非常规金属中的超导性

非常规材料是一类电子的电荷中心不在原子上的新材料体系。从能带表示理论分析上看，非常规电子材料的占据态能带无法分解为原子价电子能带表示的和，而存在电子电荷中心在空位上的非常规电子态，通常会出现阻挫表面/边界态。

Fig. 1 The crystal structures, electronic structures, and phonon dispersions of 1H-MX2.

(a) 1H/2H相的MX2的的晶体结构。(b, c) 1H相的NbSe2和TaS2的能带结构；(d, e) 不同的电子展宽计算出来的1H相的NbSe2和TaS2的声子谱；蓝色表示低温声子谱，红色表示高温声子谱

这一独特的性质使得这类材料在高化学活性、低功函数、强氢亲和力和电催化等功能材料方面，具有广泛的应用前景。另外一方面，六角结构双层堆垛的过渡金属硫化物2H-MX₂体系由于电荷密度波态和超导态的共存及其相互竞争而广受人们的关注。它们的这些物理性质虽然已经被广泛报道，但是其中的超导起源还没有揭示。

Fig. 2 The schematic of band hybridization, orbital-resolved band structures, and the phonon dispersions and vibration modes.

来自中国科学院物理研究所/北京凝聚态物理国家研究中心HM-T03组王志俊研究员指导博士后杨志龙，博士生盛昊昊等人，通过第一性原理、能带表示理论分析以及电声耦合计算，提出了1H/2H相的过渡金属硫化物MX₂的费米能级处具有分数占据的非常规电子结构，从而诱导出超导态。

Fig. 3 Electronic structures and phonon dispersions after electron doping.

他们的研究结果表明：1.这些体系的单层1H相结构中，费米能级处存在着一条电子电荷中心在空位上的、表示为的能带。2.计算声子谱发现，该类结构在高温下是稳定存在的；而在低温下，声子谱会存在软模态，预示着电荷密度波态的出现。同时计算解释了电子掺杂可以稳定晶体结构,有效地抑制低温下的电荷密度波态。3.基于BCS理论，电声耦合计算的结果表明：NbSe₂费米能级处半填充的、电荷中心位于空位上的电子态会造成比较强的电声耦合和声子软模，对超导态的贡献巨大。4.作者所预测的两个具有超导电性的非常规金属材料：单层TaNS材料和2H相的TaN₂块体材料，超导临界温度分别为10 K和26 K。

Fig. 4 Electronic structures, phonon dispersions, and electron-phonon couplings in TaNS monolayer.

该研究揭示了一类具有分数填充的空位能带结构的非常规金属材料，为探寻超导材料提供了一个有效的平台。相关论文近期发布于npj Computational Materials 10: 25 (2024)。

Fig. 5 Electronic structures, phonon dispersions, and electron-phonon couplings in bulk 2H-TaN2.

Editorial Summary

Unconventional materials are a novel class of material systems, where the effective electronic charge center reside on an empty site. According to band representation theory, the elementary band representations (EBR) of occupied bands in unconventional electronic materials can’t be decomposed into localized atomic valence-electron band representations (ABR) in the atomic limit. These materials feature obstructed electronic states emerging on their boundaries. Due to such boundary states, they have wide applications in high chemical activity, low work function, strong hydrogen affinity, and electrocatalysis materials. Moreover, the hexagonal bilayer stackings of transition metal dichalcogenides (TMD), named 2H-MX₂, have received considerable attention due to the coexistence and competition of charge density wave (CDW) states and superconductivity (SC). Although these physical properties have been extensively reported, the origin of the SC has not been revealed yet.

Led by Prof. Zhijun Wang from the HM-T03 group at the Institute of Physics, Chinese Academy of Sciences/Beijing National Laboratory for Condensed Matter Physics, postdoctoral fellow Zhilong Yang, and Ph.D. candidateHaohao Sheng and others, utilized first-principles calculations, band representation theory analysis, and electron-phonon coupling (EPC) calculations to propose that the superconducting state in 1H/2H phase TMD MX₂ is mainly attributed by their unconventional electronic structure with partial filling at the Fermi level. The study reveals: 1. In the monolayer 1H-MX₂, there exists an unconventional electronic state at the Fermi level, which has an empty-site band of  EBR. 2. The calculated phonon spectrum indicates that these structures are stable at high temperatures, while at low temperatures the existence of a soft phonon mode suggests the emergence of a CDW state. It is also suggested that electron doping can suppress the CDW state at low temperatures, leading to a stable structure. 3. Based on BCS theory and EPC calculations, our calculations show that the SC in NbSe₂ is mainly attributed to the strong EPC and soft phonon mode due to the half filling of the empty-site band. 4. Moreover, the SC has been predicted in unconventional metals TaNS monolayer and 2H-TaN₂ bulk with computed T_C = 10 K and 26 K respectively. These results demonstrate that the unconventional metals with partial filling of the empty-site band offer an attractive platform to search for superconductors. Top of FormBottom of FormTop of FormBottom of FormThis article was recently published in npj Computational Materials 10,: 25 (2024).

原文Abstract及其翻译

Superconductivity in unconventional metals (非常规金属中的超导性)

Zhilong Yang, Haohao Sheng, Zhaopeng Guo, Ruihan Zhang, Quansheng Wu, Hongming Weng, Zhong Fang& Zhijun Wang

Abstract Based on first-principles calculations, we demonstrate that 1H/2H-phase transition metal dichalcogenides MX₂ (M = Nb, Ta; X = S, Se, Te) are unconventional metals, which have an empty-site band of  elementary band representation at the Fermi level. The computed phonon dispersions indicate the stability of the system at high temperatures, while the presence of the soft phonon mode suggests a phase transition to the charge density wave state at low temperatures. Based on the Bardeen-Cooper-Schrieffer theory and computed electron-phonon coupling, our calculations show that the superconductivity (SC) in NbSe₂ is mainly attributed to the soft phonon mode due to the half filling of the empty-site band. Accordingly, the SC has been predicted in unconventional metals TaNS monolayer and 2H-TaN₂ bulk with computed T_C = 10 K and 26 K respectively. These results demonstrate that the unconventional metals with partial filling of the empty-site band offer an attractive platform to search for superconductors.

摘要通过第一性原理计算，我们发现1H/2H相的过渡金属硫化物MX₂(M = Nb, Ta; X= S, Se, Te)是一类非常规金属，其费米能级处存在着一条电子电荷中心在空位上的、表示为的能带。声子谱计算显示：该体系在高温下是结构稳定的；而在低温下存在声子软模，预示着低温下会出现电荷密度波态。基于BCS理论和电声耦合计算的结果表明NbSe₂的超导电性主要是由该声子软模贡献，其表现出强的电声耦合强度。我们认为具有强电声耦合的声子软模主要是由费米能级处半填充且电荷中心处于空位上的电子态造成的。据此，我们进一步预测了两个具有超导电性的非常规金属材料，即TaNS单层材料和2H相的TaN₂体材料，超导临界温度分别为10 K和26 K。这些研究结果表明：这类具有分数填充的、电荷中心在空位上的能带结构的非常规金属为探寻超导材料提供了一个有效的平台。