Exploiting the fascinating properties of materials near soft mode phase transitions is an emerging concept in the quest to increase thermoelectric efficiency [1,2]. The underlying idea is that soft phonons may lead to low lattice thermal conductivity, while possibly preserving high electronic conductivity. In this talk I will focus on the unusual electronic transport properties of n-type PbTe, which is a classic thermoelectric material that exists near a soft optical mode phase transition. I will show that scattering of its conduction band states due to soft transverse optical phonons is by far the weakest scattering mechanism [3]. Soft phonons thus play the key role in the high thermoelectric figure of merit of n-type PbTe: they do not degrade its electronic transport properties although they strongly suppress the lattice thermal conductivity.

Another intriguing topic is the connection between thermoelectricity and topology: several topological insulators happen to be excellent thermoelectric materials, such as Bi2Te3 [4]. Topological insulators are a new form of quantum matter whose surface states are protected from backscattering due to disorder. Despite this protection, topological surface states may be scattered by phonons [5]. Here I will show our calculations of deformation potentials for surface states in Bi2Te3 due to coupling to coherent A1g modes driven by photoexcitation. Our computed deformation potential values agree well with those obtained from time-resolved ARPES measurements and time-resolved Bragg diffraction.