摻鈣釔鋇銅氧薄膜相圖強關聯特性之研究

Abstract

在本論文中，我們利用X光吸收光譜、超快激發探測光譜和其他基本量測來研究強關聯電洞摻雜釔鈣鋇銅氧超導體的電子結構與超快動力學，並探討超導漲落的物理機制和Hubbard模型的適用性。而利用本實驗室自製的控氧系統，我們將可應用極少的樣品來量測不同電洞濃度的釔鈣鋇銅氧超導體特性。 首先，藉由一系列電阻率隨溫度的量測搭配電阻率曲率製圖方法所繪製成的廣範圍摻雜濃度的釔鈣鋇銅氧相圖，我們觀測到偽能隙隨著載子濃度的演化並且在pc ~0.21有量子臨界點的產生。而此層狀超導體從Ginzburg–Landau模型所獲得摻雜相依性的超導漲落溫度與能斯特效應的比較確立了超導漲落為能斯特效應的起因。然而，因為一些在銅氧化物電阻量測上的細節無法用現存的模型理解，所以一個更精確的載子動力學的理論是必要的。 再者，為了檢測Hubbard模型的適用性，我們量測廣範圍摻雜濃度的釔鈣鋇銅氧中O K-和Cu L-edge X光吸收光譜。不同於平均場理論所預測的線性變化，我們發現Zhang–Rice singlets的光譜權重隨著載子濃度的斜率持續地變化增加至p = 0.23且並無飽和的跡象。另外，我們也應用了超快時間解析光譜當作另一種輔助的工具來探討費米能附近的電子結構和upper Hubbard band在過摻雜摻鈣釔鋇銅氧中的演化。從波長640奈米的瞬時反射率的變化中，我們可以清楚地確定upper Hubbard band依然存在於高摻雜濃度。所以，與先前X光吸收光譜所得到的結果一樣，Hubbard模型的圖像仍是適用於在過摻雜的範圍。

In this dissertation, the mechanisms of superconducting fluctuations and the applicability of the Hubbard model in a strongly correlated hole-doped Y1−xCaxBa2Cu3O7−δ (Ca-doped YBCO) superconductor are discussed in detail. The electronic structures and ultrafast dynamics are studied via X-ray absorption spectroscopy (XAS), ultrafast pump–probe spectroscopy, and other basic measurements. Using our self-made annealing system, the characteristics of the Ca-doped YBCO with various hole concentrations can be found for a few samples. First of all, comprehensive ρ(T) measurements and the consequent resistivity curvature mapping of Ca-doped YBCO thin films elucidate a phase diagram for the entire doping range of p = 0.08–0.21. The evolution of the pseudogap was observed with an accompanying quantum critical point at pc ≈ 0.21. The temperature of the superconducting fluctuations Tf as a function of p was also calculated using the Ginzburg–Landau model for layered superconductors. Comparisons between Tf and the Nernst temperature establish a superconducting (SC) fluctuation origin for the Nernst effect. However, because some of the details in ρ(T) cannot be fully explained by the existing models, a more sophisticated theory for carrier dynamics in cuprates is indeed necessary. Moreover, in order to examine the applicability of the Hubbard model, the XAS spectra of the O K- and Cu L-edges, with a wide doping range from zero doping to overdoping, are investigated. In contrast to mean field theory, we find that the spectral weight (S) of the Zhang–Rice singlets deviates from linear dependence on the doping level p. The slope of S versus p changes continuously throughout the entire doping range with no sign of saturation up to p = 0.23. For exploring the electronic structure near the Fermi energy and the evolution of the upper Hubbard band (UHB), the ultrafast time-resolved spectroscopy was also utilized as a complementary tool. The change in sign of the transient reflectivity signal (R/R) obtained at a wavelength of 640 nm clearly identifies the existence of the UHB at high doping levels. Therefore, the picture of the Hubbard model remains appropriate deep into highly overdoped regime, which is consistent with our previous XAS results.