Possible High T_{c} Superconductivity in La_{3}Ni_{2}O_{7} under High Pressure through Manifestation of a Nearly Half-Filled Bilayer Hubbard Model.

Inspired by a recent experiment showing that La_{3}Ni_{2}O_{7} exhibits high T_{c} superconductivity under high pressure, we theoretically revisit the possibility of superconductivity in this material. We find that superconductivity can take place, which is somewhat similar to that of the bilayer Hubbard model consisting of the Ni 3d_{3z^{2}-r^{2}} orbitals. Although the coupling with the 3d_{x^{2}-y^{2}} orbitals degrades superconductivity, T_{c} can still be high enough to understand the experiment thanks to the very high T_{c} reached in the bilayer Hubbard model.

Model Construction and a Possibility of Cupratelike Pairing in a New d

Effective models are constructed for a newly discovered superconductor (Nd,Sr)NiO_{2}, which has been considered as a possible nickelate analog of the cuprates. Estimation of the effective interaction, which turns out to require a multiorbital model that takes account of all the orbitals involved on the Fermi surface, shows that the effective interactions are significantly larger than in the cuprates. A fluctuation exchange study suggests occurrence of d_{x^{2}-y^{2}}-wave superconductivity, where the transition temperature should be lowered from the cuprates due to the larger interaction.

Monte Carlo study of cuprate superconductors in a four-bandd-pmodel: role of orbital degrees of freedom.

Understanding the various competing phases in cuprate superconductors is a long-standing challenging problem. Recent studies have shown that orbital degrees of freedom, both Cuegorbitals and Oporbitals, are a key ingredient for a unified understanding of cuprate superconductors, including the material dependence. Here we investigate a four-bandd-pmodel derived from the first-principles calculations with the variational Monte Carlo method, which allows us to elucidate competing phases on an equal footing. The obtained results can consistently explain the doping dependence of superconductivity, antiferromagnetic and stripe phases, phase separation in the underdoped region, and also novel magnetism in the heavily-overdoped region. The presence ofporbitals is critical to the charge-stripe features, which induce two types of stripe phases withs)-wave andd-wave bond stripe. On the other hand, the presence ofdz2orbital is indispensable to material dependence of the superconducting transition temperature (Tc), and enhances local magnetic moment as a source of novel magnetism in the heavily-overdoped region as well. These findings beyond one-band description could provide a major step toward a full explanation of unconventional normal state and highTcin cuprate supercondutors.

Bond-length distributions in ionically bonded materials with decomposition by coordination environment.

The distributions are analysed of the bond lengths between cations and anions in nitrides, oxides and fluorides for which structural data are available in the Crystallography Open Database (https://www.crystallography.net/cod/). The distributions are decomposed according to analysis of the coordination environments of the cations, which are offered in the tool ChemEnv [Waroquiers, George, Horton, Schenk, Persson, Rignanese, Gonze & Hautier (2020). Acta Cryst. B76, 683-695]. The distributions show how well the concept of ionic radius works. Chemical trends can be observed from the obtained distributions. For example, the bond-length distribution for Sc-O is similar to that for Zr-O. The distributions for Mo-O and V-O show noteworthy similarity. Resources for reproducing all of the results are available at https://github.com/tkotani/CEBond, with additional results for nitrides and fluorides.

Two-orbital model explains the higher transition temperature of the single-layer Hg-cuprate superconductor compared to that of the La-cuprate superconductor.

In order to explore the reason why the single-layered cuprates, La(2-x)(Sr/Ba)(x)CuO4 (T(c)≃40 K) and HgBa2CuO(4+δ) (T(c)≃90 K) have such a significant difference in T(c), we study a two-orbital model that incorporates the d(z2) orbital on top of the d(x2-y2) orbital. It is found, with the fluctuation exchange approximation, that the d(z2) orbital contribution to the Fermi surface, which is stronger in the La system, works against d-wave superconductivity, thereby dominating over the effect of the Fermi surface shape. The result resolves the long-standing contradiction between the theoretical results on Hubbard-type models and the experimental material dependence of T(c) in the cuprates.

Direct theoretical evidence for weaker correlations in electron-doped and Hg-based hole-doped cuprates.

Many important questions for high-Tc cuprates are closely related to the insulating nature of parent compounds. While there has been intensive discussion on this issue, all arguments rely strongly on, or are closely related to, the correlation strength of the materials. Clear understanding has been seriously hampered by the absence of a direct measure of this interaction, traditionally denoted by U. Here, we report a first-principles estimation of U for several different types of cuprates. The U values clearly increase as a function of the inverse bond distance between apical oxygen and copper. Our results show that the electron-doped cuprates are less correlated than their hole-doped counterparts, which supports the Slater picture rather than the Mott picture. Further, the U values significantly vary even among the hole-doped families. The correlation strengths of the Hg-cuprates are noticeably weaker than that of La2CuO4. Our results suggest that the strong correlation enough to induce Mott gap may not be a prerequisite for the high-Tc superconductivity.