超導電性是材料對直流電電阻為零、并能排除磁通量的一種狀態。目前對于超導體形成背后的微觀機制尚未得到澄清和統一，但鎳基氧化物R_1-xSr_xNiO₂的發現為復雜氧化物中的超導理論提供了新的測試平臺。Sr_1-xK_xBiO₃和Ba_1-xK_xBiO₃是理想的化合物，可用于測試第一性原理模擬技術，它們表現出氧化物的復雜性，并且是采用鈣鈦礦結構的氧化物超導體之一。

由法國諾曼底大學CRISMAT實驗室的Julien Varignon博士研究表明，SCAN泛函足以捕捉在空穴摻雜時SrBiO₃從絕緣態到金屬態的特性轉變過程，并揭示超導現象背后的機制和先決條件。

Fig. 2 Ground state properties of SrBiO3.

作者通過將第一性原理DFT結果映射到涉及相關晶格畸變的Landau模型上，證明了產生絕緣相的根源是在塊體基態中本征不穩定的Bi⁴⁺陽離子歧化成了Bi³⁺和Bi⁵⁺陽離子。該過程伴隨產生B_oc這種呼吸模式的扭曲，其振幅被八面體旋轉進一步增強。

在弱摻雜含量（x=0.0625~0.125）下，空穴被困在晶格上，中間態被局域在帶隙中，最終材料形成了半導體。在中等摻雜含量（x = 0.1875~0.375）時，材料形成了金屬相，然而由于呼吸模式與八面體旋轉耦合，呼吸模式仍然存在于基態，盡管這種結構畸變本身并不愿意在材料中自發產生。這種模式的存在使費米能級附近的能帶產生了一些小的帶隙。而當摻雜量x=0.4375時，能帶結構中不再有這些帶隙，這個摻雜含量讓人聯想到實驗報告的超導相（x=0.45-0.6）。在x≥0.4375附近，由于八面體旋轉，呼吸模式在材料中趨于穩定，其振動可在材料中形成自旋配對的電子和空穴，即庫珀對。

Fig. 4 Miscibility of K within the SrBiO3 structure.

因此，作者的結果表明，接近產生自旋電子和空穴的晶格不穩定性是鉍酸鹽超導性的先決條件，與實驗觀察到的超導相有界摻雜含量完全一致。在x=0.4375的超導相內，從模擬中提取了與呼吸模式B_oc相關的電子–聲子耦合常數λ為1.22，B_oc頻率為66 meV，與實驗值（λ=1.3±0.2和ω=62 meV）非常一致。當摻雜量較大時，費米能級的態密度減小，呼吸模的頻率升高；然而隨著x的增加（上限為0.625），略為增加的還原電子–聲子矩陣元中仍保留有非零Tc。

Fig. 5 Trends in electronic and structural properties upon hole doping SrBiO3.

因此，這項研究（i）驗證了使用SCAN-DFT研究復雜氧化物超導體中的摻雜效應；（ii）要求檢查超導鎳酸鹽和其他氧化物超導體中的歧化效應，以確定鉍酸鹽中已確定的機制是否也與這些新發現的氧化物超導體相關。該文近期發表于npj Computational Materials? 9: 30 (2022)。

Editorial Summary

Superconductivity is a state in which a material has zero resistance to direct current and can discharge magnetic flux. The microscopic mechanism behind superconductor formation has not yet been clarified and unified, but the discovery of nickel-based oxides R_1-xSr_xNiO₂provides a new testbed for the theory of superconductivity in complex oxides.Sr_1-xK_xBiO₃and Ba_1-xK_xBiO₃ are ideal compounds for testing first-principles simulation techniques, they exhibit the complexity of oxides, and are one of the oxide superconductors using a chalcogenide structure.?

