Speaker
Description
The discovery of superconductivity in Ruddlesden-Popper bilayer nickelates has drawn enormous attention. We provide a unified theoretical explanation based on the two-component scenario for many experimental observations on bulk and thin film La$_3$Ni$_2$O$_7$, including the different superconducting transition temperatures, the nontrivial normal states, the effects of doping, and the Kondo effect in non-superconducting samples. For large interlayer superexchange, our theory predicts two different superconducting domes separated by a valence bond solid state for nearly half-filled $d_{z^2}$ orbital. The breaking of valence bonds by inner apical oxygen vacancies leads to Kondo effect. For small interlayer coupling, superconductivity emerges around half filling with $T_c$ less sensitive to $d_{z^2}$ doping, corresponding to thin films. Upon $d_{z^2}$ doping, the normal state shows a crossover from Fermi liquid to non-Fermi liquid and weak insulating behaviors, consistent with recent experiments. Our theory predicts ambient-pressure superconductivity may emerge in bulk La$_3$Ni$_2$O$_7$ by reducing the interlayer magnetic coupling.