Institute of Fundamental Technological Research

We calculated the structural, electronic, and magnetic properties of FeSe within density-functional theory at
the generalized gradient approximation level. First, we studied how the bandwidth of the d-bands at the Fermi
energy is renormalized by adding simple corrections: Hubbard U , Hund’s J, and by introducing long-range
magnetic orders. We found that introducing either a striped or a staggered dimer antiferromagnetic order brings
the bandwidths—which are starkly overestimated at the generalized gradient approximation level—closer to
those experimentally observed. Second, for the ferromagnetic, the striped, checkerboard, and staggered dimer
antiferromagnetic order, we investigate the change in magnetic formation energy with local magnetic moment
of Fe at a pressure up to 6 GPa. The bilinear and biquadratic exchange energies are derived from the Heisenberg
model and noncollinear first-principles calculations, respectively. We found a nontrivial behavior of the spin-
exchange parameters on the magnetization, and we put forward a field-theory model that rationalizes these results
in terms of two-dimensional spin and orbital fluctuations. The character of these fluctuations can be either that
of a standard density wave or a topological vortex. Topological vortices can result in mesoscopic magnetization
structures.

PbTe is a leading thermoelectric material at intermediate temperatures, largely thanks to its low lattice thermal conductivity. However, its efficiency is too low to compete with other forms of power generation. This efficiency can be effectively enhanced by designing nanostructures capable of scattering phonons over a wide range of length scales to reduce the lattice thermal conductivity. The presence of grain boundaries can reduce the thermal conductivity to ∼0.5 W m −1 K−1 for small acancy concentrations and grain sizes. However, grains anneal at finite temperature, and equilibrium and metastable grain size distributions determine the extent of the reduction in thermal conductivity. In the present work, we propose a phase-field model informed by molecular dynamics simulations to study the annealing process in PbTe and how it is affected by the presence of grain boundaries and voids. We find that the thermal conductivity of PbTe is reduced by up to 35% in the porous material at low temperatures. We observe that a phase transition at a finite density of voids governs the kinetics of impeding grain growth by Zener pinning.

The temperature renormalization of the bulk band structure of a topological crystalline insulator, SnTe, is calculated using first-principles methods. We explicitly include the effect of thermal-expansion-induced modification of electronic states and their band inversion on electron-phonon interaction.We show that the direct gap decreases with temperature, as both thermal expansion and electron-phonon interaction drive SnTe towards the phase transition to a topologically trivial phase as temperature increases. The band gap renormalization due to electron-phonon interaction exhibits a nonlinear dependence on temperature as the material approaches the phase transition, while the lifetimes of the conduction band states near the band edge show a nonmonotonic behawior with temperature. These effects should have important implications on bulk electronic and thermoelectric transport in SnTe and other topological insulators.