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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Civale, L., F. de la Cruz, and J. Luzuriaga. "Critical temperature and density of states in amorphous Zr70Cu30." Solid State Communications 48, no. 4 (1983): 389–391.
Abstract: By means of thermal heat treatment of amorphous Zr70Cu30 it is shown that the induced decrease in critical temperature is followed by a reduction in the electronic density of states of the same magnitude as that obtained by changing the Cu concentration. This result indicates that the density of states is the fundamental microscopic parameter determining the superconducting behavior.
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de la Cruz, F. "Structural relaxation: low temperature properties.", 269. Proceedings of the Latin American Symposium of Physics of Amorphous Systems – v1. Brazil, 1984.
Abstract: We discuss the changes in transport and superconducting properties of amorphous Zr70Cu30,
induced by thermal relaxation The experimental results are used to investigate the
relation between the microscopic parameters and the observed physical properties It
is shown that the density of eletronic states determines the shift Tc as well as the
variation of the electrical resistivity It is necessary to assume strong hybridization
between s and d bands to understand the eletrodynamic response of the superconductor
(Author)
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Decca, R. S., H. D. Drew, B. Maiorov, J. Guimpel, and E. J. Osquiguil. "Inducing superconductivity at a nanoscale: Photodoping with a near- field scanning optical microscope." Journal of Microscopy 194, no. 2-3 (1999): 407–411.
Abstract: The local modification of an insulating GdBa2Cu3O6.5 thin film, made superconducting by illumination with a near-field scanning optical microscope (NSOM), is reported. A 100-nm aperture NSOM probe acts as a sub- wavelength light source of wavelength ?(exc)=480-650 nm, locally generating photocarriers in an otherwise insulating GdBa2-Cu3O6.5 thin film. Of the photogenerated electron-hole pairs, electrons are trapped in the crystallographic lattice, defining an electrostatic confining potential to enable the holes to move. Reflectance measurements at ? = 1.55 ?m at room temperature show that photocarriers can be induced and constrained to move on a ?? 200 nm scale for all investigated ?(exc). Photogenerated wires present a superconducting critical temperature T(c) = 12 K with a critical current density J(c) = 104 A cm-2. Exploiting the flexibility provided by photodoping through a NSOM probe, a junction was written by photodoping a wire with a narrow (? 50 nm) under-illuminated gap. The strong magnetic field modulation of the critical current provides a clear signature of the existence of a Josephson effect in the junction.
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Decca, R. S., H. D. Drew, E. Osquiguil, B. Maiorov, and J. Guimpel. "Anomalous proximity effect in underdoped YBa2Cu3O6+x Josephson junctions." Physical Review Letters 85, no. 17 (2000): 3708–3711.
Abstract: Experiments were carried out to probe the underdoped insulating material by the Josephson effect. Junctions were fabricated by exploiting the capability of locally photodoping insulating RBa2Cu3O6+x (R = rare earth) material. The existence of an anomalously large proximity effect was confirmed. The critical current of the junctions was consisted with the conventional Josephson relationship.
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Dhalle, M., W. Boon, E. Osquiguil, C. Van Haesendonck, Y. Bruynseraede, J. Kwarciak, and O. Van Der Biest. "Correlation between critical current and resistivity in YBa2Cu3O7." Physica C: Superconductivity and its applications 185-189, no. PART 4 (1991): 2447–2448.
Abstract: Critical current versus temperature measurements were carried out in ceramic YBa2Cu3O7 samples. The samples were all prepared using the same sintering procedure, but starting from powders obtained by different wet and dry methods. All samples showed a linear Jc vs. T behaviour, typical of Josephson weak-link coupling, with a slope that depends on the preparation method. Using the Ambegaokar-Baratoff relation for Jc, the difference in slopes can be attributed to a change in the characteristic value of the junction resistance. This value turns out to be clearly correlated with the resistivity in the normal state.
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Durán, C., P. Esquinazi, J. Luzuriaga, and E. H. Brandt. "Bc1 of high-Tc La1.8Sr0.2CuO4 and amorphous Zr70Cu30 superconductors measured by a vibrating reed." Physics Letters A 123, no. 9 (1987): 485–488.
Abstract: The lower critical field Bc1 of the high-Tc superconductor La1.8Sr0.2CuO4 (Tc=40 K, Bcless-than-or-equals, slant7.7 mT) and of amorphous Zr70Cu30 (Tc=2.6K, Bc1less-than-or-equals, slant3.8 mT) is obtained from the resonance frequency ?/2? of a vibrating reed made of these materials. At applied magnetic fields Ba<Bc1, ?(Ba)=?i(Ba) as predicted for an ideally diamagnetic reed. Above Bc1 we observe a reversible linear law ?2??2inot, vert, similarB2a?B2c1. This allows a simple definition of Bc1 even in ceramic superconductors into which flux starts to penetrate at Bamuch less-thanBc1 due to their granular structure.
