Ryan Herchig

Electrocaloric effect in ferroelectric nanowires from atomistic simulations.

Electrocaloric effect is presently under active investigation owing to both the recent discoveries of giant electrocaloric effects and its potential for solid state cooling applications. We use first-principles-based direct simulations to predict the electrocaloric temperature change in ferroelectric ultrathin nanowires. Our findings suggest that in nanowires with axial polarization direction the maximum electrocaloric response is reduced when compared to bulk, while the room temperature electrocaloric properties can be enhanced by tuning the ferroelectric transition temperature. The potential of ferroelectric nanowires for electrocaloric cooling applications is discussed.

Emergence of ferroelectricity in antiferroelectric nanostructures

First-principles-based finite-temperature simulations are used to predict the emergence of ferroelectricity in antiferroelectric nanostructures made of PbZrO3. The phenomenon is expected to occur in antiferroelectric nanodots, nanowires, and thin films with good surface charge compensation and can be explained by the recently proposed surface effect. Our computations provide a microscopic insight into the equilibrium phases, phase competition, and electrical properties of PbZrO3 nanostructures. The dependence of these properties on the electrical boundary conditions and nanostructure size is investigated.

Isentropic magnetoelectric coupling in planar heterostructures

Thermodynamical expressions are derived to calculate isentropic magnetoelectric and electromagnetic couplings in ferroelectric/ferromagnetic heterostructures. The expressions predict that the isentropic magnetoelectric and electromagnetic effects are reversible and comparable in magnitude and can be tuned in different ways. The expressions are applied to estimate isentropic couplings in commonly grown planar heterostructures of La0.7Sr0.3MnO3/Ba1-xSrxTiO3 and La0.7Ca0.3MnO3/Ba1-xSrxTiO3 with x= 0.0, 0.1, and 0.5 using a combination of first-principles computational data and experimental data from the literature. The isentropic coupling coefficients of up to 10−9 s/m are predicted close to room temperature for La0.7Ca0.3MnO3/BaTiO3 heterostructures. Our findings show a way to tune the isentropic coupling coefficients in a particular temperature range by engineering heterostructures with favorable transition temperatures.

Unusual soft mode dynamics in ferroelectric PbTiO3 nanowire under different mechanical boundary conditions

First-principles-based atomistic simulations are used to investigate equilibrium phases and soft mode dynamics in ultrathin ferroelectric PbTiO3 nanowire with poor surface charge compensation subjected to a wide range of mechanical boundary conditions. The presence of the depolarizing field along the nanowires transverse directions leads to the appearance of a unique high-frequency hard phonon mode that can be used to characterize electrical boundary conditions. This mode is insensitive to the mechanical load. Hydrostatic pressure was found to significantly influence the Curie point and ferroelectric soft modes in the nanowire. Uniaxial stress applied either along axial or transverse nanowires direction is capable of inducing polydomain flux-closure phases that have a unique “dynamical” fingerprint. In such phases, the modes that originate from the soft modes of bulk PbTiO3 become hard, which could open a way to potential identification of such nanodomain phases. In all cases, uniaxial stress significan...

An unusual route to polarization reversal in ferroelectric ultrathin nanowires

Ferroelectric nanowires are promising candidates for miniaturized ferroelectric devices. Some potential nanoscale applications of the nanowires, such as ultra high density ferroelectric memory, utilize their reversible polarization. To meet the ever increasing demand for low energy consumption, it is extremely desirable to reduce the operational fields associated with polarization reversal. In this Letter, we use first-principles-based simulations to explore an unusual route to polarization reversal that utilizes a combination of relatively low bias field and THz pulsed radiation. Such an approach allows for lower operational fields and may lead to other potential applications such as THz radiation sensing and remote switches.