Session C09: Dielectric and Ferroic Oxides - Elastic Phenomena

New functionalities from gradient couplings: Flexoelectricity and more

Flexoelectricity, the coupling between an inhomogeneous deformation and the electrical polarization, has emerged a "hot" topic in modern materials science due to its cross-cutting relevance to many phenomena of fundamental and technological interest. Understanding the intriguing physics that governs its behaviour at the nanoscale is crucial to harnessing the potential of strain gradients in practical applications, and such a progress requires a substantial support from theory. Due to the inherent breakdown of lattice periodicity that a strain gradient entails, however, first- principles calculations of flexoelectricity remain technically challenging at several levels. In this talk, I will discuss the recent methodological developments that have made such calculations feasible, and their application to realistic materials systems. In particular, I will focus on manifestations of the flexoelectric effect in SrTiO3, either at the level of the bulk crystal, surfaces or ferroelastic twin boundaries. More generally, I will emphasize the opportunities that spatial gradients (e.g. a nonuniform polarization, strain or magnetization field) offer for materials design, by enabling functionalities that would be otherwise forbidden in a uniform crystalline phase.   

Strain-induced nonlinear susceptibility enhancement in thin film Ba1-xSrxTiO3

The search for improved optoelectronic devices and ferroelectric field effect transistors demands the ability to manipulate optical properties in ferroelectric thin films. Ferroelectrics are materials with built-in electric polarization P, which breaks inversion symmetry and creates a strong electro-optic coefficient. Second harmonic generation (SHG) is used as a non-invasive optical method to characterize second order nonlinear susceptibility χ⁽²⁾, a property closely related to P, in thin film Ba_1-xSr_xTiO₃ (BSTO). Through lattice mismatch with the SrTiO₃(001) substrate, one is able to tune compressive strain induced at the substrate-film interface by changing the Sr molar ratio x in the BSTO alloy. An enhancement in χ⁽²⁾ is observed at x=0.3, corresponding to an approximate strain of 1.5% at the interface, which is indicative of the SHG sensitivity to strain via elongation of P, as well as a fine tuning in the strain relaxation mechanism resulting from misfit dislocations.   

Mechanical Control of Quantum Tunneling and its Implication for Nanoscale Flexoelectricity

Flexoelectricity refers to a coupling between electrical polarization and strain-gradient. This electromechanical phenomenon is allowed by all material symmetry and is most profound at the nanoscale. Nanoscale flexoelectricity enables functional control of ferroelectric polarization/domain configuration, photovoltaic response, and defect distribution in oxide thin films. It also holds the potential for lead-free micro- and nano-electromechanical device applications. Accordingly, flexoelectricity has emerged as a field of active experimental and theoretical research. One of the pressing challenges, however, is developing a means to characterize flexoelectricity at the nanoscale. To this end, we studied quantum tunneling through an ultra-thin dielectric film as a function of mechanically-induced flexoelectric polarization. The tunneling transport exhibits a systematic change with polarization, which we attribute, based on the WKB modeling to flexoelectric polarization-driven modification of the tunneling barrier. Furthermore, we discuss how this mechanical approach enables quantifying the flexoelectric coefficient at the nanoscale.   

Flexocaloric effects in ferroic materials

Flexoferroic materials are materials with interplay between a ferroic property such as polarization or magnetization and the gradient of a mechanical strain. Flexoelectricity is one of the most studied effects due to the fact that this effect is allowed by symmetry in any material. We develop the thermodynamics of the flexoferroic materials and apply the results to study flexocaloric effects in ferroic solids. Flexocaloric effect is the reversible component of the thermal response of a solid to deformation bending (in general inhomogeneous straining). The two limits of interest correspond to bending performed in isothermal and adiabatic conditions. In the first case, the flexocaloric effect is quantified by the change of entropy of the material, which is related to the heat exchanged with the surroundings. In the second case it is quantified by the corresponding change of temperature. Recently a number of studies have considered the possibility of a flexocaloric effect in ferroelectric materials. We address the nature of this phenomenon in a variety of ferroic materials.   