Dr. Julien Varignon from Laboratoire CRISMAT, Normandie Université, France, showed that the SCAN functional is sufficient to capture the trends in insulating to the metal character of SrBiO₃ upon hole doping and to reveal the mechanism and prerequisites behind the appearance of superconductivity. By mapping the first-principles DFT results on a Landau model involving the relevant lattice distortions, the insulating phase is shown to be reached by disproportionation effects associated with an intrinsic instability of Bi⁴⁺ cations to disproportionation to Bi³⁺/Bi⁵⁺cations in the bulk ground state. This is accompanied by a breathing mode distortion B_oc whose amplitude is further enhanced by the octahedra rotations. At weak doping content (x?=?0.0625 to 0.125), holes are trapped on the lattice and intermediate states are localized in the band gap, ultimately resulting in a semiconducting behavior. At intermediate doping content (x?=?0.1875 to 0.375), a metallic phase is reached but the breathing mode is still present in the ground state due to its coupling with octahedral rotations, despite the fact that the structural distortion alone is not willing to spontaneously pop up in the material. The presence of this mode induces small gaps in the bands dispersing around the Fermi level. No gaps are anymore identified in the band structure at x?=?0.4375, a doping content reminiscent of the superconducting phase reported experimentally (x?=?0.45–0.6). Around x?≥?0.4375, the breathing mode is found on the verge of becoming stable in the material due to octahedral rotations and thus its vibration can form spin-paired electrons and holes in the material, i.e., Cooper pairs. These results thus suggest that the proximity of a lattice instability producing spin-paired electrons and holes is a prerequisite for superconductivity in the bismuthates, in sharp agreement with the bounded doping content observed experimentally for the superconducting phase. Within the superconducting phase at x?=?0.4375, an electron-phonon coupling constant λ associated with the breathing mode B_oc of 1.22 and a B_oc frequency of 66?meV are extracted from the simulations, in sharp agreement with the experimental values (λ?=?1.3?±?0.2 and ω?=?62?meV, respectively). At larger doping content, the breathing mode frequency becomes harder and the density of states at the Fermi level decreases but a slightly increased reduced electron-phonon matrix element with increasing x preserves a non-zero T_c up to x?=?0.625.?

Their study thus (i) validates the use of SCAN-DFT for studying doping effects in complex oxide superconductors and (ii) calls for inspection of disproportionation effects in superconducting nickelates and other oxide superconductors to see if the identified mechanism in bismuthates is also relevant for these newly identified oxide superconductors.?This article was recently published in npj Computational Materials 9: 30 (2022).

原文Abstract及其翻譯

Origin of superconductivity in hole doped SrBiO₃bismuth oxide perovskite from parameter-free first-principles simulations (無參數第一性原理研究空穴摻雜SrBiO₃氧化鉍鈣鈦礦超導性的起源)

Julien Varignon?

Abstract The recent discovery of nickel oxide superconductors have highlighted the importance of first-principles simulations for understanding the formation of the bound electrons at the core of superconductivity. Nevertheless, superconductivity in oxides is often ascribed to strong electronic correlation effects that density functional theory (DFT) cannot properly take into account, thereby disqualifying this technique. Being isostructural to nickel oxides, Sr_1-xK_xBiO₃?superconductors form an ideal testbed for unveiling the lowest theory level needed to model complex superconductors and the underlying pairing mechanism yielding superconductivity. Here I show that parameter-free DFT simulations capture all the experimental features and related quantities of Sr_1-xK_xBiO₃?superconductors, encompassing the prediction of an insulating to metal phase transition upon increasing the K doping content and of an electron-phonon coupling constant of 1.22 in sharp agreement with the experimental value of 1.3?±?0.2. The proximity of a disproportionated phase is further demonstrated to be a prerequisite for superconductivity in bismuthates.

摘要最近發現的鎳氧化物超導體凸顯了第一原理模擬對于理解超導核心的束縛電子的形成的重要性。然而，氧化物中的超導性通常歸因于強電子相關效應，而這種效應通常無法在密度泛函理論(DFT)的框架中準確模擬。Sr_1-xK_xBiO₃超導體與鎳氧化物具有等結構，為揭示復雜超導體模型所需的最低理論水平以及產生超導性的潛在配對機制提供了理想的試驗臺。在這里，我展示了無參數DFT模擬捕獲了Sr_1-xK_xBiO₃超導體的實驗特性和相關量，包括在增加K含量時絕緣態到金屬態的相變預測，以及與實驗值為1.3±0.2完全一致的電子–聲子耦合常數1.22。本工作還進一步證明了接近歧化相是形成鉍酸鹽超導性的先決條件。