Only Perception: 解開近藤（Kondo）效應之謎

2008-09-23

解開近藤（Kondo）效應之謎

Unlocking the secret of the Kondo Effect
http://www.physorg.com/news141290015.html

September 22, 2008

(PhysOrg.com) -- 一個包括來自於倫敦大學學院（UCL）倫敦奈米科技中心以及 IBM Almaden 研究中心研究者的團隊，在理解基礎物理學某種使人困惑的現象上鍛造出一項突破：近藤效應（Kondo effect，譯註：與蕭敬騰無關，http://en.wikipedia.org/wiki/Kondo_effect）。這些發現今日報告在科學期刊 Nature Physics 線上版。

近藤效應，是物理學中許多粒子表現出猶如單一物體（單一一個量子力學的物體）的少數例子之一，困擾世上科學家已有數十年。當單個磁性原子位於一金屬內部，金屬的自由電子會「遮蔽（screen）」這個原子。在這種狀況下，一大群環繞這個原子的電子變成受到磁化。

有時候，若此金屬被冷卻到非常低溫，原子自旋會進入一種所謂的「量子疊加（quantum superposition）態」。在這種狀態中，其北極同時指向二種相反的方向。結果，環繞此自旋的整個電子雲也將同時被磁化成二種方向。

現在，利用同樣團隊在 2007 年所開發的技術，研究者證明，要預測近藤效應何時發生是可能的 -- 而且能了解為什麼。關鍵結果為磁性原子當前環境（immediate surroundings）的幾何學。藉由仔細研究這種幾何學如何影響該原子的磁力矩（或「自旋」），近藤效應的浮現現在能被預測與理解。

Dr. Cyrus Hirjibehedin，IBM 團隊成員之一，他現在是 UCL 的講師以及 LCN 的學術職員之一，表示："這項結果代表著我們對此基礎物理現象之理解的進展，而且對於未來的奈米等級磁性裝置可能有重要的後果。"

※ 相關報導：

＊ The role of magnetic anisotropy in the Kondo effect
http://dx.doi.org/10.1038/nphys1072
http://www.nature.com/nphys/journal/vaop/ncurrent/abs/nphys1072.html

Alexander F. Otte, Markus Ternes, Kirsten von Bergmann,
Sebastian Loth, Harald Brune, Christopher P. Lutz,
Cyrus F. Hirjibehedin & Andreas J. Heinrich
Nature Physics Published online: 21 September 2008
doi: 10.1038/nphys1072

In the Kondo effect, a localized magnetic moment is screened by forming a correlated electron system with the surrounding conduction electrons of a non-magnetic host. Spin S=1/2 Kondo systems have been investigated extensively in theory and experiments, but magnetic atoms often have a larger spin. Larger spins are subject to the influence of magnetocrystalline anisotropy（磁晶異向性）, which describes the dependence of the magnetic moment's energy on the orientation of the spin relative to its surrounding atomic environment. Here we demonstrate the decisive role of magnetic anisotropy in the physics of Kondo screening. A scanning tunnelling microscope is used to simultaneously determine the magnitude of the spin, the magnetic anisotropy and the Kondo properties of individual magnetic atoms on a surface. We find that a Kondo resonance emerges for large-spin atoms only when the magnetic anisotropy creates degenerate ground-state levels that are connected by the spin flip of a screening electron. The magnetic anisotropy also determines how the Kondo resonance evolves in a magnetic field: the resonance peak splits at rates that are strongly direction dependent. These rates are well described by the energies of the underlying unscreened spin states.