Defect-Mediated Nucleation and Dynamics across the Phase Transition in the Excitonic Insulator Candidate Ta2NiSe5

Materials are rarely static; they breathe, shift, and sometimes undergo sudden transformations that change how they conduct electricity or interact with light. In the world of condensed matter physics, scientists study these changes, known as phase transitions, to understand how the microscopic arrangement of atoms dictates the behavior of the macroscopic world. Some transitions happen smoothly, like ice melting into water, where the change is uniform across the entire sample. Others are more abrupt and complex, involving the sudden appearance of new structures that grow and spread, much like cracks forming in a frozen lake. One material that has sparked intense debate among researchers is Ta2NiSe5, a crystal that sits at the boundary between a semiconductor and a semimetal. At a specific temperature of roughly 326 Kelvin, this material is supposed to switch from a low-temperature state, where it acts as a semiconductor, to a high-temperature state where it conducts electricity more freely. For years, scientists have argued over the nature of this switch: does it happen as a smooth, continuous evolution across the whole material, or does it proceed through the nucleation and growth of distinct regions, a process often described as martensitic-like?

To settle this question, a team of researchers from the Federal University of Minas Gerais in Brazil turned their attention to the surface of Ta2NiSe5 crystals. Instead of looking at the material as a whole, which averages out local details, they used a technique called scanning tunneling microscopy and spectroscopy. This method allows them to map the surface of a material with atomic precision, measuring not just the shape of the atoms but also the electronic states available to them at every single point. By heating the sample in small increments and watching what happened in real space, they could see exactly how the transition unfolded, rather than just inferring it from bulk measurements. Their work reveals that the transition is far from uniform. Instead of the entire crystal changing at once, the material exists in a state of coexistence where islands of the old structure and the new structure sit side by side, separated by sharp boundaries that move and evolve over time.

The researchers focused on three specific types of locations on the crystal surface: perfectly clean areas, spots near isolated point defects where an atom is missing, and the edges of steps where the surface drops down. In the pristine, defect-free regions, they observed that the new high-temperature phase did not appear randomly. Instead, it nucleated, or began to form, in a highly directional manner. The new phase grew preferentially perpendicular to the long chains of atoms that make up the crystal structure. It was as if the transformation was constrained by the material's own internal geometry, spreading out in a specific direction while respecting the alignment of the atomic chains. This anisotropic growth confirmed that the transition is not a simple, uniform shift but a complex process governed by the local arrangement of atoms.

However, the story changed dramatically when the researchers looked at areas with imperfections. Near a single point defect, where a missing atom creates a local disruption in the lattice, the behavior was entirely different. The defect acted as a powerful trigger, causing the new phase to nucleate right around it and spread outward in all directions, effectively breaking the directional constraint seen in the clean areas. The transition happened much faster here, with the new phase covering a larger area in the same amount of time compared to the pristine regions. Similarly, at the edges of steps in the crystal surface, the new phase began to form along the edge, but the growth was asymmetric. The phase spread more readily into the upper terrace of the step than the lower one, suggesting that the physical boundary of the step itself influenced how the transformation moved.

By tracking these changes over time, the team mapped the evolution of the material with a level of detail previously impossible. They found that at the transition temperature, the material does not instantly flip from one state to another. Instead, it lingers in a mixed state for extended periods, sometimes over an hour, with distinct domains of the old and new phases separated by well-defined walls. These walls move slowly, driven by a process of nucleation and growth rather than a continuous, smooth change. The researchers measured the fraction of the surface covered by each phase at different times and found that the speed of the transition depended heavily on the local environment. Defects accelerated the process significantly, while step edges introduced a delay and an asymmetry in how the new phase spread.

This detailed view of the surface transition challenges the traditional view of the material's behavior. While bulk measurements, which average over the entire volume of the crystal, suggest a smooth, second-order transition, the surface reveals a much more dynamic and heterogeneous reality. The presence of distinct domains and the time-dependent evolution of their boundaries point toward a martensitic-like mechanism, where the transformation proceeds through the formation and expansion of specific regions rather than a uniform shift. The study demonstrates that local disorder, such as missing atoms or surface steps, plays a critical role in lowering the energy barrier for the new phase to form, effectively acting as seeds for the transformation.

The implications of these findings extend beyond just understanding one specific material. They highlight the importance of looking at the nanoscale details of phase transitions, where the rules can differ significantly from what is observed in the bulk. For materials like Ta2NiSe5, which are candidates for use in advanced electronic devices such as polarization-sensitive photodetectors, knowing how defects and surface conditions influence the transition is crucial. The ability to control or predict where and how these transitions occur could be key to designing more efficient and reliable devices. By revealing the true, messy, and dynamic nature of the phase transition, this work provides a clearer picture of how complex quantum materials behave, moving the field from abstract theories to concrete, visualized reality.