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Abstract: Vortex images of superconducting material Tl-1223 thin film taken with a scanning SQUID microscope are presented. Among the vortices images, coexistence of vortex–antivortex pairs is observed at low field region. Short distances of the pairs indicate the existence of strong pinning centers in this material. [Copyright &y& Elsevier]

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. [Copyright &y& Elsevier]

Electron irradiation effects on MgB2 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. [Copyright &y& Elsevier]

Electron irradiation effects on MgB2 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. [ABSTRACT FROM AUTHOR]

Abstract: We have been investigating for neutron detection with the MgB2 transition edge sensor (TES). For the purpose, we have been developing a low noise measurement system for the detection. To confirm the performance of the detecting sensor, alpha ray detection from an americium-241 (241Am) alpha-ray source was achieved. A short microfabricated sample with 10μm length and 1μm width is used to improve the S/N ratio. The detection is achieved under a constant current condition in the range between 1 and 6μA bias current, and the resistivity changes at the sample due to the alpha ray irradiation is detected just on the transition edge. [Copyright &y& Elsevier]

Abstract: We have investigated spatial configurations of vortices in superconducting disks of an amorphous MoGe film. A scanning SQUID microscope allows us to image vortices magnetically in the disks. Results on the disks with 56μm in diameter demonstrate that quasi-symmetric distributions imposed by the circle geometry appear for vorticity up to five. Meanwhile, for vorticity more than six, vortices form shell structures, in which one vortex (two vortices) is located in the center and the others form a ring around. [Copyright &y& Elsevier]

Abstract: A scanning SQUID microscope is a powerful tool for direct observations of vortices. Magnetic sensitivity is high enough, but spatial resolution is limited by the size of pickup coils. To improve the spatial resolution, a trial usage of microprobe on the pickup coil was achieved. Obtained images are expanded about four times, and the attached microprobe acts as a magnetic lens. [Copyright &y& Elsevier]

Abstract: Pinning properties of superconducting material Tl-1223 thin film are investigated. Vortex images are compared between a pristine and high-energy gold ion irradiated sample. This comparison implies which pinning center is effective for this material between large pinning centers and columnar defect. Large pinning centers are better for this material. [Copyright &y& Elsevier]

Direct observation of vortex states confined in mesoscopic regular triangle dots of amorphous Mo–Ge thin films was made with a scanning superconducting quantum interference device microscope. The observed magnetic images illustrate clearly how vortices are distributed over the triangle dots by forming not only commensurate triangular clusters, but also unique patterns imposed by incommensurability. We discuss the results in terms of vortex shells and construct the packing sequence of vortices in the multiple shell structure. [ABSTRACT FROM AUTHOR]

We report on multiterminal transport measurement in YBa2Cu3Oy films, where a vortex flow channel is artificially introduced in a strong pinning environment with columnar defects. The samples were prepared by partially masked heavy-ion irradiation. Two kinds of phase transitions with different transition temperatures, TVG and TBG, which correspond to the vortex glass transition and the Bose glass transition, are observed for the unirradiated region and the irradiated one, respectively. At high field, the situation of TVG, where the flux flow in unirradiated channel is confined between Bose glass solids of the irradiated region, is realized. The vortex velocity distribution is almost homogeneous inside the channel. Around the boundary, however, we find that the vortices flowing in the unirradiated channel drag the vortices of the Bose glass solid in the irradiated region. [Copyright &y& Elsevier]

Vortex phase diagrams have been studied on heavy-ion irradiated YBa2Cu3Oy films by the measurements of current–voltage (I–V) characteristics, resistance R and critical current density Jc as a function of magnetic field B parallel to the columnar defects. In the scaling analyses of the I–V data, we find the non-universal nature of the glass transition. The dynamic critical exponent z, which is almost constant at high fields, shows an anomalous peak and the glass transition line Tg(B) reveals an upturn behavior at B∼Bφ/3 with Bφ the matching field, where we also observe step-like behavior both in R(B) and Jc(B) curves. These results indicate the existence of a temperature-independent boundary in the B–T plane at which the dissipation mechanism changes. [Copyright &y& Elsevier]

Vortex phase diagrams have been studied on heavy-ion irradiated YBa2Cu3Oy films by the measurements of current–voltage (I–V) characteristics, resistance R and critical current density Jc as a function of magnetic field B parallel to the columnar defects. In the scaling analyses of the I–V data, we find the non-universal nature of the glass transition. The dynamic critical exponent z, which is almost constant at high fields, shows an anomalous peak and the glass transition line Tg(B) reveals an upturn behavior at B∼Bφ/3 with Bφ the matching field, where we also observe step-like behavior both in R(B) and Jc(B) curves. These results indicate the existence of a temperature-independent boundary in the B–T plane at which the dissipation mechanism changes. [ABSTRACT FROM AUTHOR]

