Type-II <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"><mml:mrow><mml:mi>t</mml:mi><mml:mo>−</mml:mo><mml:mi>J</mml:mi></mml:mrow></mml:math> model and shared superexchange coupling from Hund's rule in superconducting <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"><mml:msub><mml:mi mathvariant="normal">La</mml:mi><mml:mn>3</mml:mn></mml:msub><mml:msub><mml:mi mathvariant="normal">Ni</mml:mi><mml:mn>2</mml:mn></mml:msub><mml:msub><mml:mi mathvariant="normal">O</mml:mi><mml:mn>7</mml:mn></mml:msub></mml:math>
Hanbit Oh, Ya-Hui Zhang
Abstract
80 $K$ superconductivity was recently observed in a La${}_{3}$Ni${}_{2}$O${}_{7}$ under pressure. A pressing challenge is to unravel the intriguing pairing mechanism within these materials. Here, the authors point out a novel role of Hund's rule to transmit the superexchange coupling of the ${d}_{{z}^{2}}$ orbital to the ${d}_{{x}^{2}\ensuremath{-}{y}^{2}}$ orbital, which is a key factor for understanding the remarkably high ${T}_{c}$. The transmitted Hund coupling makes the ${d}_{{x}^{2}\ensuremath{-}{y}^{2}}$ orbital feel a large interlayer spin coupling but no interlayer hopping, a situation not explored before. Within one-orbital mean-field theory, the authors find robust $s$-wave pairing even at 50% hole doping. Moreover, they propose a new analytical framework, the so-called type-II $t$-$J$ model, which they argue is the minimal model to incorporate all essential ingredients, including large Hund's coupling and two orbital effects. This work offers a new model, develops a pioneering method, and identifies intriguing pairing mechanisms in the emerging nickelate field.