Alex gave a plenary lecture at the 19th International Conference on Photoacoustic and Photothermal Phenomena in Bilbao, Spain

**PHOTOACOUSTICS IN SEARCH FOR THE PHONON MEAN FREE PATH**

A. A. Maznev

Department of Chemistry, Massachusetts Institute of Technology, Cambridge MA 02139, USA.

Phonon mean free path (MFP) controls thermal transport in most non-metallic solids. Traditionally, solid state physics textbooks estimated the thermal phonon MFP, based on the elementary kinetic theory of thermal conductivity, as 3*k*/*vC _{v}*, where

*k*is the lattice thermal conductivity,

*v*speed of sound and

*C*lattice specific heat. This estimate yields, for example, a MFP of ~40 nm for Si at room temperature. However, ample experimental evidence obtained in micro/nanoscale heat transport studies has shown that the textbook estimates of the phonon MFP are oftentimes wrong. For example, room temperature thermal transport in Si has been shown to significantly deviate from the Fourier law at distances ~1 µm [1], indicating that long-MFP phonons play a much larger role in heat transport than previously thought. In recent years, the interest to the phonon MFP has greatly intensified in the context of practical applications such as thermal management of microelectronic devices and designing low thermal conductivity thermoelectric materials.

_{v}Photoacoustic and photothermal techniques have been at the forefront of the current surge of interest to phonon-mediated heat transport [2]. In this talk, I will give an overview of recent experimental work aimed at getting insight into the phonon MFP. On the one hand, photothermal techniques have been used to measure phonon size effects, with a characteristic length controlled either by the geometry of the photothermal source [1] of by the dimension of a nanostructure [3]. We will see how size effect data can be used to “reconstruct” the phonon MFP distribution. On the other hand, picosecond photoacoustic techniques have been employed to measure the phonon MFP directly. While these measurements have not yet provided phonon MFP data across the entire Brillouin zone, a number of insights into phonon-phonon and electron-phonon interactions have been obtained [4,5]. We will take a look at recent experimental developments, including those enabled by coherent x-ray and EUV sources, touch on the spectacular progress in theory which is now capable of calculating the phonon MFP from first principles, and discuss unresolved problems and challenges for future research.

[1] J. A. Johnson, A. A. Maznev, J. Cuffe, J. K. Eliason, A. J. Minnich, T. Kehoe, C. M. Sotomayor Torres, G. Chen, and K. A. Nelson, Phys. Rev. Lett. 110, 025901 (2013).

[2] D. G. Cahill, P. V. Braun, G. Chen, D. R. Clarke, S. Fan, K. E. Goodsen, P. Keblinski, W. P. King, G. D. Mahan, A. Majumdar, H. J. Maris, S. R. Phillpot, E. Pop, and L. Shi, Appl. Phys. Rev. 1, 011305 (2014).

[3] J. Cuffe, J. K. Eliason, A. A. Maznev, K. C. Collins, J. A. Johnson, A. Shchepetov, M. Prunnila, J. Ahopelto, C. M. Sotomayor Torres, G. Chen, and K. A. Nelson, Phys. Rev. B 91, 245423 (2015).

[4] R. Legrand, A. Huynh, B. Jusserand, and B. Perrin, Phys Rev. B 93, 184304 (2016).

[5] B. Liao, A. A. Maznev, K. A. Nelson, and G. Chen, Nat. Commun. 7, 13174 (2016).