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Guimpel, J., and F. de la Cruz. "Electrical resistivity of amorphous Zr70Cu30 and the Kondo like model." Solid State Communications 44, no. 7 (1982): 1045–1046.
Abstract: Electrical resistivity measurements of amorphous Zr70Cu30 as a function of the concentrations of two level systems show that a Kondo like theory cannot explain the observed temperature dependence.
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Herbsommer, J. A., J. Luzuriaga, and S. - W. Cheong. "Vortex glass melting in single crystal La1.825 Sr0.075CuO4." Physica C 258, no. 1-2 (1996): 169–174.
Abstract: The vortex phase diagram in single crystalline La1.85Sr0.075CuO4 has been studied using an AC-susceptibility technique. A peak in the out-of-phase (x?) component of the susceptibility indicates a transition from a pinned flux lattice (FLL) to an unpinned one. This peak is frequency dependent for all the values of the magnetic field measured (0.01 to 4 T), and this, as well as the general behavior found in the cuprates, has prompted us to interpret our data as evidence for a vortex-glass to liquid transition in the FLL. The activation energies obtained can be fitted to a theory developed by Vinokur et al. Measurements with the magnetic field at an angle with the Cu-O planes may also be understood qualitatively within this framework.
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Herbsommer, J. A., J. Luzuriaga, L. Civale, G. Nieva, G. Pasquini, H. Lanza, and P. Levy. "Angular variation of pinning near the irreversibility temperature in single crystal Y Ba2Cu3O7-? with splayed columnar defects." Physica C 304, no. 1-2 (1998): 112–118.
Abstract: We 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 108 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.
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Lederman, D., E. Osquiguil, G. Nieva, J. Guimpel, J. Hasen, Y. Bruynseraede, and I. K. Schuller. "Photoinduced enhancement of superconductivity." Journal of Superconductivity 7, no. 1 (1994): 127–130.
Abstract: The photoinduced enhancement of superconductivity in RBa2Cu3Ox (R=rare earth or yttrium) and PryR1-yBa2Cu3Ox was explored through temperature-dependent resistivity, Hall coefficient and mobility, and x-ray diffraction measurements. The increases in Tc are enhanced near the metal-insulator transition, although photoinduced changes always exist in oxygendeficient samples. Several explanations, including intergrain Josephson coupling, photoassisted oxygen ordering, and the trapping of photogenerated electrons in oxygen vacancies, are discussed.
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Libbrecht, S., E. Osquiguil, B. Wuyts, M. Maenhoudt, Z. X. Gao, and Y. Bruynseraede. "Effect of room temperature annealing on the properties of oxygen- deficient YBa2Cu3Ox films." Physica C: Superconductivity and its applications 206, no. 1-2 (1993): 51–58.
Abstract: Annealing at room temperature of quenched oxygen-depleted YBa2Cu3Oxn (6.5?xn<6.8) films, produces a clear increase in the critical temperature Tc and current density Jc, while the normal state resistivity ?{variant}xx and Hall coefficient RH decrease as a function of annealing time, indicating an increase in carrier density. The enhancement of the critical current density Jc is more pronounced at high fields and temperatures due to a shift of the irreversibility line, which scales with the increasing Tc. The observed changes are accompanied by corresponding changes in the crystalline structure, which can be related to oxygen ordering effects.
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Luzuriaga, J., M. - O. Andre, and W. Benoit. "Frequency and amplitude response of the flux-line lattice to mechanical perturbation in ceramic YBa2Cu3O7." Physica C: Superconductivity and its applications 201, no. 3-4 (1992): 257–262.
Abstract: The mechanical response of the flux-line lattice has been measured with a low-frequency forced pendulum in ceramic YBa2Cu3O7. A dissipation peak observed in temperature sweeps is frequency-independent 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.
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Maiorov, B., and E. Osquiguil. "Vortex solid state in YBa2Cu3O7-? twinned crystals." Physical Review B – Condensed Matter and Materials Physics 64, no. 5 (2001): 0525111–0525114.
Abstract: We report on the scaling of transport properties around the vortex melting in YBa2Cu3O7-? oriented-twin single crystals in applied magnetic fields between 1 T and 18 T. We find that for the whole measured field range the linear resistivity scales as ?(t,?)?tsyF
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Maiorov, B., and E. Osquiguil. "Bose-glass-like phases in oriented-twin YBa2Cu3O7-? crystals." Journal of Low Temperature Physics 135, no. 1-2 (2004): 131–134.
