核磁共振在強關聯電子系統中物理的探討

Abstract

隨著材料科學的進步, 最近許多新穎材料中出現的新的物理現象, 常是傳統凝態 理論所無法完全描述的。一般認為物質中電子之間的強交互作用力及電子與所處局 部環境的相互影響, 扮演著重要的角色。實驗工具像是核磁共振光譜儀, 可以直接探 測到電子局部行為的工具, 就顯得額外重要。例如, 超導體中的有序參數的對稱性就 可由核磁共振光譜輕易的鑑別, 但是其它工具像是電阻或磁化率等巨觀的量測就有其 限制。台灣現有的核磁共振系統, 大多設計為化學, 生物, 或是醫學方面的用途, 並 不適合凝態物理實驗需求。據瞭解目前台灣可用於凝態領域方面的系統, 可能只有一 台 (一般所稱solid-state NMR 大多不符合凝態物理實驗的需求)。因此, 我們提出這 個為期三年的研究計畫, 就是希望建立一個專門為凝態物理研究目地的核磁共振光譜 儀。 我們所提出的凝態核磁共振光譜儀, 將是具有研究物質在各種極端物理環境下的 量測能力, 例如: 高磁場 (7 tesla), 低溫 (1.5 K), 和高壓(3 GPa)。所以研究的物理及 材料範圍可以較為寬廣。另外, 核磁共振光譜儀是可與其它巨觀量測工具相互補的, 例如, 可協助其他用巨觀工具所量測到的不明物理相變做微觀的鑑別。因此, 可預見 的是此一凝態核磁共振光譜儀將會使台灣在凝態物理實驗的研究體系更加完整, 且更 具競爭力。 此外, 我們並提出利用此核磁共振光譜儀來研究這三種不同的新穎材料: 重費米 子金屬材料, 過渡金屬氧化物, 及奈米材料。前兩類材料是所謂的強關聯電子系統, 因為這類物質中, 電子之間有很強的交互作用力。但在過渡金屬氧化物中, 除電子之 間的作用力外, 電子又有電子自旋, 軌域, 及晶格運動之間的相互影響而產生各種複 雜的物理現象。另一方面, 當物質的尺寸小到奈米等級的時候, 由於量子效應增強, 奈米材料常會出現與塊材不一樣的新奇特性。因此, 我們所提的計畫就是要用核磁共 振光譜儀來探討這三類物質中所出現的各種不同多體物理及量子現象。因為只有當 我們完全瞭解這些現象背後的物理, 新的可應用的材料才有可能快速的被開發。

As material science continues making progress, many new materials have been discovered and display novel phenomena that cannot be predicted from conventional theories. Electrons with strong interactions and their couplings to the local environment often play dominant roles. An experimental technique such as nuclear magnetic resonance (NMR) that can probe the local electron behavior directly, in contrast to the bulk measurements (e.g., resistivity and specific heat, etc.), is exclusively essential to condensed-matter research. For example, the Cooper pairing symmetry in a superconductor can be easily determined by NMR, but often cannot be probed by many bulk measurement tools. In Taiwan, most of NMR facilities are located in the institutes for the chemistry, biology, and medical purposes. Their NMR configurations are usually not suitable for the condensed-matter experiments. We, therefore, propose a three-year research project to establish one of the few NMR research laboratories especially designed for the condensed-matter research in Taiwan. As NMR is an important complement to many bulk measurement techniques, it is foreseeable that this proposed NMR system will make the condensed-matter research network more complete and more competitive in Taiwan. Our proposed NMR system will be capable of doing experiments under different extreme conditions, i.e., high magnetic field (7 tesla), low temperature (1.5 K), and high pressure (3 GPa), so that many exotic phases in materials under different physical conditions can be explored. With this versatile NMR system, a variety of materials such as superconductors and magnetic materials, etc. can also be studied. In addition, we propose NMR research in the following systems: heavy-fermion metals, transition metal oxides, and nanosized materials. The first two systems are often categorized as strongly-correlated systems because the strong electron-electron interactions exist in these materials so that various correlated behavior and cooperative phenomena are emerging. In addition to the electron interactions, the intricate couplings among spin, charge, and lattice also occur in the transition metal oxides so that these materials often have a complex phase diagram. On the other hand, materials in nanometer size also reveal unusual phenomena that are different from those in bulk size, due to the quantum size effect. Our goal is to understand the complicated many-body and quantum interactions behind these novel phenomena by using NMR, so that new functional materials can be discovered. This is one of the major challenges in current materials research.