POSTER SESSION 1PO2: Vortex Physics

Monday, Feb. 21, 2:30 p.m. – 4:30 p.m., Hall D (GRB)

1PO2-1 Third harmonic ac susceptibility of superconductors with finite thickness

M.J. Qin and C.K. Ong, Centre for Superconducting and Magnetic Materials and Department of Physics, National University of Singapore, Lower Kent Ridge Road, Singapore 119260

Presenting Author: M.J. Qin

The magnetic moment, flux and current profiles, hysteresis loops, ac susceptibilities of superconductors with finite thickness in perpendicular magnetic field have been calculated by means of the Brandt method, which can be applied to arbitrary cross section and arbitrary current voltage characteristics E(j). Detailed results are presented for third harmonic ac susceptibilities in a strip with rectangular cross sections 2a x 2b and power-law electric field E(j)=Ec(j/j_c)ⁿ, thus taking into account the flux creep effects.

Third harmonic ac susceptibilities of infinite long superconductors in parallel magnetic field, which are usually used to compare with experimental data, have also been calculated by numerically solving the flux creep equation. The results indicate that when compared with experimental data, the results of infinite long samples are not a good approximation and those of the samples with finite thickness should be used.

1PO2-2 Ac susceptibility of superconductors with geometric barrier

M.J. Qin and C.K. Ong, Centre for Superconducting and Magnetic Materials and Department of Physics, National University of Singapore, Lower Kent Ridge Road, Singapore 119260

Presenting Author: M.J. Qin

The irreversibility of penetration and exit of magnetic flux lines due to geometric barrier has been observed in superconductors with arbitrary shape and material laws H(B,r) and E(J,B,r) when a perpendicular magnetic field B_z(r,t) is applied (Brandt method). The current density J(r,t), magnetic induction B(r,t) and hence the magnetic moment of a superconducting strip have been obtained. A hysteresis loop is observed in the absence of volume pinning, clearly showing the effects of the geometric barrier.

The ac response of the strip with geometric barrier has been studied when the perpendicularly applied field is sinusoidal B_a(r,t)=B₀sin(w t). ac susceptibilities of strips with different thickness and different geometric barriers have been calculated and compared with those without geometric barrier.

1PO2-3 Transverse dynamic vortex correlation in a Bi₂Sr₂CaCu₂O_8+d Corbino disk

Yuri Eltsev, Koichi Nakao, Susumu Shibata, and Naoki Koshizuka, Superconductivity Research Laboratory, ISTEC, 10-13 Shinonome 1-chome, Koto-ku, Tokyo 135-0062, Japan

Presenting Author: Y. Eltsev

We report on the electrical transport measurements in Bi₂Sr₂CaCu₂O_8+d single crystal in a Corbino disk geometry. In this contact configuration current entering the sample at the center of disk and leaving it at the perimeter generates current density decreasing as 1/r with radius. The Lorentz force on the vortices//c-axis induced by this current pushes them along closed circular orbits, and radial transverse vortex-vortex velocity correlation may be resolved by measuring radial potential distribution. At 0.01<B<0.2T, in the vortex liquid state, we observed strong deviation from 1/r vortex velocity distribution. The voltage drop taken in the outer part of sample is found to be higher than that measured across equally spaced potential contacts in the inner part of disk. This result indicates that due to transverse vortex correlation flux lines near perimeter move with a higher velocity than those close to the center. Observed nonlocal voltage response strongly depends on the value of driving force decreasing with the increase of transport current. At higher fields B>0.2T the radial potential distribution is well discribed by local conductivity model suggesting that flux lines move independently.

This work was supported by the New Energy and Industrial Technology Development Organization (NEDO).

1PO2-4 The Effects of Uranium Doping and Thermal Neutron Irradiation on the Pinning Properties of Ag/Bi-2223 Tapes

Damian G. Marinaro 1, Shi X. Dou 1, Josip Horvat 1, Yuan C. Guo 1, and John Boldeman 2. 1 Institute for Superconducting and Electronic Materials, University of Wollongong, Wollongong, NSW, 2522, Australia. 2 Australian Nuclear Science and Technology Organisation, New Illawarra Road, Lucas Heights, NSW, 2234, Australia.

Presenting Author: D.G. Marinaro

Ag/Bi-2223 multifilamentary tapes doped with small quantities of UO₄ powder were prepared by the powder-in-tube process. Thermal neutron irradiation of these tapes induces fission of the ²³⁵U atoms, with the resulting fission fragments creating randomly splayed defects. An observed increase in J_c after irradiation as well as a shift in the irreversibility line to higher fields, in comparison to undoped samples, implies an improvement in the flux pinning of the tapes as a result of this process.

