揭秘Ising超导体NbSe2：电声耦合与自旋涨落

在过去的二十年里，单原子层的生长和剥离技术取得了突破性进展，为科学发现和技术创新开辟了新时代。继石墨烯之后，过渡金属二硫属化物（TMDs）受到了广泛关注，成为众多新颖量子现象的宝库。近年来，一种名为Ising超导性的现象在NbSe₂等材料中被观察到，这种现象主要由自旋-轨道耦合和中心反演对称性破缺驱动。

Fig. 1 Electron–phonon interaction and intra/inter-pocket Eliashberg spectral function. 

Ising超导体与其他层状材料（如拓扑材料、单层TMDs和二维磁性层状材料）之间的邻近效应和界面效应有望被应用在量子信息存储和自旋电子学等器件上。最近，关于这类材料，除了占主导地位的电声耦合（EPC）之外，人们还关注了自旋涨落（SF）在Ising超导现象中可能发挥的作用，讨论了三重态占主导的可能性，并引出了一个关于单重态-三重态Leggett振荡的猜想。然而，这些假设尚未经过定量检验。

Fig. 2 Superconducting gap without and with spin-fluctuation interaction.

来自美国弗吉尼亚州乔治梅森大学物理与天文学系的S. Das等，基于第一性原理计算，针对Ising超导体NbSe₂进行了详细研究，探讨了EPC和SF的作用。作者发现：(1)EPC具有强烈的各向异性，主要来自于K-K’散射，因此排除了三重态对称性作为激发态的可能性；(2)超导性由SF显著削弱，但各向异性保持不变；(3)作者发现了Leggett模式的可能性，不是在单重态-三重态之间，而是在s₊₊– s_±通道中。

Fig. 3 Schematic illustration of the Ising superconductor pairing symmetry.

作者通过对电声耦合和自旋涨落贡献的自洽分析，估计了平均的BCS能隙ΔBCS、Leggett模式频率Ω_L、以及Ω_L/2ΔBCS，与实验结果比较接近。本研究为理解NbSe₂及其他Ising超导体的超导现象提供了较为全面的定量分析，为今后的实验和理论研究提供了有力支持。相关论文近期发布于npj Computational Materials9: 66 (2023)。

Fig. 4 Analytical solution of the two-gap BCS equations.

Editorial Summary

Unraveling the Ising Superconductivity in NbSe₂: Electron-Phonon Coupling and Spin Fluctuations

Revolutionary progress in the growth and exfoliation of single atomic layers over the last two decades has led to a new era of scientific discoveries and technological innovation. Following graphene, transition metal dichalcogenides (TMDs) have taken the spotlight, as a treasure trove for a plethora of novel quantum phenomena. In recent years, a phenomenon called Ising superconductivity has been observed in materials such as NbSe₂, driven primarily by spin-orbit coupling and the absence of inversion symmetry. The proximity and interface effects between Ising superconductors and other layered materials, such as topological materials, single-layer TMDs, and two-dimensional magnetic layered materials, could lead to interesting device applications for quantum information storage and spintronics. Recently, attention was called to the possible role that spin fluctuations (SF) play in this phenomenon, in addition to the dominant electron–phonon coupling (EPC); the possibility of a predominantly triplet state was discussed and led to a conjecture of viable singlet–triplet Leggett oscillations. However, these hypotheses have not been put to a quantitative test. 

S. Das et al. from the Department of Physics and Astronomy, George Mason University, USA, conducted an in-depth study on the Ising superconductor NbSe₂, and explored the roles of EPC and SF based on first-principles calculations. The authors found that: (1) EPC is strongly anisotropic, largely coming from the K−K’ scattering, and therefore excludes triplet symmetry even as an excited state; (2) superconductivity is substantially weakened by SF, but anisotropy remains as above; and, (3) the possibility of a Leggett mode was found, not in a singlet–triplet but in an s++ – s± channel. Based on the self-consistent analytical solution and first-principles calculations including electron–phonon coupling and spin-fluctuation contribution, the authors estimated the average BCS gap ΔBCS, Leggett mode frequency Ω_L, and the ratio Ω_L/2ΔBCS, showing good agreement with experimental results. This study provides a fairly comprehensive quantitative analysis for understanding the superconductivity in NbSe₂ and other Ising superconductors, offering valuable insights for future experimental and theoretical research.Thisarticle was recently published in npj Computational Materials 9: 66 (2023).

原文Abstract及其翻译

Electron-phonon coupling and spin fluctuations in the Ising superconductor NbSe₂ (Ising超导体NbSe₂中的电声耦合与自旋涨落)

S. Das, H. Paudyal, E. R. Margine, D. F. Agterberg & I. I. Mazin

Abstract

Ising superconductivity, observed in NbSe₂ and similar materials, has generated tremendous interest. Recently, attention was called to the possible role that spin fluctuations (SF) play in this phenomenon, in addition to the dominant electron–phonon coupling (EPC); the possibility of a predominantly triplet state was discussed and led to a conjecture of viable singlet–triplet Leggett oscillations. However, these hypotheses have not been put to a quantitative test. In this paper, we report first principle calculations of the EPC and also estimate coupling with SF, including full momentum dependence. We find that: (1) EPC is strongly anisotropic, largely coming from the K−K′  scattering, and therefore excludes triplet symmetry even as an excited state; (2) superconductivity is substantially weakened by SF, but anisotropy remains as above; and, (3) we do find the possibility of a Leggett mode, not in a singlet–triplet but in an s₊₊ – s_±channel.

摘要

在NbSe₂和类似材料中观察到的Ising超导性引起了极大的关注。最近，除了占主导地位的电声耦合（EPC）之外，人们还关注了自旋涨落（SF）在这一现象中可能发挥的作用，讨论了三重态占主导的可能性，并引出了一个关于单重态-三重态Leggett振荡的猜想。然而，这些假设尚未经过定量检验。在本文中，我们报道了电声耦合的第一性原理计算，还估算了与自旋涨落的耦合，包括完全动量依赖性。我们发现：(1) EPC呈强烈的各向异性，主要来自于K-K’散射，因此，即使作为激发态，也排除了三重态对称性；(2)超导性受到自旋涨落的显著削弱，但各向异性仍然存在；(3)我们确实发现了Leggett模式的可能性，不是在单重态-三重态之间，而是在s₊₊– s_±通道中。