We report the effects of heavy-ion irradiation in (Ba 0.6 K 0.4 )Fe 2 As 2 single crystals including its dose dependence. We found that the suppression of T c is weak up to a certain dose of irradiation. Critical current density ( J c ) under self-field is strongly enhanced up to 15 MA cm −2 at 2 K by the introduction of defects through irradiations with different energies and ion species. This upper limit is in agreement with the previous report for that in cuprate superconductors. The dose dependence of J c follows B Φ 1/2 , which can be explained by the simple scenario on the interaction between a driven vortex and discontinuous columnar defects. [ABSTRACT FROM AUTHOR]

Abstract: We report effects of particle energy on physical properties of Co-doped BaFe2As2 in the case of Xe and Au irradiations. The irradiation-induced suppression of T c is found to be related with several factors, such as defect structures and the thickness of the crystal. Heavy-ion irradiation induces enhancement of critical current density J c , and a weak dip in the hysteresis loop. The fish-tail effect is completely suppressed by a modest density of defects. The enhancement of J c is closely related to the defect structure. Namely, splayed structure in columnar defects, inevitably introduced at lower energies, results in a strong enhancement of J c by forced entanglement of vortices. [Copyright &y& Elsevier]

Abstract: Magnetic relaxations in Ba(Fe1− x Co x )2As2 before and after swift particle irradiations are measured. The normalized relaxation rate S shows a clear suppression in a pristine sample at low fields below the self-field. Heavy-ion irradiations suppress these anomalies considerably, although there still remain weak features depending on ion species and/or energies. In a proton irradiated sample, the low-field anomaly of S survives although it is smeared compared with that in a pristine sample. All these results indicate that the low-field anomaly of S is suppressed by defects, and correlated defects have stronger effects. Possible origins of the low-field anomaly of S are discussed. [Copyright &y& Elsevier]

Abstract: Critical current density was measured for oxygen-controlled (Bi, Pb)-2223 single crystals before and after the irradiation with gold ions in a magnetic field parallel to the irradiation-induced defects along the c-axis. Eleven specimens prepared in different annealing conditions were measured. The condensation energy density of each specimen was evaluated from the observed critical current density by using the summation theory of pinning forces of columnar defects and the flux creep theory. It was found that the specimen heat-treated at 1 atm in oxygen atmosphere has the highest condensation energy density among all specimens. Hence, it is speculated that the optimum oxygen pressure for the anneal is around 1atm. [Copyright &y& Elsevier]

Abstract: The introduction of the c-axis-correlated disorder is a promising technique for the improvement of the critical current density J c for of the REBa2Cu3O y films and tapes. In order to understand the vortex pinning state diagram of typical c-axis-correlated disorder, the angular dependence of J c was measured in detail for Y123 films with columnar defects induced by heavy-ion irradiation. We found that the peak in the angular dependence of J c at the parallel external field to the c-axis becomes sharp and large with increasing a magnetic field across the matching magnetic field at various temperatures. Hence, the c-axis-correlated pinning works well even in much higher magnetic fields than the matching magnetic field. Those behaviors suggest that the interstitial vortices, which locate in between vortices pinned by the c-axis-correlated disorders, are also trapped through the elastic interaction of the vortices. [Copyright &y& Elsevier]

Abstract: Effect of heavy ion irradiation was investigated for DyBCO coated conductor fabricated by ISD (inclined substrate deposition) process to study the flux pinning property by columnar defects. It was found that the critical current density in the specimen with large columnar defects increased sufficiently at high magnetic fields. On the other hand, the enhancement of the critical current density was not sufficient for the case of small columnar defects. The obtained results were analyzed using the flux creep-flow model and the most probable value of the virtual critical current density in the ideal creep free case was estimated for columnar defects. These results were compared with the value expected by the statistical summation theory and agreement was obtained. This theoretical result predicts that introduction of columnar defects with larger radius of higher density is effective for improvement of the critical current density. [Copyright &y& Elsevier]

