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]

石墨烯的电子能带结构中的各向同性狄拉克锥
2012年4月24日,麻省理工学院在其官方首页报道了唐-崔瑟豪斯理论 (Tang-Dresselhaus Theory),该机构的科学家唐爽崔瑟豪斯夫人 (Mildred Dresselhaus) 在各项异性狄拉克锥方面取得突破,将引领半导体行业的芯片设计和热电能源领域。

典型的狄拉克锥材料包括石墨烯拓扑绝缘体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·华莱士英语P. R. Wallace (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] 麻省理工学院唐爽崔瑟豪斯夫人(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]

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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 xk 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 xk yk 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]

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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] )。

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参考文献

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