Presentation Information
[PCP1-01]Theoretical Analysis of the Crystal and Electronic Structure of Thin-Film La3Ni2O7
*Kensei Ushio1, Shu Kamiyama2, Yuto Hoshi1, Ryota Mizuno3, Masayuki Ochi2,3, Kazuhiko Kuroki2, Hirofumi Sakakibara4 (1. Faculty of Eng., Tottori Univ. (Japan), 2. Department of Physics, University of Osaka, (Japan), 3. Forefront Research Center, University of Osaka, (Japan), 4. Advanced Mechanical and Electronic System Research Center(AMES), (Japan))
Keywords:
high-temperature superconductivity,first-principles calculation,nickelate
Purpose
Since the discovery of superconductivity in bulk La3Ni2O7 at a transition temperature of Tc ~ 80K in 2023, bilayer nickelates has attracted considerable attention. Theoretically, the strongly hybridized orbitals between the two layers in La3Ni2O7 are believed to enhance s+- wave superconductivity. However, since superconductivity in bulk La3Ni2O7 requires applying high pressure of approximately 10 to 20 GPa.On this background, Hwang's group has reported superconductivity at around Tc ~ 40 K in La3Ni2O7 thin film under ambient pressure, which immediately drew significant attention. In their experiments, thin films were grown on SrLaAlO4 (SLAO), LaAlO3 (LAO), and (LaAlO3)0.3(Sr2TaAlO6)0.7 substrates. Superconductivity was observed only in SLAO-based films, suggesting that the strain from the substrate may play a crucial role. Motivated by these findings, we investigated the substrate dependence of superconductivity through theoretical calculations.
Method
We determine the crystal structure using first-principles band calculations. During this process, the a and b axes are fixed to the substrate's lattice constants, while the c axis and atomic positions are optimized. An effective model is then derived from the optimized crystal structure using the maximally localized Wannier orbital method and first-principles band calculations. Superconductivity is evaluated using the fluctuation exchange approximation (FLEX) for the obtained model.
Results
Among three cases of substrates, the film grown on SLAO substrate has a Ni-O-Ni bond angle that is closest to 180 degree. The presence or absence of Gamma pockets depends on the structure, but analysis of the gap function confirmed that s+- wave symmetry is robust under all substrate conditions.
Consideration
The transition temperature of thin films, as reported in experiments, is approximately half that of bulk materials under high pressure. Our consideration is as followings. The experimental structure of thin films tends to suppress the hopping integral tperp between the two layers, which leads to a decrease in the transition temperature. Therefore, subtle structural differences can be an important factor in determining superconducting properties.
Conclusion
We have shown that structural changes caused by the substrate affect the electronic structure and details of the band structure in La3Ni2O7 thin films, while s+- wave symmetry is robust. However, the significant decrease in transition temperature is likely due to the weakening of interlayer bonding, depending on the experimentally obtained structural characteristics.
Since the discovery of superconductivity in bulk La3Ni2O7 at a transition temperature of Tc ~ 80K in 2023, bilayer nickelates has attracted considerable attention. Theoretically, the strongly hybridized orbitals between the two layers in La3Ni2O7 are believed to enhance s+- wave superconductivity. However, since superconductivity in bulk La3Ni2O7 requires applying high pressure of approximately 10 to 20 GPa.On this background, Hwang's group has reported superconductivity at around Tc ~ 40 K in La3Ni2O7 thin film under ambient pressure, which immediately drew significant attention. In their experiments, thin films were grown on SrLaAlO4 (SLAO), LaAlO3 (LAO), and (LaAlO3)0.3(Sr2TaAlO6)0.7 substrates. Superconductivity was observed only in SLAO-based films, suggesting that the strain from the substrate may play a crucial role. Motivated by these findings, we investigated the substrate dependence of superconductivity through theoretical calculations.
Method
We determine the crystal structure using first-principles band calculations. During this process, the a and b axes are fixed to the substrate's lattice constants, while the c axis and atomic positions are optimized. An effective model is then derived from the optimized crystal structure using the maximally localized Wannier orbital method and first-principles band calculations. Superconductivity is evaluated using the fluctuation exchange approximation (FLEX) for the obtained model.
Results
Among three cases of substrates, the film grown on SLAO substrate has a Ni-O-Ni bond angle that is closest to 180 degree. The presence or absence of Gamma pockets depends on the structure, but analysis of the gap function confirmed that s+- wave symmetry is robust under all substrate conditions.
Consideration
The transition temperature of thin films, as reported in experiments, is approximately half that of bulk materials under high pressure. Our consideration is as followings. The experimental structure of thin films tends to suppress the hopping integral tperp between the two layers, which leads to a decrease in the transition temperature. Therefore, subtle structural differences can be an important factor in determining superconducting properties.
Conclusion
We have shown that structural changes caused by the substrate affect the electronic structure and details of the band structure in La3Ni2O7 thin films, while s+- wave symmetry is robust. However, the significant decrease in transition temperature is likely due to the weakening of interlayer bonding, depending on the experimentally obtained structural characteristics.