Bogoliubov Fermi Surfaces in Superconductors with Broken Time-Reversal Symmetry
Bogoliubov Fermi Surfaces in Superconductors with Broken Time-Reversal Symmetry
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DOI：
10.1103/physrevlett.118.127001
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发表时间：
2017-03-24
影响因子：
8.6
通讯作者：
Timm, C.
中科院分区：
文献类型：
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作者：
Agterberg, D. F.;Brydon, P. M. R.;Timm, C.
一般认为，在没有无序或外加磁场的情况下，超导激励间隙存在三种可能的类型：无节点间隙、有点节点的间隙和有线节点的间隙。在这里，我们表明，对于自发破缺时间反转对称性的偶宇称节点超导态，低能激发谱通常不属于这些类别中的任何一个；相反，它扩展了博戈柳波夫费米面。这些费米面可以可视化为通过将点或线节点分别“膨胀”成球体或环面而生成的二维表面。这些膨胀的节点在拓扑上受到 Z(2) 不变量的间隙保护，我们用普法夫函数给出该不变量。我们还表明，具有这些费米表面的超导态可以是能量稳定的。我们理论中的一个关键因素是，不止一个频段参与配对。由于具有破缺时间反转对称性的偶宇称超导性的所有候选材料都是多带系统，因此我们预计这些 Z(2) 保护的费米表面将无处不在。
It is commonly believed that, in the absence of disorder or an external magnetic field, there are three possible types of superconducting excitation gaps: The gap is nodeless, it has point nodes, or it has line nodes. Here, we show that, for an even-parity nodal superconducting state which spontaneously breaks time-reversal symmetry, the low-energy excitation spectrum generally does not belong to any of these categories; instead, it has extended Bogoliubov Fermi surfaces. These Fermi surfaces can be visualized as two-dimensional surfaces generated by "inflating" point or line nodes into spheroids or tori, respectively. These inflated nodes are topologically protected from being gapped by a Z(2) invariant, which we give in terms of a Pfaffian. We also show that superconducting states possessing these Fermi surfaces can be energetically stable. A crucial ingredient in our theory is that more than one band is involved in the pairing; since all candidate materials for even-parity superconductivity with broken time-reversal symmetry are multiband systems, we expect these Z(2) -protected Fermi surfaces to be ubiquitous.