ICT2014 Abstracts

Chemical bonds are one the most fundamental factors in solids that bridge and govern the structure and property relation. It is well known that chemical bonds have strong influence on electronic band structure of semiconductors, which, in turn, control the electron transport, or electrical conductivity. A parallel but much less studied and understood topic is their impact on the behavior of lattice dynamics and phonon transport of semiconductors. Recently we have shown that a delicate competition of ionic and covalent bonding character produces partially localized low-frequency phonon modes, a soft transverse optical phonon mode, and a positive temperature coefficient for the band gap in PbTe [1]. In this talk, I will present our latest studies that reveal atomistic mechanisms leading to lattice dynamics found in experiments of Bi₁₄Rh₃I₉, a narrow bandgap topological insulator, which has a very unique insulating phonon blocking layer with BiI4 nanowire array. Extremely weak chemical bonds between the nanowires cause low-lying intralayer vibrational modes down to 15 cm^-1 and large Grüneisen parameters. A second case in point is CdSb that has one weak Cd-Sb bond per Sb due to the unbalanced electron shell. There exists a cleavage plane along the weak bonds. The optic mode for the cleaving motion is at 27 cm^-1. The longitudinal acoustic phonons interact with the optic mode and low room temperature thermal conductivity ~ 1.0 W/m-K is obtained.

1. Yi Zhang, Xuezhi Ke, Paul R. C. Kent, Jihui Yang, and Changfeng Chen, "Anomalous Lattice Dynamics near the Ferroelectric Instability in PbTe", Phys. Rev. Lett. 107, 175503 (2011).