利用原子級掃描探針技術觀察范德瓦耳斯鐵磁材料中近藤晶格與電荷密度波之交互作用

Abstract

電荷與自旋自由度之間的交互作用，一直是凝態物理中的核心議題之一。近藤交互作用（Kondo interactions）雖然能促進局域自旋矩與傳導電子之間的耦合，然而傳統上近藤系統（Kondo system）大多與 f 電子相關，而 f 電子高度局域化的特性限制了其對電子能帶結構（band structure）的影響，並使得電荷與磁性之間的有序連結更加困難。 電荷序（charge order）與近藤晶格（Kondo lattice）共存的直接實驗證據一直未被觀測到，儘管在過去十多年裡早已被理論學家所預測。近期一種范德瓦耳斯（van der Waals）鐵磁材料 Fe5GeTe2 的發現填補了這項空白。由於其遷移性磁性（itinerant magnetism）的本質，此 d 電子近藤系統成為研究電荷－自旋耦合的理想平台，連結了磁序與電荷序。在本論文中，我們利用掃描式穿隧顯微術與光譜技術（scanning tunneling microscopy and spectroscopy, STM/STS）來探究 Fe5GeTe2 的原子尺度形貌與電子結構，闡明其背後的物理原理。 論文的第一部分，我們進行樣品的特性分析，探討其磁性與結構。 Fe5GeTe2 在磁化率－溫度曲線（M–T curves）中展現出明顯的磁各向異性（magnetic anisotropy）與多重相變點。此外，其中一個特定的鐵位置—— Fe(I) 原子，不僅對居里溫度的提升至關重要，也為材料引入了顯著的結構複雜性。我們發現源自 Fe(I) 原子不同排列構型而導致的各種周期性結構，而這些結構變化也造成了電性的差異。 在第二部分，我們呈現了 Fe5GeTe2 電子行為中更引人注目的現象。我們觀測到其電子結構中出現一個 √3×√3 R30° 調變（modulation），並伴隨相位反轉，這一現象證實了電荷密度波（charge density wave, CDW）的形成。同時，在費米能量（Fermi energy）附近出現的法諾共振（Fano resonance），證明了其近藤晶格的行為。此外， Fe(I) 缺陷處形成的近藤空穴（Kondo hole）則突顯了 Fe(I) 原子在促進近藤交互作用中的關鍵角色。更值得注意的是，我們發現電荷密度波與近藤晶格之間具有同調的振盪行為，顯示了兩者之間的強耦合關聯。我們亦建立了簡化模型來說明這兩種現象如何交織影響。 總結而言，本論文對 Fe5GeTe2 進行了原子級的深入研究，凸顯了 Fe(I) 原子在引發結構多樣性與介導電荷密度波－近藤晶格耦合中扮演的關鍵角色。我們的發現強調了 Fe5GeTe2 作為研究低維鐵磁系統中關聯有序現象的有力平台，並增進了對 d 電子重費米子系統（d-electron heavy fermion system）中近藤物理學的理解。

The interplay between charge and spin degrees of freedom is one of the central themes in condensed matter physics. While Kondo interactions facilitate the coupling between local spin moments and conduction electrons, traditional Kondo systems are associated with f-electrons, whose localized nature limits their influence on the electronic band structure and impedes the connection between charge and magnetic orderings. Direct experimental evidence of the coexistence of charge order and Kondo lattice has remained elusive for more than a decade, despite theoretical predictions. The recent discovery of Fe5GeTe2, a van der Waals ferromagnetic material, has bridged this gap. Due to its itinerant magnetic nature, this d-electron Kondo system offers an ideal platform for investigating charge-spin coupling. Here, we use scanning tunneling microscopy and spectroscopy (STM/STS) to probe the atomic-scale topographic and electronic landscape of Fe5GeTe2, elucidating its underlying principles. In the first part of this thesis, we perform sample characterization, examining both magnetic and structural properties. In its magnetization–temperature (M–T) curves, Fe5GeTe2 exhibits strong magnetic anisotropy and multiple transition points. One particular iron site, the Fe(I) atom, which is crucial for enhancing the Curie temperature, also introduces structural complexities to the material. We identify distinct periodic arrangements originating from different configurations of Fe(I) atoms within the crystal. These structural variations further result in contrasting electronic properties. Beyond this structural diversity, we reveal even more intriguing phenomena arising from Fe5GeTe2’s electronic behaviors, described in the second part of this thesis. We observe a √3×√3 R30° modulation in the electronic structure, accompanied by phase inversion, demonstrating the formation of a charge density wave (CDW). Simultaneously, the emergence of a Fano resonance line shape near the Fermi energy confirms its Kondo lattice behavior. In addition, the formation of Kondo holes at Fe(I) vacancy sites emphasizes the critical role of Fe(I) atoms in facilitating Kondo interactions. More intriguingly, we discover coherent oscillatory behavior between the CDW and the Kondo lattice, revealing their strong correlation. A simplified model is further developed to illustrate how the two phenomena are intertwined. In summary, this thesis presents a thorough investigation of Fe5GeTe2 at the atomic scale, highlighting the key role played by the Fe(I) atoms in inducing structural diversity and mediating the CDW-Kondo lattice coupling. Our findings establish Fe5GeTe2 as a promising platform for studying correlated orderings in low-dimensional ferromagnetic systems and deepen the understanding of Kondo physics in d-electron heavy fermion systems.