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IV-VI materials are some of the most efficient bulk thermoelectric materials due to their proximity to soft-mode phase transitions, which leads to low lattice thermal conductivity. It has been shown that the lattice thermal conductivity of PbTe can be considerably reduced by bringing PbTe closer to the phase transition e.g. via lattice expansion. However, the effect of soft phonon modes on the electronic thermoelectric properties of such system remains unknown. Using first principles calculations, we show that the soft zone center transverse optical phonons do not deteriorate the electronic thermoelectric properties of PbTe driven closer to the phase transition via lattice expansion due to external stress, and thus enhance the thermoelectric figure of merit. We find that the optical deformation potentials change very weakly as the proximity to the phase transition increases, but the population and scattering phase space of soft phonon modes increase. Nevertheless, scattering between electronic states near the band edge and soft optical phonons remains relatively weak even very near the phase transition., 8 pages, 7 figures

GeTe is a well-known ferroelectric and thermoelectric material that undergoes a structural phase transition from a rhombohedral to the rocksalt structure at $\ensuremath{\sim}600--700$ K. We present a first-principles approach to calculate the thermal expansion of GeTe in the rhombohedral phase up to the Curie temperature. We find the minimum of the Helmholtz free energy with respect to the structural parameters, including the internal atomic displacement, in a manner similar to the traditional Gr\"uneisen theory, explicitly accounting for the variation of the static elastic energy with respect to all structural parameters. We obtain the temperature dependence of the structural parameters of rhombohedral GeTe in very good agreement with experiments. In particular, we correctly reproduce a negative volumetric thermal expansion of GeTe near the phase transition. We show that the negative thermal expansion is induced by the coupling between acoustic and soft transverse optical phonons, which is also responsible for the low lattice thermal conductivity of GeTe.

Department of Education and Learning, Northern Ireland (Investigators Programme No. 15/1A/3160.)

The major obstacle in the design of materials with low lattice thermal conductivity is the difficulty in efficiently scattering phonons across the entire frequency spectrum. Using first principles calculations, we show that driving PbTe materials to the brink of the ferroelectric phase transition could be a powerful strategy to solve this problem. We illustrate this concept by applying tensile [001] strain to PbTe and its alloys with another rock-salt IV-VI material, PbSe; and by alloying PbTe with a rhombohedral IV-VI material, GeTe. This induces extremely soft optical modes at the zone center, which increase anharmonic acoustic-optical coupling and decrease phonon lifetimes at all frequencies. We predict that PbTe, Pb(Se,Te) and (Pb,Ge)Te alloys driven close to the phase transition in the described manner will have considerably lower lattice thermal conductivity than that of PbTe (by a factor of 2-3). The proposed concept may open new opportunities for the development of more efficient thermoelectric materials., 9 pages, 3 figures

It has been argued that the experimentally observed limitation of heat transport through boron nitride nanotubes is due to intershell scattering rather than to inefficient heat transfer to inner shells. Using an atomistic Green's function calculation, we present evidence that on the contrary, intershell or any other type of scattering along the nanotubes is not the main limiting mechanism, and heat conduction restricted to a few layers is responsible for the low thermal conductivities experimentally measured. Our results also indicate that anharmonic scattering in boron nitride is relatively weak, which might lead to considerably larger thermal conductivity for well-contacted nanotubes than previously reported.

The intraband optical properties of quantum dots comprising one electron and up to two Mn ions, subjected to an axial magnetic field, have been investigated theoretically using the configuration-interaction method. We have shown that the electron-Mn and the $\mathrm{Mn}\text{\ensuremath{-}}\mathrm{Mn}$ exchange interaction mechanisms induce splitting of the energy levels for zero magnetic field, and lead to ground-state transitions and anticrossing of the states for finite magnetic fields. This results in the presence of interesting effects in the intraband absorption spectrum, such as the appearance of new absorption lines and anticrossing features, absent in the single-particle spectrum. Our analysis predicts that both the electron--Mn ion and the $\mathrm{Mn}\text{\ensuremath{-}}\mathrm{Mn}$ exchange interactions may be probed by means of intraband magneto-optical spectroscopy.

A theoretical model of electron transport in quantum cascade lasers subjected to a magnetic field is developed. The Landau level electronic structure was calculated from the envelope-function Schr\"odinger equation within the effective-mass approximation. The electron transport in a magnetic field was modeled using the self-consistent rate-equation description for the full period of the cascade and its interaction with adjacent periods. The scattering processes included in the model are electron\char21{}longitudinal-optical-phonon, electron\char21{}longitudinal-acoustic-phonon, and electron-electron scattering. All these processes show oscillatory behavior with magnetic field, and their interplay determines the electron transport and the output characteristics of quantum cascade lasers in magnetic field. The model was applied to investigate the influence of magnetic field on the performance of a $\mathrm{GaAs}∕\mathrm{AlGaAs}$ quantum cascade laser emitting at $\ensuremath{\lambda}\ensuremath{\approx}11.4\phantom{\rule{0.3em}{0ex}}\mathrm{\ensuremath{\mu}}\mathrm{m}$ [P. Kruck et al., Appl. Phys. Lett. 76, 3340 (2000)]. The calculated results show good overall agreement with the available experimental data.