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-<a:math xmlns:a="http://www.w3.org/1998/Math/MathML" display="inline"><a:mrow><a:msub><a:mrow><a:mi>E</a:mi></a:mrow><a:mrow><a:mi>F</a:mi></a:mrow></a:msub></a:mrow></a:math> 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 <c:math xmlns:c="http://www.w3.org/1998/Math/MathML" display="inline"><c:mrow><c:msub><c:mi>CeRh</c:mi><c:mn>2</c:mn></c:msub><c:msub><c:mi>As</c:mi><c:mn>2</c:mn></c:msub></c:mrow></c:math> has stimulated extensive attention due to its unusual <e:math xmlns:e="http://www.w3.org/1998/Math/MathML" display="inline"><e:mrow><e:mi>H</e:mi><e:mtext>−</e:mtext><e:mi>T</e:mi></e:mrow></e:math> phase diagram consisting of two-phase superconductivity, antiferromagnetic order, and possible quadrupole-density wave orders. However, the critical near-<g:math xmlns:g="http://www.w3.org/1998/Math/MathML" display="inline"><g:mrow><g:msub><g:mrow><g:mi>E</g:mi></g:mrow><g:mrow><g:mi>F</g:mi></g:mrow></g:msub></g:mrow></g:math> 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 <i:math xmlns:i="http://www.w3.org/1998/Math/MathML" display="inline"><i:mrow><i:mo stretchy="false">(</i:mo><i:mi>DFT</i:mi><i:mo stretchy="false">)</i:mo><i:mo>+</i:mo><i:mi>U</i:mi></i:mrow></i:math> calculations. With bulk-sensitive soft x ray, we reveal quasi-2D hole and electron pockets near the <m:math xmlns:m="http://www.w3.org/1998/Math/MathML" display="inline"><m:mrow><m:msub><m:mrow><m:mi>E</m:mi></m:mrow><m:mrow><m:mi>F</m:mi></m:mrow></m:msub></m:mrow></m:math>. On the other hand, under VUV light, the Ce flat bands are resolved with the <o:math xmlns:o="http://www.w3.org/1998/Math/MathML" display="inline"><o:mrow><o:mi>c</o:mi><o:mtext>−</o:mtext><o:mi>f</o:mi></o:mrow></o:math> hybridization persisting up to well above the Kondo temperature. Most importantly, we observe a symmetry-protected fourfold Van Hove singularity (VHS) coexisting with the <q:math xmlns:q="http://www.w3.org/1998/Math/MathML" display="inline"><q:mrow><q:mi>Ce</q:mi><q:mtext> </q:mtext><q:mtext> </q:mtext><q:mtext> </q:mtext><q:mn>4</q:mn><q:msubsup><q:mrow><q:mi>f</q:mi></q:mrow><q:mrow><q:mn>5</q:mn><q:mo>/</q:mo><q:mn>2</q:mn></q:mrow><q:mrow><q:mn>1</q:mn></q:mrow></q:msubsup></q:mrow></q:math> flat bands at the <s:math xmlns:s="http://www.w3.org/1998/Math/MathML" display="inline"><s:mi>X</s:mi></s:math> 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 (<u:math xmlns:u="http://www.w3.org/1998/Math/MathML" display="inline"><u:mrow><u:mn>1</u:mn><u:mo>/</u:mo><u:mn>2</u:mn></u:mrow></u:math>, <w:math xmlns:w="http://www.w3.org/1998/Math/MathML" display="inline"><w:mrow><w:mn>1</w:mn><w:mo>/</w:mo><w:mn>2</w:mn></w:mrow></w:math>, 0). Uniquely, it will also result in a new type of <y:math xmlns:y="http://www.w3.org/1998/Math/MathML" display="inline"><y:mi>f</y:mi></y:math>-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 <ab:math xmlns:ab="http://www.w3.org/1998/Math/MathML" display="inline"><ab:mrow><ab:msub><ab:mi>CeRh</ab:mi><ab:mn>2</ab:mn></ab:msub><ab:msub><ab:mi>As</ab:mi><ab:mn>2</ab:mn></ab:msub></ab:mrow></ab:math> but also open up new prospects for exploring the novelties of many-body systems with <cb:math xmlns:cb="http://www.w3.org/1998/Math/MathML" display="inline"><cb:mi>f</cb:mi></cb:math>-VHS hybridization. Published by the American Physical Society 2024