Abstract: We experimentally study the dynamical response of the superconducting vortex system near the solid-liquid transition by applying forces parallel and perpendicular to a planar random defect array. Our results show that for magnetic fields above a certain critical field Hcr'? 4 T the solid glassy vortex phase in YBa2Cu3O7-? crystals with oriented twin planes does not correspond to the so-called Bose-glass phase.
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Maiorov, B., A. V. Silhanek, F. De la Cruz, E. Osquiguil, A. Rydh, and Rapp. "Comment on “strong vortex liquid correlation from multiterminal measurements on untwinned YBa2Cu3O7-? single crystals” [2] (multiple letters)." Physical Review Letters 88, no. 13 (2002).
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Nakamura, O., I. Chan, J. Guimpel, and I. Schuller. "New buffer layer for high?temperature superconducting ceramics on sapphire: LaBa2Cu3Oy/Ag bilayers." Applied Physics Letters 59, no. 10 (1991): 1245.
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Nakamura, O., J. Guimpel, F. Sharifi, R. Dynes, and I. Schuller. "Synthesis and properties of a-axis and b-axis oriented GdBa2Cu3O7-d high Tc thin films." Applied Physics Letters 61, no. 21 (1992): 2598.
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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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Osquiguil, E., V. L. P. Frank, and F. de la Cruz. "Two dimensional collective flux pinning in melt spun superconducting amorphous Zr70Cu30." Solid State Communications 55, no. 3 (1985): 227–230.
Abstract: We have measured the critical currents in melt spun amorphous Zr70Cu30 and their evolution with heat treatment. We found that the pinning forces may be interpreted in terms of the Larkin-Ovchinnikov two dimensional collective pinning theory.
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Osquiguil, E., M. Maenhoudt, B. Wuyts, Y. Bruynseraede, D. Lederman, G. Nieva, J. Guimpel, and I. K. Schuller. "Photoexcitation effects in YBa2Cu3Ox." Journal of Alloys and Compounds 195, no. C (1993): 667–670.
Abstract: We show through photoexcitation experiments carried out on YBa2Cu3Ox c-axis oriented films that photoinduced phenomena are not restricted to insulating samples. Superconducting films with critical temperatures varying between 2 K and 40 K show a clear enhancement of the superconducting transition after illumination. The increase in Tc is accompanied by a corresponding decrease in the resistivity ?, and an increase in the Hall coefficient RH, indicative of an increased carrier density. Furthermore, the carrier mobility is also affected by illumination.
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Rigal, L. B., D. C. Schmadel, H. D. Drew, B. Maiorov, E. Osquiguil, J. S. Preston, R. Hughes, and G. D. Gu. "Magneto-optical evidence for a gapped Fermi surface in underdoped YBa 2Cu3O6+x." Physical Review Letters 93, no. 13 (2004).
Abstract: An evidence presented that supports partial gapping of the Fermi surface by a measurement of the Hall frequency from high frequency magneto-optical measurements on underdoped YBa2Cu3O6+x (YBCO) was discussed. It was found that the Hall frequency was obtained independently of the unknown k-space dependence of the carrier scattering rates that complicated the dc Hall effect. It was observed that the frequency dependence of the Hall angle was Drude-like and indicated a quasielastic relaxation process for optimal and underdoped samples. The results show that there was an increase of the Hall frequency in the pseudogap state.
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Rodr. "Role of anisotropy in the vortex lattice of superconductors high-Q oscillator measurements in 2H-NbSe2 and La1.825Sr0.075CuO4." Physica C: Superconductivity and its applications 229, no. 1-2 (1994): 59–64.
Abstract: Measurement of the response of the flux-line lattice in NbSe2 and La1.825Sr0.075CuO4 show important differences between these two materials. In particular, we have studied the magnetic-field and angular dependence of the response of a high-Q mechanical oscillator in fields of up to 1 T. The features seen in NbSe2 seem to be well explained in terms of a change in the pinning regime, usually termed the “peak effect” in the critical current, using the collective-pinning model of Larkin and Ovchivnikov within Ginzburg-Landau anisotropic theory. On the other hand the behavior found in the high-Tc material LSCO seems to fall naturally into a description which takes into account the possibility of phase transitions in the vortex lattice and the quasi-two-dimensional character of the superconductivity.
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Rodr. "Role of anisotropy in the vortex lattice of superconductors high-Q oscillator measurements in 2H-NbSe2 and La1.825Sr0.075CuO4." Physica C: Superconductivity and its applications 229, no. 1-2 (1994): 59–64.
