Charge-transfer properties and electron dynamics in ferromagnetic CoS2

We investigated the element-specific electronic structure and charge-carrier dynamics of a single-crystal ferromagnet <a:math xmlns:a="http://www.w3.org/1998/Math/MathML"><a:msub><a:mi>CoS</a:mi><a:mn>2</a:mn></a:msub></a:math> with complementary x-ray spectroscopy techniques. Hard x-ray photoemission (HAXPES) is used to provide crucial information on the bulk electronic structure and chemical bonding in <b:math xmlns:b="http://www.w3.org/1998/Math/MathML"><b:msub><b:mi>CoS</b:mi><b:mn>2</b:mn></b:msub></b:math> that is compared against the isoelectronic paramagnet <c:math xmlns:c="http://www.w3.org/1998/Math/MathML"><c:msub><c:mi>CoSe</c:mi><c:mn>2</c:mn></c:msub></c:math>. The Co <d:math xmlns:d="http://www.w3.org/1998/Math/MathML"><d:mrow><d:mn>1</d:mn><d:mi>s</d:mi></d:mrow></d:math> core-level line shows several satellite features for <e:math xmlns:e="http://www.w3.org/1998/Math/MathML"><e:msub><e:mi>CoS</e:mi><e:mn>2</e:mn></e:msub></e:math>, showing explicit charge-transfer processes and local screening of the core hole by S ligands, whereas no such features are observed in <f:math xmlns:f="http://www.w3.org/1998/Math/MathML"><f:msub><f:mi>CoSe</f:mi><f:mn>2</f:mn></f:msub></f:math>. The satellite structures indicate the electronic configuration of divalent <g:math xmlns:g="http://www.w3.org/1998/Math/MathML"><g:msup><g:mrow><g:mi>Co</g:mi></g:mrow><g:mrow><g:mn>2</g:mn><g:mo>+</g:mo></g:mrow></g:msup></g:math> as a combination of <h:math xmlns:h="http://www.w3.org/1998/Math/MathML"><h:mrow><h:msup><h:mi>d</h:mi><h:mn>8</h:mn></h:msup><h:mi mathvariant="normal">Ḻ</h:mi></h:mrow></h:math> and <j:math xmlns:j="http://www.w3.org/1998/Math/MathML"><j:mrow><j:msup><j:mi>d</j:mi><j:mn>9</j:mn></j:msup><j:msup><j:mi mathvariant="normal">Ḻ</j:mi><j:mn>2</j:mn></j:msup></j:mrow></j:math> in addition to the nominal ionic <l:math xmlns:l="http://www.w3.org/1998/Math/MathML"><l:msup><l:mrow><l:mi>d</l:mi></l:mrow><l:mn>7</l:mn></l:msup></l:math> state, where <m:math xmlns:m="http://www.w3.org/1998/Math/MathML"><m:mi mathvariant="normal">Ḻ</m:mi></m:math> represents an S <o:math xmlns:o="http://www.w3.org/1998/Math/MathML"><o:mrow><o:mn>3</o:mn><o:mi>p</o:mi></o:mrow></o:math> hole. We employ resonant Auger spectroscopy across the S <p:math xmlns:p="http://www.w3.org/1998/Math/MathML"><p:mi>K</p:mi></p:math>-edge for <q:math xmlns:q="http://www.w3.org/1998/Math/MathML"><q:msub><q:mi>CoS</q:mi><q:mn>2</q:mn></q:msub></q:math> to obtain electron delocalization times to adjacent Co atomic sites. The fast carrier dynamics are attributed to strongly screened Coulomb interactions and hence a facile carrier delocalization. The strong hybridization formed between the Co <r:math xmlns:r="http://www.w3.org/1998/Math/MathML"><r:mrow><r:mn>3</r:mn><r:mi>d</r:mi></r:mrow></r:math> and S <s:math xmlns:s="http://www.w3.org/1998/Math/MathML"><s:mrow><s:mn>3</s:mn><s:mi>p</s:mi></s:mrow></s:math> states with pronounced charge-transfer character reflects a self-doped system with a finite density <t:math xmlns:t="http://www.w3.org/1998/Math/MathML"><t:mi>n</t:mi></t:math> of holes at the sulfur site (<u:math xmlns:u="http://www.w3.org/1998/Math/MathML"><u:msup><u:mi mathvariant="normal">Ḻ</u:mi><u:mi>n</u:mi></u:msup></u:math>), in line with recent models that indicate a negative charge-transfer energy for <w:math xmlns:w="http://www.w3.org/1998/Math/MathML"><w:msub><w:mi>CoS</w:mi><w:mn>2</w:mn></w:msub></w:math>. In addition to HAXPES data, we also report on experimental and theoretical <x:math xmlns:x="http://www.w3.org/1998/Math/MathML"><x:mi>L</x:mi></x:math>-edge x-ray absorption and x-ray magnetic circular dichroism data for <y:math xmlns:y="http://www.w3.org/1998/Math/MathML"><y:msub><y:mi>CoS</y:mi><y:mn>2</y:mn></y:msub></y:math> that demonstrate multiconfiguration effects in the excitation process. To enable a direct comparison of the experimental spectra, we used density functional theory calculations to obtain the projected density of states to describe the ground-state electronic structure. The existence of fast carrier dynamics and strong charge-transfer properties, demonstrated in this study, highlights the unique nature of <z:math xmlns:z="http://www.w3.org/1998/Math/MathML"><z:msub><z:mi>CoS</z:mi><z:mn>2</z:mn></z:msub></z:math> with a wide potential in topological spintronics applications and integration in energy-related device platforms.