超導材料銣鎢青銅礦(RbxWO3, 0.17 ≤ x ≤ 0.33)之x光吸收能譜研究

Abstract

近年來，超導材料銣鎢青銅礦(RbxWO3)引起科學家們的興趣，歸因其複雜的結構變化，導致不同程度的電子-聲子交互作用，連結超導相變溫度的改變。X光繞射能譜顯示銣鎢青銅礦為六角晶相結構，但在銣含量較少(x ≤ 0.23)的情況下，其出現混合晶相與六角晶相共存態 。其次，x光吸收近邊結構能譜顯現鎢的5d軌域受到八面體對稱(Oh)的影響，分裂成eg與t2g簡併態，其分裂能與t2g簡併態的半高寬在銣含量0.23與0.27時出現反曲點，這意味著鎢氧八面體隨掺雜銣含量的多寡，展現不同程度的扭曲狀態，呼應超導相變溫度的變化趨勢。再者，延伸x光吸收精細結構能譜指出鎢氧八面體受到摻雜銣的影響，中心鎢原子有位移的現象產生。綜上所述，我們認為摻雜不同含量的銣，造成鎢氧八面體的局部扭曲，調制鎢5d軌域之eg與t2g簡併態電子結構，進而影響超導相變溫度的變化。

Rubidium tungsten bronzes (RbxWO3, 0.17 ≤ x ≤ 0.33) have recently attracted much attention due to their complex structural phase transitions, strong electron-phonon coupling, and intriguing superconducting properties. X-ray powder diffraction patterns show hexagonal tungsten bronze (HTB) phase in x ≥ 0.23 samples. The coexistence of intergrowth tungsten bronze (n-ITB, n = 2, 3) and HTB phases are observed in x< 0.23 samples. Furthermore, x-ray absorption near-edge structure spectra show that the octahedral splitting energy of eg and t2g states in tungsten 5d orbital and band width of t2g states exhibit two inflection points in x = 0.23 and 0.27 samples, indicating that WO6 octahedra undertake different degrees of local distortions. Additionally, x-ray extend absorption fine structure spectra show the distortion occurs at x = 0.23 due to the off-center displacement of tungsten ion. All of these observables suggest that rubidium doping induces the local lattice distortion of the WO6 octahedra, leading to the modification of the electronic structures of eg and t2g states in tungsten 5d orbital, thereby accounting for the characteristic changes of superconducting transition temperature of these materials.