillators start to experience a significant drag force, which varies non-linearly with velocity. We present an empirical way of modeling such systems, and calculate the expected resonant response of such oscillators near the fundamental frequency as a function of the applied driving force. We apply the model to the crossover from linear to non-linear drag forces and compare with previous models as well as selected experimental data on the transition to turbulence in oscillatory flow of 4He.Ítem Acceso Abierto Pinning forces and defects as measured by critical currents in amorphous superconducting materials(American Physical Society, 1987-03-01) Luzuriaga, J.; Gerencia Física. Departamento Materia Condensada. División Bajas TemperaturasAnalysis of the available data on critical currents in amorphous superconductors shows a striking similarity in the defects responsible for the pinning force in all samples in which two-dimensional flux pinning can be observed. Order-of-magnitude calculations show that this is consistent with the existence of disclinations in glasses as has been proposed in different theoretical papers by Rivier, Nelson, Kleman, and Sadoc. The evolution of pinning forces with annealing also shows a similar behavior in all the published data, and the trend is toward a state with fewer or smaller disclinations. If disclinations are identified with two-level systems (TLS) this trend is consistent with a decrease of the scattering of phonons by TLS which is observed upon annealing in thermal conductivity measurements.Ítem Acceso Abierto Possible visualization of a superfluid vortex loop attached to an oscillating Beam(Springer Nature, 2015-02-00) Zemma, E.; Tsubota, M.; Luzuriaga, J.; Gerencia Física. Departamento Materia Condensada. División Bajas TemperaturasVisualization using tracer particles is a relatively new tool available for the study of superfluid turbulence and flow, which is applied here to oscillating objects submerged in the liquid. We report observations of a structure seen in videos taken from outside a cryostat filled with superfluid helium at 2 K, which is possibly a vortex loop attached to an oscillator. The feature, which has the shape of an incomplete arch, is visualized due to the presence of solid H2 tracer particles and is attached to a beam oscillating at 38 Hz in the liquid. It has been recorded in videos taken at 240 frames per second, fast enough to take ~6 images per period. This makes it possible to follow the structure, and to see that it is not rigid. It moves with respect to the oscillator, and its displacement is in phase with the velocity of the moving beam. Analyzing the motion, we come to the conclusion that we may be observing a superfluid vortex attached to the beam and decorated by the hydrogen particles. An alternative model, considering a solid hydrogen filament, has also been analyzed, but the observed phase between the movement of the beam and the filamentary structure is better explained by the superfluid vortex hypothesis.