“Despite the extremely small length scales and fast timescales involved, we are now in a position to record ‘movies’ of phase transitions developing in quantum materials in real space and real time,” said first author Aaron Sternbach, a former Columbia physics PhD student and postdoc who is now an assistant professor studying optically driven quantum materials at the University of Maryland.
VO2 can shift between an insulator and a metal at close to room temperature, which makes it a promising candidate for practical uses in technology. One trigger for the transition is light, which Sternbach and his colleagues showed is not an instantaneous switch. Rather, they found that the transition occurs by forming tiny patches of metal that then grow and merge nonuniformly across the sample—albeit, in just trillionths of a second.
The movies Sternbach and his colleagues recorded with their experimental setup enabled them to visualize the insulator-to-metal transition developing in select regions and growing throughout the sample. "It is desirable to drive the phase transition with the smallest amount of energy possible." he explained "However, when a minimum of energy is used, metal actually forms more efficiently in tiny regions of the sample. It is very exciting that we can now see this happening.”
Co-author Andrew Millis, a theoretical physicist at Columbia, noted that the results are exciting on many levels. “The new ability to see how insulator-to-metal transitions actually happen will be invaluable in the design and prototyping of actual devices,” he said. “The results obtained on this material provide a fascinating challenge to our theoretical understanding of the transition, which has heretofore been based on models of spatially homogeneous materials.”
Read More: A. J. Sternbach, T. Slusar, F. L. Ruta, S. Moore, X. Chen, M. K. Liu, H. T. Kim, A. J. Millis, R. D. Averitt, and D. N. Basov. Inhomogeneous Photosusceptibility of VO2 Films at the Nanoscale. Physical Review Letters (2024). DOI: 10.1103/PhysRevLett.132.186903