| dc.description.abstract | In pristine Bi2212, the electromagnetic coupling determines the C44C_{44}C44? of the stack of 2D vortices as the field increases. In the presence of columnar defects, the 2D vortices in adjacent CuO bilayers are pinned along the track, resulting in a large enhancement of the CnC_nCn? of the stack. Thus, the role played by electromagnetic coupling in causing the minimum at Hm(T)H_m(T)Hm?(T) in the pristine state is absent in the post-irradiated state, as the correlation between the pancake vortices in adjacent layers becomes very strong in the presence of columnar defects.
The RF dissipation is observed to disappear at a temperature TfT_fTf?. This marks the onset of strong single-flux-line pinning, i.e., when the normal core size of the flux line becomes commensurate with the effective defect diameter.
At low fluences of 250 MeV ion irradiation, the signatures of oxygen-deficient weak links in the as-grown Bi2212 single crystal disappear immediately after irradiation. This is attributed to the displacement of oxygen into the surrounding oxygen-deficient regions under the impact of the ions. The reappearance of these features 60 days after storing at 300 K suggests that at these low dosages, the elimination of oxygen-deficient regions is only temporary. These results, along with the observed recovery in the resistivity measurements of [24, 25], point to the non-trivial role of such displaced oxygen in determining the losses in the Bi2212 system.
The magnetic-field-dependent RF losses in the CuO bilayers of Bi2212 single crystals begin nearly 20 K above the nominal TcT_cTc? of this phase. The lack of broadening of P(H)P(H)P(H) and the non-increasing saturation fields between 102 K and TcT_cTc?, and the absence of any change in the intensity of the signals in successively cleaved crystals, point to the fact that the Bi2223 phase is not responsible for this onset. Its origin lies in the fluctuating Cooper pairs in the bilayers. The pairing remains very weak until TcT_cTc? and is suppressed by the application of fields as low as 10 G. Of course, it would be ideal to perform the above experiment in pure Bi2212 single-phase crystals. Since such crystals are not easily available, it is fair to believe that these results constitute possible, if not conclusive, evidence for superconducting fluctuations in the CuO bilayers of Bi2212.
The above results point to the crucial role played by the modulating field in causing the fine structure. It implies that the fine structure is not simply caused by fluxons nucleating in the Josephson junctions [8] or in the loops [1] by an increasing field, but it is assisted by the modulating field as well. This is evident in the amplitude and frequency dependence in the field modulation experiment and the absence of fine structure on frequency modulation.
In conclusion, it is demonstrated that frequency modulation offers a technique to record the direct dissipation P(H)P(H)P(H), free of any complications of magnetic field modulation in HTSC. While the frequency modulation technique yielded excellent results in large-size ceramic samples, it had limitations on its sensitivity when very small single-crystal samples were studied. In the latter, the dissipation is lower and hence higher sensitivity is required to detect the weak signals. It is desirable to enhance the sensitivity of frequency modulation so that weak signals from single crystals can be recorded. This limitation on sensitivity is also the reason for employing magnetic field modulation to study the Bi2212 crystals in the foregoing chapters. | |
