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Excitonic insulators host a condensate of electron-hole pairs at equilibrium, giving rise to collective many-body effects. Although several materials have emerged as excitonic insulator candidates, evidence of long-range coherence is lacking and the origin of the ordered phase in these systems remains controversial. Here, using ultrafast pump-probe microscopy, we investigate the possible excitonic insulator Ta 2 NiSe 5 Below 328 K, we observe the anomalous micrometer-scale propagation of coherent modes at velocities of ~10 5 m/s, which we attribute to the hybridization between phonon modes and the phase mode of the condensate. We develop a theoretical framework to support this explanation and propose that electronic interactions provide a substantial contribution to the ordered phase in Ta 2 NiSe 5 These results allow us to understand how the condensate's collective modes transport energy and interact with other degrees of freedom. Our study provides a unique paradigm for the investigation and manipulation of these properties in strongly correlated materials., (Copyright © 2021 The Authors, some rights reserved; exclusive licensee American Association for the Advancement of Science. No claim to original U.S. Government Works. Distributed under a Creative Commons Attribution License 4.0 (CC BY).)

A new technique of high-resolution micro-Raman thermometry using anatase TiO2 microparticles (0.5-3 μm) is presented. These very high spatial resolution measurements (280 nm) reveal temperature gradients even within individual microparticles. Potential applications of this technique are demonstrated by probing the temperature distribution of a micro-fabricated heater consisting of a thin silicon nitride (Si-N) membrane with a gold coil on top of the membrane. Using TiO2 microparticle micro-Raman thermometry, the temperature from the outer edge of the coil to the inner portion was measured to increase by ~40 °C. These high spatial resolution microscopic measurements were also used to measure the temperature gradient within the 20 μm wide Si-N between the gold heating coils. 2D numerical simulations of the micro heater temperature distribution are in excellent agreement with the experimental measurements of the temperatures. These measurements illustrate the potential to extend applications of micro-Raman thermometry to obtain temperature details on a sub-micrometer spatial resolution by employing microparticles.