image: (a) The low-energy electron orbitals of La3Ni2O7 form symmetric/antisymmetric molecular orbitals under the influence of interlayer coupling. (b) The Mott limit of La3Ni2O7 is described by a molecular Mott insulator corresponding to two antisymmetric molecular orbitals formed by the dx2-y2 (blue) and dz2 (green) orbitals, while the symmetric dx2-y2 orbital (red) contributes the self-doping effect.
Credit: ©Science China Press
The problem of high-temperature superconductivity is hailed as the "crown jewel" in the field of condensed matter physics research. The mechanism of high-temperature superconductivity remains one of the most challenging yet fascinating research topics in modern condensed matter physics. As the first discovered high-temperature superconducting materials, the physical mechanisms of copper-oxide superconductors are widely believed to be closely related to the electronic correlations and the Mott insulating nature of the parent compounds. Consequently, doping a Mott insulator is considered a core pathway for achieving high-temperature superconductivity.
In 2023, La3Ni2O7 was discovered to exhibit a superconducting transition temperature exceeding 80K under high pressure, setting the highest record for nickel-based superconducting systems since their discovery in 2019. With a unique bilayer crystal structure, the mechanism behind the superconducting La3Ni2O7 draws great attention in the field of condensed matter physics. The discovery of La3Ni2O7 not only serves as a new platform for investigating the properties of high-temperature superconductivity but also provides a unique perspective for advancing the understanding of the general underlying mechanisms of high-temperature superconductivity.
Professor Fu-Chun Zhang from the Kavli Institute for Theoretical Sciences at the University of Chinese Academy of Sciences and Professor Kun Jiang from the Institute of Physics, Chinese Academy of Sciences, along with their team, proposed that the Mott limit of this material can be described by a self-doped molecular Mott insulator. The model is based on the interplay between strong correlation effects and interlayer coupling. The physical origin associated with the superconductivity is similar to that of doped Mott insulators in copper-based superconductors.
In La3Ni2O7, the top and bottom nickel-oxygen planes are connected via apical oxygen atoms residing in the middle. The significant interlayer coupling causes localized atomic orbitals to form symmetric and antisymmetric molecular orbitals. With strong onsite interactions, the Mott limit of the material can be described by two nearly degenerate antisymmetric molecular orbitals, formed by the dx2-y2 orbital and the dz2 orbital, respectively, as shown in Fig. 1(a). Meanwhile, the higher-energy symmetric dx2-y2 molecular orbital leads to the self-doping effect, which eventually gives rise to the superconductivity through the mechanism analogous to that of doped Mott insulators. Based on this framework, the authors derived the low-energy effective theory for La3Ni2O7 and analyzed properties such as its superconducting pairing symmetry using renormalized mean-field calculations.
Journal
National Science Review