Pressure Effects on High Temperature Superconductors, and Physical Properties of V_1-xTi_xSe₂ Single Crystals

High critical current density (Jc) and small magnetic relaxation are crucial for technological applications of high temperature superconductors. So far, improved Jc has been reported, based on the pinning of vortices via local structural inhomogeneities in superconductors, which are induced by chemical doping, irradiation, and inclusion of non-superconducting secondary phases. In cuprates and iron-based superconductors (IBSCs), the superconducting state is induced mainly through chemical doping/substitution (x) and high pressure. Chemical doping (addition of holes or electrons) can change the lattice parameters and introduce internal chemical pressure, which modulate the electronic band structure and the density of states at the Fermi level through changes to the carrier concentration. High pressure is a well-known clean and effective tuning technique to explore superconductivity. In an IBSC, the most significant effect of high pressure is directly causing a reduction in the cell volume or a structural phase transition. Through changing the chemical bond lengths and angles, structural phase transitions have multiple effects on the various physical properties, including superconductivity. Structural phase transitions can change the crystal lattice size, which is a dominant factor in interplane and intraplane charge transfers. Thus, pressure can efficiently alter the carrier concentration and charge type at the Fermi level, which is critical to both the normal state and superconductivity. Magnetic exchange coupling can also be manipulated by high pressure, since the ferromagnetic or antiferromagnetic exchange interactions are largely determined by the bond length and angle, which dictate the wave function overlap between adjacent magnetic atoms. The pressure effect has shown enormous and unique advantages in the study of superconductivity, particularly for iron-based superconductors. Most technological applications of high temperature superconductors, however, do not involve polycrystalline crystals or films, but rather so-called coated conductors (CCs), which are films epitaxially grown on long-length highly-textured buffered metallic tapes. Nevertheless, little is known about the effects of in-situ hydrostatic pressure on these systems. So, this thesis reports the systematic study of in-situ hydrostatic pressured induced improvement of critical current density and vortex pinning in state-of-the-art Y(Dy)Ba2Cu3O7- coated conductors. The fundamental significance of the in-situ pressure induced significant enhancement of flux pinning or Jc is that there is still plenty of room to further improve the supercurrent carrying capability for both Fe-based superconductors and yttrium barium copper oxide (YBCO) coated conductors.