The ease with which domain walls (DWs) in ferroelectric materials can be written and erased provides a versatile way to dynamically modulate heat fluxes. In this work we evaluate the thermal boundary resistance (TBR) of 180$^{\circ}$ DWs in prototype ferroelectric perovskite PbTiO$_3$ within the numerical formalisms of nonequilibrium molecular dynamics and nonequilibrium Green's functions. An excellent agreement is obtained for the TBR of an isolated DW derived from both approaches, which reveals the harmonic character of the phonon-DW scattering mechanism. The thermal resistance of the ferroelectric material is shown to increase up to around 20%, in the system sizes here considered, due to the presence of a single DW, and larger resistances can be attained by incorporation of more DWs along the path of thermal flux. These results, obtained at device operation temperatures, prove the viability of an electrically actuated phononic switch based on ferroelectric DWs
We calculate the thermal conductivity, κ, of the recently synthesized hexagonal diamond (lonsdaleite) Si using first-principles calculations and solving the Boltzmann Transport Equation. We find values of κ which are around 40% lower than in the common cubic diamond polytype of Si. The trend is similar for [111] Si nanowires, with reductions of the thermal conductivity that are even larger than in the bulk in some diameter range. The Raman active modes are identified, and the role of mid-frequency optical phonons that arise as a consequence of the reduced symmetry of the hexagonal lattice is discussed. We also show briefly that popular classic potentials used in molecular dynamics might not be suited to describe hexagonal polytypes, discussing the case of the Tersoff potential.
We demonstrate a fully electric control of the heat flux, which can be continuously modulated by an externally applied electric field in PbTiO3, a prototypical ferroelectric perovskite, revealing the mechanisms by which experimentally accessible fields can be used to tune the thermal conductivity by as much as 50% at room temperature.
We present a theoretical study of the lattice thermal conductivity of SrTiO 3 in its antiferrodistortive ferroelastic phase and of its dependence on an applied external electric field, via electrophononic couplings. The calculations are done by using second-principles density-functional theory and the full solution of the Boltzmann transport equation. Our results allow, on one hand, to identify and explain deviations from the usual temperature dependence of the thermal conductivity, revealing Poiseuille flow and a rare umklapp transport regime, in agreement with recent experimental results [Martelli et al., Phys. Rev. Lett. 120, 125901 (2018)]; on the other hand, they show that an external electric field, by reducing the symmetry of the lattice, activates different phonon-phonon scattering processes and thus yields a reduction of the thermal conductivity, supporting the generality of a heat control strategy previously reported by some of us [Seijas-Bellido et al., Phys. Rev. B 97, 184306 (2018)].
Using a combination of equilibrium classical molecular dynamics (within the Green-Kubo formalism) and the Boltzmann transport equation, we study the effect of strain on the ZnO thermal conductivity focusing in particular on the case of hydrostatic and uniaxial strain. The results show that in the case of hydrostatic strain up to ±4%, we can obtain thermal conductivity variations of more than 100%, while for uniaxial strains the calculated thermal conductivity variations are comparatively less pronounced. In particular, by imposing uniaxial compressive strains up to −4%, we estimate a corresponding thermal conductivity variation close to zero. The mode analysis based on the solution of the Boltzmann transport equation shows that for hydrostatic strains, the thermal conductivity variations are mainly due to a corresponding modification of the phonon relaxations times. Finally, we provide evidence that for uniaxial compressive strains the contribution of the phonon relaxations time is balanced by the increase of the group velocities leading to a thermal conductivity almost unaffected by strain.
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