以時間解析飛秒光譜研究六方晶系結構鈥錳氧單晶之載子動力學

Abstract

在本論文中，我們利用具有時間解析的激發探測實驗去研究多鐵性六方晶系鈥錳氧單晶的超快載子動力學。首先，我們量測六方晶系鈥錳氧單晶的基本特性，例如用X光θ-2θ繞射確認晶體結構及用超導量子干涉儀進一步確定磁矩隨溫度的排列情形。在此材料系統中，所謂”多鐵”是指具有電性(鐵電)與磁性(反鐵磁)以及彈性等多個有序特性的共存，且在該材料中磁電、磁彈間存在著強的耦合(Coupling)作用，即施加電場可影響磁性，反之加磁場又可影響其電的特性。這些有序參數間的交互作用引發許多的有趣現象，近年來引發科學家的研究熱潮。

從激發探測實驗結果中，透過波長可調的飛秒光譜實驗在鈥錳氧單晶中觀察到反鐵磁有序與電子結構會有強烈的耦合情形，該材料中存在的磁長程有序會造成錳離子3d軌域在尼爾溫度有異常的藍移現象，同時在尼爾溫度之上因短程有序的出現造成反射率變化(ΔR/R)的振幅對溫度變化之斜率有顯著的改變。接著，我們在鈥錳氧單晶中透過激發光所造成的光致熱彈效應(thermoelastic effect)在尼爾溫度觀察到異向性的磁彈耦合行為，進而揭示了電性、彈性與磁性不同自由度間的耦合情形。其中光致熱彈效應在ab平面上造成反射率變化被我們定義的”負”分量所主宰，而沿著c方向傳遞的應力形變波則造成反射率變化中的”振盪”分量，兩個分量皆在尼爾溫度出現異常的變化。最後，從反射率變化中的振盪分量隨溫度的變化，我們認為週期的變化來自於鐵電的極化率隨溫度的改變。

In this dissertation, we study the characteristics of ultrafast carrier dynamics in hexagonal HoMnO3 single crystals via time-resolved pump-probe experiments. First of all, we measured the fundamental properties of hexagonal HoMnO3, such as crystal structure by the X-ray diffraction (XRD) θ-2θ pattern, the arrangement of magnetic moment by the superconducting quantum interference device (SQUID). Multiferroic materials with coexistence of several ferroic orders (ferroelectric, ferromagnetic, or ferroelastic) have attracted great attention of scientists recently. The magnetoelectric response is the appearance of an electric polarization upon applying a magnetic field and/or the appearance of a magnetization upon applying an electric field.

In pump-probe results, we demonstrate that the strong coupling between the electronic structure and antiferromagnetic (AFM) ordering in hexagonal HoMnO3 single crystals can be simultaneously delineated by wavelength-dependent femtosecond spectroscopy. The emergence of long-range and short-range magnetic ordering are unambiguously revealed in association with an abnormal blue-shift of Mn3+ 3d level around the Néel temperature and the slope change of the temperature-dependent ΔR/R near transition, respectively. Furthermore, the coupling among the magnetization, polarization, and strain degree of freedoms has been simultaneously disclosed in the hexagonal HoMnO3 single crystals through the photo-induced anisotropic ultrafast thermoelastic dynamics. The thermoelastic effect in the ab-plane associated with the giant magnetoelastic coupling around the Néel temperature (TN) is intimately correlated to the evolvement of a negative component in the transient reflectivity changes (ΔR/R) obtained from temperature-dependent pump-probe experiments. Moreover, the variation of the period of the oscillation component in ΔR/R caused by a strain pulse propagating along c-axis exhibits an abrupt drop around TN, presumably due to the antiferromagnetic ordering-induced ferroelectricity.