Talk by Steffen Bötzel

Superconductivity and Spin Density Waves in Bilayer Nickelates

Abstract

Bilayer nickelates have recently emerged as a promising platform for studying unconventional superconductivity in systems with multiorbital character and pronounced interlayer coupling. At ambient pressure, bulk La$_3$Ni$_2$O$_7$ exhibits a double-stripe antiferromagnetic order, with the two NiO$_2$ layers within each bilayer aligned antiferromagnetically. Under high pressure, superconductivity with critical temperatures of up to 80 K has been reported. More recently, superconductivity has also been observed in thin films at ambient pressure, with $T_{\rm c}$ reaching 60 K.

A distinctive feature of bilayer nickelates is their exceptionally strong interlayer coupling, which can naturally give rise to interlayer Cooper pairing. This leads to an $s_{\pm}$ superconducting state in which the order parameter changes sign between the bonding and antibonding bilayer bands. The interlayer nature of the pairing is also reflected in several characteristic properties of the superconducting state.

In this presentation, I will review the main experimental developments and current theoretical understanding of superconductivity and spin-density-wave order in bilayer nickelates. From a weak-coupling perspective, I will show how mirror-symmetry odd spin fluctuations naturally account for the ambient-pressure double-stripe magnetic order while simultaneously providing the pairing interaction responsible for interlayer superconductivity under pressure. Finally, I will discuss different experimental signatures of the proposed $s_{\pm}$ state, including collective spin excitations (spin resonance) and the effect impurity scattering.

Host

Prof. Brian M. Andersen