非常規材料是一類電子的電荷中心不在原子上的新材料體系。從能帶表示理論分析上看，非常規電子材料的占據態能帶無法分解為原子價電子能帶表示的和，而存在電子電荷中心在空位上的非常規電子態，通常會出現阻挫表面/邊界態。

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

(a) 1H/2H相的MX2的的晶體結構。(b, c) 1H相的NbSe2和TaS2的能帶結構；(d, e) 不同的電子展寬計算出來的1H相的NbSe2和TaS2的聲子譜；藍色表示低溫聲子譜，紅色表示高溫聲子譜

這一獨特的性質使得這類材料在高化學活性、低功函數、強氫親和力和電催化等功能材料方面，具有廣泛的應用前景。另外一方面，六角結構雙層堆垛的過渡金屬硫化物2H-MX₂體系由于電荷密度波態和超導態的共存及其相互競爭而廣受人們的關注。它們的這些物理性質雖然已經被廣泛報道，但是其中的超導起源還沒有揭示。

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

來自中國科學院物理研究所/北京凝聚態物理國家研究中心HM-T03組王志俊研究員指導博士后楊志龍，博士生盛昊昊等人，通過第一性原理、能帶表示理論分析以及電聲耦合計算，提出了1H/2H相的過渡金屬硫化物MX₂的費米能級處具有分數占據的非常規電子結構，從而誘導出超導態。

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

他們的研究結果表明：1.這些體系的單層1H相結構中，費米能級處存在著一條電子電荷中心在空位上的、表示為的能帶。2.計算聲子譜發現，該類結構在高溫下是穩定存在的；而在低溫下，聲子譜會存在軟模態，預示著電荷密度波態的出現。同時計算解釋了電子摻雜可以穩定晶體結構,有效地抑制低溫下的電荷密度波態。3.基于BCS理論，電聲耦合計算的結果表明：NbSe₂費米能級處半填充的、電荷中心位于空位上的電子態會造成比較強的電聲耦合和聲子軟模，對超導態的貢獻巨大。4.作者所預測的兩個具有超導電性的非常規金屬材料：單層TaNS材料和2H相的TaN₂塊體材料，超導臨界溫度分別為10 K和26 K。

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

該研究揭示了一類具有分數填充的空位能帶結構的非常規金屬材料，為探尋超導材料提供了一個有效的平臺。相關論文近期發布于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及其翻譯

Superconductivity in unconventional metals (非常規金屬中的超導性)

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.

摘要通過第一性原理計算，我們發現1H/2H相的過渡金屬硫化物MX₂(M = Nb, Ta; X= S, Se, Te)是一類非常規金屬，其費米能級處存在著一條電子電荷中心在空位上的、表示為的能帶。聲子譜計算顯示：該體系在高溫下是結構穩定的；而在低溫下存在聲子軟模，預示著低溫下會出現電荷密度波態。基于BCS理論和電聲耦合計算的結果表明NbSe₂的超導電性主要是由該聲子軟模貢獻，其表現出強的電聲耦合強度。我們認為具有強電聲耦合的聲子軟模主要是由費米能級處半填充且電荷中心處于空位上的電子態造成的。據此，我們進一步預測了兩個具有超導電性的非常規金屬材料，即TaNS單層材料和2H相的TaN₂體材料，超導臨界溫度分別為10 K和26 K。這些研究結果表明：這類具有分數填充的、電荷中心在空位上的能帶結構的非常規金屬為探尋超導材料提供了一個有效的平臺。