Spin dynamics and possible topological magnons in the nonstoichiometric pyrochlore iridate Tb2Ir2O7 studied by RIXS

We report on a resonant inelastic x-ray scattering study on a single crystal of a nonstoichiometric pyrochlore iridate <a:math xmlns:a="http://www.w3.org/1998/Math/MathML"><a:mrow><a:msub><a:mi>Tb</a:mi><a:mrow><a:mn>2</a:mn><a:mo>+</a:mo><a:mi>x</a:mi></a:mrow></a:msub><a:msub><a:mi>Ir</a:mi><a:mrow><a:mn>2</a:mn><a:mo>−</a:mo><a:mi>x</a:mi></a:mrow></a:msub><a:msub><a:mi mathvariant="normal">O</a:mi><a:mrow><a:mn>7</a:mn><a:mo>−</a:mo><a:mi>y</a:mi></a:mrow></a:msub></a:mrow></a:math> (<c:math xmlns:c="http://www.w3.org/1998/Math/MathML"><c:mrow><c:mi>x</c:mi><c:mo>≃</c:mo><c:mn>0.25</c:mn></c:mrow></c:math>) that magnetically orders at <d:math xmlns:d="http://www.w3.org/1998/Math/MathML"><d:mrow><d:msub><d:mi>T</d:mi><d:mi mathvariant="normal">N</d:mi></d:msub><d:mo>≃</d:mo><d:mn>50</d:mn></d:mrow></d:math> K. We observe a propagating gapped magnon mode at low energy and model it using a Hamiltonian consisting of a Heisenberg exchange <f:math xmlns:f="http://www.w3.org/1998/Math/MathML"><f:mrow><f:mo>[</f:mo><f:mi>J</f:mi><f:mo>=</f:mo><f:mn>16.2</f:mn><f:mo>(</f:mo><f:mn>9</f:mn><f:mo>)</f:mo><f:mspace width="0.16em"/><f:mi>meV</f:mi><f:mo>]</f:mo></f:mrow></f:math> and Dzyaloshinskii-Moriya interactions <h:math xmlns:h="http://www.w3.org/1998/Math/MathML"><h:mrow><h:mo>[</h:mo><h:mi>D</h:mi><h:mo>=</h:mo><h:mn>5.2</h:mn><h:mo>(</h:mo><h:mn>3</h:mn><h:mo>)</h:mo><h:mspace width="0.16em"/><h:mi>meV</h:mi><h:mo>]</h:mo></h:mrow></h:math>, which shows the robustness of interactions despite Tb stuffing at the Ir site. Strikingly, the ratio <j:math xmlns:j="http://www.w3.org/1998/Math/MathML"><j:mrow><j:mi>D</j:mi><j:mo>/</j:mo><j:mi>J</j:mi><j:mo>=</j:mo><j:mn>0.32</j:mn><j:mo>(</j:mo><j:mn>3</j:mn><j:mo>)</j:mo></j:mrow></j:math> supports possible nontrivial topological magnon band crossing. This material may thus host coexisting fermionic and bosonic topology, with potential for manipulating electronic and magnonic topological bands thanks to the <k:math xmlns:k="http://www.w3.org/1998/Math/MathML"><k:mrow><k:mi>d</k:mi><k:mtext>−</k:mtext><k:mi>f</k:mi></k:mrow></k:math> interaction.