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.

中文翻译：

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两相超导体中近 EF 平带和范霍夫奇点的共存


量子多体系统，特别是那些具有大近 $E_F$ 密度态的系统，因增强的电子相关性而表现出丰富的相图而闻名。最近发现的局部非中心对称重费米子超导体 ${\mathrm{CeRh}}_2{\mathrm{As}}_2$ 因其不寻常的由两相超导性、反铁磁序和可能的四极密度波序组成的 $H\text{−}T$ 相图而引起了广泛关注。然而，关键的近 $E_F$ 电子结构在实验上仍然难以捉摸。在这里，我们通过结合软 X 射线和真空紫外 (VUV) 角分辨光发射光谱测量和原子分辨密度泛函理论 $(\mathrm{DFT})+U$ 计算来提供这一关键信息。利用体敏感软 X 射线，我们揭示了 $E_F$ 附近的准二维空穴和电子袋。另一方面，在 VUV 光下，Ce 平带随着 $c\text{−}f$ 杂交持续到远高于近藤温度而被解析。最重要的是，我们观察到对称保护的四重范霍夫奇点（VHS）与 $X$ 点处的 $\mathrm{Ce}4f_{5/2}^1$ 平带共存，据我们所知，这是从未有过的。之前有报道过。这种罕见的共存预计将导致区域边缘处的高态密度、奇宇称相的大上临界场以及向量为（ $1/2$ , 0)。独特的是，它还将产生一种新型的 $f$ -VHS 杂交，改变 VHS 和平带的顺序和精细电子结构。 我们的研究结果不仅提供了对 ${\mathrm{CeRh}}_2{\mathrm{As}}_2$ 中多相性质的重要见解，而且还为探索 $f$ -VHS 杂交多体系统的新颖性开辟了新的前景。