狄拉克锥
狄拉克锥是一种特殊二维材料中的电子能带结构,在此结构中,电子具有像光一样的相对论性质。科研人员认为狄拉克锥可能是通向未来超级芯片、量子计算机、超导和桌面相对论技术的路径。[1][2][3][4]
典型的狄拉克锥材料包括石墨烯、拓扑绝缘体、铋锑薄膜和其他新型纳米材料。[1][5][6] 这些特殊二维材料中电子的能量和动量具有线性的色散关系,因此其费米能级附近的电子能带结构呈现出上下两个锥体,分别代表电子和空穴。两个锥体的顶端刚好相连,形成“零带隙”的半金属相.
狄拉克锥的名字来源于狄拉克方程,由保罗·狄拉克 (Paul Dirac) 提出,用以统一描述物质的量子力学效应和相对论效应。狄拉克锥可以是各向同性,也可是各向异性的。石墨烯中存在各向同性的狄拉克锥,由飞利浦·华莱士 (P. R. Wallace) 于1947提出[7],并由诺贝尔物理学奖得主安德烈·海姆 (Andre Geim) 和康斯坦丁·诺沃肖洛夫 (Konstantin Novoselov) 于2005年首次在实验中观察到。[8] 麻省理工学院的唐爽和崔瑟豪斯夫人(Mildred Dresselhaus)于2012年在其唐-崔瑟豪斯理论 (Tang-Dresselhaus Theory) 中首次提出了系统性构建各向异性狄拉克锥的方法。[9][10][11]
描述
在量子力学中,狄拉克锥描述 [12]价带和导带的能量在二维晶格k空间中,除了零维狄拉克点所在的位置外,其他任何动量的价带和导带能量都不相等。由于是锥型,电传导可以用无质量费米子的电荷载流子来描述,在理论上这种情况可由相对论性的狄拉克方程来处理。 [13]无质量费米子可以导致各种奇異的量子霍尔效应、或是拓扑材料中的磁电效应和超高载流子迁移率。 [14] [15]在 2008-2009 年实验上使用角分辨光电子能谱(ARPES) 对钾-石墨插层化合物KC 8 [16]和几种铋基合金的狄拉克锥進行了观察。[17] [18] [15]
狄拉克锥是二维材料 (像是单层石墨烯)或拓扑绝缘体的表面态的特征。狄拉克锥在材料中是线性色散关系,由能量与晶体动量的两个分量k x和k y來描述。然而,这个概念可以扩展到三维材料,其中狄拉克半金属由能量与k x 、 k y和k z的线性色散关系來定义。在动量空间中,色散关系为超圆锥体,它具有双重简并能带,也在狄拉克点相交。 [15]狄拉克半金属同时包含时间反演对称性和空间反演对称性;当其中一个对称性被破坏时,狄拉克点可以分裂成两个外尔点,材料变成外尔半金属。 [19] [20] [21] [22] [23] [24] [25] [26] [27] [28] [29] 在2014年,实验上利用ARPES对狄拉克半金属砷化镉 的能带结构进行了直接观测。 [30] [31] [32]
模拟系统
已在许多物理系统实现狄拉克点,例如等离子体学、声子学或纳米光子学(微腔、 [33]光子晶体[34] )。
參看
参考文献
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