高溫超導銅氧化物Y1-xCaxBa2Cu3Oy和Y1-xPrxBa2Cu4O8之光譜研究

Abstract

我們研究摻雜不同電洞濃度之銅氧化物超導體Y1-xCaxBa2Cu3Oy與次摻雜之Y1-xPrxBa2Cu4O8的光譜特性，這些資訊有助於我們了解在不同相圖區域的樣品之內部電子結構、晶格動力學及雙磁振子激發的變化。 首先，藉由分析Y1-xCaxBa2Cu3Oy的全頻光譜，我們發現，隨著電洞濃度增加，紅外光活性聲子逐漸被屏蔽，電漿邊界有藍位移的趨勢，尤其重要的是光學電導率顯現高頻往低頻的權重轉移現象，與Y1-xCaxBa2Cu3Oy的電性相轉變有緊密的關連性。此外，總和定理的分析顯示過摻雜樣品的有效電荷數目偏離Tc與電洞濃度的關係曲線。 其次，我們分析Y1-xCaxBa2Cu3Oy的拉曼散射光譜，實驗結果顯示電子-聲子交互作用引起O(2,3)反向振動模的不對稱，隨著電洞濃度增加，頻率往低頻偏移至過摻雜而趨緩，有趣地是，僅有理想摻雜樣品的聲子自洽能受到超導臨界溫度之影響。另一方面，我們觀察到高頻拉曼散射光譜呈現雙磁振子的激發峰，隨著電洞濃度增加，雙磁振子的峰值往低頻偏移且半高寬變大，顯示其超交換能變小，衰減參數變大，反鐵磁性短程有序的相干長度變短。 最後，我們觀察到Y0.9Pr0.1Ba2Cu4O8樣品的雙磁振子峰值，介於YBa2Cu4O8與PrBa2Cu4O8的雙磁振子激發峰之間。

We present the optical reflectivity and Raman-scattering measurements of Y1-xCaxBa2Cu3O7 thin films and Y1-xPrxBa2Cu4O8 polycrystals. The room-temperature optical conductivity spectrum of the YBa2Cu3O6 is typical of an insulator, showing only phonons in the far-infrared and several electronic absorption bands at higher frequencies. With increasing oxygen concentration, there is a large spectral weight transferred from high-to low-frequency region. Similar behavior is also observed in the Ca doping. There are three important changes in the Raman phononic spectra:(i) a harding of the A1g c-axis bridging oxygen phonon with increasing oxygen content; (ii) a softening and broadening of the B1g planar oxygen phonon after Ca doping; (iii) as the temperature is lowered, the B1g planar oxygen phonon in YBa2Cu3O7 shows remarkable self-energy effects when the superconductiving gap opens. Moreover, the observed B1g two-magnon excitation peak near 2700 cm-1 in YBa2Cu3O6 is broadened and weakened with increasing oxygen concentration or Ca content. For the Y0.7Ca0.3Ba2Cu3O6 film, no two-magnon excitation is visible, indicating an antiferromagnetic correlation length is less than twice the lattice parameter in the heavily overdoped phase regime. Interestingly the high-frequency part of B1g Raman-scattering spectrum of Y0.9Pr0.1Ba2Cu4O8 exhibits a band peaked at ~2650 cm-1, whose position is located between the YBa2Cu4O8 and PrBa2Cu4O8.