Observation of an amplitudelike mode in the excitonic insulator Ta 2 Pd 3 Te 5

This is the authors' abstract. We don't add key points for this paper.

The excitonic insulator candidate <a:math xmlns:a="http://www.w3.org/1998/Math/MathML"> <a:mrow> <a:msub> <a:mi>Ta</a:mi> <a:mn>2</a:mn> </a:msub> <a:msub> <a:mi>Pd</a:mi> <a:mn>3</a:mn> </a:msub> <a:msub> <a:mi>Te</a:mi> <a:mn>5</a:mn> </a:msub> </a:mrow> </a:math> has emerged as a promising platform to explore macroscopic quantum condensation and its collective excitations. Using polarization-resolved Raman spectroscopy, we identify a symmetry-selective electronic mode that softens and undergoes linewidth renormalization across the characteristic temperature <b:math xmlns:b="http://www.w3.org/1998/Math/MathML"> <b:mrow> <b:msup> <b:mi>T</b:mi> <b:mo>*</b:mo> </b:msup> <b:mo>∼</b:mo> <b:mn>100</b:mn> <b:mspace width="0.16em"/> <b:mi mathvariant="normal">K</b:mi> </b:mrow> </b:math> . This mode, most prominent in the <e:math xmlns:e="http://www.w3.org/1998/Math/MathML"> <e:msub> <e:mi>B</e:mi> <e:mrow> <e:mn>3</e:mn> <e:mi>g</e:mi> </e:mrow> </e:msub> </e:math> channel, is assigned to vestigial collective excitations associated with excitonic correlations, as evidenced by its order-parameter-like temperature evolution and symmetry dependence. In parallel, a damped Drude-like response exhibits a suppressed scattering rate and maximum intensity at <f:math xmlns:f="http://www.w3.org/1998/Math/MathML"> <f:msup> <f:mi>T</f:mi> <f:mo>*</f:mo> </f:msup> </f:math> , while phonon intensities display symmetry-dependent anomalies, signaling anisotropic band reconstructions and the spontaneous opening of a collective condensation gap. These observations provide compelling spectroscopic evidence for collective amplitude excitations in <g:math xmlns:g="http://www.w3.org/1998/Math/MathML"> <g:mrow> <g:msub> <g:mi>Ta</g:mi> <g:mn>2</g:mn> </g:msub> <g:msub> <g:mi>Pd</g:mi> <g:mn>3</g:mn> </g:msub> <g:msub> <g:mi>Te</g:mi> <g:mn>5</g:mn> </g:msub> </g:mrow> </g:math> , establishing this material as a bulk excitonic insulator.