Orbital Symmetry and Orbital Excitations in High-Tc Superconductors

• Published in: Condensed matter 2019-06, Vol.4 (2), p.46
• Main Authors: Oleś, Andrzej M., Wohlfeld, Krzysztof, Khaliullin, Giniyat
• Format: Article
• Language: English
• Subjects: Bismuth strontium calcium copper oxide; charge-transfer insulator; Cooper pairing; Copper; Cuprates; Electron spin; electron-phonon; Entangled states; Excitation; Fermi surface; Group 5A compounds; Heterostructures; high-Tc superconductivity; orbital excitation; orbital symmetry; spin-orbit coupling; spin-orbital entanglement; Strontium; Superconductivity; Superconductors; Symmetry
• ISSN: 2410-3896; 2410-3896
• DOI: 10.3390/condmat4020046

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 − δ , Ba 2 CuO 4 − δ , 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 2 g symmetry contribute in Sr 2 RuO 4 . Finally, electrons with both t 2 g 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.