Modification of Iron d-orbitals in Superconducting FeSe Crystal - by Raman Spectra Study

鐵d軌域的改變在超導體硒鐵的拉曼光譜研究

熊欣怡(Hsin-I Hsiung)

指導教授 ： 吳茂昆

國立清華大學/理學院/物理學系/碩士(2013年)

https://doi.org/10.6843/NTHU.2013.00552

摘要

在此論文中，我們利用拉曼光譜研究硒化鐵在不同溫度下，從低頻率的聲子 訊號到高頻 (2000 cm-1) 電子躍遷的光譜變化。在130 cm-1 左右的聲子頻率在硒化鐵結構轉變溫度(~ 90 K)以上迅速下降，聲子訊號在181 cm-1 左右的半高寬也在這個溫度以上漸漸變寬。在高頻的部分，我們發現了電子軌域分裂的現象。從電子拉曼散射的訊號中，1519 cm-1 的訊號在130 K 以下會分裂成1517 cm-1 和1543 cm-1 兩個訊號，而這兩個訊號為鐵的電子拉曼散射從dzx/dyz 到dx2-y2 軌域的訊號，其能量差隨著溫度降低而逐漸變小。這個結果顯示出在130 K 左右，鐵的dzx 和dyz 軌域會分裂，隨著溫度降低這兩個軌域漸漸靠近而達成平衡。此外，另一個有趣的現象是1004 cm-1 的訊號在130 K 以下才會出現。而超快光學在硒化鐵的研究上也指出在130 K 左右會有一個能隙出現，符合我們的實驗結果，顯示出電子軌域重新排列的溫度高於結構轉變的溫度。我們以3 %的銅取代部分鐵原子，產生沒有超導性也無結構轉變的銅硒化鐵來做比較。從拉曼光譜的結果顯示，銅硒化鐵在高頻沒有電子拉曼散射的訊號，顯示其電子軌域沒有分裂。從我們的實驗結果顯示出，電子軌域的重新排列在研究鐵基超導體的超導機制上為一個重要關鍵。

關鍵字

超導 ； 硒化鐵 ； 拉曼 ； 軌域

並列摘要

We have studied the Raman spectra of superconducting FeSe crystals at different temperatures. The spectral range covers the phonon modes at low frequency regime to electron Raman scattering at high frequency regime, up to 2000 cm-1. The phonon mode near 130 cm-1 shows a sharp decrease in the frequency, and the line-width of the mode near 181 cm-1 increases above 90 K, which is a consequence of the structural transition as reported in x-ray and neutron diffraction experiments. At high frequency region, the mode at 1519 cm-1 splits into two modes as temperature below 130 K. These two modes are attributed to the electronic Raman scattering from zx/yz to (x2-y2) d-orbitals of Fe. The energy difference of these two modes decreases as temperature decreases. These behaviors indicate that the dzx and dyz orbitals split at ~130 K and then become closer to achieve balance as temperature decreases. Surprisingly, the other Raman mode near 1004 cm-1 appears only below the same temperature. These results indicate that a modification of d-orbital electron state of iron happens at temperature much higher than the structural transition, which was also observed in the transient optical reflectivity study, implicating a precursor order in iron-based superconductor. In comparison with non-superconducting Cu0.03Fe0.97Se crystal, it shows no structural distortion, and the crystal field splitting cannot be observed in Raman spectrum. This indicates that the orbital modification could be an important clue for understanding the mechanism of superconductivity in iron-based superconductors.

並列關鍵字

superconductor ； FeSe ； Raman ； iron ； orbital

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