以二維飛秒光譜研究複雜自適性材料之相分離現象

Abstract

本計畫首先將建立一套以飛秒脈衝雷射為光源，同時結合光激發-探測技術、近場光學顯微技術以及低溫技術的量測系統，使我們的超快光譜不但具有飛秒級的時間解析能力，更能在低溫下擁有奈米級的空間解析能力，如此對於現今在複雜自適性物質(complex adaptive matter)中正在發燒的「相分離」議題，提供另一個研究及思考的方向，甚至提供更具代表性及說服力之的實驗結果。由於可見光或近紅外光在一般複雜自適性物質，如高溫超導銅氧化物、龐磁阻、多鐵性錳氧化物…等，其穿透深度約有數數千埃 (.)，相較於現今許多量測方法的數十埃(.) 深度而言(如：掃瞄式穿遂電子顯微鏡…等)，本方法所得之結果較能真實揭露材料的本質，而排除表面效應所造成的影響。另外，本計劃希望在未來三年內，利用過去幾年我們在超快動力學的經驗，分析這些具空間解析的二維飛秒光譜，嘗試從中找出各現象(或相)的特徵弛緩動力行為以及其所對應的物理量為何，進而釐清這些系統中複雜的磁性與電性性質。相信從動力學角度出發，勢必能對現今物理學家所關心的許多議題做出重大貢獻，例如：為何不同相會共存在某些特定的材料？相的共存是彼此競爭後的妥協，或是相互作用衍生的結果？甚至高溫超導發生的物理機制…等。

This proposal is intended to develop a low temperature pump-probe system with spatial resolution in nanometer-scale by combining with the near-field scanning optics microscope (NSOM). Then, this system could be applied to study the hot issue, coexistence of phases (or phase separation), in complex adaptive matters, such as high Tc superconducting cuprates, heavy fermion metallic compounds, colossal magnetoresistances, multiferroics manganites, et al.. Recently, many significant investigations about the coexistence of multiferroic phases (or phase separation) have been revealed by tunneling electron microscope, scanning tunneling microscope, photoemission spectroscope, and so on. However, the intrinsic nature characteristics in bulk could not be truly disclosed by these results due to the measuring signal is completely from the charged particles whose penetration depth is only several nanometers. Thus, the surface conditions (or surface effects) should be included in this kind of measurement. Owing to the penetration depth of visible or near-infrared photon is larger than the charged particle by at least a factor of 100, the surface effects could be easily ruled out in optical pump-probe measurements. Inspired from the experience of studying the ultrafast dynamics of photoinduced quasiparticles in past several years, the 2D femtosecond spectroscopy with the information about relaxation dynamics can be a desirable way to explore the characteristic physical quantity (or order parameter) belonged to each phase. Once the characteristic order parameters have been identified, the complicated ferroelectric and ferromagnetic properties could be clarified in such kind of materials. In that case, more convictive data about the coexistence of multiferroic phases could be provided to address some questions in the complex adaptive matters. For instance, why would these two seemingly irrelevant phases (or phenomena) coexist in the some particular materials? Is the coexistence of multiphases a result of competition or compromise between different interactions among different quasiparticles? It is hoped that the success in this proposal would greatly enhance our current comprehension of complex adaptive matters in condensed matter physics and may help us to realize the mechanism of high Tc superconductors.