Comparing first-principles density functionals plus corrections for the lattice dynamics of YBa2_2Cu3_3O6_6

The enigmatic mechanism underlying unconventional high-temperature superconductivity, especially the role of lattice dynamics, has remained a subject of debate. Theoretical insights have long been hindered due to the lack of an accurate first-principles description of the lattice dynamics of cuprates. Recently, using the r2SCAN meta-GGA functional, we were able to achieve accurate phonon spectra of an insulating cuprate YBa$_2$Cu$_3$O$_6$, and discover significant magnetoelastic coupling in experimentally interesting Cu-O bond stretching optical modes [Phys. Rev. B 107, 045126 (2023)]. We extend this work by comparing PBE and r2SCAN performances with corrections from the on-site Hubbard U and the D4 van der Waals (vdW) methods, aiming at further understanding on both the materials science side and the density functional side. We demonstrate the importance of vdW and self-interaction corrections for accurate first-principles YBa2 Cu3 O6 lattice dynamics. Since r2SCAN by itself partially accounts for these effects, the good performance of r2SCAN is now more fully explained. In addition, the performances of the Tao-Mo series of meta-GGAs, which are constructed in a different way from SCAN/r2SCAN, are also compared and discussed.Comment: arXiv admin note: text overlap with arXiv:2210.0656

Understanding the Quantum Oscillation Spectrum of Heavy-fermion Compound SmB6

SmB6 is a mysterious compound that is electrically insulating but yet it exhibits quantum oscillations, which are a telltale signature of the metallic state. Adding to the enigma is the possibility that SmB6 is a topological Kondo insulator. Here, we report first-principles, parameter-free all-electron electronic-structure calculations on SmB6, which yield the band structure and crystal-field splittings within the f-electron complex in accord with experiments. Predicted energies of several magnetic phases where charge, spin and lattice degrees of freedom are treated on an equal footing are found to be extremely close, indicating the key role of spin fluctuations in SmB6. Our results show that the topological Kondo state of SmB6 is robust regardless of its magnetic configuration. The Fermi surfaces derived from our predicted ground state explain the experimentally observed bulk quantum oscillations, and our large calculated effective mass of electrons at the Fermi surface explains how the material is essentially insulating, with a measured specific heat that is in excellent agreement with our calculations.Comment: 17 pages, 7 figures and 1 tabl