Coexistence of near-EF Flat Band and Van Hove Singularity in a Two-Phase Superconductor

Quantum many-body systems, particularly, the ones with large near-$E_F$ density states, are well known for exhibiting rich phase diagrams as a result of enhanced electron correlations. The recently discovered locally noncentrosymmetric heavy fermion superconductor ${\mathrm{CeRh}}_2{\mathrm{As}}_2$ has stimulated extensive attention due to its unusual $H\text{−}T$ phase diagram consisting of two-phase superconductivity, antiferromagnetic order, and possible quadrupole-density wave orders. However, the critical near-$E_F$ electronic structure remains experimentally elusive. Here, we provide this key information by combining soft-x-ray and vacuum ultraviolet (VUV) angle-resolved-photoemission-spectroscopy measurements and atom-resolved density-functional-theory $(\mathrm{DFT})+U$ calculations. With bulk-sensitive soft x ray, we reveal quasi-2D hole and electron pockets near the $E_F$. On the other hand, under VUV light, the Ce flat bands are resolved with the $c\text{−}f$ hybridization persisting up to well above the Kondo temperature. Most importantly, we observe a symmetry-protected fourfold Van Hove singularity (VHS) coexisting with the $\mathrm{Ce}4f_{5/2}^1$ flat bands at the $X$ point, which, to the best of our knowledge, has never been reported before. Such a rare coexistence is expected to lead to a large density of states at the zone edge, a large upper critical field of the odd-parity phase, as well as spin and/or charge instabilities with a vector of ($1/2$, $1/2$, 0). Uniquely, it will also result in a new type of $f$-VHS hybridization that alters the order and fine electronic structure of the VHS and flat bands. Our findings provide not only key insights into the nature of multiple phases in ${\mathrm{CeRh}}_2{\mathrm{As}}_2$ but also open up new prospects for exploring the novelties of many-body systems with $f$-VHS hybridization.