CeRu₂: Peak Effect or FFLO?

The Laves phase pyrochlore CeRu₂ has recently sparked renewed interest in the quantum materials community as a magnetic superconductor with flat bands observed near the Fermi energy from ARPES measurements. However, it is still controversial what role the flat bands play in the “exotic” heavy-fermion behavior of CeRu₂ and whether it induces the FFLO state characterized by a spatially modulated superconducting gap. The FFLO state is expected to appear in magnetization measurements as a secondary hysteresis loop below the superconducting transition temperature. A secondary explanation instead argues that the peak effect, which arises from the vortex pinning dynamics due to disorder, is the origin of the hysteresis loop. We will discuss thermal transport as a probe into the nature of the vortices in CeRu₂ and describe which scenario is more likely.