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Title: Fe(Se,Te)超導材料之光譜性質研究
Optical properties of Fe(Se,Te) superconducting materials
Authors: 劉祥麟
Hsiang-Lin Liu
Mei-Chun Yeh
Keywords: FeSe
Iron-based Superconductors
Optical properties
Issue Date: 2014
Abstract: 我們研究Fe(Se,Te)超導材料的光譜性質,樣品包含FeSe單晶、FeSe薄膜及Fe1.05Se0.447Te0.553單晶,其中,以脈衝雷射成長c軸取向FeSe薄膜於(100)MgO基板,膜厚約為300 nm,超導相轉變溫度約為7.6 K。FeSe與Fe1.05Se0.447Te0.553單晶則為(001)晶面,超導相轉變溫度分別為8 K與12.5 K。 首先,我們測量室溫橢圓偏光光譜,觀察到FeSe單晶樣品具有4個吸收峰,分別為1.7 eV、2.7 eV、4.0 eV及6.2 eV。1.7 eV吸收峰對應Fe2+在3d軌域的d-d 電子躍遷;2.7 eV、4.0 eV及6.2 eV則為Se 4p到Fe 3d的電子躍遷。對照FeSe單晶,FeSe薄膜清楚觀察到1.7 eV及6.2 eV吸收峰,Fe1.05Se0.447Te0.553單晶則以1.7 eV、2.7 eV及6.2 eV吸收峰較明顯。我們推論對應FeSe單晶相近頻率的吸收峰位置,其吸收峰之電子躍遷行為與FeSe單晶相同。 其次,我們研究FeSe薄膜的拉曼散射光譜,其顯示4個拉曼峰,頻率位置為107 cm-1、180 cm-1、192 cm-1和238 cm-1,分別對應Eg(1)、A1g、B1g及Eg(2) 振動模,高頻 2300 cm-1 B1g對稱性拉曼峰屬於雙磁振子激發。隨著溫度的升高,A1g與B1g振動模約在475 K以上消失,550 K時,FeSe薄膜完全轉變為α-Fe2O3。相較之下,隨著溫度的降低,在低溫8 K、90 K、220 K三個溫度點皆觀察到拉曼峰值位移量的改變。A1g與B1g對稱性拉曼峰於220 K藍移量開始減緩,推測與短程有序軌道 (或電荷) 有關;接著在結構相轉變溫度90 K處,再一次有藍移量緩和發生;我們於超導相轉變溫度8 K,觀察到拉曼峰值有紅移現象。再者,B1g拉曼峰在室溫與低溫6 K之位移變化量高達8 cm-1,與A1g拉曼峰的位移量1.2 cm-1有明顯差距。B1g拉曼峰的頻率位移量異常現象與聲子-自旋耦合效應有關,隨著溫度的降低,Fe2+由高自旋態轉變為低自旋態,穩定的低自旋態導致系統的總能量下降,晶格結構更穩定,鍵能增強,藍移展現明顯。 最後,我們量測Fe1.05Se0.447Te0.553單晶的低溫拉曼散射光譜,並與FeSe薄膜相比較,發現同樣在三個溫度點:240 K、90 K(結構相轉變)、12.5 K(超導相轉變) 拉曼峰頻率位移量改變。然而,B1g與A1g拉曼峰在室溫與低溫的位移變化量沒有明顯差距,我們推測取代的Te離子改變Fe2+系統自旋狀態。
We present spectroscopic ellipsometry and Raman-scattering measurements of superconducting Fe(Se,Te) materials. The c-axis oriented FeSe thin films with thickness of 300 nm were deposited on the MgO substrates using pulsed laser deposition. Its superconducting transition temperature is about 7.6 K. FeSe and Fe1.05Se0.447Te0.553 single crystals with (001) orientation show the superconducting transition temperatures at about 8 K and 12.5 K. First, the absorption spectrum of FeSe single crystaldetermined from spectroscopic ellipsometry analysis shows four bands at about 1.7, 2.7, 3.9, and 6.2 eV. The optical absorption near 1.7 eV is due to on-site Fe2+ d-d excitations. Other high energy peaks above 2 eV are associated with the charge-transfer transitions between Se 4p and Fe 3d states. Corresponding FeSe single crystal, FeSe thin films was observed 1.7 eV and 6.2 eV absorption peak, and Fe1.05Se0.447Te0.553 single crystal was observed with 1.7, 2.7, and 6.2 eV absorption peak, and the electronic transitions with the same behavior corresponding to FeSe single crystals. Second, room-temperature Raman-scattering spectrum of FeSe thin film exhibits four phonon modes at about 107, 180, 192, and 238 cm-1, displaying symmetries of Eg(1), A1g, B1g, and Eg(2), respectively. Furthermore, the B1g excitation peak near 2300 cm-1 is associated with two-magnon scattering. Additionally, with increasing temperature, A1g and B1g phonon modes disappear above 475 K. At 550 K, Raman-scattering spectrum transfers to that of α-Fe2O3. In contrast to high temperature Raman-scattering spectra, A1g and B1g phonon modes exhibit anomalies with decreasing temperature in frequency at 220, 100, and 8 K that coincide with the onset of short-range charge and/or orbital orders, structural and superconducting phase transitions. Notably, a very large hardening (~ 8 cm-1) of the B1g phonon mode is possibly due to spin-phonon coupling. With decreasing temperature, Fe2+ ions change from the high spin state into low spin state. Finally, temperature-dependent Raman-scattering spectra of Fe1.05Se0.447Te0.553 single crystal show similar phonon anomalies at 240, 90, and 12.5 K. Interestingly, no large hardening of the B1g phonon mode can be seen, suggesting the Te doping disrupts the spin state transitions in Fe2+ ions.
Other Identifiers: GN060141019S
Appears in Collections:學位論文

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