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中翻日翻譯社

不外，在2D材估中，這會造成大差異。雙層CrGeTe₃在0.3 T(T：Tesla，磁通量密度單元)的磁場下，雙層CrGeTe₃的T_c是44 K(K：kelvin，絕對溫度)，0.065 T的磁場下是28 K，及零磁場下不及4K：翻譯Zhang及同寅們將此觀測後果解釋為揭露了，2D與3D磁鐵間的一種首要差別：在3D中，磁性是由自旋與臨近自旋間的交流彼此感化所閣下，而2D磁性是由材料的磁各向異性所左右。自旋的優先選擇是以特定方向成一向線翻譯

在針對CrGeTe₃的研究中，Zhang及同僚們發現了，磁改變溫度T_c光鮮明顯取決於施加的磁場。在磁材料的諸多研究中，施加小磁場來不變磁矩，是一種典型作法。在多量材料中，小磁場對於主要屬性不會造成大差別。

But now two teams—one led by Xiang Zhang of the University of California, Berkeley翻譯社 and the other a collaboration between the groups of the University of Washington’s Xiaodong Xu and MIT’s Pablo Jarillo-Herrero—have observed clear signatures of ferromagnetism in two chromium-based 2D materials. And they’ve both uncovered unexpected new effects.

Of all the types of condensed-matter behavior that have been observed in two-dimensional materials, ferromagnetism has been notably absent.

The past decade has seen an explosion in the study of atomically thin materials with diverse properties. Graphene is a semimetal, hexagonal boron nitride is an insulator翻譯社 molybdenum disulfide is a semiconductor, and so on.

But in 2D materials, it does: The T_c of a bilayer of CrGeTe₃ is 44 K under a 0.3 T field, 28 K under a 0.065 T field, and less than 4 K under zero field. Zhang and colleagues interpret the observation as revealing a key difference between 2D and 3D magnets: Magnetism in 3D is governed by the exchange interaction between spins and their neighbors翻譯社 whereas 2D magnetism is governed by a material’s magnetic anisotropy翻譯社 the spins’ preference to align in a particular direction.

In work on CrGeTe₃, Zhang and colleagues discovered a striking dependence of the magnetic transition temperature T_c on the applied magnetic field. Applying a small field to stabilize the magnetic moments is a typical practice in studies of magnetic materials; in bulk materials, the field doesn’t make much difference to the properties of interest.

在二維材料中，凝聚物(冷凝物)已被觀測的所有類型行為中，鐵磁性一向特別缺少翻譯

 
(圖援引自原文)

Xu、Jarillo-Herrero及同寅們探究了CrI₃。他們發現，在低於45K的溫度下，單層的CrI₃強烈自發磁化，其實不比多量該種材料61K的T_c溫度低太多翻譯當檢視分歧厚度的樣本時，令他們感應訝異。

曩昔十年，在具有多種屬性之極綿力料的研究已經泛起激增。石墨烯是一種半金屬、六方氮化硼是一種絕緣體、二硫化鉬是一種半導體，等等翻譯

如同三層樣本，單層樣本是鐵磁性的。不外，雙層樣本除被施加大於0.65T的磁場以外，沒有展現淨鐵磁性。此些研究人員斷言，雙層樣本傾向於反鐵磁挨次，因為一層加快自旋，另外一層減速自旋。該種與層有關的效應緣由仍然不詳。

Whether long-range magnetic order could survive in the 2D limit was an open and fundamental question.

翻譯：許東榮

原文網址：http://physicstoday.scitation.org/do/10.1063/PT.6.1.20170608a/full/

Xu, Jarillo-Herrero, and colleagues examined CrI₃. They found robust spontaneous magnetization in monolayer CrI₃ at temperatures below 45 K翻譯社 not too much lower than the material’s bulk T_c of 61 K. Their surprise came when they looked at samples of different thicknesses.

Monolayer samples were ferromagnetic翻譯社 as were trilayers. But bilayers exhibited no net ferromagnetism unless a field greater than 0.65 T was applied. The researchers concluded that bilayers tend toward antiferromagnetic order翻譯社 with one layer spin up and the other spin down. The reason for the layer-dependent effect is still unknown.

長距離的磁挨次可否延續存在於2D境界中，是一項未解決且根基的問題翻譯

不過，今朝兩支團隊(一支由美國加州大學柏克萊分校Xiang Zhang所帶領，另外一支是華盛頓大學Xiaodong Xu小組與麻省理工學院Pablo Jarillo-Herrero小組間的共同研究團隊)，在兩種以鉻為根蒂根基的2D材料中，已觀測到鐵磁性的清楚特徵。亦即，這兩團隊已揭穿了意想不到的新效應。