Current-density implementation for calculating flexoelectric coefficients

The flexoelectric (FxE) effect, where polarization is induced by a strain gradient, is universal in all insulators. As devices shrink to the micro and nano scale, large strain gradients can occur, and therefore the FxE effect can play a significant role in their electrical and mechanical properties; also, the FxE effect can be exploited for novel device design paradigms such as piezoelectric ``meta-materials'' constructed from nonpiezoelectric constituents, or mechanical switching of ferroelectric polarization. One of the crucial limitations to understanding and exploiting the FxE effect is the lack of an efficient first-principles methodology to calculate all of the components of the bulk FxE tensor; the transverse and shear components in particular are problematic. In this work we develop such a methodology based on density functional theory to calculate the full bulk, clamped-ion FxE tensor from a single unit cell by calculating the current-density response to the adiabatic displacement of atoms from a long-wavelength acoustic phonon. We benchmark our methodology on simple systems of isolated nobel gas atoms, and apply it to calculate the clamped-ion flexoelectric constants for a variety of technologically important cubic materials.   

Giant Photostriction in Perovskite SrIrO3 Thin Films

Photostrictive effect depicts a direct light-matter interaction that exhibits a reversible mechanical deformation in a material under light illumination. In this work, we investigated the giant visible-light-induced deformation of complex oxide SrIrO₃ (SIO), taking advantages of its strong spin-orbit coupling, sizable crystal field and high absorption across the visible spectrum at room temperature. By using highly structural sensitive and contactless in-situ Raman spectroscopy, the red-shift behaviors of all the peaks in SIO Raman spectrum were observed as increasing excitation laser intensities. The local strain states of SIO under laser illumination were analyzed by means of the phonon deformation potential theory. A giant photon-induced strain in SIO thin films was unveiled after deducting the local heating effect by considering the Stokes/anti-Stokes Raman intensity ratio. We found that SIO shows significant photostriction compared to conventional semiconductors, polymers and perovskite oxides under the same experiment configuration. The strong photostrictive effect and stability of SIO at room temperature paves a promising route towards new applications and multifunctionalities of photon-driven devices.   

Origin of Negative Longitudinal Piezoelectric Effect

Piezoelectrics are a class of high-coupling functional materials that can convert mechanical energy to electrical energy and vice versa. The longitudinal piezoelectric coefficients along the polar axis are almost always positive. Using density functional theory calculations, we demonstrate that several hexagonal ABC ferroelectrics possess significant negative longitudinal piezoelectric effect: the lattice contracts along the direction of an applied electric field. By screening through a first-principles-based database of piezoelectrics for 941 inorganic crystalline compounds, we find that such counterintuitive piezoelectric effect is a general phenomenon. We find that the electric polarization of several ABC ferroelectrics increases with increasing hydrostatic pressure, making them appealing for high-pressure applications. The origin of negative longitudinal piezoelectric response relies on the strong ionic bonds associated with small effective charges and rigid potential energy surfaces.   

On the Strain-Dependent Multiferroic Phase Diagram of SrMnO3

Computational [1] and experimental [2] work suggested that epitaxial strain can produce ferroelectricity (FE), and ferromagnetism (FM), in the otherwise paraelectric antiferromagnet SrMnO₃. Strain engineering thus allows for controlling the ferroic phases and their critical temperatures (T_C) in SrMnO₃. Here, computational methods combining density functional theory with Monte Carlo simulations of the Heisenberg Hamiltonian, as well as molecular dynamics with an effective Hamiltonian for FE, are used to investigate the strain-temperature ferroic phase diagram of SrMnO₃. Analysis of the strain dependent exchange interactions provides insight into the sequence of magnetic ordering. While the FM state is energetically unfavoured in the non-polar structure at all strains, a strong dependence of the out-of-plane exchange interaction on the Mn-O-Mn bond angle causes FM to be favoured by FE at high (~4-5%) strain.
While the FE T_C increases with strain after the FE onset (around 3% strain) the magnetic T_C is more insensitive to strain. This enables a situation with coincident FE and magnetic T_C’s and potentially enhanced magnetoelectric coupling. This might be of interest, e.g. in exploring multicaloric effects. [1] Phys. Rev. Lett. 104, 207204 (2010). [2] Nat. Nanotech. 10, 661 (2015).   