Magnetic relaxation measurements were conducted on these samples to elucidate the effective pinning potential (U_eff) as a function of current density, calculated from the time derivative of magnetisation (M):U_eff=-k_BT.[ln½ dM/dt½ -C]/g(T), where C is a constant and g(T)=(1-(T/T_c)²) is a temperature scaling function. Changes in the pinning potential for different doping levels and neutorn fluences were examined. A transition from weak pin dominated flux creep to creep donimated by strong pinning centres at long times is observed. From the behaviour of the pinning potential, the nature of the flux dynamics within the samples will be discussed. Current voltage curves, taken with magnetic field H parallel to the c-axis, are also used.

1PO2-5 Weak Pinning Phenomena in Liquid State in Bi₂Sr₂CaCu₂O_8+d with Columnar Defects

Kazuhiro Kimura 1, Ryo Koshida 1, Satoru Okayasu 3, Masao Sataka 3, Yukio Kazumata 4, Wai K. Kwok 5, George W. Crabtree 5, and Kazuo Kadowaki 1,2. 1 Institute of Materials Science, University of Tsukuba, Tsukuba, 305-8573, Japan. 2 CREST. 3 Japan Atomic Energy Research Institute. 4 Japan Advanced Technology. 5 Argonne National Laboratory.

Presenting Author: K. Kimura

Systematic magnetization measurements on as-grown and columnar defected single crystal Bi₂Sr₂CaCu₂O_8+d (BF = 0.005 - 1 T) have been studied by using a SQUID magnetometer. The magnetization hysteresis (M-H) curves in all samples show anomalous weak irreversible behavior in the vortex liquid regime above the irreversibilty line, H_irr, but below a characteristic field, H_F. This anomalous weak hysteretic behavior is also detected by temperature sweep measurements. In as-grown samples H_F lies below the critical point, where the first-order vortex lattice melting transition undergoes the second-order transition. H_F strongly depends on the disorder level of the samples and systematically shifts to higher temperature and higher field region with increasing the number of columnar defects, while does not depend on the smaple size as well as the sample shape. The magnetization relaxation is checked between H_irr and H_F in zero-field-cool conditions and only slow relaxation was observed. The field-cool magnetization was also identical with the zero-field one. These results strongly suggest existence of a possible new phase in the liquid state between H_irr and H_F, which may appear more clearly and more efficiently with disorders.

1PO2-6 Pinning Property Change of High-Energy Heavy-Ion Irradiated Bi-2212 Single Crystals due to Thermal Annealing after Irradiation

Koji Ogikubo 1, Takayuki Terai 1, Kenji Yamaguchi 2, and Michio Yamawaki 1. 1 Dept. Quantum Engineering and Systems Science, The University of Tokyo, Bunkyo-ku, Tokyo 113-8656, Japan. 2 Nuclear Engineering Research Laboratory, The University of Tokyo, Tokai-mura, Ibaraki 319-1106, Japan.

Presenting Author: K. Ogikubo

Change in pinning properties of Bi₂Sr₂CaCu₂O_8+x(Bi-2212) single crystals due to high-energy heavy-ion irradiation is reported. Bi-2212 single crystal specimens (2mm x 2mm x 0.1mm) prepared by the floating-zone method were irradiated with high-energy heavy-ions (200MeV Au, 510MeV Kr and 460MeV Xe using a tandem accelerator and an AVF cyclotron accelerator at JAERI (Japan Atomic Energy Research Institute). The fluences were from 1x10¹⁰ to 5x10¹¹ cm^-2. After irradiation, the specimens were annealed in air at 673K for 1h to 24h. Magnetic relaxation measurement was carried out for the specimens before and after annealing with a vibrating sample magnetometer (VSM) at 20K and 40K with the applied magnetic field of 0.01T parallel to the c-axis of the specimens.

For Au-irradiated specimens, the magnetization decreased almost logarithmically with time in all cases. For the specimen irradiated with Au ions at the fluence of 5x10¹⁰ cm^-2, the magnetic relaxation rate of the specimen did not change even by thermal annealing up to 3h. On the other hand, for the specimen irradiated with Au ions at the fluence of 1x10¹¹ cm^-2, the magnetic relaxation rate decreased about 10% by thermal annealing of 1h. This difference is attributed to the difference in the recovery behavior of olumnar defects for both cases. The change of the pinning potential will also be reported.

1PO2-7 Interplay of pancake vortices and columnar defects in heavy-ion irradiated Bi₂Sr₂CaCu₂O₈

R.J. Drost 1, C.J. van der Beek 2, M. Konczykowski 2, and P.H. Kes 1. 1 Kamerlingh Onnes Laboratorium, Leiden University, P.O. Box 9504, 2300 RA Leiden, The Netherlands. 2 Laboratoire des Solides Irradiés, Ecole Polytechnique, 91128 Palaiseau, France.

