X光吸收光譜對Cd2Re2O7相轉變之研究

Abstract

近年來Cd2Re2O7被廣泛的研究與討論。Cd2Re2O7是所謂的pyrochlore oxides，最早被人發現在200K具有一個二階相變，而後又被提出在120K具有另外一個一階相變，近年又發現在~1K的時候Cd2Re2O7是超導體；這是pyrochlore oxides 系統中第一個被發現具有超導現象的樣品。本碩士論文主要研究 Cd2Re2O7 單晶在經過結構相變後電子結構的變化。實驗方面，我們量測了不同溫區的的單晶繞射與Re L-edge 延伸X光吸收光譜精細結構(EXAFS) 譜圖並做傅立葉轉換，探討Cd2Re2O7在溫度變化下晶體結構的改變；並利用同步輻射高度極化的光源，進行不同溫度的 O K-edge 近邊緣X光吸收光譜(XANES)，研究結構相變下電子結構的變化。發現結構相變導致了電子結構的改變。理論計算方面，我們利用VASP(Vienna Ab-initio Simulation Package)軟體計算不同溫區的能帶結構，以了解費米能階(Fermi level)附近與費米能階以上空軌域的能態密度(Density of state)。

ABSTRACT Cd2Re2O7 has been attracting a lot of attention and investigated extensively recently. It is the first superconductor found in pyrochlore oxides family with transition temperature ~ 1 K and type-II superconductor characteristics. In this thesis, the relation among the electronic structure, phase transition and some physical properties of Cd2Re2O7 single crystals have been investigated. We conducted the X-ray diffraction as well as Re L2,3-edge extended X-ray absorption fine structure (EXAFS) at different temperatures to investigate the local structure of Cd2Re2O7 and temperature-dependent O K-edge X-ray absorption near edge structure (XANES) to probe its electronic structure. A spectral weight transfer via the temperature variation O K-edge XANES spectra of Cd2Re2O7 is clearly observed. We also performed the band structure calculations to understand the DOS near the Fermi level of Cd2Re2O7 at different temperatures.