以X光吸收光譜學檢驗Hubbard理論模型

Abstract

本論文中利用同步輻射中心極化光源進行O K-edge和Cu L-edge X光近緣吸收光譜(X-ray absorption near edge structure, XANES)實驗，樣品為脈衝雷射蒸鍍系統(Pulse Laser Deposition System, PLD)所製備的Y1-xCaxBa2Cu3Oy (x=0、0.3、0.4，6.0≦y≦6.95)薄膜。本實驗探討Y1-xCaxBa2Cu3Oy薄膜的銅氧平面費米能階(Fermi level；EF)附近能帶之光譜權重(spectral weight)做討論。量測載子(電洞)濃度0≦p≦0.23區域間的樣品以分析其電洞之分佈，進而重新檢視D. C. Peets et al. (Phys. Rev. Lett. 103, 087402 (2009)) 中對Hubbard model理論預測的質疑。 我們並無觀察到D. C. Peets等人於論文中所宣稱在載子濃度p＞0.21處Zhang-Rice band 光譜權重有飽和的情形，而是持續有增長的趨勢，故本實驗認為Hubbard model理論預測在0≦p≦0.23區域間仍可適用。 本論文亦補充李佩茹學姊實驗論文[26]中缺少的低載子濃度YBa2Cu3Oy薄膜的銅氧平面費米能階(Fermi level；EF)附近能帶之O K-edge光譜權重(spectral weight)，於偽能隙(Pseudogap)上下溫區是否有所差異，進而檢證Philip Phillips et al., Phys. Rev. Lett. 99, 046404 (2007)所提出高溫超導體中Hidden charge 2e boson model之新理論。實驗結果為光譜權重有所變化，與理論定性相符。

In this thesis, We measured the x-ray absorption spectroscopy (XAS) at O K- and Cu L-edges of Y1-xCaxBa2Cu3Oy (x=0, 0.3, 0.4, 6.0≦y≦6.95) thin films which were fabricated by pulse laser deposition technique. We studied the spectral weight at the doping level 0≦p≦0.23 to check the Hubbard model, and compared the results to the theoretical trend proposed by D. C. Peets et al., Phys. Rev. Lett. 103, 087402 (2009). It is concluded that Zhang-Rice band spectrum weight still grows with the p. The experimental results suggest that the Hubbard model be applicable at least up p=0.23. We also measure the temperature dependent XAS on YBa2Cu3Oy (0.5≦p≦0.092) above and below the pseudogap temperature T*. The experimental results qualitatively support the theory of the Hidden charge 2e boson.