Presenting Author: P.H. Kes

The competition between intervortex interaction, pinning energy and entropy has been studied in detail in heavy-ion irradiated Bi₂Sr₂CaCu₂O_8+d single crystals by measuring the magnetization in the reversible regime on crystals that have been irradiated with various doses Bf and under different angles a with respect to the c-axis. The reversible magnetization is uniquely determined by the thermodynamic free energy which is composed of the three above mentioned contributions. These can be determined seperately from the analyses of the experiments. It is shown that the entropy contribution plays a minor role in pristine crystals and in irradiated crystals at fields close to and above Bf. It becomes very dominant at low fields in crystals with high Bf. It turns out that quite detailed information can be obtained on pancake vortex alignment along the columnar defects. The correct values for the pinning energy can be determined. They scale with the irradiation angle and temperature in a way expected for core interaction. The d-wave symmetry of the order parameter causes an increase of the effective radius of the amorphous defect core.

1PO2-8 Effect of the microscopic correlated-pinning landscape on the macroscopic critical current density in YBCO films

R. Gerbaldo 1, G. Ghigo 1, L. Gozzelino 1, E. Mezzetti 1, B. Minetti 1, C. Camerlingo 2, and A. Rovelli 3. 1 INFM- UdR Torino-Politecnico; INFN - Sez. Torino, Department of Physics, Politecnico di Torino, C.so Duca degli Abruzzi 24, I-10129 Torino, Italy. 2 IC-CNR, Arco Felice, Italy. 3 I.N.F.N. Laboratorio Nazionale del Sud, Catania.

Presenting Author: E. Mezzetti

To study the microscopic vortex static and dynamics in HTSC systems, many investigations have been carried out on periodic arrays of pinning sites. Under such conditions, commensurability effects show up, so that high-stability vortex configurations are produced and persist under an increasing current or external field. Vortex matching effects have also recently been obtained in different superconducting systems including Josephson junctions and superconducting networks. In this paper we show that in good quality YBCO films, the static properties of the vortices (in particular the J_c vs. B curves) are determined by a continuum-spectrum matching of the vortex lattice to the in plane island-structure of the film. As far as static properties are investigated, the structure actually behaves as a 1D network of Josephson junctions. Correlated defects, with nanometric distances and pseudo-periodic distribution, modulate this network. As a probe to study, modify and tune the distribution length of the cells in the network, we use nanometric size columnar defects, at different doses [1]. By means of a suitable model we fit the experimental J_c vs. B data to calculate the average length of the cells and the length distribution, before and after irradiation [2]. The results are compared with the length distributions obtained by means of HRXRD technique. A good agreement with the Josephson junction model is obtained.

[1] E. Mezzetti et al., Phys. Rev. B (1999), 7623.

[2] G. Ghigo et al., IWCC Simposium paper - Supercond. Sci. Technol., in press.

1PO2-9 Fluctuation Induced Conductivity Studies of 100 MeV Oxygen Ion Irradiated Pb-Doped Bi-2223 Superconductors

Tamalika Banerjee 1,2, Ravi Kumar 2, D. Kanjilal 2, S. Ramasamy 1. 1 School of Physical Sciences, University of Madras, Guindy Campus, Chennai 600 025, TN, India. 2 Nuclear Science Centre, Aruna Asaf Ali Marg, P.O. Box 10502, New Delhi 110 067, India.

Presenting Author: T. Banerjee

The movement of flux lines inside a superconductor when subjected to an external field causes dissipation of energy and impedes their usage for commercial applications. One way of arresting this flux creep is to introduce microstructural defects which is best realized by irradiating these materials with Swift Heavy Ion (SHI) irradiation. As a matter of fact, SHI irradiation proves to be a major tool in tailoring the transport properties of high temperature superconductors for use in flux flow devices. The superconducting and normal state properties of these materials are modified by such heavy ion irradiation. In this work, 100 MeV oxygen beam has been used to irradiate Pb-doped Bi-2223 superconductors with fluences in the range of 1013 ions/cm² to 1014 ions/cm². Resistivity measurements show that both the grains as well as the grain boundaries have been affected as a result of such irradiation manifested in the decrease of T_c (0), increase of Delta-T_c and room temperature resistivity. The differences in the normal state resistivity at different fluences have also been discussed here. An analysis of the excess conductivity has been used within the framework of the Aslmazov-Larkin (AL) and Lawrence-Doniach (LD) theory. The critical exponent was determined from the plot of the logarithmic derivative of excess conductivity versus the reduced temperature. In almost all cases, a dimensional crossover from 2D to 3D is observed using AL theory. The changing values of the coherence length and the Josephson interlayer coupling strength is also estimated from LD theory. The impact of SHI on this superconductor leading to the build up of the second phase and the changes in the fluctuation induced excess conductivity will be presented in this work.