利用偏振解析拉曼散射研究層狀鉍鍶鈣銅氧之晶格對稱性及熱穩定性

Abstract

鉍鍶鈣銅氧（BSCCO）是層狀銅氧化物家族中具有高溫超導潛力的材料之一，其臨界溫度高於液氮溫度（77 K）。本論文專注於最廣泛研究的Bi2Sr2CaCu2O8+x （Bi-2212），我們製備了高品質的Bi-2212樣品，並使用拉曼光譜研究其晶格振動模態。透過在平行和垂直偏振散射下進行晶體取向和偏振解析的測量，以及進行對稱性分析，我們闡明了聲子振動模式的特性。大部分拉曼活性模式可歸因於三斜晶系Ci結構中的Ag對稱性，與通常認定的正交D2h結構有所不同。然而在垂直偏振光譜中，我們觀察到幾個聲子模式呈現異常的對稱性破壞，形成了二重對稱性，這與Ag聲子模態的典型四重對稱性不同，其異常行為歸因於非零的拉曼張量元素和各異向性晶體所引起的光學異向吸收與雙折射效應。論文的第二部分，我們利用拉曼光譜研究了Bi-2212在不同實驗條件下的化學穩定性。拉曼光譜顯示，未老化的Bi-2212樣品在高達500 ℃的熱處理後其振動模式保持不變。然而，老化的樣品在退火過程中易受損，並產生了新的副產物（例如CuO、BiOx）。我們的研究結果為未來基於BSCCO的元件製程提供了有價值的資訊。

Bismuth strontium calcium copper oxide (BSCCO) is one of the promising high-temperature superconductors in the family of layered copper oxides, which exhibit high critical temperature (Tc) above liquid nitrogen temperature (77 K). In this thesis, we focus on the most generally studied family of Bi2Sr2CaCu2O8+x (Bi-2212). We fabricate high-quality, layered Bi-2212 samples and perform Raman spectroscopy to study their lattice vibration modes. By measuring the crystal-orientation- and polarization-resolved spectra in both parallel (HH) and crossed (HV) polarization scattering geometries and performing symmetry analysis, we clarify the characteristics of the phonon vibration modes. In the back-scattering geometry, most of the Raman active modes can be attributed to Ag symmetry in the triclinic Ci structure, in contrast to the commonly assumed orthorhombic D2h structure. Interestingly, in HV scattered spectra, we observe an anomalous symmetry-breaking to a two-fold symmetry from several phonon modes, in contrast to the typically four-fold symmetry of Ag phonons. We attribute this anomalous behavior to the birefringence effect and optical absorption of the anisotropic crystal. In the second part of the thesis, we employ Raman spectroscopy to study the chemical stability of Bi-2212 in various experimental conditions. The Raman spectra show that the vibrational modes of the freshly cleaved Bi-2212 flakes remained unchanged after thermal treatment up to 500 ℃. However, the aged Bi-2212 samples are prone to damage as evidenced by new byproducts (e.g., CuO, BiOx) during thermal annealing. Our results provide valuable information for fabricating BSCCO-based devices in the future.