Session N69: Polarization Switching and Phase Transition in Ferroelectrics

Large electrocaloric effects in PST multilayer capacitors over a wide range of temperatures including room temperature

Electric field-driven ferroelectric phase transitions result in thermal changes known as electrocaloric effects. Large electrocaloric effects arise in thin films because breakdown fields are large, and when many thin films are combined into a multilayer capacitor (MLC) then one has a macroscopic working body for pumping heat in a prototype cooling device. I will describe electrocaloric MLCs of PbSc_0.5Ta_0.5O₃ (PST) that outperform the Gd working bodies in magnetocaloric prototypes. These MLCs display large voltage-driven changes of temperature (up to 5.5 K) over a wide range of starting temperatures (over 3 K across 176 K) [Nature 575 (2019) 468]. I will also highlight other recent work: electrocaloric cooling cycles with true regenation via field variation [PRX 9 (2019) 041002]; quasi-indirect measurements of electrocaloric temperature change via comparison of adiabatic and isothermal electrical polarization data [APL Materials 9 (2021) 010701]; and a review article on magnetocaloric, electrocaloric and mechanocaloric effects [Science 370 (2020) 797].   

Emergent antiferroelectric-to-ferroelectric transition in NaNbO3 membranes

NaNbO₃ is one of the most complicated perovskite systems, exhibiting antiferroelectricity at room temperature and a rich spectrum of temperature-driven phase transitions. Although NaNbO₃ has been studied for decades, previous work primarily focused on its bulk form and a significant lack of understanding still exists regarding the structure and property evolution of NaNbO₃ thin films. In this work, we study the effects of thickness on ferroic ordering and nano-mechanical properties of NaNbO₃ thin-film membranes. Using pulsed laser deposition and epitaxial lift-off techniques, we synthesize high crystalline quality, crack-free NaNbO₃ membranes with thickness ranging from 10 to 160 nm and millimeter lateral scale.  We probe an intriguing antiferroelectric-to-ferroelectric phase transition with reducing membrane thickness using a combination of characterization techniques including X-ray diffraction, piezoresponse force microscopy, and Raman spectroscopy. This emergent phase transition leads to a non-monotonic thickness dependence of Young’s modulus including a both strain gradient dominated response and a surface elasticity dominated response in the ferroelectric phase regime below 40 nm, as well as another surface dominated response in the mixed phase regime above 40 nm.   

Tuning Negative Capacitance state inPbZr0.2Ti0.8O3/SrTiO3Heterostructures via Layer Thickness Ratio

The negative capacitance (NC) effect in a ferroelectric (FE) can be stabilized by interfacing with a dielectric (DE) layer for developing steep slope transistors. In this study, we examine the transition from FE to DE ground state in PbZr_0.2Ti_0.8O₃/SrTiO₃ (PZT/STO) heterostructures using the transient NC measurements. We deposit epitaxial PZT/STO bilayers with a total thickness of 100 nm on 10 nm LaNiO₃ buffered (001) STO substrates. With decreasing FE-DE thickness ratio (r), the remanent polarization of the stack decreases monotonically and exhibits an abrupt drop at the critical ratio of r_c = 8-7, which is consistent with the Landau theory modeling of the free energy profile. Upon polarization switching, we observe the charge damping associated with the transient NC mode within 2 μs. Modeling the charge switching dynamics in the r = 8 sample reveals a domain wall motion limited behavior, which evolves to a multidomain state in the r = 7 sample. The transient NC mode is quenched in the r = 6 sample, consistent with the suppressed ferroelectric order. Our study provides critical material information for designing complex oxide-based NC field-effect transistors for low-power nanoelectronics.      

