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arXiv 1602.02778cond-mat.str-el
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自旋-轨道耦合系统中由对称性破缺序得到的拓扑超导相

Topological superconducting phases from inversion symmetry breaking order in spin-orbit-coupled systems

Yuxuan Wang, Gil Young Cho, Taylor L. Hughes, Eduardo Fradkin

摘要:

我们分析了超导不稳定性的量子临界点附近的反转对称性破缺秩序。我们首先证明了反转对称破缺序的涨落导致两个简并超导(SC)不稳定性,一个在$s$波通道中,另一个在时间反演不变的奇宇称配对通道中(最简单的情况与$^3$ He-B相的情况相同).值得注意的是,我们发现,不像许多著名的例子,对凝聚体的对称性的选择是独立的动量空间结构的集体模式,介导的配对相互作用。我们发现,这种简并是一个保守的费米子螺旋,$χ$的存在的结果,和两个简并通道对应于偶数和奇数组合的SC序参量与$χ=\pm1$。结果,该系统具有扩大的对称性$U(1)\times U(1)$,每个$U(1)\times U(1)$对应于螺旋度$χ$的一个值。由于扩大的对称性,这个系统承认奇异的拓扑缺陷,如分数量子涡旋,我们表明有一个马约拉纳零模约束在其核心。我们讨论了如何扩大的对称性可以解除小的扰动,如库仑相互作用或费米面分裂的存在下,打破反演对称性,我们表明,由此产生的超导状态可以是拓扑或平凡的参数。的$U(1)\times U(1)$对称性恢复在拓扑和平凡SC状态之间的相位边界,并允许拓扑不同的SC相位之间的过渡,而不会消失的序参量。我们提出了一个全球性的超导状态的相图,并讨论了可能的实验影响。

英文摘要:

We analyze the superconducting instabilities in the vicinity of the quantum-critical point of an inversion symmetry breaking order. We first show that the fluctuations of the inversion symmetry breaking order lead to two degenerate superconducting (SC) instabilities, one in the $s$-wave channel, and the other in a time-reversal invariant odd-parity pairing channel (the simplest case being the same as the of $^3$He-B phase). Remarkably, we find that unlike many well-known examples, the selection of the pairing symmetry of the condensate is independent of the momentum-space structure of the collective mode that mediates the pairing interaction. We found that this degeneracy is a result of the existence of a conserved fermionic helicity, $χ$, and the two degenerate channels correspond to even and odd combinations of SC order parameters with $χ=\pm1$. As a result, the system has an enlarged symmetry $U(1)\times U(1)$, with each $U(1)\times U(1)$ corresponding to one value of the helicity $χ$. Because of the enlarged symmetry, this system admits exotic topological defects such as a fractional quantum vortex, which we show has a Majorana zero mode bound at its core. We discuss how the enlarged symmetry can be lifted by small perturbations, such as the Coulomb interaction or Fermi surface splitting in the presence of broken inversion symmetry, and we show that the resulting superconducting state can be topological or trivial depending on parameters. The $U(1)\times U(1)$ symmetry is restored at the phase boundary between the topological and trivial SC states, and allows for a transition between topologically distinct SC phases without the vanishing of the order parameter. We present a global phase diagram of the superconducting states and discuss possible experimental implications.

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