Macroscopic polarization in ferroelastic SrTiO3 via gradient-mediated couplings

We study the microscopic mechanisms that are responsible for the
the polarity of ferroelastic twin boundaries (TBs) in tetragonal SrTiO3.
We carry out our investigation by using a new multiscale approach that starts from
the first-principles results, and uses them in order to build an effective
continuum Hamiltonian by taking the long-wavelength limit of the
relevant lattice-dynamical data.
Such a methodology has allowed us to identify three main gradient-mediated
mechanisms for polarity:
(i) flexoelectricity, which originates from the strain gradient at the TB;
(ii) a "rotopolar" coupling, which comes directly from the gradients of the
octahedral tilts;
(iii) a previously overlooked tri-linear coupling, which is mediated
by an antiferroelectric displacement of the Ti atoms.
Remarkably, the rotopolar coupling bears a strong analogy with the
mechanism that generates a polarization in cycloidal magnets, thereby
allowing for a macroscopic breakdown of inversion symmetry even in a
periodic sequence of parallel twins.   

Direct evidence of phonon instability driving geometric improper ferroelectricity in multiferroic YMnO3

In improper ferroelectrics, the anharmonic coupling between a stable zone-center polar phonon mode and unstable zone-boundary non-polar modes is theoretically proposed to drive the spontaneous polarization. However, the exact mechanism of the transition driven by the coupling between two phonon modes remains to be observed. We have performed comprehensive T-dependent phonon dispersion measurements in the archetypal improper ferroelectric, YMnO₃, including the behavior across the ferroelectric transition, using inelastic neutron scattering and single-crystal x-ray diffraction. In addition, we performed both 0K and finite temperature first-principles lattice dynamics simulations, including anharmonic renormalization effects. Both experiments and simulations reveal the zone-boundary soft-mode, and its temperature dependent anharmonic coupling to the zone-center polar mode, providing direct evidence for the previously proposed mechanism of geometric improper ferroelectricity.   

Observation of a Topological Transition Point During Dynamic Failure of Brittle Solids

We observed a topologic phase transition in the course of dynamic failure of brittle films of several minerals under indentation loads. Spatiotemporal distribution of the stress field was monitored with a nano-second resolution using multi-array piezo-electric transducers. From this data , we construct a dynamic acoustic crystal and observe a topological transition point in the acoustic band structure where the band inverts. We recognized three main features in the transition from pre-failure to post-failure regimes: existence of a solitonic state where multiple solitons –some of them with supersonic velocity- nucleate, propagate and collide; onset of nucleation of solitons- moving domain walls between distinct topological mechanical phases- is manifested in Van Hove singularities and the kinetic energy of the acoustic crystal shows an inverse symmetric form. We show that the inversion of symmetry occurs due to interactions of two types of solitons having opposite group velocities; one raises the energy while the other one lowers the kinetic energy of the acoustic crystal. The annihilation points of these two types of solitons are neutral points (i.e., Dirac-points of the crystal).   

Optically Induced Phase Transition in Compressively Strained BiFeO3 on LaAlO3

The structural and electronic properties of multiferroics are highly sensitive to external perturbations when the material has a strain state that places it near the phase boundary between two structural phases. Recent advances in epitaxial growth techniques have allowed epitaxial strain to be used to drive BiFeO₃ to the phase boundary between tetragonal (T-like) and rhombohedral (R-like) phases. Here, we report an optically induced phase transition from the R-like phase to T-like phase following illumination by an ultrafast optical pulse. The transition is observed in a time-resolved synchrotron x-ray diffraction study of a compressively strained BiFeO₃ thin film grown on a LaAlO₃ substrate. The experimental signatures of the phase transition are an increase in the intensity of the x-ray reflection associated with the T-like phase and an accompanying decrease in the intensity of the R-like phase. We have conducted a study of optically induced strain and resolved the intensity changes in BiFeO₃ at the submicron-scale through the use of x-ray nanobeams and x-ray full-field imaging. These probes reveal that (i) the intensity changes are proportional to the magnitude of the strain and (ii) the degree of the phase transition varies across the lateral extent of the BiFeO₃ layer.