Crystal growth and diffuse scattering of tungsten bronze Ba2RFeNb4O15 ferroelectrics

Tetragonal tungsten bronzes represent a particular, non-perovskite structural motif that can display relaxor ferroelectric properties, the most well-known example being Sr_1-xBa_xNb₂O₆, which has an unfilled (1-2-6) structure that contains random cation vacancies. More recently, a group of filled tungsten bronzes with the formula Ba₂RFeNb₄O₁₅ (R=Rare Earth) have also been shown to display relaxor ferroelectric behavior. These are of particular interest as a comparison to SBN because, lacking the random cation vacancies of the 1-2-6 structure, they may reveal which local structural features are common to both filled and unfilled structures and which are characteristic only of the cation vacancies. In this work, we carried out crystal growth experiments, frequency-dependent dielectric susceptibility measurements, and diffuse X-ray scattering experiments on Ba₂RFeNb₄O₁₅ (R=Pr, Nd) as well as Sr_1-xBa_xNb₂O₆ for a means of comparison. By identifying features that are common and disparate from these crystals, we attempt to correlate them with the dielectric properties.   

Quantum criticality in ferroelectric-paraelectric heterostructures

Phase transition between ferroelectricity and quantum paraelectricity via non-thermal tuning parameters can lead to quantum critical behaviour and associated emergent phenomena. Ferroelectric quantum critical systems are, however, rare despite the abundance of ferroelectric materials. In this work, we discuss the possibility of achieving tunable quantum paraelectricity in ferroelectric-paraelectric heterostructures, where the quantum temperature - the temperature below which the onset of an effective ferroelectricity is suppressed due to quantum fluctuations - can be tuned by the thickness ratio. This raises the prospect of observing quantum phase transition and a quantum critical region in such systems, with the thickness ratio as the tuning parameter.  The quantum critical region also offers unexpected prospects in the field of ferroelectric quantum criticality. Going forward, experiments geared at elucidating if a quantum phase transition is indeed present in the zero-temperature limit in these heterostructures and at extracting the scaling laws that characterize the ferroelectric quantum critical regime will shed more light on this exciting area.   

Probing Polar and Dielectric Materials via Impurity Qubit Relaxometry

A qubit sensor with an electric dipole moment acquires an additional contribution to its relaxation rate when it is placed in the vicinity of a polar or dielectric material, as a consequence of electric noise arising from polarization fluctuations in the sample. In this talk, we characterize this relaxation rate as a function of experimentally tunable parameters such as sample-probe distance, probe-frequency, and temperature, and demonstrate that it offers a novel window into probing dielectric properties of insulating materials over a wide range of length-scales and frequency-scales. We establish the feasibility of our proposal with a specific qubit of choice and illustrate its ability to probe questions ranging from collective modes in long-range interacting systems, to phase transitions and disorder-dominated phenomena in relaxor ferroelectrics. Our proposal paves the way for a novel table-top probe of dielectric and polar materials, in a parameter regime complementary to existing tools and techniques.   

Structural phases and thermodynamics of BaTiO3 from an integrated machine learning model

Modeling the ferroelectric transition of any given material requires three key ingredients: (1) a model of the potential energy surface, that describes the energetic response to a structural distortion; (2) the free energy surface sampled at the relevant, finite-temperature conditions; and; (3) the polarization of individual configurations that determines, through averaging over samples, the observed polarization and the phase transitions. To this aim, we introduce an integrated machine-learning framework, based on a combination of an interatomic potential and a vector model for microscopic polarization, which we use to run Molecular Dynamics simulations of ferroelectrics with the same level of accuracy of the underlying DFT method, on time and length scales that are not accessible to direct ab-initio modeling. This allows us to uncover the microscopic nature of the ferroelectric transition in barium titanate (BaTiO3). The presence of an order-disorder transition is the main driver of ferroelectricity, while the coupling between symmetry breaking and cell distortions determines the presence of partly-ordered (tetragonal and orthorhombic) phases. The framework also allows us to reconstruct the temperature-dependent BaTiO3 phase diagram, with first-of-its-kind accuracy.   

