K. Oka

Possible non-resonant r.f. absorption evidence for superconducting fluctuations above TC in Bi2Sr2CaCu2O8 single crystals

Abstract The magnetic field dependent r.f. (20 MHz) power dissipation, P ( H ) vs H , for H r.f. ‖ c ‖ H in Bi 2 Sr 2 CaCu 2 O 8 (Bi 2 2 1 2) single crystals shows an onset at T ∼ 102 K, nearly 20 K above the nominal T C (∼82 K). In contrast, for H r.f. ‖ ab , H ‖ c , the onset of losses occurs at the T C . One possible reason for the onset at ∼102 K could be the Bi 2 Sr 2 Ca 2 Cu 3 O 8 (Bi 2 2 2 3) inclusions with a T C ∼ 100–110 K. However, our non-resonant r.f. absorption studies cast doubt on this conclusion and point towards an alternative explanation in terms of superconducting fluctuations above the T C of Bi 2 2 1 2 single crystals. Support for this is provided by the characteristic temperature dependence of the magnetic field dependent r.f. dissipation.

Oxygen displacement in a 107Ag17+ ion irradiated Bi2Sr2CaCu2O8 single crystal

We have studied the magnetic field (H∥c) dependent rf dissipation (Hrf∥a) in an as-grown Bi2Sr2CaCu2O8 single crystal prior to and after irradiation with 250 MeV 107Ag17+ ions. In a comparison of the responses from the as-grown crystal with an air-annealed crystal, features due to oxygen deficient regions acting as weak links in the former are identified. These features disappear immediately after irradiation of the as-grown crystal. We attribute such behavior to the displacement of oxygen from columnar tracks to deficient regions thus eliminating the weak links. Losses from the same irradiated as-grown crystal stored at 300 K for 60 days show that the features similar but not identical to those observed in the pristine state have reappeared implying that the displaced oxygen is in a metastable configuration in the deficient regions and hence is mobile due to thermal effects even at 300 K.We have studied the magnetic field (H{parallel}c) dependent rf dissipation (H{sub rf}{parallel}a) in an as-grown Bi{sub 2}Sr{sub 2}CaCu{sub 2}O{sub 8} single crystal prior to and after irradiation with 250 MeV {sup 107}Ag{sup 17+} ions. In a comparison of the responses from the as-grown crystal with an air-annealed crystal, features due to oxygen deficient regions acting as weak links in the former are identified. These features disappear immediately after irradiation of the as-grown crystal. We attribute such behavior to the displacement of oxygen from columnar tracks to deficient regions thus eliminating the weak links. Losses from the same irradiated as-grown crystal stored at 300 K for 60 days show that the features similar but not identical to those observed in the pristine state have reappeared implying that the displaced oxygen is in a metastable configuration in the deficient regions and hence is mobile due to thermal effects even at 300 K. {copyright} {ital 1997 American Institute of Physics.}

Vortex dynamics at rf frequencies in Bi2Sr2CaCu2O8 single crystals

Abstract We have studied the low magnetic field high temperature region of the H–T phase diagram of Bi2Sr2CaCu2O8 single crystals using the technique of non-resonant rf response at a frequency of 20 MHz. With Hrf‖a, H‖c, the isothermal magnetic field scans below TC show that the frequency f(H) of the tank circuit decreases continuously with increase in H before saturating at H∼HD(T). Such a decrease in f(H) reflects increasing rf penetration into the weakly screened region between CuO bilayers. The saturation of f(H) at its lowest value for H∼HD(T) indicates complete rf penetration (and hence the disappearance of field dependence) due to the vanishing of the screening rf currents Irfc in those regions or equivalently when the phase coherence between adjacent superconducting layers vanishes. Therefore HD(T) represents the decoupling of the adjacent superconducting bilayers, and hence also a 3D to 2D decoupling transition of the vortex structure. Simultaneous monitoring of the field dependent rf power dissipation P(H) shows a maximum in dP/dH at HD(T). The observed HD(T) line in many crystals is in excellent agreement with the (1/t−1) behavior proposed for decoupling.

Suppressed rf dissipation in 107Ag17+ ion irradiated Bi2Sr2CaCu2O8 single crystals by enhanced flux line tilt modulus

We have studied the isothermal, magnetic field (H‖c) dependent rf power P(H) dissipation (Hrf‖a) in the superconducting state of Bi2Sr2CaCu2O8 single crystals prior to and after irradiation with 250 MeV 107Ag17+ ions. In the pristine state, P(H) shows an initial decrease with increase in field, reaches a minimum at HM(T) and increases monotonically for H>HM(T). This behavior arises when the electromagnetic coupling between the pancake vortices in adjacent CuO layers becomes dominant on increasing the field and minimizes the distortions of the flux lines by confining the 2D vortices. In the post irradiated state, such an initial decrease and the minimum in P(H) is not observed but only a much reduced rf dissipation that monotonically increases with field from H = 0 onwards is seen. We attribute this difference to the strong enhancement of the tilt modulus C44 of the flux lines on irradiation when the pancake vortices in adjacent CuO bilayers are pinned along the track forming a well-stacked flux line in the field direction (‖c). We have also observed that the rf dissipation disappears at a certain temperature Tsf, at which the normal core of the flux line becomes commensurate with the columnar track diameter.

Non-resonant rf absorption evidence for reentrant melting of vortex lattice in Bi2Sr2CaCu2O8 single crystals

We have studied the magnetic field dependent rf (20 MHz) losses in

Nonresonant microwave absorption study of intrinsic Josephson coupling in Bi2Sr2CaCu2O8 single crystals.

Bi_2Sr_2CaCu_2O_8

Bulk vortex matter inBi2Sr2CaCu2O8+δusing Corbinol disk contacts

single crystals in the low field and high temperature regime. Above

Bulk vortex matter in Bi2Sr2CaCu2O8+δ using Corbinol disk contacts

H_C_l

Bulk vortex matter in Bi2 Sr2 CaCu2 O8+delta using Corbinol disk contacts

the dissipation begins to decrease as the field is increased and exhibits a minimum at

Superconducting and structural properties of PrBa 2Cu 3O x under high pressure

H_M>H_C_l