Tuning superconductivity and spin-vortex instabilities in CaKFe4As4 through in-plane antisymmetric strains

Lattice strains of appropriate symmetry have served as an excellent tool to explore the interaction of superconductivity in the iron-based superconductors with orthorhombic-nematic and stripe spin-density-wave (SSDW) order. In this Letter, we contribute to a broader understanding of the coupling of strain to superconductivity and competing normal-state orders by studying <a:math xmlns:a="http://www.w3.org/1998/Math/MathML"><a:mrow><a:msub><a:mi>CaKFe</a:mi><a:mn>4</a:mn></a:msub><a:msub><a:mi>As</a:mi><a:mn>4</a:mn></a:msub></a:mrow></a:math> under large, in-plane strains of <b:math xmlns:b="http://www.w3.org/1998/Math/MathML"><b:msub><b:mi>B</b:mi><b:mrow><b:mn>1</b:mn><b:mi>g</b:mi></b:mrow></b:msub></b:math> and <c:math xmlns:c="http://www.w3.org/1998/Math/MathML"><c:msub><c:mi>B</c:mi><c:mrow><c:mn>2</c:mn><c:mi>g</c:mi></c:mrow></c:msub></c:math> symmetry. In contrast to the majority of iron-based superconductors, pure <d:math xmlns:d="http://www.w3.org/1998/Math/MathML"><d:mrow><d:msub><d:mi>CaKFe</d:mi><d:mn>4</d:mn></d:msub><d:msub><d:mi>As</d:mi><d:mn>4</d:mn></d:msub></d:mrow></d:math> exhibits superconductivity with a relatively high transition temperature of <e:math xmlns:e="http://www.w3.org/1998/Math/MathML"><e:mrow><e:msub><e:mi>T</e:mi><e:mi>c</e:mi></e:msub><e:mo>∼</e:mo><e:mn>35</e:mn><e:mspace width="0.28em"/><e:mi mathvariant="normal">K</e:mi></e:mrow></e:math> in proximity of a noncollinear, tetragonal, hedgehog spin-vortex crystal (SVC) order. Through experiments and calculations, we demonstrate an anisotropic in-plane strain response of <h:math xmlns:h="http://www.w3.org/1998/Math/MathML"><h:msub><h:mi>T</h:mi><h:mi>c</h:mi></h:msub></h:math> and the favored SVC configuration in <i:math xmlns:i="http://www.w3.org/1998/Math/MathML"><i:mrow><i:msub><i:mi>CaKFe</i:mi><i:mn>4</i:mn></i:msub><i:msub><i:mi>As</i:mi><i:mn>4</i:mn></i:msub></i:mrow></i:math>. This supports a scenario, in which the change in spin fluctuations dominates the strain response of superconducting <j:math xmlns:j="http://www.w3.org/1998/Math/MathML"><j:msub><j:mi>T</j:mi><j:mi>c</j:mi></j:msub></j:math>. Overall, by suggesting moderate <k:math xmlns:k="http://www.w3.org/1998/Math/MathML"><k:msub><k:mi>B</k:mi><k:mrow><k:mn>2</k:mn><k:mi>g</k:mi></k:mrow></k:msub></k:math> strains as an effective parameter to change the stability of SVC and SSDW, we outline a pathway to a unified phase diagram of iron-based superconductivity. Published by the American Physical Society 2024