Luzuriaga, J., A. BadíaMajós, G. Nieva, J. L. Giordano, C. Lopez, A. Serquis, and G. Serrano. "Magnetic relaxation induced by transverse flux shaking in MgB2 superconductors." Superconductor Science and Technology 22 (2009): 15021.
Abstract: We 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.

Luzuriaga, J., A. BadíaMajós, G. Nieva, J. L. Giordano, C. Lopez, A. Serquis, and G. Serrano. "Experimental and numerical study of transverse flux shaking in MgB2 superconductors." In Journal of Physics: Conference Series, 012009. Vol. 167., 2009.
Abstract: Magnetization measurements in the mixed state of thick strips of carbon nanotube
doped MgB
2
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.

Conte, V., J. L. Giordano, J. J. Zarate, and J. Luzuriaga. "Superconducting Devices for Radioastronomy; First Steps in Chile: SNSjunction Fabrication.", 424–427. Vol. 1., 2010.
Abstract: We present results of the microfabrication of Josephson Junctions (JJs) of SuperconductorNormalsuperconductor (SNS) type, as a first step in the acquisition of the knowhow of superconducting devices applied to radioastronomy. A twojunction SQUID was built using a Nb/Al bilayer deposited on a Si wafer. The procedure needs only one mask and a single UV exposure (i.e., one photolithography step). We show the micrograph and the observed non linear characteristic IV curve at 4K of the first device fabricated in the framework of ALMA Grant 31090010.

Luzuriaga, J. "Mean free path in soccer and gases." European Journal of Physics 31, no. 5 (2010): 1071–1076.
Abstract: The 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. Â© 2010 IOP Publishing Ltd.

Zemma, E., and J. Luzuriaga. "Measurements of Turbulence Onset and Dissipation in Superfluid Helium with a Silicon Double Paddle Oscillator." Journal of Low Temperature Physics 166, no. 34 (2012): 171–181.
Abstract: We have studied experimentally the response of a silicon single crystal double paddle oscillator submerged in superfluid helium from the lambda point to 1.55 K. Measuring the resonance frequency and dissipation on three modes of this high Q system allows us to study the dissipation at the onset of turbulence in the flow around the paddle. The critical velocity V c for turbulence onset decreases with temperature. If we use the density of the normal component of the superfluid to obtain a Reynolds number Re associated with V c we find a value which is largely temperature independent. This result is different from the behavior previously found by other authors below 1 K, where the quantized vorticity (extrinsic nucleation) is observed at velocities more than an order of magnitude greater. In our temperature range, we conclude that the transition is governed by the normal fraction acting as a classical fluid. The laminar regime shows a dissipation that is proportional to the viscous drag calculated by well known formulas for an object oscillating in a liquid. We also find a decrease in resonance frequency in the turbulent regime which is clearly observed but hard to reproduce from run to run.

Zemma, E., and J. Luzuriaga. "Anomalous Trajectories of H2 Solid Particles Observed Near a Sphere Oscillating in Superfluid Turbulent 4He." Journal of Low Temperature Physics 173, no. 12 (2013): 71–79.
Abstract: Using 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.

Zemma, E., and J. Luzuriaga. "Turbulent Flow Around an Oscillating Body in Superfluid Helium: Dissipation Characteristics of the Nonlinear Regime." Journal of Low Temperature Physics 172, no. 34 (2013): 256–265.
Abstract: By examining the resonance curves of an oscillator submerged in superfluid liquid helium, it is found that their shape is affected by two distinct dissipation regimes when the amplitude is large enough to generate turbulence in the liquid. In a resonance curve, the central part close to resonance, may be in a turbulent regime, but the response is of much lower amplitude away from the resonance frequency, so that the oscillation can still be in the linear regime for frequencies not exactly at resonance. This introduces an ambiguity in estimating the inverse quality factor Q âˆ’1 of the oscillator. By analyzing experimental data we consider a way of matching the two ways of estimating Q âˆ’1 and use the information to evaluate the frictional force as a function of velocity in a silicon paddle oscillator generating turbulence in the superfluid.

Zemma, E., J. Luzuriaga, and S. Babuin. "Analysis of motion of solid hydrogen tracer particles in oscillating superfluid flows." In Journal of Physics: Conference Series, 012029. Vol. 568., 2014.
Abstract: We 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 ReÎ´ based on the viscous penetration depth Î´. 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 ReÎ´, 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.

Schmoranzer, D., M. J. Jackson, and J. Luzuriaga. "On the Nonlinear Damping of Mechanical Oscillators in Flows of 4He." Journal of Low Temperature Physics 175, no. 1 (2014): 97–103.
Abstract: In 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 nonlinearly 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 nonlinear drag forces and compare with previous models as well as selected experimental data on the transition to turbulence in oscillatory flow of 4He.

Godfrin, H., J. Luzuriaga, E. Collin, and J. Guimpel, eds. Frontiers of Low Temperature Physics ULT2014., 2014.
Abstract: Abstract book for the UltraLow Temperatures conference held at S.C. de Bariloche as a satellite to the LT27.