Dielectric response of BaTiO3 from an integrated machine learning model

Modeling the finite-temperature behavior of ferroelectric materials from first principles has always been challenging due to the large supercells and long simulation times required for adequate sampling.  Here we demonstrate the use of an integrated machine learning (ML) model of the potential energy and polarization surfaces of barium titanate (BaTiO₃) to overcome these difficulties and run long MD simulations with DFT accuracy. The integrated ML model allows us to study the microscopic nature of the paraelectric-ferroelectric transition and uncover surprising new insights, e.g. that the long-range, “needle-like” correlations observed previously can arise from a purely short-range model with no explicit long-range terms. Finally, we compute the frequency-dependent dielectric response function, finding a spectrum qualitatively similar that obtained with previous effective-Hamiltonian simulations as well as to experimentally measured profiles, with some remaining discrepancies that we trace back to the underlying DFT model. We expect this integrated, generally applicable modeling technique to become a valuable tool for elucidating the ferroelectric behavior of a wide variety of materials.   

Square Tensile Strain Effect on Epitaxial BaTiO3 Film

Strain engineering for the heteroepitaxy film is a technique to improve the ferroelectric properties or create a novel ground state. The symmetry and misfit lattice constant of substrates strongly affect feasible materials and physical properties of the epitaxial heterostructures. So far, prevalent perovskite oxide substrates, such as cubic SrTiO₃ and orthorhombic R(Al/Sc/Lu)O₃ (R = rare earth), have been prominently employed. Recently, we have successfully grown a new perovskite oxide single crystal and synthesized epitaxial BaTiO₃ film on the lab-made substrate. In this talk, we will present the physical properties of the square tensile strained BaTiO₃ film and discuss the square tensile strain effect on BaTiO₃ based on the density functional theory calculations.   

Tilt-driven antiferroelectricity in PbZrO3

PbZrO₃ (PZO) is, by far, the best-known antiferroelectric (AFE) material. During its AFE phase transition, its atomic structure undergoes a complex set of distortions, resulting in the ↑↑↓↓ displacement pattern of Pb ions and in seemingly unrelated O₆ octahedra tilts. The physical origin of such complex antiferroelectricity in PZO has been a subject of debates over the last years. Here we combine ab-initio simulations with Landau theory modelling to study the physics behind the AFE phase transition in PZO. We show that the atomic distortions accompanying it condense during a locked-in incommensurate phase transition and are essentially spatial modulations of polarization, P, and of antiphase O₆ octahedra tilts, φ. We further demonstrate that the AFE state in PZO is stabilized by the trilinear rotopolar gradient coupling between the modulated P and φ, having form W Pφ(∇φ). Our theoretical framework combines the defining features of two main existing models of PZO into a single physical mechanism: locked-in incommensurate modulations and trilinear coupling between distortions. The proposed mechanism is promising for description of actual incommensurate phases observed in PZO-like perovskites, such as PLZST and PbHfO3.   

Switching a polar metal via strain gradients

Although rare, spontaneous breakdown of inversion symmetry sometimes occurs in a material which is metallic: these are commonly known as polar metals or ferroelectric metals. Their “polarization”, however, is difficult to switch via an electric field, which limits the experimental control over band topology. Here we shall investigate, via first-principles theory, flexoelectricity as a possible way around this obstacle with the well known polar metal LiOsO₃ . The flexocoupling coefficients are computed for this metal with high accuracy with an approach based on real-space sums of the interatomic force constants. A Landau-Ginzburg-Devonshire-type first-principles Hamiltonian is built and a critical bending radius to switch the material is estimated, whose order of magnitude is comparable to that of BaTiO₃. Our work opens exciting new avenues in the search for new functionalities in polar metals, and in the field of flexoelectricity too. Our results constitute the first successful calculation of flexocoupling coefficients in metals, and constitutes therefore an important advance in first-principles methodologies as well.   

Elastic Fluctuations can drive Ferroelectric Transitions Incommensurate

We consider a Ginzburg-Landau theory for a ferroelectric phase transition whose primary order parameter (electrical polarization) is coupled to elastic strain through electrostriction. We show that the strain coupling leads to a quartic term in the effective Hamiltonian that describes long-range, anisotropic interactions between fluctuations in the order parameter. At the level of mean field theory plus Gaussian fluctuations, we demonstrate that the ferroelectric instability is driven to a finite wavevector, so that the transition from the disordered phase to the fully ordered ferroelectric phase must occur via an incommensurate phase.