Spectral line

Quantum mechanics

Soft modes

Raman spectroscopy

Materials science

Phase transition

Electronic structure

Phonon

Semiconductor

Ferroelectricity

Physics

Exciton

Dielectric

Mathematics

Homogeneous space

Condensed matter physics

Geometry

Molecular electronic transition

Optoelectronics

Insulator (electricity)

Physical Review Research

Ta2NiSe5 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 Tc. This suggests a picture of joint structural and electronic orders driving the phase transition.Received 10 July 2020Revised 30 October 2020Accepted 11 November 2020DOI:https://doi.org/10.1103/PhysRevResearch.2.042039Published by the American Physical Society under the terms of the Creative Commons Attribution 4.0 International license. Further distribution of this work must maintain attribution to the author(s) and the published article's title, journal citation, and DOI. Open access publication funded by the Max Planck Society.Published by the American Physical SocietyPhysics Subject Headings (PhySH)Research AreasExcitonsPhase transitionsTechniquesRaman spectroscopyCondensed Matter & Materials Physics

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>

Phononic soft mode behavior and a strong electronic background across the structural phase transition in the excitonic insulator Ta2NiSe5