Abstract :

Microscopic quantum mechanical interactions among energy carriers such as phonons and electrons govern the macroscopic thermal and electronic properties of crystalline solids, such as thermal and electrical conductivity, and electronic mobility. Understanding the interactions of energy carriers among themselves and with material imperfections is essential to discover new materials and engineer new devices for a wide variety of energy applications such as thermoelectrics, optoelectronics and energy storage.
In this talk, I will first describe my experimental research at Caltech to answer an important nanoscale phonon transport problem that has remained unsolved for decades: “Do THz-frequency thermal phonons reflect specularly from atomically rough surfaces, thereby preserving their phase? Or do they scatter diffusely and lose it?”. By implementing a non-contact optical experiment called the transient grating (TG) on suspended thin silicon (Si) membranes, and by first-principles analysis of the TG experimental data, I will show that thermal phonons are exquisitely sensitive to the surface roughness of just a few atomic planes on the Si membrane, and that our experimental and computational machinery enables us to obtain the first measurements of the specular phonon reflection probability as a spectral function of phonon wavelength [1].
Next, I will discuss my computational research at Boston College, where I am developing new first-principles tools to analyze the thermal and electronic properties of novel materials, for which the conventional phonon theory fails drastically. As an example, I will describe a curious case of thermal transport in boron arsenide (BAs), where the lowest order scattering processes involving three phonons are unusually weak and four-phonon scattering due to higher-order anharmonicity affects the thermal conductivity significantly [2]. I will also show that this competition between three and four-phonon scattering results in an unusual non-monotonic pressure dependence of the thermal conductivity in BAs [3].
Finally, I will motivate my future research directions using our recent first principles prediction of anomalously strong electron-phonon coupling in metals with nested Fermi surfaces [4] and the role of non-equilibrium phonon populations and four-phonon scattering in affecting the ultrafast carrier dynamics and steady-state carrier transport in common III-V semiconductors.

[1] Navaneetha K. Ravichandran, Hang Zhang & Austin Minnich, Physical Review X 8 (4), 041004, 2018
[2] Fei Tian, Bai Song, Xi Chen, Navaneetha K. Ravichandran et al., Science 361 (6402), 582-585, 2018
[3] Navaneetha K. Ravichandran & David Broido, Nature Communications 10 (827), 2019
[4] Chunhua Li, Navaneetha K. Ravichandran, Lucas Lindsay & David Broido, _Physical Review Letters 121 (17), 175901, 2018

Here is my brief bio:
I obtained my Dual Degree (B. Tech and M. Tech) in Mechanical Engineering from the Indian Institute of Technology, Madras. I obtained my Masters in Space Engineering and PhD in Mechanical Engineering from Caltech, working with Prof. Austin Minnich. For my PhD, I worked on experimentally investigating phonon boundary scattering in thin silicon membranes using the transient grating experiment. I am currently a postdoctoral fellow at Boston College, where I am working with Prof. David Broido on developing a predictive first-principles computational framework to capture the thermal and electronic properties of materials at high temperatures and extreme environmental conditions.