Abstract: Pinning properties such as the critical current density and the irreversibility field of superconducting (Bi,Pb)-2223 single crystals with different O2 annealing conditions were investigated before and after a heavy ion irradiation which nucleated columnar defects parallel to the c-axis. The critical current density was enhanced by a factor of 3 to 4 after the irradiation due to the strong pinning effect by the columnar defects. In addition, the irreversibility field was also improved, and the improvement becomes larger at higher temperatures. The enhancement of the pinning properties was larger in O2 annealed specimen, suggesting an improvement of the dimensionality of the superconductivity. The experimental results were theoretically analyzed using the flux creep-flow model. It was found that the theoretical prediction can explain the experimental results of the critical current density and the irreversibility field. [Copyright &y& Elsevier]

Abstract: The critical current density in Bi-2212 superconductors with various anisotropies irradiated by heavy ions was investigated in the medium temperature region to understand the effects of defect size and the anisotropy of the superconductor. It was found that the critical current density and the irreversibility field were larger for the specimen with larger defect and/or with smaller anisotropy. Introduction of stronger pinning centers and the optimization of the doping condition to improve the dimensionality are desired for further improvement of the critical current properties. [Copyright &y& Elsevier]

Abstract: The critical current density was measured at low temperatures for Bi-2212 single crystals with small anisotropy prepared by oxygen doping and by addition of Pb. The anisotropy parameters of these specimens were about 66. The Pb-doped specimen showed the peak effect up to T/T c =0.7. The critical current density at high fields of these specimens was significantly improved as well as the irreversibility field than specimens with larger anisotropy. [Copyright &y& Elsevier]

The condensation energy which determines the pinning strength is estimated from the pinning property of the columnar defects for Bi-2212 superconductor in various doped states of oxygen. It is found that the condensation energy increases drastically with decreasing anisotropy parameter. In the low temperature region the condensation energy density of Bi-2212 increases abnormally with decreasing temperature and this suggests a drastic change in the superconductivity in the block layer. [Copyright &y& Elsevier]

We studied angular dependence of vortex states in Sr2RuO4 single crystals through magnetization measurements using micro-Hall probe. In a pristine sample, the width of magnetization hysteresis loop decreases strongly when the field is applied at large angle from the c-axis. Through angular dependence of the upper critical field, the in-plane anisotropy is estimated. In heavy-ion irradiated samples, the critical current density shows remarkable increase in a limited angle range, suggesting the presence of enhanced pinning due to introduction of columnar defects. In all samples, irreversible magnetization shows dip structures close to zero field. We discuss this dip structure in relation with the presence of time-reversal symmetry-breaking domains. [Copyright &y& Elsevier]

Change in critical current density of Bi2Sr2CaCu2O8+x (Bi-2212) single crystals due to high-energy heavy-ion (200 MeV Au or 200 MeV Ag) irradiation followed by thermal annealing treatment was investigated. After irradiation at the fluence of 5×1011 cm−2 for both Au ions and Ag ions, the specimens were annealed in air. Different annealing time dependencies of critical current density (Jc) were observed between the measurement temperatures of 20 and 40 K for both Au- and Ag-irradiated specimens. Jc gradually decreased with the increase of annealing time at 20 K. On the other hand, it gradually increased at 40 K. Such different dependencies of Jc are considered to be attributed to the different kind of defects that contribute dominantly to the pinning effect at each temperature. [Copyright &y& Elsevier]

A 50 μm thick slice of (Cu,C)Ba2Ca3Cu4Oy (Cu:1234) bulk superconductor grown by high-pressure synthesis technique was irradiated with high-energy (240 MeV) Au15+ ions (1011 cm−2). Complimentary DC magnetization measurements up to 5 T showed an increase in critical current density by two orders of magnitude. AC susceptibility studies were performed in DC magnetic fields up to 14 T and at various temperatures, using various AC field amplitudes (up to 15 Oe) and frequencies (up to 10 kHz). Critical current densities at various temperatures and fields were estimated from critical state model. Using the third harmonic response of the sample we were able to separate different contributions to the dissipation: inter-grain regions, grains without columnar defects (heavy-ions penetration depth is about one quarter of sample’s thickness), and grains with columnar defects. In irradiated grains, we could observe a peak-effect in the field dependence of critical current density at high DC fields and temperatures between 85 and 105 K, suggesting the possibility of the highest irreversibility field at liquid nitrogen temperature. [Copyright &y& Elsevier]

