The archetypal high temperature superconductor Bi₂Sr₂CaCu₂O[subscript[8+δ]] has been extensively investigated. However, until now, little has been known about the behaviour of the magnetic vortices inside the Vortex Glass and liquid state. µSR measurements have shown a negative skewness for the field probability distributions in these regimes. Such a negative skewness has only recently been explained as being a direct consequence of three-body correlations between vortices in a similar layered superconductor. With a new understanding and knowledge of the physics of these systems, it is instructive to re-visit the superconductor Bi₂Sr₂CaCu₂O[subscript[8+δ]] to explain the evolution of these three-body correlations occurring here. Comparing this with the ion-irradiated superconductors (of the same Bi₂Sr₂CaCu₂O[subscript[8+δ]] material), allows one to observe how three-body correlations between vortices evolve differently to that in the pristine material. Moreover, in the region of the macroscopic irreversibility line, entropically driven disorder exists below the matching field B[subscript[ϕ]] , whilst there is the appearance of relatively straight vortex lines at fields above B[subscript[ϕ]] . Such phenomena suggest a significant di fference in the evolution of three-body correlations compared with the unirradiated material. There has been much work conducted on the interplay between superconductivity and magnetism in materials of reduced dimensions. Work presented here on the ferromagnet/superconductor trilayer system (Permalloy/Nb/Permalloy) has shown a novel magnetic profile at the interface between the ferromagnetic and superconducting boundary, where, contrary to what is expected, the magnetism appears to be significantly suppressed at the interface before increasing towards the centre of the Nb layer.

<p>This thesis is focused on studies of substituted Hg-based superconducting copper oxides ((Hg_1-x<i>M</i>_x)Ba₂Ca_n-1Cu_nO2_n+2+δ). These compounds are promising objects of investigation, not only from a fundamental point of view but also because of their high values of superconducting transition temperature (<i>T</i>_c) and irreversibility field (<i>H</i>_irr).</p><p>The first part of the thesis is devoted to optimization of the synthesis procedure for Hg-based cuprates. The influence of different parameters (<i>T</i>,<i> t</i>, <i>p</i>(Hg),<i> p</i>(O₂)) on the synthesis of these compounds in sealed silica tubes was studied. Optimal conditions yielded samples containing up to 95% of HgBa₂Ca₂Cu₃O_8+δ (Hg-1223). The formation of solid solutions with the formula (Hg_1-xCu_x)Ba₂Ca₂Cu₃O_8+δ (where x <= 0.5) was also established. Another technique was developed, using LiF as a flux, for synthesis of samples containing up to 90% of the HgBa₂CaCu₂O_6+δ (Hg-1212) phase.</p><p>The second part concerns synthesis and studies of oxyfluorides using Hg-1212 and Hg-1223 as starting materials together with XeF₂ as a fluorinating agent. It was found that oxyfluorides of both phases have a parabolic dependence of T_c vs. <i>a</i> parameter as well as enhanced <i>T</i>_c values (Δ<i>T</i> ≈ 3-4 K) in comparison with optimally doped non-fluorinated analogues. The crystal structure of Hg-1223 oxyfluoride was studied by X-ray powder and neutron diffraction methods. It is suggested that chemical modification of the crystal structure leads to a decrease in Cu-O distance without noticeable change in Cu-O-Cu angle (in the (CuO₂) layers), which may be the significant factors influencing this <i>T</i>_c increase. Hg-1223 oxyfluoride was also studied under high pressure for first time. It was found that this compound has a record-high <i>T</i>_c value (≈ 166 K) at <i>P</i> ≈ 23 GPa.</p><p>The last part describes the investigation of substituted Hg-based superconductors in the series (Hg_0.9M_0.1)Ba₂CuO_4+δ {(Hg,<i>M</i>)-1201}, where <i>M</i> = Tl, Pb, W, Mo, Nb and V. A comprehensive study of these compounds by various methods (X-ray powder diffraction, EDX, IR-, EXAFS- and XANES -spectroscopy) indicated that the change of charge carrier doping level is a crucial factor determining the irreversibility line. (Hg_0.9Mo_0.1)Ba₂CuO_4+δ showed the most improved irreversibility line position among the (Hg,<i>M</i>)-1201 compounds studied in this series.</p>

No description available.

No description available.

No description available.

No description available.

No description available.

No description available.

No description available.

No description available.