E.R. Yacoby

Magnetic properties of YNi2B2C superconductor

Abstract We report on a study of the magnetic properties of the intermetallic superconductor YNi 2 B 2 C in a wide range of magnetic fields (up to 8 T) and temperatures (4.4−16 K). The results yield the characteristic length scales and fields for this compound: λ (0) ≈ (3.5 ± 0.5) × 10 −5 cm, ξ(0) ≈ (10 ±2) × 10 −7 cm, H c1 (0) ≈ 83 G and H c2 (0)≈ 47600 G. We compare our results with those obtained by other groups and add new information on thermodynamic properties, in particular on the lower critical field, and on irreversible magnetic features such as the critical current, pinning force and magnetic relaxation. We conclude that YNi 2 B 2 C is a type-II superconductor with relatively weak pinning centers.

Irreversibility line in YBa2Cu3O7 samples: A comparison between experimental techniques and effect of electron irradiation

Abstract We have investigated the irreversibility line (IRL) of a ceramic YBa 2 Cu 3 O 7 sample by probing the onset of irreversibility in four DC and AC techniques. All techniques yield a line which obeys a power law H=A( 1−T T c ) n but A and n depend on the technique. Also, the location of the peak in the AC signal obeys this power-law but it does not scale with the onset of irreversibility. We have also investigated the effect of two doses of 2.5 MeV electron irradiation on the IRL and find that after irradiation both the transition temperature T c and the irreversibility temperature T irr decrease. The decrease in T irr scales with the decrease in T c . We propose that this scaling indicates that electron-irradiation does not induce pinning centers stronger than those present in the unirradiated sample.

Flux pinning enhancement near Tc in YBa2Cu3O7 crystals irradiated by 5.3 GeV Pb ions

Abstract We have studied the effect of 5.3 GeV Pb irradiation on irreversible magnetic properties of YBa 2 Cu 3 O 7 crystal, using an ultra-sensitive local Hall-probe technique. In this article we briefly summarize our previous results and present new data at 90 K (=0.97 T c ) which demonstrate that this type of irradiation enhances flux pinning in the vicinity of T c .

Critical current density, reversible and irreversible magnetization in YBa2Cu4O8 and Y0.9Ca0.1Ba2Cu4O8

Abstract DC magnetic measurements on the high- T c superconductors YBa 2 Cu 4 O 8 (124, T c =80 K) and Y 0.9 Ca 0.1 Ba 2 Cu 4 O 8 ( T c =87K) are reported. The irreversible lines of 124 and ceramic YBa 2 Cu 3 O 7 (123) are quite similar. The results show that there is no significant difference between the critical currents of the 124 phase and those of the 123 phase. Conversions of 124 into 123+CuO does not change the grain size of the material but enhances its critical current density.

Flux pinning by columnar defects in high-temperature superconducting crystals

Abstract Y-Ba-Cu-O:123 and Bi-Sr-Ca-Cu-O:2212 crystals were irradiated with 5.8 GeV Pb ions. The columnar defects, produced by this irradiation are expected to yield the strongest possible pinning energy. Indeed, there is an apparent increase in the width of the magnetization curves. However, the results of magnetic relaxation experiments indicate the energy barrier for flux creep from the columnar defects much smaller than expected pinning energy. This effect is explained in terms of nucleation of vortex loop mechanism of flux creep. The estimate of pinning energy of columnar defect can be obtained from measurement of the angular dependence of the magnetization curves.

Flux creep and related phenomena in high temperature superconductors

Abstract Irreversible features, which dominate the magnetic data in high temperature superconductors, are reviewed. Special attention is given to two phenomena: (i) The temperature and field dependence of the relaxation rate and its implication on measurement of critical fields. (ii) The field dependence of the magnetization which exhibits scaling features below the irreversibility line. The discrepancy in the barrier heights from magnetic relaxation and from resistivity measurements is discussed.

Flux-reorientation in irradiated YBa2Cu3O7 and Bi2Sr2CaCu2O8 crystals

Pinning by columnar defects in irradiated YBa 2 Cu 3 O 7 (YBCO) and Bi 2 Sr 2 CaCu 2 O 8 (BSCCO) crystals is most efficient when the field is parallel to the defect. However, in the low field limit the remanent magnetization is independent of the direction of the field, indicating that the fluxons flop towards the columnar defect. The flux-flop is almost temperature independent in YBCO and strongly temperature dependent in BSCCO due to the different nature of the vortices

Angular dependence of the magnetization curves and interlayer Josephson coupling in Bi2Sr2CaCu2O8

Magnetization curves were measured for Bi2Sr2CaCu2O8 crystals which were irradiated with 5.8 GeV Pb ions along the c direction or at 45° with respect to it. Each ion produces continuous columnar that crosses the sample and yields a unidirectional pinning center. The density of the defects in the various samples ranges from 1010 to 2.5 X 1011 ions/cm2. In all the irradiated samples the width of the magnetization curves is found to be the largest for fields along the defects indicating that when the field is at other directions the vortices are tilted away from the defects. This manifests the important role of the Josephson coupling between the CuO2 planes. We also report on accelerated flux escape around zero magnetization and around zero field, particularly in samples with a high dose of irradiation.

Effects of irradiation on magnetization curves in high temperature superconductors

Irradiation of high temperature superconductors yields effects that reveal significant information concerning the pinning mechanism. After electron irradiation highly mobile defects migrate to the surface and cause a significant reduction in the irreversible magnetization and thus demonstrates the importance of surface barriers. The realization of the role of the surface barriers resolve some of the most bothering puzzles in the magnetic behavior of high temperature superconductors. Irradiation with heavy ions which produces columnar defects induces “flux-flop” in the low field limit, from a direction determined by the field towards the defect direction. This phenomenon enables an experimental evaluation of the energy of a fluxon trapped by a columnar defect.

Unidirectional pinning in irradiated Bi2Sr2CaCu2O8 (invited)

Bi2Sr2CaCu2O8 crystals were irradiated with heavy ions (Pb, Xe) to produce columnar defects along the c direction or at 45° with respect to it. We describe the dependence of the critical currents, irreversibility fields, pinning force, and magnetic relaxation rates on the dose and on the type of ions. The irreversible properties of the Pb‐irradiated samples are more enhanced than those of the Xe‐irradiated sample at low temperatures, and vice versa at high temperatures. At low temperatures the efficiency of the columnar defects is enhanced with the ion dose but, at high temperatures, the maximum dose (2.5×1011 ions/cm2) is less efficient than intermediate doses. The unidirectional nature of the columnar pinning centers serves to demonstrate that the vortices maintain linelike features. For most of the irradiated samples, the pinning force density data are reduced to a single curve below 45 K. The relaxation rates at low temperatures reflect thermal reduction of the effective pinning barriers.