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.