Investigation of the high-Tc phase diagram using optical spectroscopies and high pressures.

摘要

Superconductivity is the collective, macroscopic motion of all the free electrons in a solid. Its derivation from the metallic state, in which free electrons move and scatter independently of each other, has been one of the triumphs of condensed matter physics in the last century. In conventional superconductors, coupling of the electrons to lattice vibrations (phonons) is the mechanism behind the energy gap characterizing the superconducting state. In copper-oxide superconductors, with much higher transition temperatures inexplicable by the current theoretical framework, the mechanism has been a matter of much debate.; Other than the high Tc, these superconductors are characterized by an energy gap of d-wave symmetry. In this thesis, I will first discuss Angle-Resolved Photoemission Spectroscopy (ARPES) experiments on Bi2 Sr2CaCu2O8 (Tc ∼ 94K). We found anisotropic electron-phonon coupling of a d-wave form; we present a calculation that extends the theory done for the isotropic superconductor Pb to account for the anisotropy.; The high-Tc superconductors are also unique because they evolve from anti-ferromagnetic insulating states rather than good metals. The second part of the thesis will discuss Raman Spectroscopy and Transport measurements on insulating Bi2Sr2YCu2O8 under pressures of up to 35GPa. A new-pressure axis of the high-Tc phase diagram is introduced, characterized by a critical pressure which bears resemblance to the putative critical point at optimal doping. This critical pressure ties together electronic, spin, and lattice degrees of freedom.