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superconductivity in chromium nitrides Pr3Cr10-xN11 with strong electron correlations

National Science Review 2020年第1期7卷 21-26页

作者： Wei Wu Kai Liu Yanjie Li Zhenhai Yu Desheng Wu Yuting Shao Shihang Na Gang Li Ruizhen Huang Tao Xiang Jianlin Luo Beijing National Laboratory for Condensed Matter Physics and Institute of Physics Chinese Academy of Sciences School of Physical Sciences University of Chinese Academy of Sciences Department of Physics and Beijing Key Laboratory of Opto-electronic Functional Materials & Micro-nano Devices Renmin University of China School of Physical Science and Technology Shanghai Tech University Songshan Lake Materials Laboratory Kavli Institute for Theoretical Sciences

Exploration of superconductivity in Cr-based compounds has attracted considerable interest because only a few Cr-based superconductors(CrAs, A2Cr3As3and ACr3As3（A = K, Rb, Cs, Na）) have been discovered so far and they show an unconventional pairing mechanism. We report the discovery of bulk superconductivity at 5.25 K in chromium nitride in Pr3Cr10-xN11with a cubic lattice structure. A relatively large upper critical field of Hc2（0）～12.6 T is determined, which is larger than the estimated Pauli-paramagnetic pair-breaking magnetic field. The material has a large electronic specific-heat coefficient of 170 mJ K-2mol-1—about 10 times larger than that estimated by the electronic structure calculation,which suggests that correlations between 3d electrons are very strong in Pr3Cr10-xN11, and thus quantum fluctuations might be involved. Electronic structure calculations show that the density of states at the Fermi energy are contributed predominantly by Cr 3d electrons, implying that the superconductivity results mainly from the condensation of Cr 3d electrons. Pr3Cr10-xN11represents a rare example of possible unconventional superconductivity emerging in a 3D system with strong electron correlations. Nevertheless,clarification of the specific pairing symmetry needs more investigation.

关键词： Cr-based superconductors nitrides strong electron correlations unconventional superconductivity

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superconductivity with peculiar upper critical fields in quasi-one-dimensional Cr-based pnictides

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Chinese Physics B 2018年第10期27卷 15-26页

作者： Guang-Han Cao Zeng-Wei Zhu Department of Physics Zhejiang University Hangzhou 310027 China Wuhan National High Magnetic Field Center School of Physics Huazhong University of Science and Technology Wuhan 430074 China

The discovery of superconductivity in quasi-one-dimensional Cr-based pnictides A2Cr3As3（A = alkali metals） has generated considerable research interest, primarily owing to their reduced dimensionality, significant electron correlations,and possible unconventional superconductivity. The upper critical field（Hc2） provides important information on the superconducting pairing. In this paper, we first briefly overview the latest research progress on the Cr-based ***, we introduce typical Hc2（T） behaviors of type-II superconductors in relation with the pair-breaking *** a description of the measurement method for Hc2, we focus on the analysis of Hc2 data, especially for the temperature and angle dependence, in K2Cr3As3 crystals. The result indicates（i） an absence of Pauli-paramagnetic pair breaking for field perpendicular to the Cr3As3 chains, and（ii） a unique threefold modulation for the in-plane Hc2Φ profile. Finally we conclude with remarks on the possible unconventional superconducting pairing symmetry.

关键词： Cr-based superconductors upper critical fields unconventional superconductivity

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Robustness of s-wave pairing symmetry in iron-based superconductors and its implications for fundamentals of magnetically driven high-temperature superconductivity

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Frontiers of physics 2016年第5期11卷 79-89页

作者： Jiangping Hu Jing Yuan Institute of Physics Chinese Academy of Sciences Beijing 100190 China Department of Physics Purdue University West Lafayette IN 47907 USA Collaborative Innovation Center of Quantum Matter Beijing 100871 China

Based on the assumption that the superconducting state belongs to a single irreducible representation of lattice symmetry, we propose that the pairing symmetry in all measured iron-based superconductors is generally consistent with the A1g s-wave. Robust s-wave pairing throughout the different families of iron-based superconductors at different doping regions signals two fundamental principles behind high-To superconducting mechanisms：（i） the correspondence principle： the short-range magnetic-exchange interactions and the Fermi surfaces act collaboratively to achieve high-Tc superconductivity and determine pairing symmetries; （ii） the magnetic-selection pairing rule： supercon- ductivity is only induced by the magnetic-exchange couplings from the super-exchange mechanism through cation-anion-cation chemical bonding. These principles explain why unconventional high- Tc superconductivity appears to be such a rare but robust phenomena, with its strict requirements regarding the electronic environment. The results will help us to identify new electronic structures that can support high-Tc superconductivity.

