Anisotropic hybridization dynamics in the quasi-one-dimensional Kondo lattice CeCo$lt;sub$gt;2$lt;/sub$gt;Ga$lt;sub$gt;8$lt;/sub$gt; revealed by ultrafast optical spectroscopy

We investigate the ultrafast dynamics of the quasi-one-dimensional Kondo lattice CeCo₂Ga₈ using optical pump-probe spectroscopy. Time-resolved pump-probe reflectivity measurements reveal a strong anisotropy in the photoinduced response, which is a direct consequence of the material&#x2019;s unique electronic structure. The temperature dependence of the relaxation dynamics provides evidence for the formation of two distinct hybridization gaps that appear at different temperatures in the heavy fermion state. A direct gap of <inline-formula><math xmlns:mml="http://www.w3.org/1998/Math/MathML" altimg="c:\heptemp\HML208361145.png" baseline="-3.5" display="inline" id="mathml_M1" overflow="scroll"><mml:mrow class="MJX-TeXAtom-ORD"> <mml:mstyle><mml:mrow><mml:mn>2</mml:mn><mml:msub><mml:mrow><mml:mi mathvariant="normal">&#x0394;</mml:mi></mml:mrow><mml:mrow class="MJX-TeXAtom-ORD"><mml:mi>d</mml:mi><mml:mi>i</mml:mi><mml:mi>r</mml:mi></mml:mrow></mml:msub><mml:mo>&#x2248;</mml:mo></mml:mrow></mml:mstyle></mml:mrow></math></inline-formula> 50 meV that persists up to <inline-formula><math xmlns:mml="http://www.w3.org/1998/Math/MathML" altimg="c:\heptemp\HML20836958.png" baseline="-0.5" display="inline" id="mathml_M3" overflow="scroll"><mml:mrow class="MJX-TeXAtom-ORD"> <mml:mstyle mathcolor="khaki"><mml:msup><mml:mi>T</mml:mi><mml:mo>&#x2020;</mml:mo></mml:msup><mml:mo>&#x2248;</mml:mo></mml:mstyle></mml:mrow></math></inline-formula> 90 K, well above the coherence temperature <inline-formula><math xmlns:mml="http://www.w3.org/1998/Math/MathML" altimg="c:\heptemp\HML208361146.png" baseline="-0.5" display="inline" id="mathml_M5" overflow="scroll"> <mml:mrow class="MJX-TeXAtom-ORD"><mml:mstyle><mml:msup><mml:mi>T</mml:mi><mml:mo>&#x2217;</mml:mo></mml:msup><mml:mo>&#x2248;</mml:mo></mml:mstyle></mml:mrow></math></inline-formula> 20 K. We attribute this higher-temperature gap to the hybridization fluctuations. An indirect gap of <inline-formula><math xmlns:mml="http://www.w3.org/1998/Math/MathML" altimg="c:\heptemp\HML208361147.png" baseline="-3.5" display="inline" id="mathml_M7" overflow="scroll"><mml:mrow class="MJX-TeXAtom-ORD"> <mml:mstyle><mml:mn>2</mml:mn><mml:msub><mml:mi mathvariant="normal">&#x0394;</mml:mi> <mml:mrow class="MJX-TeXAtom-ORD"><mml:mi>i</mml:mi><mml:mi>n</mml:mi><mml:mi>d</mml:mi></mml:mrow></mml:msub><mml:mo>&#x2248;</mml:mo></mml:mstyle></mml:mrow></math></inline-formula> 14 meV opens closer to <inline-formula><math xmlns:mml="http://www.w3.org/1998/Math/MathML" altimg="c:\heptemp\HML20836961.png" baseline="-0.5" display="inline" id="mathml_M9" overflow="scroll"><mml:mrow class="MJX-TeXAtom-ORD"> <mml:mstyle mathcolor="khaki"><mml:msup><mml:mi>T</mml:mi><mml:mo>&#x2217;</mml:mo></mml:msup></mml:mstyle></mml:mrow></math></inline-formula>, signifying the development of long-range coherence in the heavy fermion state. Furthermore, we find that the hybridization gap can be suppressed with increasing pump fluence, indicating a delicate interplay between photoexcitation and the coherent heavy fermion state. Our results provide insights into the interplay of Kondo physics and low dimensionality in CeCo₂Ga₈, and establish ultrafast optical spectroscopy as a sensitive probe of anisotropic hybridization in heavy fermion materials.