Phononic soft mode behavior and a strong electronic background across the structural phase transition in the excitonic insulator <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"><mml:mrow><mml:msub><mml:mi>Ta</mml:mi><mml:mn>2</mml:mn></mml:msub><mml:mi>Ni</mml:mi><mml:msub><mml:mi>Se</mml:mi><mml:mn>5</mml:mn></mml:msub></mml:mrow></mml:math>
Min-Jae Kim, Armin Schulz, T. Takayama, Masahiko Isobe, H. Takagi, S. Kaiser
Abstract
Ta 2 NiSe 5 became one of the most investigated candidate materials for hosting an excitonic insulator ground state. Many studies describe the corresponding phase transition as a condensation of excitons breaking a continuous symmetry. This view got challenged recently pointing out the importance of the loss of two mirror symmetries at a structural phase transition that occurs together with the semiconductor-excitonic insulator transition. For such a scenario an unstable optical zone-center phonon at low energy is proposed to drive the transition. Here we report on the experimental observation of such a soft mode behavior using Raman spectroscopy. In addition we find a novel spectral feature, likely of electronic or joint electronic and phononic origin, that is clearly distinct from the lattice dynamics and that becomes dominant at T c . This suggests a picture of joint structural and electronic orders driving the phase transition.