單層薄膜FeSe/STO之非絕熱超導現象聲子模型

Abstract

單層薄膜FeSe/STO已經成為近來追求高溫超導性的重要材料，其超導相變溫度Tc至少可達到60~70K。在此同時，其簡單的材料結構有助於我們簡化理論上需要考慮的條件，為理解庫柏對(Cooper pair)的配對機制提供了希望。最近的實驗表明，其超導性有可能是來自於單層FeSe薄膜與極性基質STO之間的強電子聲子交互作用(electron-phonon interaction)。我們以此為出發點，希望對此類型材料建立其超導性的聲子模型。實驗上所觀測到的強光學聲子與金屬(主要是聲學聲子)在定性上是不同的，因為在此種材料中，絕熱性(adiabaticity)已經喪失了。電子在超導口袋(superconducting pockets)中的時間尺度跟聲子大致相同。 本篇論文利用量子場論之路徑積分方法，在非絕熱(nonadiabatic)體系中建立此類二維材料的Eliashberg聲子模型。我們利用強光學聲子，計算出電子的能帶重構(band reconstruction)，以及相變溫度Tc。最後將我們的結果與絕熱(adiabatic)極限下，傳統BCS聲子模型的McMillan 公式的估計值作比較。

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.