Defect-Mediated Nucleation and Dynamics across the Phase Transition in the Excitonic Insulator Candidate Ta2NiSe5
Ta2NiSe5 is a quasi-one-dimensional material that exhibits a structural and electronic phase transition from a low-temperature monoclinic (semiconductor) to a high-temperature orthorhombic (semimetal) phase at approximately TC = 326 K. Here, we used variable-temperature scanning tunneling microscopy and spectroscopy to resolve the phase transition spatially, identifying the distinct spectroscopic signatures of the monoclinic and orthorhombic phases in pristine regions and near isolated point-defects and step edges. Although the phase transition of Ta2NiSe5 is generally regarded as second-order, it has previously been described as exhibiting martensitic-like characteristics. This implies that the transformation may proceed via spatial phase coexistence and domain boundaries rather than through a continuous evolution. Our surface-sensitive measurements confirm this scenario, retrieving the coexistence and evolution of monoclinic and orthorhombic domains in real space. Upon heating through TC, we find that the two phases coexist as spatially segregated regions over extended timescales, separated by well-defined boundaries that evolve via localized nucleation and growth, rather than a spatially uniform transformation. In pristine regions, the orthorhombic phase nucleates anisotropically, perpendicular to the Ta-Ni-Ta chains, whereas point-defects and step edges act as local nucleation centers that promote the transition and suppress this intrinsic anisotropy. These results provide direct real-space visualization of how surface-specific structural and electronic variations, together with local disorder, modify the martensitic-like phase transition in Ta2NiSe5.