We studied angular dependence of vortex states in Sr2RuO4 single crystals through magnetization measurements using micro-Hall probe. In a pristine sample, the width of magnetization hysteresis loop decreases strongly when the field is applied at large angle from the c-axis. Through angular dependence of the upper critical field, the in-plane anisotropy is estimated. In heavy-ion irradiated samples, the critical current density shows remarkable increase in a limited angle range, suggesting the presence of enhanced pinning due to introduction of columnar defects. In all samples, irreversible magnetization shows dip structures close to zero field. We discuss this dip structure in relation with the presence of time-reversal symmetry-breaking domains. [ABSTRACT FROM AUTHOR]

Change in critical current density of Bi2Sr2CaCu2O8+x (Bi-2212) single crystals due to high-energy heavy-ion (200 MeV Au or 200 MeV Ag) irradiation followed by thermal annealing treatment was investigated. After irradiation at the fluence of 5×1011 cm−2 for both Au ions and Ag ions, the specimens were annealed in air. Different annealing time dependencies of critical current density (Jc) were observed between the measurement temperatures of 20 and 40 K for both Au- and Ag-irradiated specimens. Jc gradually decreased with the increase of annealing time at 20 K. On the other hand, it gradually increased at 40 K. Such different dependencies of Jc are considered to be attributed to the different kind of defects that contribute dominantly to the pinning effect at each temperature. [ABSTRACT FROM AUTHOR]

A 50 μm thick slice of (Cu,C)Ba2Ca3Cu4Oy (Cu:1234) bulk superconductor grown by high-pressure synthesis technique was irradiated with high-energy (240 MeV) Au15+ ions (1011 cm−2). Complimentary DC magnetization measurements up to 5 T showed an increase in critical current density by two orders of magnitude. AC susceptibility studies were performed in DC magnetic fields up to 14 T and at various temperatures, using various AC field amplitudes (up to 15 Oe) and frequencies (up to 10 kHz). Critical current densities at various temperatures and fields were estimated from critical state model. Using the third harmonic response of the sample we were able to separate different contributions to the dissipation: inter-grain regions, grains without columnar defects (heavy-ions penetration depth is about one quarter of sample’s thickness), and grains with columnar defects. In irradiated grains, we could observe a peak-effect in the field dependence of critical current density at high DC fields and temperatures between 85 and 105 K, suggesting the possibility of the highest irreversibility field at liquid nitrogen temperature. [ABSTRACT FROM AUTHOR]

We observed the arrangements of vortex lines inside Bi2Sr2CaCu2O8+δ (Bi-2212) thin films with tilted columnar defects by using Lorentz microscopy with our 1 MV holography electron microscope. It was found that while vortex lines were trapped along tilted columnar defects irrespective of the direction of the applied magnetic field at temperatures above 19 K, vortex lines stood up perpendicularly to the film plane below 12 K. This unusual behavior of vortices was attributed to the different pinning mechanism due to densely distributed atomic-size defects in Bi-2212. [Copyright &y& Elsevier]

Abstract: We report the formation of columnar defects in Co-doped BaFe2As2 single crystals with different heavy-ion irradiations. The formation of columnar defects by 200MeV Au ion irradiation is confirmed by transmission electron microscopy and their density is about 40% of the irradiation dose. Magneto-optical imaging and bulk magnetization measurements reveal that the critical current density J c is enhanced in the 200MeV Au and 800MeV Xe ion irradiated samples while J c is unchanged in the 200MeV Ni ion irradiated sample. We also find that vortex creep rates are strongly suppressed by the columnar defects. We compare the effect of heavy-ion irradiation into Co-doped BaFe2As2 and cuprate superconductors. [ABSTRACT FROM AUTHOR]

The vortex order–disorder phase transition line in (La0.937Sr0.063)2CuO4 exhibits a steep concave decrease throughout the whole temperature range. This unusual behavior is explained postulating that in (La0.937Sr0.063)2CuO4, both thermal and disorder-induced fluctuations take part in destabilizing the vortex lattice. Irradiation of the samples with electrons causes a significant decrease in both the magnitude of the transition field and the curvature of the transition line. These results are interpreted as caused by the enhanced role of disorder-induced fluctuations as compared with thermal fluctuations. [Copyright &y& Elsevier]

Abstract: We report effects of heavy-ion irradiation in Ba(Fe1−x Co x )2As2 single crystals. The columnar defects with about 40% of the irradiation dose are confirmed by transmission electron microscopy. Magneto-optical imaging and bulk magnetization measurements reveal strong enhancement of the critical current density in the irradiated region. The vortex creep rate is also strongly suppressed by the columnar defects. Effects of heavy-ion irradiation into Ba(Fe1−x Co x )2As2 and cuprate superconductors are compared. [ABSTRACT FROM AUTHOR]