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 twodimensional flux pinning can be observed. Orderofmagnitude 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 twolevel systems this trend is consistent with a decrease of the scattering of phonons by TLS which is observed upon annealing in thermal conductivity measurements.

Luzuriaga, J., M.  O. Andre, and W. Benoit. "Frequency and amplitude response of the fluxline lattice to mechanical perturbation in ceramic YBa2Cu3O7." Physica C: Superconductivity and its applications 201, no. 34 (1992): 257–262.
Abstract: The mechanical response of the fluxline lattice has been measured with a lowfrequency forced pendulum in ceramic YBa2Cu3O7. A dissipation peak observed in temperature sweeps is frequencyindependent between 1 mHz and 5 Hz. Dissipation depends strongly on applied torque, and for fixed temperatures this dependence is well fitted by a rheological model of extended dry friction. If the model is extended to take account of thermal activation, however, it does not agree with the measured frequency independence, which is hard to explain within simple models of thermal activation.

Luzuriaga, J., M.  O. Andre, and W. Benoit. "Mechanical response of the flux lines in ceramic YBa2Cu3O7." Physical Review B 45, no. 21 (1992): 12492–12497.
Abstract: We have studied the mechanical response of the fluxline lattice (FLL) in ceramic samples of YBa2Cu3O7 by means of a lowfrequency 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 zerofieldcooling or fieldcooling procedure was followed. Measurements of the internal friction and elastic modulus as a function of the applied stress at constant temperature show amplitudedependent 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.

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.

Luzuriaga, J., C. D'Ovidio, and F. de la Cruz. "Ideal two dimensional flux pinning induced by annealing in superconducting amorphous Zr75Rh25." Solid State Communications 57, no. 9 (1986): 753–756.
Abstract: By annealing amorphous Zr75Rh25 prepared by melt spinning an ideal behaviour of the critical current was observed, in which the measured pinning forces are seen to follow the two dimensional LarkinOrchivnikov theory. In the region where this theory is valid the changes in the pinning forces can be explained by the changes in the superconducting matrix and it can be assumed that the pinning centers do not change with the heat treatment.

Luzuriaga, J., and F. de la Cruz. "Reversible magnetization of surface superconductivity." Solid State Communications 25, no. 8 (1978): 605–607.
Abstract: Direct measurements of the surface superconducting magnetization of the PbTl system shows that this state is a thermodynamic reversible one.

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.

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.

NÃºÃ±ez Regueiro, M., P. Esquinazi, M. A. Izbizky, C. Durán, D. Castello, and J. Luzuriaga. "Tunneling systems in high temperature superconductors." Annales de Physique 13, no. 5 (1988): 401–406.
Abstract: We have measured the thermal conductivity, sound attenuation and relative variation of sound velocity of compounds of the LaSrCuO and RBa2Cu3O7 families, where R is a rareearth. We have found that all these properties can be quantitatively correlated within a tunneling system model similar to that used in amorphous materials. This interpretation would naturally explain the anomalous linear term of the low temperature specific heat observed in these materials.

NÃºÃ±ez 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 153155, no. PART 2 (1988): 1016–1017.
Abstract: We have neasured the sound velocity, internal friction, thermal conductivity and specific heat of Labased 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.
