新穎量子物質之物理特性研究-子計畫二：新穎量子物質之電子動力學研究

Abstract

本子計畫將建立一套可在不同溫度、不同磁場下進行光激發中紅外（及 兆赫波)探測之光譜系統，用以研究拓樸絕緣體中表面態和鐵基超導體中超 導能隙之超快動力學。近來，三維拓樸絕緣體表面態[狄拉克錐(Dirac cone)] 的存在已被ARPES、STM...等實驗證實，但這些探測方式相當複雜且昂貴， 無法作立即性的快速檢測，這勢必大大限制了拓樸絕緣體的應用與發展。 因此，本計晝將採用GaSe非線性晶體產生光子能量小於塊材能隙(以Bi2Se3 為例約300 meV)的中紅外光(60-165 meV)以避開塊材效應，而能真正且乾淨 的探測到表面態的Dirac cone ；再配合激發探測之技術，則可進一步觀察及 研究拓樸絕緣體Dirac cone中電子的動力學行為。另外，我們也將利用拓樸 絕緣體產生兆赫波(THz)，並分析此兆赫波輻射是否能用於探測表面態的特 性。本計晝第二部分將運用光子能量較小的兆赫波(1.6-20.7 meV)來研究鐵 基超導體(11系列-FeSe)之超導能隙，而利用控制兆赫波的偏振來探測不同 晶軸上之超導能隙，即可解析出鐵基超導體中超導能隙之對稱性及在THz 波段之複數介電常數、複數折射率、複數電導率等重要電磁參數。而研究 超導能隙及古柏對(Cooper pairs)在受光脈衝破壞後的弛緩過程，也將有助於 我們了解鐵基超導之超導機制，甚至高溫超導體發生超導的物理機制。

This proposal is intended to study the temperature-dependent and magnetic field-dependent ultrafast dynamics of surface state in topological insulators and superconducting gap in iron-base superconductors by optical pump mid-IR (or THz) probe spectroscopy. Recently, the Dirac-cone surface state in topological insulators has been detected by ARPES and STM. However, these experiments are too expensive and complicate to instantly investigate the properties of the Dirac-cone surface state in topological insulators. The applications of topological insulators will come to a standstill. Owing to the photon energy of mid-IR (60-165 meV) generated by GaSe crystals is smaller than the bulk band gap (〜300 meV in Bi2Se3) in topological insulators, therefore, we are going to develop the mid-IR spectroscopy to avoid the excitation in bulk state and purely probe the Dirac-cone surface state in topological insulators. Combined with optical pump, the ultrafast dynamics of electrons in Dirac-cone surface state can be unambiguously revealed. Additionally, the topological insulators will be used as the THz emitter. The information about Dirac-cone surface state carried on the output THz wave will be extracted in this study.

In the second part of this proposal, we will study the superconducting-gap symmetry of 11-type FeSe superconductors using polarized THz time-domain spectroscopy, whose photon energy (1.6-20.7 meV) covered the magnitude of superconducting gap. By rotating the polarization of THz wave, the superconducting gap can be probed along each orientation of FeSe crystals and then the symmetry of superconducting gap can be further revealed. Moreover, the complex dielectric constant, refractive index, and conductivity of FeSe superconductors in THz region can be also obtained from the THz time-domain spectra. Finally, the optical pump THz probe spectroscopy will be applied to investigate the recovering dynamics of superconducting gap and Cooper pairs. This study would greatly enhance our current comprehension of the superconducting mechanism in iron-based superconductors and may help us to realize the superconducting mechanism of high Tc superconductors.