Abstract: Measurement of the response of the flux-line lattice in NbSe2 and La1.825Sr0.075CuO4 show important differences between these two materials. In particular, we have studied the magnetic-field and angular dependence of the response of a high-Q mechanical oscillator in fields of up to 1 T. The features seen in NbSe2 seem to be well explained in terms of a change in the pinning regime, usually termed the “peak effect” in the critical current, using the collective-pinning model of Larkin and Ovchivnikov within Ginzburg-Landau anisotropic theory. On the other hand the behavior found in the high-Tc material LSCO seems to fall naturally into a description which takes into account the possibility of phase transitions in the vortex lattice and the quasi-two-dimensional character of the superconductivity.
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Tutzauer, H., P. Esquinazi, M. E. de la Cruz, and F. de la Cruz. "Production of ribbon quenched samples using an arc furnace." Review of Scientific Instruments 51, no. 6 (1980): 855–856.
Abstract: We describe a new device to produce ribbons of disordered metals by ultrarapid quenching combining an arc furnace with a rotating cylinder. The characterization of samples of disordered La70 Cu30 and La76Au24 prepared using this method shows good agreement with those obtained by other authors for the same alloys.
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Valenzuela, S. O., B. Maiorov, E. Osquiguil, and V. Bekeris. "Elastic-to-plastic crossover below the peak effect in the vortex solid of YBa2Cu3O7 single crystals." Physical Review B – Condensed Matter and Materials Physics 65, no. 6 (2002): 605041–605044.
Abstract: We report on transport and ac susceptibility studies below the peak effect in twinned YBa2Cu3O7 single crystals. We find that disorder generated at the peak effect can be partially inhibited by forcing vortices to move with an ac driving current. The vortex system can be additionally ordered below a well-defined temperature where elastic interactions between vortices overcome pinning-generated stress and a plastic-to-elastic crossover seems to occur. The combined effect of these two processes results in vortex structures with different mobilities that give place to history effects.
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Vanacken, J., S. Libbrecht, M. Maenhoudt, C. Van Haesendonck, E. Osquiguil, and Y. Bruynseraede. "The magnetic moment relaxation in oxygen-deficient YBa2Cu3Ox." Physica C: Superconductivity and its applications 197, no. 1-2 (1992): 9–14.
Abstract: The magnetic moment relaxation in polycrystalline YBa2Cu3Oxmaterial has been studied using zero-field-cooled measurements. The temperature dependence of the relaxation rate shows a field-dependent maximum. When the oxygen content is reduced, the maximum shifts towards lower temperatures. This shift is directly related to a decrease of the intragrain critical current density in the oxygen-deficient material, inducing a decrease of the field H* for complete flux penetration. The characteristic activation energies for depinning in the oxygen-deficient material are below 20 meV.
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Vanacken, J., E. Osquiguil, and Y. Bruynseraede. "Irreversibility line and critical currents in oxygen deficient YBa2Cu3Ox ceramics." Physica C: Superconductivity and its applications 183, no. 1-3 (1991): 163–166.
Abstract: The influence of oxygen concentration on the irreversibility line and critical current of polycrystalline YBa2Cu3Ox was determined using field and zero-field cooled magnetic measurements and magnetic hysteresis data. Although the field dependence of Tirr(H) is unchanged, the irreversibility line shifts to lower temperatures for decreasing oxygen content and the critical current density is strongly reduced.
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Watt. "Photoinduced changes in Raman spectra of YBa2Cu3O6.4 films." Journal of Superconductivity 7, no. 1 (1994): 131–136.
Abstract: The evolution of Raman spectra with illumination has been studied in YBa2Cu3O6.4 films at temperatures between 5-300 K. Low laser power has always been used to avoid local overheating, which was controlled by measuring the local temperature by the Stokes/anti-Stokes ratio. Three important photoinduced effects have been found: (i) the enhancement of the intensity of the observed phonon modes: (Cu(2) at 141 cm-1, O(2)-O(3) at 338 cm-1, and O(4) at 488 cm-1), which may be related to the ordering of oxygen vacancies, (ii) the increase of the electronic scattering background for low Raman frequencies ?, which is in agreement with the enhancement of the static conductivity ?(??0) after illumination, and (iii) the suppression of the intensity of the two-magnon band, which may be caused by the increase of charge carriers due to photodoping.
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Wuyts, B., Z. X. Gao, S. Libbrecht, M. Maenhoudt, E. Osquiguil, and Y. Bruynseraede. "Growth of particle-free YBa2Cu3O7 films by off-axis sputtering." Physica C: Superconductivity and its applications 203, no. 3-4 (1992): 235–239.
Abstract: The preparation of good YBa2Cu3O7 thin films is often hindered by the production of non-stoichiometric particles which degrade the superconducting properties of the layer. A systematic study of films prepared by off-axis single target magnetron sputtering shows that the presence of copper-rich particles dramatically decreases the critical current density but does not affect the critical temperature. The formation of these particles can be avoided by carefully adjusting the position of the substrate with respect to the target. Films without off-stoichiometric particles have excellent superconducting properties.
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