Speaker
Description
The discovery of the bilayer nickelate La$_3$Ni$_2$O$_7$ as a high-temperature superconductor in bulk and thin film samples has raised fundamental questions about the origin of its unusually high transition temperature. While superconductivity emerges at high pressure or compressive strain, the normal state at ambient conditions displays spin stripe order with wavevector $Q=(\pi/2,\pi/2)$. In this talk, we propose a microscopic Hamiltonian that captures the structural evolution from orthorhombic to nearly tetragonal symmetry with increasing pressure, and argue that both stripe magnetism and superconductivity emerge from the interplay of Hund's coupling $J_H$ and interlayer superexchange $J_z$. Our DMRG calculations show that $(\pi/2,\pi/2)$ stripe order arises at sizable $J_H$ from a hidden quasi-one-dimensionality, while superconductivity emerges upon increasing $J_z$ in the crossover between the two regimes. Complementary RPA calculations on a tight-binding model fitted to ARPES data further support the pivotal role of $J_H$: in the strong Hund's coupling regime $s_\pm$-wave pairing dominates over competing $d$-wave tendencies, while $(\pi/2,\pi/2)$ magnetic ordering is already seeded by Fermi surface nesting at the bare level and further enhanced by $J_H$.