关键词： iron-based superconductors cuprates unconventional superconductivity high-temperature superconductors

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Theoretical studies of superconductivity in doped BaCoSO

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Frontiers of physics 2018年第3期13卷 141-147页

作者： Shengshan Qin Yinxiang Li Qiang Zhang Congcong Le Jiangping Hu Beijing National Laboratory for Condensed Matter Physics and Institute of Physics Chinese Academy of Sciences Beijing 100190 China University of Chinese Academy of Science Beijing 100049 China Kavli Institute of Theoretical Sciences University of Chinese Academy of Sciences Beijing 100049 China Collaborative Innovation Center of Quantum Matter Beijing 100190 China

We investigate superconductivity that may exist in the doped BaCoSO, a multi-orbital Mott insulator with a strong antiferromagnetie ground state. The superconductivity is studied in both t-J type and Hubbard type multi-orbital models by mean field approach and random phase approximation （RPA） analysis. Even if there is no C4 rotational symmetry, it is found that the system still carries a d-wave like pairing symmetry state with gapless nodes and sign changed superconducting order parameters on Fermi surfaces. The results are largely doping insensitive. In this superconducting state, the three t2g orbitals have very different superconducting form factors in momentum space. In particular, the intra-orbital pairing of the dx2-y2 orbital has an s-wave like pairing form factor. The two methods also predict very different pairing strength on different parts of Fermi surfaces. These results suggest that BaCoSO and related materials can be a new ground to test and establish fundamental principles for unconventional high temperature superconductivity.

关键词： unconventional superconductivity pairing symmetry

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Effects of carrier density and interactions on pairing symmetry in a t_(2g) model

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Chinese Physics B 2024年第1期33卷 223-230页

作者：李云霄西文翰董召阳姚子健于顺利李建新 National Laboratory of Solid State Microstructures and Department of Physics Nanjing UniversityNanjing 210093China Nanjing University of Science and Technology Nanjing 210094China Department of Physics Nanjing Normal UniversityNanjing 210023China Collaborative Innovation Center of Advanced Microstructures Nanjing UniversityNanjing 210093China

By utilizing the fluctuation exchange approximation method,we perform a study on the superconducting pairing symmetry in a t_(2g) three-orbital model on the square *** the tight-binding parameters of the model are based on Sr_(2)RuO_(4),we have systematically studied the evolution of superconducting pairing symmetry with the carrier density and interactions,making our findings relevant to a broader range of material *** a moderate Hund’s coupling,we find that spin fluctuations dominate the superconducting pairing,leading to a prevalent spin-singlet pairing with a d_(x^(2)-y^(2))-wave symmetry for the carrier density within the range of n=1.5-4 per *** reducing the Hund’s coupling,the charge fluctuations are enhanced and play a crucial role in determining the pairing symmetry,leading to a transition of the pairing symmetry from the spin-singlet d_(x^(2)-y^(2))-wave to the spin-triplet ***,we find that the superconducting pairings are orbital *** the carrier density changes from n=4 to n=1.5,the active orbitals for superconducting pairing shift from the quasi-two-dimensional orbital dxy to the quasi-one-dimensional orbitals d_(xz) and d_(yz).

关键词： unconventional superconductivity pairing mechanism Fermi surface three-orbital model

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Effects of pressure and doping on Ruddlesden-Popper phases La_(n+1)Ni_(n)O_(3 n+1)

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Journal of Materials Science & Technology 2024年第18期185卷 147-154页

作者： Mingxin Zhang Cuiying Pei Qi Wang Yi Zhao Changhua Li Weizheng Cao Shihao Zhu Juefei Wu Yanpeng Qi School of Physical Science and Technology ShanghaiTech UniversityShanghai 201210China ShanghaiTech Laboratory for Topological Physics ShanghaiTech UniversityShanghai 201210China Shanghai Key Laboratory of High-resolution Electron Microscopy ShanghaiTech UniversityShanghai 201210China

