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京都大学 / 新制・課程博士 / 博士(理学) / 甲第23696号 / 理博第4786号 / 新制||理||1685(附属図書館) / 京都大学大学院理学研究科物理学・宇宙物理学専攻 / (主査)教授 松田 祐司, 教授 柳瀬 陽一, 教授 石田 憲二 / 学位規則第4条第1項該当 / Doctor of Science / Kyoto University / DFAM

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A method of performing accurate oxygen analysis on cuprate based superconducting materials was established using electron probe micro analysis (EPMA). A range of YBa₂Cu₃O_δ ceramics with varying oxygen concentration were used to test the method. Using YBCO as a reference material, a suitable standard for oxygen analysis of Bi₂Sr₂CaCu₂O_δ (BiSCCO-2212) materials was obtained. This standard was used to perform full elemental analysis of a range of BiSCCO-2212 crystals, post annealed in pO₂'s between 10^-5-2atm. When the average Cu valence of each crystal was calculated and plotted as a function of the critical temperature (T_c) for each crystal, it was shown that BiSCCO-2212 materials conformed to the 'universal' trend illustrated by most other HTS and did not exhibit anomalous behaviour as had been previously believed. The phase assemblage and superconducting properties of BiSCCO-2212 Ag-clad multifilamental wires, prepared using the powder-in-tube (PIT) method by BICC, were studied as a function of a time/temperature profile. pO₂ of the processing atmosphere was found to be the predominant factor in determining the stoichiometry of the 2212 phase within wires. The phase assemblage is not simply a function of pO₂ as previously believed and can be controlled, in part, by the post annealing temperature. Homogenisation of the phase assemblage in BiSCCO-2212 Ag-clad wires can be achieved by prolonged heating (96hrs) at an appropriate temperature. An investigation into the 10K superconducting BiSCCO phase has shown the Sr-rich solid solution to extend towards the ideal stoichiometry of 2:2:1 (Bi:Sr:Cu) with increasing pO₂. Using a combination of high pO₂ (60atm) to achieve the appropriate Bi:Sr stoichiometry followed by post annealing in N₂ to adjust the oxygen content, it was possible to prepare single-phase ceramics of stoichiometry Bi_2.11(2)Sr_1.90(2)Cu_0.99(2)O_δ with a T_c=10.5K(5).

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Using time-resolved photo-induced reflectivity, we reported for the first time a systematic work on the ultrafast response of Bi₂Sr₂CaCu₂O_8+δ (BSCCO-2212) and Tl₂Ba₂CuO_6+δ (TBCO-2201), measurements of detwinned YBa₂Cu₃O_7-δ (YBCO-123) single crystal with the electric field ̲E parallel to the a and b-axis and high-resolution measurements of the rising edge dynamics of YBCO-123 thin films. We identified similar photo-induced responses for BSCCO, TBCO and for YBCO with ̲E ⊥ ̲b, which indicates that we observed a universal response of HTSC coming from the CuO2 superconducting planes. This latter dynamics is composed of three different components corresponding to the superconducting, pseudogap and normal state. A bi-molecular model has been put forward to explain the linear temperature dependence of the decay rate; the model implies that the re-formation of the condensate is limited by the rate at which quasiparticle interact. Moreover, we observed superconducting fluctuations up to 13K above T_c and a divergence of the long-lived component magnitude at very low temperature, which is explained by a cw heating model. In the pseudogap state, we have several indications that the negative peak observed between T_c and T* has a different origin from that of the superconducting signal below T_c. We argued that the probe mechanism of the pseudogap signal is electronic excitations of the pseudogap correlations. In the normal state, the observed dynamics is similar to that of simple metals. In the second part of this thesis, the rising edge dynamics of YBCO has been resolved in time. The model developed to interpret the results implies that the hot quasiparticles relaxation time down to the Fermi energy is 55fs. In this context, we proved that the Mazin model cannot explain both the oscillatory and the non-oscillatory part of the dynamics in YBCO. Finally, in YBCO-123, a new response has been observed with ̲E ∥ ̲b. We argued that the origin of this component is intraband transitions. This dynamics is solely responsive to the pseudogap, coming from the difference in scattering rate between pre-formed pairs and quasiparticles in the Drude reflectivity. The response with ̲E ⊥ ̲b exhibits a strong a-b plane anisotropy in its long-lived component, which can be interpreted as a d-wave gap symmetry using the thermally-activated model.

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