以拉曼散射光譜研究 Sr2Y(Ru1-xCux)O6 與 Fe(Se,Te) 超導材料之晶格-電荷-自旋多重耦合效應

Abstract

本論文研究 Sr2Y(Ru1-xCux)O6 與 Fe(Se,Te) 超導材料的變溫拉曼散射光譜，經由分析拉曼特徵峰隨各種變因的改變，探討晶格結構和磁性及超導態的相關性。第一部份中，我們研究銅離子的摻雜與溫度對於 Sr2YRuO6 晶格結構的影響，實驗結果顯示，隨著銅離子的摻雜量增加，拉曼峰半高寬變大，代表晶格內部無序度擴增。同時，二階與一階拉曼散射峰的強度比值亦隨著銅離子摻雜量增加而變大，由樣品的光學電導率證實，此為入射光子能量 (2.3 eV) 接近樣品吸收峰 (2.35 eV) 造成的共振拉曼效應增強所致。當樣品的溫度降低，在 Sr2YRuO6 的弱鐵磁耦合溫度 (TC = 135 K) 以及釕離子的尼爾溫度 (TN, Ru = 23 K) 附近，我們觀察到由自旋-聲子交互作用所引起的拉曼峰頻率藍移及半高寬變窄的現象。此外，x = 0.4 超導樣品的 Y-O 與 Ru-O 八面體氧離子的伸張振動模於超導相變溫度 (Tc = 30 K) 以下顯現由自洽能效應所造成的聲子軟化。另外，二階與一階拉曼散射峰的強度比值隨著溫度的降低而上升，與低溫下自陷態激子的生命期增加有關。 第二部份，FeSe 單晶樣品的室溫非偏振拉曼散射光譜具有 4 個主要的拉曼峰，其頻率位置位在 104 cm-1、178 cm-1、193 cm-1 以及 255 cm-1，分別屬於 Eg(1)、A1g、B1g 及 Eg(2) 對稱性，另在高頻區域顯現具有 B1g 對稱性的寬廣雙磁振子散射訊號，其峰值中心位置約在 2222 cm-1。以羅侖茲模型分析光譜數據，B1g 對稱性高頻拉曼峰的半高寬在 230 K 以下急遽變寬，並於 90 K 以下轉為變窄，象徵著晶格內部反鐵磁域隨著溫度的下降發生改變，影響自旋-電荷交互作用，造成 FeSe 電阻率在特定溫度展現異常。此外，B1g 對稱性低頻拉曼峰在超導相變溫度 (Tc = 8.8 K) 附近頻率展現異常紅移且半高寬變窄，應與電子-聲子耦合效應有關。 最後，我們分析三種鐵同位素多晶樣品 xFeSe0.35Te0.65 (x = 54、 56，及57) 的拉曼散射光譜，隨著鐵質量數增加，A1g、B1g 以及 Eg(2) 對稱性拉曼峰的頻率紅移，表示晶格常數隨之變大，與 x 光繞射實驗結果相互呼應，進一步分析鐵離子相關的 B1g 對稱性拉曼峰的頻率位置偏移量，符合古典簡諧振子模型的預測。此外，我們發現兩個微弱的 1350 cm-1 與 1600 cm-1 拉曼峰，此為鐵離子 3d 軌域於晶格場分裂所致。有趣地是，比較三種鐵同位素樣品的 A1g 與 B1g 對稱性拉曼峰於低溫下的羅侖茲模型擬合參數，顯示在結構扭曲溫度 100 K 及超導相變溫度 13 K 附近的變化趨勢展現不一致性。

We present Raman-scattering studies of Sr2Y(Ru1-xCux)O6 and Fe(Se,Te). Our aim is to investigate the interplay among lattice, electronic, and magnetic excitations in these novel materials. First, with increasing Cu content, the linewidth of Raman-active phonon modes broadens, reflecting an increased lattice disorder. When the temperature is lowered, Ru-related phonon modes exhibit a blueshift at the weak-ferromagnetic transition temperature (TC = 135 K) and the Ru’s Neal temperature (TN, Ru = 23 K), indicating a spin-phonon interaction. In the case of the x = 0.4 sample, Y-O and Ru-O stretching modes show a softening below superconducting transition temperature (Tc = 30 K), suggesting the presence of self-energy effect. Furthermore, the intensity ratio of the second to first order Raman peaks is increasing with Cu doping, that is likely due to resonance Raman-scattering effect. With decreasing temperature, this intensity ratio shows an enhancement, which is related with the increased lifetime of “self-trapped” exciton. Second, Raman-scattering spectrum of FeSe exhibits four phonon modes at about 104, 178, 193, and 255 cm-1, displaying symmetries of Eg(1), A1g, B1g, and Eg(2). Moreover, the observed B1g two-magnon excitation near 2222 cm-1 is broadened at 230 K and then narrowing below 90 K, correlated with the variation of the resistivity data. Additionally, the B1g phonon mode shows a redshift below Tc (~ 8.8 K) driven by an electron-phonon interaction. Finally, with different iron isotope substitution in xFeSe0.35Te0.65, the peak positions of A1g, B1g, and Eg(2) phonon modes shift to lower frequencies, indicating a decreased force constant by lattice dilatation, in agreement with the observations in x-ray diffraction data. Furthermore, the variation of frequency position of B1g phonon mode is consistent with the predictions of simple spring constant model. Two high-frequency modes are observed at about 1350 and 1600 cm-1, attributed to the electronic Raman scattering from 3d-orbitals splitting of Fe2+ ion. Interestingly, the A1g and B1g Raman peaks and their linewidth exhibit irregular temperature dependence at 100 K and 13 K.