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Esquinazi, P., J. Luzuriaga, and C. Durán. "Vibrating-reed studies of flux pinning in the superconducting metallic glass Zr70Cu30." Physical Review B 37, no. 7 (1988): 3689–3692.
Abstract: With 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 in the reduced-field (b) range 5
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Luzuriaga, J. "Pinning forces and defects as measured by critical currents in amorphous superconducting materials." Physical Review B 35, no. 7 (1987): 3625–3628.
Abstract: Analysis 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 towards a state with fewer or smaller disclinations. If disclinations are identified with two-level systems this trend is consistent with a decrease of the scattering of phonons by TLS which is observed upon annealing in thermal conductivity measurements.
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Herbsommer, J. A., G. Nieva, and J. Luzuriaga. "Interplay between pinning energy and vortex interaction in YBa2Cu3O7-delta with oriented twin boundaries in tilted magnetic fields: Bitter decoration and tilt-modulus measurements." Physical Review B 62, no. 5 (2000): 3534–3541.
Abstract: We have performed Bitter decoration and ac susceptibility measurements in single-crystal YBa2Cu3O7-delta 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 de 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 degrees they form a staircase pattern, and in this case pinning by the twin boundaries remains effective up to 75 degrees. We observe vortex chains in twin-free-zones of the sample for tilted fields at 65 degrees and 40 degrees. 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.
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Esquinazi, P., J. Luzuriaga, C. Durán, D. A. Esparza, and C. D'Ovidio. "Anomalies in the internal friction and sound velocity in the high-temperature superconductor La1.8Sr0.2CuO4." Physical Review B 36, no. 4 (1987): 2316–2318.
Abstract: The 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.
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de la Cruz, F., H. J. Fink, and J. Luzuriaga. "Temperature dependence of the superconducting giant-vortex state. Theory and Experiment." Physical Review B 20, no. 5 (1979): 1947–1959.
Abstract: When a type-I superconductor with a surface nucleation field Hc3(T)>Hc(T) (thermodynamic critical field) is thermally cycled in an axially applied magnetic field H0 between the temperatures T(Hc3) and about T(Hc), experiments show that the magnetization changes reversibly. The latter is diamagnetic near T(Hc3) but can be paramagnetic just above T(Hc). This behavior is explained by assuming that the fluxoid quantum number b is fixed at the transition from the normal to the superconducting state and retained at lower temperatures. The value of b is determined almost entirely by the flux at the transition which is enclosed by a contour located at a distance ξ1.7 from the surface inside the cylinder (ξ is the coherence length). The temperature variation of the order parameter f at the surface of the cylinder, the magnetization m, and the temperature at which m=0 for f≠0 are calculated for R>>ξ. Conservation of the fluxoid quantum number, while T is varied causes the two opposing surface currents to become imbalanced. This is the source of the observed para- and diamagnetism.
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Martinez, E. N., P. Esquinazi, and J. Luzuriaga. "Measurements of the superfluid transition in helium by means of a vibrating reed." American Journal of Physics 58, no. 12 (1990): 1163–1168.
Abstract: A relatively simple method is presented that has been used in an undergraduate experimental course for observing the superfluid transition in 4He. The experimental technique consists of submerging a vibrating reed in liquid helium and observing the changes in frequency and damping (energy dissipation) as the temperature is regulated above and below the superfluid transition. The results of the measurements are interpreted by means of a simplification of the acting hydrodynamic forces, and it is seen that most of the relevant physics is well described by the approximations.
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Herbsommer, J. A., G. Nieva, and J. Luzuriaga. "Reproducibility of the disordered vortex structure in twinned single crystals of YBCO." Physica C 341 (2000): 1087–1088.
Abstract: Bitter decorations of the vortices in twinned single crystal YBCO have been obtained to study the disordered structure formed on successive cooldowns of the sample under the same conditions of field and temperature. Between succesive decorations, the iron dots could be removed completely, giving a surface clean enough for further decoration. It was found that successive realizations of the disordered vortex state were very similar at long range, and differed mostly on short scales, compared to the average vortex separation. We have quantified the correlation between successive structures, and double sided decorations have been also performed to compare with the extent of spatial correlation in the applied field direction. The coincidence of the position of the vortices in successive realizations of the structure, is unexpected, and indicates an unusual behavior for a glassy system. Our technique makes it possible to image the position of the vortices in detail and repeatedly, providing us with a model amorphous system in which to study the reproducibility of the disordered structure.
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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. © 2010 IOP Publishing Ltd.
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Arce, R., L. Civale, J. Luzuriaga, J. Guimpel, and F. de la Cruz. "Surface normal regions in superconducting Zr70Cu30 induced by thermal relaxation." Solid State Communications 48, no. 12 (1983): 1027–1030.
Abstract: Penetration depth measurements show that thermal heat treatment in amorphous Zr70Cu30 induces a normal region 5000 Å thick at the surface of ultrarapid quenched ribbons. Upper critical field measurements indicate that the new induced phase is a normal one while the rest of the sample remains as a homogeneous superconducting phase.
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Núñez Regueiro, M., P. Esquinazi, M. A. Izbizky, C. Duran, D. Castello, J. Luzuriaga, and G. Nieva. "Evidence of tunneling systems through thermal and acoustic measurements in ceramic materials." Physica C: Superconductivity and its applications 153-155, no. PART 2 (1988): 1016–1017.
Abstract: We have neasured the sound velocity, internal friction, thermal conductivity and specific heat of La-based and (Y/Eu)- based ceramics. The low temperature data can be understood in terms of the Tunneling Model assuming the existence of Tunneling Systems (TS). From these measured properties we obtain values for the parameter C=P?2/?v2, where P is the density of states of TS, ? the coupling constant between phonons and TS, ? the mass density and v the sound velocity close to those obtained for amorphous and “orientational” disordered crystalline materials. Our data would indicate that the TS are intrinsic defects independent of the sample porosity.
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