1PO2-90 Pinning regimes in YBaCuO single crystals below the melting line
Sergio O. Valenzuela and Victoria Bekeris, Laboratorio de Bajas Temperaturas, Depto. de Física, F.C.E.y N., Universidad Nacional de Buenos Aires, Pabellón I, Ciudad Universitaria, 1428, Buenos Aires, Argentina
Presenting Author: S.O. Valenzuela
We performed minor hysteresis loops in YBaCuO single crystals by superimposing a small ripple to a large dc magnetic field. The hysteresis loops are reconstructed both from the harmonic components of the susceptibility and from the time dependent signal in the susceptometer secondary coil. For temperatures and fields below the thermodynamic melting line, the hysteresis loops indicate that different parts of the sample could be in different pinning regimes depending on its thermomagnetic history. A linear regime in the inner region, where vortices remain pinned, and a nonlinear regime occurring in the outer zone, where the higher currents drive the vortices out of the pinning sites and assist the vortex structure in ordering, locally reducing the critical current density.
1PO2-91 Influence of a driving force on the pinning of a field-cooled vortex lattice
Alexey V. Pan and Pablo Esquinazi, Department of Superconductivity and Magnetism, Institute for Experimental Physics II, University of Leipzig, Linnéstrasse 5, D-04103 Leipzig, Germany
Presenting Author: A.V. Pan
We have investigated the influence of very small driving currents on the elastic coupling of the field cooled (FC) state of the flux line lattice (FLL) in 400 nm thick YBa2Cu3O7 superconducting films. The measurement of the elastic coupling has been performed with the vibrating reed technique loading the sample in situ with dc transport currents. We found that the FLL cooled without a driving force is not in an equilibrium state. Results obtained for magnetic fields applied at 0° and 30° relative to the CuO2 planes, show an enhancement of the elastic coupling of the films for high enough external drive. This result implies a plastic reassembling of the FLL to a stronger pinned configuration. This pinning enhancement is observed at driving currents orders of magnitude lower than the critical one and at temperatures and magnetic fields far below the thermally activated depinning transition. We found a 25% increase of the elastic coupling with the external drive in agreement with numerical simulations done by E.H. Brandt, Phys. Rev. Lett. 50 (1983) 1599.
1PO2-92 Nonlinear microwave response of an YBCO single crystal in an applyed magnetic field
V.V. Bolginov, V.M. Genkin, G.I. Leviev, and L.V. Ovchinnikova, Institute of Solid State Physics RAS, Chernogolovka, Moscow District, Russia, 142432
Presenting Author: V.V. Bolginov
The microwave second harmonics generation by an YBCO single crystal in a magnetic field is studied. The signal is found to be connected with dc screening current in an applied magnetic field. It is demonstrated that the field dependence of the harmonics intensity yields directly the pinning current dependence on a magnetic field. It is found that the experimental data can be interpreted in terms of the generalized critical current model that takes into account the vortices diffusion and the absence of a barrier preventing vortices from escape from a sample. It is shown that near the sample surface for decreasing fields the state is realized where the screening current density is much smaller against the critical density. The experiment shows that vortices are not radiation sources at a double frequency. The relationship between the harmonics radiation mechanism in the Meissner phase and modulation of the order parameter by a microwave magnetic field (Ginzburg-Landau nonlinearity) is discussed. It is found that by measuring the temperature dependence of the harmonics radiation intensity in the Meissner state one can obtain, with a fair accuracy, the temperature dependence of the magnetic field penetration depth.
1PO2-93 High-frequency Electromagnetic Response in the Mixed State of YBa2Cu3Oy
Yoshishige Tsuchiya 1, Katsuya Iwaya 1, Tetsuo Hanaguri 2, Haruhisa Kitano 1, Atsutaka Maeda 1,4, Terukazu Nishizaki 3, Kenji Shibata 3, and Norio Kobayashi 3. 1 Department of Basic science, The University of Tokyo, 3-8-1, Komaba, Meguro-ku, Tokyo 153-8902, Japan. 2 Department of Advanced Materials Science, The University of Tokyo, 7-3-1, Hongo Bunkyo-ku, Tokyo 113-8656, Japan. 3 Institute for Materials Research, Tohoku University, Katahira, Aoba-ku, Sendai 980-8577, Japan. 4 CREST, 4-1-8, Honcho, Kawaguchi, 332-0012, Japan.
