Ferroelectric Domain Walls in PbTiO₃ Are Effective Regulators of Heat Flow at Room Temperature

Abstract

Achieving efficient spatial modulation of phonon transmission is an essential step on the path to phononic circuits using "phonon currents". With their intrinsic and reconfigurable interfaces, domain walls (DWs), ferroelectrics are alluring candidates to be harnessed as dynamic heat modulators. This paper reports the thermal conductivity of single-crystal PbTiO 3 thin films over a wide variety of epitaxial-strain-engineered ferroelectric domain configurations. The phonon transport is proved to be strongly affected by the density and type of DWs, achieving a 61% reduction of the room-temperature thermal conductivity compared to the single-domain scenario. The thermal resistance across the ferroelectric DWs is obtained, revealing a very high value (≈5.0 × 10 −9 K m 2 W −1 ), comparable to grain boundaries in oxides, explaining the strong modulation of the thermal conductivity in PbTiO 3 . This low thermal conductance of the DWs is ascribed to the structural mismatch and polarization gradient found between the different types of domains in the PbTiO 3 films, resulting in a structural inhomogeneity that extends several unit cells around the DWs. These findings demonstrate the potential of ferroelectric DWs as efficient regulators of heat flow in one single material, overcoming the complexity of multilayers systems and the uncontrolled distribution of grain boundaries, paving the way for applications in phononics.

Reduction of thermal conductivity in ferroelectric SrTiO3 thin films

“…The strain fields associated with ferroelastic DWs in BiFeO 3 , Pb(Zr 0.3 Ti 0.7 )O 3 , and PbTiO 3 [12,13,15] impose a very large thermal resistance, resulting in an important reduction of the thermal conductivity of these FE materials. Our measurements cannot discriminate between the effect of the contribution of different types of DWs, i.e., purely ferroelectric 180 • and ferroelastic 90 • .…”

“…As κ 0 we have used the experimental thermal conductivity of STO on LSAT. Using a value of R ≈ 5 × 10 −9 K m 2 W −1 , the thermal boundary resistance reported for a DW in BiFeO 3 and PbTiO 3 [13,15], the average distance φ can be calculated ( Fig. 4).…”

“…The presence of domains with electrical polarization pointing along different directions will riddle the material with polarization and strain gradients extending along ferroelectric domain walls (FEDWs), which in turn are proposed to be effective phonon scatterers [10][11][12][13][14][15][16]. In this regard, strained STO offers a unique model system to quantify the effect of FEDWs on the thermal conductivity κ (T ).…”

Mechanical Softness of Ferroelectric 180° Domain Walls

“…Finally, we also investigate a PbTiO 3 thin film of 50 nm thickness epitaxially grown by reactive molecular beam epitaxy on a single-crystal SrTiO 3 substrate (the growth details can be found elsewhere [20]). Because of the large compressive stress exerted by the substrate (−1.36%), only domains with vertical (out-of-plane) polarization are allowed in the ferroelectric film [20]. After electrically polarizing two different areas of the film with a dc voltage of 5 V, we pole domains of opposite sign, as indicated by the corresponding PFM phase contrast [ Fig.…”

“…Even purely ferroelectric (i.e., nonferroelastic) domain walls also react to strain gradients introduced by external indentation [18] or by the proximity of another ferroelectric (nonferroelastic) domain wall [19]. Moreover, elastic contrast between domains and domain walls may affect the propagation and scattering of phonons-and, consequently, also the propagation of heat [20][21][22]. Also, if there is mechanical contrast in 180°domain walls, it may be used as an alternative way to mechanically read ferroelectric bits [23].…”