NaxCoO2 近緣吸收光譜之極性相依現象探討與比熱研究

Abstract

在NaxCoO2 的O K-edge X 光近緣吸收光譜中，我們觀察到隨著入 射光電場偏振方向的改變，光譜會產生一顯著的極性相依現象。這似 乎不能以一般認為的，在NaxCoO2 中eg states 簡併的說法來解釋。因此我們製備了x = 0.68 及x = 0.75 的薄膜樣品進行偏極化的XANES 實驗，觀察此現象是否與溫度相依，並以實驗數據及理論計算試圖解釋此極性相依現象的成因。在分析了Na1CoO2 系統的能態密度後，我們推測在NaxCoO2 的O K-edge 吸收光譜中pre-edge 部分不同能量處的峰值，可能並不單純緣於Co3+及Co4+的eg 軌域提供的空態，另一原因是由於被電子占據的Co 3d 軌域在空間中的不對稱性造成了O 2p 軌域與之混成後，使px,y 與pz 在能量上也被分開了。 另外，我們也對Na0.832CoO2 單晶進行了比熱及磁性量測，並且在 T = 8 K 及T = 15 K 處各發現了一個相轉變；我們也從數據中發現此15 K 以及8 K 處的相生成是相依於降溫速率的，而此相依性的產生則是決定於樣品中Na 離子機動性的存在溫區。最後，根據比熱數據中相變溫度隨外加磁場移動的行為，並且在經過磁化率的確認後，推斷15 K 及8 K 是Na0.832CoO2 的反鐵磁相變溫度。磁場中15 K 及8 K處的反鐵磁態轉變成鐵磁態。

The soft X-ray absorption spectra (XAS) of NaxCoO2 revealed marked and puzzling polarization dependence. It can not be explained by the degeneracy of eg states generally believed in NaxCoO2. We fabricated the thin films of x = 0.68 and x = 0.75 to investigate the polarization dependence of XAS. We also try to explain this phenomenon with the first principle DFT calculations. After the analysis of the DOS of NaxCoO2, we propose that the pre-edge peaks at 529 eV and 530 eV of NaxCoO2 O-K edge may be not solely due to the unoccupied states of Co3+ and Co4+ eg states, but also due to the spacial asymmetry in theoccupied Co 3d orbitals. Due to the hybridization between Co 3d & O 2p orbitals, the px,y and pz states will be non-degenerate. Besides the XANES measurements, we performed the specific heat and magnetization experiments on Na0.832CoO2 single crystal to investigate the phase diagram. It is found that there are two new phase transitions at T = 8 K or T = 15 K, respectively. The occurrence of the phase transition at T = 15 K and T = 8 K depend on the cooling rate through the temperature region that the mobility of Na ions exists. Finally, we conclude that both the 8 K and 15 K transitions are of the antiferromagnetic ordering at zero field. The transition from antiferromagnetism to ferromagnetism was observed for both ordering states. Therefore, metamagnetism was observed in both 8 K and 15 K transitions of Na0.832CoO2.