Presenting Author: Y. Tsuchiya
High-frequency electromagnetic response in the mixed state of YBa2Cu3Oy (YBCO) was investigated by measuring the complex surface impedance Zs = Rs + iXs in an intense magnetic field H // c. In high-purity untwined YBCO, we observed two kinds of anomaly in the field dependence of Zs. One corresponded to the vortex melting transition, at which the field-gradient of Rs relatively increased with increasing field. This is the behavior which is naturally expected in terms of the change in the pinning properties at the vortex melting. However, the rapid increase of Xs at the melting transition in Bi2Sr2CaCu2Oy, which we previously reported, is not observed in YBCO. Another anomaly was found below 50K, where the field-gradient of Rs decreased. This may correspond to the rapid increase of magnetization, which was reported by Nishizaki et al. The field dependence of Rs and Xs was in good agreement with the Coffey-Clem model. From the data, we estimated that the vortex viscosity h at 10K is about 5x10-7 Ns/m2. This value indicates that the core of vortices in YBCO is not in the superclean limit, but in the intermediate region between the classical and the quantum limit.
1PO2-94 Effect of the Multiplicative Noise on the Microwave Absorption in the Superconductor
Tatiana S. Shaposhnikova, Kazan Physical-Technical Institute, Kazan, 420029, Russia
Presenting Author: T.S. Shaposhnikova
The vortex oscillations induced by the external microwave field are considered. Our investigation is focused to the manifestation of the random changes of the pinning potential well. It is shown that such random fluctuations may be described in terms of the multiplicative noise in the equation of vortex motion.
The analysis of the equation of motion obtained for the vortex residing near the pinning center has shown that for the certain parameters of the noise the nonequilibrium phase transition in a flux system will occur. It is also shown that at the large noise level the vortex system becomes very sensitive to the weak microwave magnetic field. This may cause the considerable amplification of the microwave absorption in superconductor.
1PO2-95 A stochastic resonance phenomenon in single crystal Nb
M.I. Tsindlekht 1, I. Felner 1, M. Gitterman 2, B.Ya. Shapiro 2. 1 The Racah Institute of Physics, The Hebrew University of Jerusalem, Jerusalem 91904, Israel. 2 Physics Department, Bar-Ilan University, Ramat Gan 52900, Israel.
Presenting Author: M.I. Tsindlekht
We report the first experimental observation of a stochastic resonance phenomenon in a single crystal of Nb. It was found that a rectified signal on the modulation frequency (excitation was done by an amplitude modulated ac field with carrier frequency - 3.2 MHz and modulation frequency - 1.5 kHz in a geometry with the ac field parallel to the dc field and both fields parallel to the sample axis) exists only in a surface superconducting state of Nb. The amplitude dependence of the rectified signal is nontrivial. There are two thresholds at low and high amplitude of excitation. These thresholds depend on the dc field, so that there is a dc magnetic field Hn at which rectified signal becomes zero at any amplitude of excitation. At T < 7.5K Hn(T) becomes higher than Hc3 extracted from magnetization curve. We believe that Hn, rather than Hc3 measured by SQUID, defines the boundary between the surface superconducting phase and paramagnetic phase. The theoretical analysis based on a two level model of a stochastic resonance and which provides a qualitative explanation of the experimental results will be discussed.
1PO2-96 Smallness of the Transverse Force on Moving Vortices in YBa2Cu3O7–d
J.T. Markert, K. Mochizuki, and E.E. Judge, Department of Physics, University of Texas at Austin, Austin, TX 78712 USA
Presenting Author: J.T. Markert
Theoretical predictions of the transverse force on moving vortices (the Magnus force) vary widely; for example, two recent theories differ in their predictions by three orders of magnitude [1,2]. To impose limits on such theories, we have designed and built an apparatus to directly measure the transverse force on moving vortices. The system uses a high-Q single-crystal silicon mechanical oscillator with a minimum detectable force resolution of Fmin = 3x10–13 N/Hz1/2. A superconducting single crystal (in this case, YBa2Cu3O7–d ) is mounted on the oscillator; a small (diameter = 0.5 mm) permanent magnet applies a field to the center of the crystal, avoiding edge-pinning effects. The permanent magnet is oscillated using a set of piezoelectric transducers, providing a periodic force; the direction of the force can be varied to make the oscillator sensitive to longitudinal or transverse forces. Temperature and sample material are the main parameters tuned to vary the degree of pinning. At low temperatures, we have directly measured "large" pinning forces of approximately 3x10–7 N to better than 1%. At higher temperatures, we observe the phase shift indicative of the transition to viscous motion of the vortices, and the expected decrease in the longitudinal pinning and viscous forces as Tc is approached. However, any transverse force is much smaller than recently estimated [3]. Our data support theories where the transverse force is much smaller than the full hydrodynamic value.
