Dr. Navaneetha Ravichandran\, Boston College

\nTitle: Phonon scattering from material boundaries and higher-ord er anharmonicity

\nAbstract:

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

In the first p art of my talk\, I will describe my experimental research at Caltech to an swer an important nanoscale phonon transport problem that has remained uns olved for decades: “Do THz-frequency thermal phonons reflect specularly fr om atomically rough surfaces\, thereby preserving their phase? Or do they scatter diffusely and lose it?”. By implementing a novel non-contact optic al experiment called the transient grating (TG) on suspended thin silicon (Si) membranes\, and by rigorous first-principles analysis of the TG exper imental 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.

\nIn the second part of my talk\, I will discuss my computational research at Boston College\, where I am d eveloping new first-principles tools to analyze the thermal properties of novel materials\, for which the conventional phonon theory fails drastical ly. I will begin by describing a curious case of thermal transport in boro n arsenide (BAs)\, where the lowest order scattering processes involving t hree phonons are unusually weak and four-phonon scattering due to higher-o rder 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 r eason: the unusually strong anharmonic bonds in NaCl. I will show that the phonons interact so strongly in NaCl that they invalidate the Peierls-Bol tzmann description of phonon transport\, even below half of the melting te mperature. To address this issue\, I have developed a new phonon renormali zation approach based on many-body theory\, which creates new “dressed-up” quasi-particles that interact weakly to admit the Peierls-Boltzmann treat ment of heat conduction. I will show that our new phonon renormalization a pproach along with higher-order four-phonon scattering enables us to get g ood agreement with several temperature-dependent measurements of phonon di spersions\, thermal expansion and thermal conductivity simultaneously.

\nb io:

\nI 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 o
n experimentally investigating phonon boundary scattering in thin silicon
membranes using the transient grating experiment. I am currently a postdoc
toral fellow at Boston College\, where I am working with Prof. David Broid
o on developing a rigorous predictive first-principles computational tool
that simultaneously works for multiple thermal and thermodynamic propertie
s of strongly anharmonic materials.

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