Recently,the discovery of superconductivity with a critical temperature Tc up to 80 K in Ruddlesden-Popper phases La_(n+1)Ni_(n)O_(3 n+1)(n=2)under pressure has garnered considerable *** to now,the superconductivity was only observed in La_(3)Ni_(2)O_(7)single crystal grown with the optical-image float-ing zone furnace under oxygen *** remains to be understood the effect of chemical doping on superconducting La_(3)Ni_(2)O_(7)as well as other Ruddlesden-Popper ***,we systematically investi-gate the effect of external pressure and chemical doping on polycrystalline Ruddlesden-Popper *** results demonstrate that the application of pressure and doping effectively tunes the transport prop-erties of Ruddlesden-Popper *** find pressure-induced superconductivity up to 86 K in La_(3)Ni_(2)O_(7)polycrystalline sample,while no signatures of superconductivity are observed in La_(2)NiO_(4)and La_(4)Ni_(3)O_(10)polycrystalline samples under high pressure up to 50 *** study sheds light on the exploration of high-Tc superconductivity in nickelates.

关键词： unconventional superconductivity High pressure Nickelates Ruddlesden-Popper phases

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A review of some new perspectives on the theory of superconducting Sr_(2)RuO_(4)

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Chinese Physics B 2021年第10期30卷 29-36页

作者： Wen Huang Shenzhen Institute for Quantum Science and Engineering Southern University of Science and TechnologyShenzhen 518055China Guangdong Provincial Key Laboratory of Quantum Science and Engineering Southern University of Science and TechnologyShenzhen 518055China

The nature of the Cooper pairing in the paradigmatic unconventional superconductor Sr_(2)RuO_(4) is an outstanding puzzle in condensed matter *** the tremendous efforts made in the past twenty-seven years,neither the pairing symmetry nor the underlying pairing mechanism in this material has been understood with clear *** is largely due to the lack of a superconducting order that is capable of interpreting in a coherent manner the numerous essential experimental *** this stage,it may be desirable to reexamine our existing theoretical descriptions of superconducting Sr_(2)RuO_(4).This review focuses on several recent developments that may provide some clues for future *** highlight three separate aspects:1)any pairing in the Eu symmetry channel,with which the widely discussed chiral p-wave is associated,shall acquire a 3D structure due to spin-orbit entanglement;2)if the reported Kerr effect is a superconductivity-induced intrinsic bulk response,the superconductivity must either exhibit a chiral character,or be complex mixtures of certain set of helical p-wave pairings;3)when expressed in a multiorbital basis,the Cooper pairing could acquire numerous exotic forms that are inaccessible in single-orbital *** implications of each of these new perspectives are briefly discussed in connection with selected experimental phenomena.

关键词： Sr_(2)RuO_(4) unconventional superconductivity pairing symmetry Kerr effect

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Cuprate-like electronic structures in infinite-layer nickelates with substantial hole dopings

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National Science Review 2024年第8期11卷 180-186页

作者： Xiang Ding Yu Fan Xiaoxiao Wang Chihao Li Zhitong An Jiahao Ye Shenglin Tang Minyinan Lei Xingtian Sun Nan Guo Zhihui Chen Suppanut Sangphet Yilin Wang Haichao Xu Rui Peng Donglai Feng Advanced Materials Laboratory State Key Laboratory of Surface Physics and Department of Physics Fudan University School of Emerging Technology University of Science and Technology of China New Cornerstone Science Laboratory University of Science and Technology of China Shanghai Research Center for Quantum Sciences National Synchrotron Radiation Laboratory and School of Nuclear Science and Technology University of Science and Technology of China

Superconducting infinite-layer（IL） nickelates offer a new platform for investigating the long-standing problem of high-temperature *** models were proposed to understand the superconducting mechanism of nickelates based on the calculated ele ctronic structure,and the multiple Fermi surfaces and multiple orbitals involved create complications and controversial *** the past five years,the lack of direct measurements of the electronic structure has hindered the understanding of nickelate *** we fill this gap by directly resolving the electronic structures of the parent compound LaNiO2and superconducting La0.8Ca0.2NiO2using angle-resolved photoemission *** find that their Fermi surfaces consist of a quasi-2D hole pocket and a 3D electron pocket at the Brillouin zone corner,whose volumes change upon Ca *** Fermi surface topology and band dispersion of the hole pocket closely resemble those observed in hole-doped ***,the cuprate-like band exhibits significantly higher hole doping in superconducting La0.8Ca0.2NiO2compared to superconducting cuprates,highlighting the disparities in the electronic states of the superconducting *** observations highlight the novel aspects of the IL nickelates,and pave the way toward the microscopic understanding of the IL nickelate family and its superconductivity.

关键词： unconventional superconductivity nickelate superconductors oxide MBE ARPES electronic structure superconducting phase diagram

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