Orbital symmetry and orbital excitations in high-Tc superconductors

Abstract

We discuss a few possibilities of high-Tc 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 x2 − y2 symmetry doped by holes. Next, several routes towards involving both eg orbital symmetries for doped holes are discussed: (i) some give superconductivity in a CuO2 monolayer on Bi2212 superconductors, Sr2CuO4−δ, Ba2CuO4−δ, while (ii) others as nickelate heterostructures or Eu2−xSrxNiO4, could in principle realize it as well. At low electron filling of Ru ions, spin-orbital entangled states of t2g symmetry contribute in Sr2RuO4. Finally, electrons with both t2g and eg 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.