Dr. Navaneetha Ravichandran, Boston College

Title: Phonon scattering from material boundaries and higher-order anharmonicity

Abstract:

Phonons, which are quantized lattice vibrations, govern the thermal and thermodynamic properties of crystalline solids. Understanding phonon properties is essential to engineer new materials for a wide variety of energy applications such as thermoelectrics, superconductors, energy storage etc., and has been a topic of intense research interest over the past several decades.

In the first part of my talk, I will 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 novel non-contact optical experiment called the transient grating (TG) on suspended thin silicon (Si) membranes, and by rigorous 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.

In the second part of my talk, I will discuss my computational research at Boston College, where I am developing new first-principles tools to analyze the thermal properties of novel materials, for which the conventional phonon theory fails drastically. I will begin by describing 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. Finally I will talk about phonons in sodium chloride (NaCl), where, once again, the conventional phonon theory fails drastically, but for a different reason: the unusually strong anharmonic bonds in NaCl. I will show that the phonons interact so strongly in NaCl that they invalidate the Peierls-Boltzmann description of phonon transport, even below half of the melting temperature. To address this issue, I have developed a new phonon renormalization approach based on many-body theory, which creates new “dressed-up” quasi-particles that interact weakly to admit the Peierls-Boltzmann treatment of heat conduction. I will show that our new phonon renormalization approach along with higher-order four-phonon scattering enables us to get good agreement with several temperature-dependent measurements of phonon dispersions, thermal expansion and thermal conductivity simultaneously.

bio:

I am originally from India. I obtained my undergraduate degree from the Indian Institute of Technology, Madras. I obtained my Masters and PhD 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 rigorous predictive first-principles computational tool that simultaneously works for multiple thermal and thermodynamic properties of strongly anharmonic materials.