[1] P. Ao and D.J. Thouless, Phys. Rev. Lett. 70, 2158 (1993).
[2] M.V. Feigel'man et al., JETP Lett. 62, 834 (1995).
[3] X.-M. Zhu, E. Brändstörm, and B. Sundqvist, Phys. Rev. Lett. 78, 122 (1997).
Supported by the U.S. National Science Foundation Grant No. DMR-9705414.
1PO2-97 Various regimes of flux motion in Bi2Sr2CaCu2O8+d single crystals
Weimin Chen, J. Jung, and J.P. Franck, Department of Physics, University of Alberta, Edmonton, Alberta, Canada T6G 2J1
Presenting Author: J.P. Franck
Different regimes of vortex motion were identified in the magnetoresistance of Bi2Sr2CaCu2O8+d single crystals: (1) thermally activated flux flow (TAFF) with constant activation energy (U), when the defect-pinning density is high; (2) TAFF-like plastic motion of highly entangled vortex liquid at low temperatures, which has an activation barrier, Upl ~ (1-T/Tc)/H1/2. This usually manifests itself as a "foot" at the end of resistive transition; (3) pure free flux flow over a wide temperature range above that of (2) in clean and optimally doped samples, as described by the Bardeen-Stephen model; or in its place, (4) a combination of (2) and (3). This analysis gives an overall picture of flux motion in Bi cuprates.
1PO2-98 Effect of Silver Doping on the Transport Properties of Epitaxial YBa2Cu3O7-x Thin Films
G. Bolaños 1, P. Prieto 1, D. Arias 2, Z. Sefrioui 2, and J. Santamaría 2. 1 Departamento de Física, Universidad del Valle A.A. 25360, Santiago de Cali, Colombia. 2 Departamento de Física Aplicada III, Universidad Complutense de Madrid, 28040-Madrid, Spain.
Presenting Author: G. Bolaños
Epitaxial YBa2Cu3O7-x (YBCO) thin films doped with 5-20 wt % Ag have been grown 'in situ' at high oxygen pressures by a dc Sputtering technique on SrTiO3 single crystals. Critical current densities, Jc, around 4.3 x 105 A/cm2 at 80 K were measured in 5 wt % Ag-doped YBCO films which has been found to be higher than the value of 1.9 x 105 A/cm2 for undoped samples measured in a magnetic field of 2T. The improvement in Jc has been interpreted using De Gennes model which establish that Ag dopping enhance the weak link between superconducting grains. The Resistivity of undoped and doped YBCO films have been measured in magnetic field up to 8 T as function of the temperature. Current-voltage (I-V) characteristics were used to investigate the Vortex Glass (VG) phase, in magnetic fields up to 6 T. The r -j curves scale according to the 3D VG model. That reveals dynamic critical exponents of n =1.4 and z=4.87 for 5 wt % Ag -doped YBCO films and n =1.0 and z=7.5 for undoped films.
This work has been supported by COLCIENCIAS under the research project Development of Electronic Devices Based on HTc - Superconducting Thin Films Contract No 1106-05-197-95.
1PO2-99 High Frequency ac Response of Vortices in Superclean Superconductors
Yasunori Mawatari, Frontier Technology Div., Electrotechnical Laboratory, 1-1-4 Umezono, Tsukuba, Ibaraki 305-8568, Japan
Presenting Author: Y. Mawatari
Linear response of vortex lines to a high-frequency ac magnetic field is investigated for superclean type-II superconductors. The analysis is based on an equation of vortex motion which consists of the viscous drag force, the Hall force (i.e., the transverse force of which direction is perpendicular to the vortex velocity), the pinning force, and the elastic interaction force of vortex lattices. Combining the equation of vortex motion with the London equation, we investigate ac penetration depth, ac magnetic field distribution, ac permeability, and vortex motion in slab superconductors.
When the Hall force is much larger than the viscous drag force as in superclean superconductors, a vortex has an effective mass, and resonance phenomena appear at a frequency below the pinning frequency. At the resonance frequency (~GHz) various striking phenomena are predicted to occur: anomaly in ac penetration depth, excitation of standing waves, and a drastic change in the frequency dependence of ac permeability. For ac magnetic field parallel to the dc magnetic field, traces of straight vortex lines are ellipses. For ac field perpendicular to the dc field, on the other hand, vortices are deformed helically and helicon resonance occurs.