| Summary: | 碩士 === 國立交通大學 === 電子物理系所 === 105 === Singled-layered FeSe on STO has become a very important material in recent quest for high Tc superconductivity. Its critical temperatures rise up at least beyond 60~70K. At the same time, its simple material structure helps us simplify the theoretical considerations, bringing the hope for understanding the mechanism of Cooper pairing. Recent experiments has shown that the superconductivity is likely to result from the strong electron-phonon interaction generated in the single-layered FeSe by the polar substrate STO. We take this as a starting point, hoping to establish a phonon model superconductivity of such materials. Strong optical phonon modes established experimentally are qualitatively different from those (mostly acoustic) in metals since the adiabaticity is lost. The time scales of electrons in a small superconducting pockets and of phonons are about the same.
In this thesis, we use the path integral method of quantum field theory to establish the Eliashberg type phonon model of such kind of two-dimensional material in the nonadiabatic regime. We calculate the band reconstruction by strong optical phonons and the phase transition temperature Tc. Finally, we compare our results with the McMillan formula of the traditional BCS phonon model in the adiabatic limit.
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