Department of Applied Physics, University of Tokyo and JST-CREST, Hongo, Bunkyo-ku, Tokyo, Japan.
J Phys Condens Matter. 2010 Apr 28;22(16):164206. doi: 10.1088/0953-8984/22/16/164206. Epub 2010 Mar 30.
We analyze and overview some of the different types of unconventional quantum criticalities by focusing on two origins. One origin of the unconventionality is the proximity to first-order transitions. The border between the first-order and continuous transitions is described by a quantum tricritical point (QTCP) for symmetry breaking transitions. One of the characteristic features of the quantum tricriticality is the concomitant divergence of an order parameter and uniform fluctuations, in contrast to the conventional quantum critical point (QCP). The interplay of these two fluctuations generates unconventionality. Several puzzling non-Fermi-liquid properties in experiments are taken to be accounted for by the resultant universality, as in the cases of Y bRh(2)Si(2), CeRu(2)Si(2) and β-Y bAlB(4). Another more dramatic unconventionality appears again at the border of the first-order and continuous transitions, but in this case for topological transitions such as metal-insulator and Lifshitz transitions. This border, the marginal quantum critical point (MQCP), belongs to an unprecedented universality class with diverging uniform fluctuations at zero temperature. The Ising universality at the critical end point of the first-order transition at nonzero temperatures transforms to the marginal quantum criticality when the critical temperature is suppressed to zero. The MQCP has a unique feature with a combined character of symmetry breaking and topological transitions. In the metal-insulator transitions, the theoretical results are supported by experimental indications for V(2 - x)Cr(x)O(3) and an organic conductor κ-(ET)(2)Cu[N(CN)(2)]Cl. Identifying topological transitions also reveals how non-Fermi liquid appears as a phase in metals. The theory also accounts for the criticality of a metamagnetic transition in ZrZn(2), by interpreting it as an interplay of Lifshitz transition and correlation effects. We discuss the common underlying physics in these examples.
我们分析和概述了一些不同类型的非常规量子临界点，重点关注两个起源。非常规性的一个起源是接近一级相变。对称破缺转变的一级和连续转变之间的边界由量子三临界点（QTCP）描述。量子三临界点的一个特征是与常规量子临界点（QCP）相反，序参量和均匀涨落同时发散。这两种涨落的相互作用产生了非常规性。实验中几个令人困惑的非费米液体性质被认为是由所得的普遍性来解释的，例如 YbRh2Si2、CeRu2Si2 和 β-YbAlB4。另一种更剧烈的非常规性再次出现在一级和连续相变的边界，但在这种情况下是拓扑相变，如金属-绝缘体和 Lifshitz 相变。这个边界，即边缘量子临界点（MQCP），属于一个前所未有的普遍类，在零温度下具有发散的均匀涨落。在非零温度下一级相变的临界点的 Ising 普遍性在临界温度被抑制到零时转化为边缘量子临界性。MQCP 具有独特的特征，具有对称破缺和拓扑相变的综合特征。在金属-绝缘体转变中，理论结果得到了 V2-xCrxO3 和有机导体 κ-(ET)2Cu[N(CN)2]Cl 的实验结果的支持。确定拓扑转变也揭示了非费米液体如何作为金属中的一个相出现。该理论还通过解释 Lifshitz 转变和相关效应的相互作用，解释了 ZrZn2 中顺磁转变的临界性。我们讨论了这些例子中的共同潜在物理。
