Fariborz Kargar1 Elias Penilla2 Ece Aytan3 Jacob Lewis3 Javier Garay2 Alexander Balandin1

1, University of California, Riverside, Riverside, California, United States
2, University of California, San Diego, San Diego, California, United States
3, University of California, Riverside, Riverside, California, United States

An ability to modify the propagation of acoustic phonons – the main heat carriers in semiconductor and electrical insulator materials – has important implications for thermoelectric devices and thermal management of electronics [1]. The thermal transport can be strongly affected in nanostructured or doped materials via the changed phonon – boundary and phonon – point defect scattering rates. However, the thermal conductivity can also be altered via the changes in the phonon group velocity [1]. In this presentation, we show on the example of neodymium (Nd) doped sapphire (Al2O3), that substitution of Al atoms with much heavier Nd atoms results in a noticeable decrease in the acoustic phonon group velocity. The samples selected for this study had different amount of Nd ion substitution (x= 0, 0.1 wt% and 0.25 wt%) [2]. The acoustic phonon spectra for each sample were measured directly using the Brillouin-Mandelstam spectroscopy (BMS) at room temperature [3]. All spectra were excited with a continuous wave solid-state diode-pumped laser operating at 532 nm wavelength. The measurements were carried out at 180ο backscattering configuration. The scattered light was collected by the same lens and directed to the six-pass tandem Fabry-Perot interferometer. The polarization of the incident beam was in the plane of the axis normal to the sample and the direction of the incident beam (p-polarized). Our BMS results clearly show that with the increase in the Nd doping, the frequency of both longitudinal acoustic (LA) and transverse acoustic (TA) phonon modes, at fixed phonon wave-vector, decreases, indicating the change in the phonon group velocities. The phonon velocity modification is in line with the preliminary thermal conductivity data. Our results suggest that phonon spectrum engineering via substitutional doping can become an important tool for tuning the thermal conductivity even of the bulk materials. This capability complements a conventional approach of changing thermal conductivity via the phonon scattering rates, and it can have important implications for thermoelectric energy conversion.

This work was made possible by the Spins and Heat in Nanoscale Electronic Systems (SHINES), an Energy Frontier Research Center funded by the U.S. Department of Energy (DOE), Office of Science, Basic Energy Sciences (BES) under Award # SC0012670. AAB also acknowledges National Science Foundation (NSF) grant #1404967 on defect engineering in materials.

[1] A. A. Balandin and D. L. Nika, “Phononics in low-dimensional materials,” Material Today, 15, 266 (2012).
[2] S. Ramirez, et al., “Thermal and magnetic properties of nanostructured densified ferrimagnetic composites with graphene - graphite fillers,” Materials Design, 118, 75 (2017).
[3] F. Kargar, et al., “Direct observation of confined acoustic phonon polarization branches in free-standing semiconductor nanowires,” Nature Communications, 7, 13400 (2016).