Anisotropic excitonic magnetism from discrete C4 symmetry in CeRhIn5

Anisotropy in strongly correlated materials is a central parameter in determining the electronic ground state and is tuned through the local crystalline electric field. This is notably the case in the <a:math xmlns:a="http://www.w3.org/1998/Math/MathML"><a:mrow><a:msub><a:mi>CeCo</a:mi><a:mi>x</a:mi></a:msub><a:msub><a:mi>Rh</a:mi><a:mrow><a:mn>1</a:mn><a:mo>−</a:mo><a:mi>x</a:mi></a:mrow></a:msub><a:msub><a:mi>In</a:mi><a:mn>5</a:mn></a:msub></a:mrow></a:math> system where the ground-state wave function can provide the basis for antiferromagnetism and/or unconventional superconductivity. We develop a methodology to understand the local magnetic anisotropy and experimentally investigate with neutron spectroscopy applied to antiferromagnetic (<b:math xmlns:b="http://www.w3.org/1998/Math/MathML"><b:mrow><b:msub><b:mi>T</b:mi><b:mi>N</b:mi></b:msub><b:mo>=</b:mo><b:mn>3.8</b:mn><b:mspace width="0.16em"/><b:mi mathvariant="normal">K</b:mi></b:mrow></b:math>) <e:math xmlns:e="http://www.w3.org/1998/Math/MathML"><e:msub><e:mi>CeRhIn</e:mi><e:mn>5</e:mn></e:msub></e:math>, which is isostructural to <f:math xmlns:f="http://www.w3.org/1998/Math/MathML"><f:mi>d</f:mi></f:math>-wave superconducting (<g:math xmlns:g="http://www.w3.org/1998/Math/MathML"><g:mrow><g:msub><g:mi>T</g:mi><g:mi>c</g:mi></g:msub><g:mo>=</g:mo><g:mn>2.3</g:mn><g:mspace width="0.16em"/><g:mi mathvariant="normal">K</g:mi></g:mrow></g:math>) <j:math xmlns:j="http://www.w3.org/1998/Math/MathML"><j:msub><j:mi>CeCoIn</j:mi><j:mn>5</j:mn></j:msub></j:math>. Through diagonalizing the local crystal field Hamiltonian with discrete tetragonal <k:math xmlns:k="http://www.w3.org/1998/Math/MathML"><k:msub><k:mi>C</k:mi><k:mn>4</k:mn></k:msub></k:math> point group symmetry and coupling these states with the random phase approximation, we find two distinct modes polarized along the crystallographic <l:math xmlns:l="http://www.w3.org/1998/Math/MathML"><l:mi>c</l:mi></l:math> and <m:math xmlns:m="http://www.w3.org/1998/Math/MathML"><m:mrow><m:mi>a</m:mi><m:mo>−</m:mo><m:mi>b</m:mi></m:mrow></m:math> planes, agreeing with experiment. The anisotropy and bandwidth, underlying the energy scale of these modes, are tuneable with a magnetic field which we use experimentally to separate in energy single and multiparticle excitations thereby demonstrating the instability of excitations polarized within the crystallographic <n:math xmlns:n="http://www.w3.org/1998/Math/MathML"><n:mrow><n:mi>a</n:mi><n:mo>−</n:mo><n:mi>b</n:mi></n:mrow></n:math> plane in <o:math xmlns:o="http://www.w3.org/1998/Math/MathML"><o:msub><o:mi>CeRhIn</o:mi><o:mn>5</o:mn></o:msub></o:math>. We compare this approach to a <p:math xmlns:p="http://www.w3.org/1998/Math/MathML"><p:mrow><p:msub><p:mi>S</p:mi><p:mi>eff</p:mi></p:msub><p:mo>=</p:mo><p:mfrac><p:mn>1</p:mn><p:mn>2</p:mn></p:mfrac></p:mrow></p:math> parametrizations and argue for the need to extend conventional SU(2) theories of magnetic excitations to utilize the multilevel nature of the underlying crystal-field basis states constrained by the local point-group <q:math xmlns:q="http://www.w3.org/1998/Math/MathML"><q:msub><q:mi>C</q:mi><q:mn>4</q:mn></q:msub></q:math> symmetry. Published by the American Physical Society 2024