H. Ikeda

Irradiation effects on MgB2 bulk samples and formation of columnar defects in high-Tc superconductor

Abstract Irradiation effects on MgB2 were studied to investigate the pinning property of this new material. It is pointed out that flux pinning at grain boundaries is important for MgB2 bulk samples. We confirmed this with electron irradiation. Degradation of inter-grain coupling by electron irradiation resulted in degradation of pinning properties. Heavy ion irradiation was also accomplished on MgB2. Columnar defects introduced by the irradiation improve pinning in higher field regions. Concerning the formation of columnar defects in high-Tc superconductors, we conducted a systematic analysis of Bi2212 single crystal with different irradiation ions. Applying the time dependent line source model to our results, only a third of the electronic losses contributed to the formation of columnar defects.

Observed Gravity Change at Syowa Station Induced by Antarctic Ice Sheet Mass Change

Continuous observations with superconducting gravimeters (SG) TT-70 #016 and CT#043 have been on-going since 1993 to monitor Earth tides and Earth’s free oscillations at a gravity observation hut in Syowa Station, Antarctica. We obtained gravity residuals from the SG CT#043 data by subtracting Earth tides, effects of atmospheric pressure changes and polar motion, and instrumental drift from the original record. The smoothed gravity residuals obtained by taking a running mean of 33 days reveal variations from –5 to +5 μgal (10–8m/s2).

Electron irradiation effects on MgB2 bulk samples

Abstract Electron irradiation effects on MgB 2 bulk samples were studied to investigate pinning mechanism of the new superconducting material. The irradiation resulted in the degradation of pinning properties, such as hysteresis curves, magnetizations, and irreversibility line. The change was brought from the irradiation damages on inter-grain boundaries because these boundaries generally tend to be more damaged with electron irradiation due to smaller binding energies among the boundaries. In other word, pinning properties of the bulk materials strongly depend on those of grain boundaries. Magnetization jumps are observed near transition temperature below 0.5 T in M ( T ) curves after zero-field cooling. This is because vortices rush into the samples through the degraded boundaries with the irradiation. The damaged boundaries cannot maintain field gradient due to a large shielding current flowing on the outer surface of the sample.

Continuous Gravity Observation with the Superconducting Gravimeter CT #043 at Syowa Station, Antarctica

In April 2003, a new superconducting gravimeter CT #043 was installed at gravity observation hut in Syowa Station and TT-70 #016 was replaced with CT #043 at the end of 2003. Before the removal of the TT-70 #016, parallel observation with CT #043 and TT-70 #016 was carried out during the period from April 18 to November 4. A tidal analysis program BAYTAP-G was applied to the output signals from the both gravimeters for harmonic analysis, and tidal amplitude factors, phases and residuals were computed. Large instrumental drift over 200 μGal per year was observed in CT #043 record, but the drift could be eliminated accurately by fitting an exponential decaying function of first order. Tidal amplitudes and phases obtained from the both gravimeters data during the parallel observation period are in good agreement with each other. The detided gravity residuals agree both in variation pattern and amplitudes of the variation. This fact means reliability of the observation by CT #043 and adequacy of the manner to eliminate the instrumental drift. Tidal factors from CT #043 data after January 2004 were also compared with those obtained by previous studies using TT-70 #016 data, and they coincided with each other. From comparison of the gravity residuals from January through December 2004 with sea level trend, the obtained response coefficient to sea level change is about 0.07 μGal/cm.

Vortex dynamics in Bi2(Sr,La)2CuO6+δ and Bi2Sr2CaCu2O8+δ single crystals with columnar defects

Abstract Vortex dynamics is studied in La-substituted Bi 2 Sr 2 CuO 6+ δ (Bi-2201) and Bi 2 Sr 2 CaCu 2 O 8+ δ (Bi-2212) single crystals irradiated with 3.8 GeV 181 Ta 37+ ions through the relation between the frequency and the loss peak temperature in the temperature dependence of the imaginary part of AC magnetic susceptibility χ ″. In unirradiated Bi-2201 and Bi-2212, the behavior of thermally activated flux flow or creep is observed. The Bose-glass behavior is observed at fields below the matching field B Φ in irradiated Bi-2212. On the other hand, the Bose-glass behavior is not observed in irradiated Bi-2201. This difference is due to the extremely weak coupling between pancake vortices along the c -axis in Bi-2201 even under the presence of columnar defects. The vortices in Bi-2201 irradiated with heavy ions are almost decoupled, resulting in the occurrence of thermally activated vortex creep.

Flux pinning potential in Bi-2223 tapes irradiated by Xe ion

Abstract We have investigated the flux pinning potential of Ag–Cu alloy sheathed Bi-2223 tapes with columnar defects parallel to the c -axis introduced by Xe-ion irradiation. We measured the magnetic relaxation of the remanent. The sample was cooled in zero field and the field above H *, where the flux front penetrated at the center of the sample. The time decay of the remanent moment was measured for about 10 5 s. The relaxation of the remanent moment showed the linear ln( t ) dependence in all observed temperature range. The change in the distribution of the activation energies with irradiation was obtained. Since the deeper pinning center were increased, it is expected that the columnar defects will act as effective pinning centers.

Phase correlation investigated by the Josephson plasma resonance in Bi2(Sr,La)2CuOy

Abstract Josephson plasma resonance (JPR) was investigated in the mixed state of Bi 2 (Sr,La) 2 CuO y (Bi-2201). The plasma frequency ω p was studied in detail, as a function of temperature, magnetic field and doping. The peculiar temperature dependence of the resonance field B p ( T ) in underdoped crystals was understood in terms of pinning effect with the assumption that JPR takes place in the whole sample at an average field, which was supported by the experimental data. B p ( T ) under the uniform field application in the temperature swept experiment showed no anomaly at the irreversibility line. ω p 2 ( B ) determined precisely using cavities was found to behave as B − μ with μ smaller than unity, which might suggest that a rather strong long-range correlation exists among vortices even in the liquid state.

Optimization Of Pb And La Co‐Doped Bi2Sr2CuO6+δ Superconductors

We obtained zero‐resistance Tc of 43.8 K for the single crystals of Pb and La co‐doped Bi2Sr2CuO6+δ. The superconductivity with the onset of about 75 K was observed, and the annealing effect was investigated.

Enhancement Of Critical Current Density Under Low Magnetic Fields Around 70 K In Bi2Sr2CaCu2Oy Superconducting Whiskers

Electrical transport measurements for Bi2Sr2CaCu2Oy whiskers with a single‐step superconducting transition were carried out at high temperature ranging from 70 to 77 K below the superconducting transition temperature (= 81.5–82.5 K) to investigate critical current density (Jc) under magnetic fields along the crystal c axis. The Jc parallel to the crystal a axis, which was estimated from the V‐I curves, enhances under several ten oersteds, and then decreases with increasing the magnetic field. This peak effect in the low‐field regime at high temperature has temperature dependence in contrast to the case of matching effects.

Transport and magnetic properties of superconducting MgB2

Abstract We have investigated the transport and magnetic properties of the recently discovered superconducting materials MgB 2 . We obtained irreversibility temperature T r ( H ) and irreversibility field H r ( T ) from magnetic measurements. The irreversibility temperature T r ( H ) was determined from the merging temperature of the magnetic moment observed in the zero field cooling process in to the one in field cooling process. We also defined the transport irreversibility temperature B c2 from resistivity measurements at zero-resistivity in the presence of magnetic field. Those results of the irreversibility lines are plotted in H – T plane. The magnetic irreversibility lines are consistent with each other, while the transport one locates at the higher temperature side.