Orbital Symmetry and Orbital Excitations in High-T_c Superconductors

We discuss a few possibilities of high- $T_c$ superconductivity with more than one orbital symmetry contributing to the pairing. First, we show that the high energies of orbital excitations in various cuprates suggest a simplified model with a single orbital of $x^2-y^2$ symmetry doped by holes. Next, several routes towards involving both $e_g$ orbital symmetries for doped holes are discussed: (i) some give superconductivity in a CuO ${}_2$ monolayer on Bi2212 superconductors, Sr ${}_2$ CuO ${}_{4-\delta }$ , Ba ${}_2$ CuO ${}_{4-\delta }$ , while (ii) others as nickelate heterostructures or Eu ${}_{2-x}$ Sr ${}_x$ NiO ${}_4$ , could in principle realize it as well. At low electron filling of Ru ions, spin-orbital entangled states of $t_{2g}$ symmetry contribute in Sr ${}_2$ RuO ${}_4$ . Finally, electrons with both $t_{2g}$ and $e_g$ orbital symmetries contribute to the superconducting properties and nematicity of Fe-based superconductors, pnictides or FeSe. Some of them provide examples of orbital-selective Cooper pairing. View Full-Text