Nature of Excitons and Their Ligand-Mediated Delocalization in Nickel Dihalide Charge-Transfer Insulators
Connor A. Occhialini, Yi Tseng, Hebatalla Elnaggar, Song Qian, Mark Blei, Seth Ariel Tongay, Valentina Bisogni, Frank M. F. de Groot, Jonathan Pelliciari, Riccardo Comin
Abstract
The fundamental optical excitations of correlated transition-metal compounds are typically identified with multielectronic transitions localized at the transition-metal site, such as <a:math xmlns:a="http://www.w3.org/1998/Math/MathML" display="inline"><a:mi>d</a:mi><a:mi>d</a:mi></a:math> transitions. In this vein, intense interest has surrounded the appearance of sharp, below-band-gap optical transitions, i.e., excitons, within the magnetic phase of correlated <c:math xmlns:c="http://www.w3.org/1998/Math/MathML" display="inline"><c:mrow><c:msup><c:mrow><c:mi>Ni</c:mi></c:mrow><c:mrow><c:mn>2</c:mn><c:mo>+</c:mo></c:mrow></c:msup></c:mrow></c:math> van der Waals magnets. The interplay of magnetic and charge-transfer insulating ground states in <e:math xmlns:e="http://www.w3.org/1998/Math/MathML" display="inline"><e:mrow><e:msup><e:mrow><e:mi>Ni</e:mi></e:mrow><e:mrow><e:mn>2</e:mn><e:mo>+</e:mo></e:mrow></e:msup></e:mrow></e:math> systems raises intriguing questions on the roles of long-range magnetic order and of metal-ligand charge transfer in the exciton nature, which inspired microscopic descriptions beyond typical <g:math xmlns:g="http://www.w3.org/1998/Math/MathML" display="inline"><g:mi>d</g:mi><g:mi>d</g:mi></g:math> excitations. Here we study the impact of charge transfer and magnetic order on the excitation spectrum of the nickel dihalides (<i:math xmlns:i="http://www.w3.org/1998/Math/MathML" display="inline"><i:mrow><i:mi>Ni</i:mi><i:msub><i:mrow><i:mi>X</i:mi></i:mrow><i:mrow><i:mn>2</i:mn></i:mrow></i:msub></i:mrow></i:math>, <k:math xmlns:k="http://www.w3.org/1998/Math/MathML" display="inline"><k:mrow><k:mi>X</k:mi><k:mo>=</k:mo><k:mi>Cl</k:mi></k:mrow></k:math>, Br, and I) using Ni-<m:math xmlns:m="http://www.w3.org/1998/Math/MathML" display="inline"><m:msub><m:mi>L</m:mi><m:mn>3</m:mn></m:msub></m:math> edge resonant inelastic x-ray scattering (RIXS). In all compounds, we detect sharp excitations, analogous to the recently reported excitons, and assign them to spin-singlet multiplets of octahedrally coordinated <o:math xmlns:o="http://www.w3.org/1998/Math/MathML" display="inline"><o:mrow><o:msup><o:mrow><o:mi>Ni</o:mi></o:mrow><o:mrow><o:mn>2</o:mn><o:mo>+</o:mo></o:mrow></o:msup></o:mrow></o:math> stabilized by intra-atomic Hund’s exchange. Additionally, we demonstrate that these excitons are dispersive using momentum-resolved RIXS. Our data evidence a ligand-mediated multiplet dispersion, which is tuned by the charge-transfer gap and independent of the presence of long-range magnetic order. This reveals the mechanisms governing nonlocal interactions of on-site <q:math xmlns:q="http://www.w3.org/1998/Math/MathML" display="inline"><q:mi>d</q:mi><q:mi>d</q:mi></q:math> excitations with the surrounding crystal or magnetic structure, in analogy to ground-state superexchange. These measurements thus establish the roles of magnetic order, self-doped ligand holes, and intersite-coupling mechanisms for the properties of <s:math xmlns:s="http://www.w3.org/1998/Math/MathML" display="inline"><s:mi>d</s:mi><s:mi>d</s:mi></s:math> excitations in charge-transfer insulators. Published by the American Physical Society 2024