Controlling ferroelectrics by light-enhanced electrostatic screening

Abstract: The possibility of controlling ferroelectrics by various forms of illumination is attracting increasing attention. For example, it has been shown that intense light of a specific frequency can be used to excite selected high-energy phonon modes, which – by means of anharmonic phonon-phonon couplings – can in turn have an impact in the spontaneous polarization facilitating (even forcing) its reversal [1,2,3]. Interestingly, a more direct coupling between light and polarization can be achieved in sliding ferroelectrics [4], where illumination may also allow us to induce dynamic magnetization [4,5].

Here I will focus on a more basic strategy to control ferroelectrics by illumination, namely, leveraging the changes we can induce by photoexciting electron-hole pairs and metallizing the material. The basic idea is simple: the photogenerated mobile charges should be able to respond to the formation of local electric dipoles and screen the long-range dipole- dipole interactions, which should in turn result in a weakening (potentially, the suppression) of the ferroelectric instability. Hence, the hypothesis is that illumination with above-bandgap light may allow us to induce a ferroelectric to paraelectric transformation.

To discuss this possibility, I will refer to first-principles studies that revealed that only a small fraction of ferroelectric compounds have their spontaneous polarization affected by metallization [6]. More specifically, we will see only materials like BaTiO3, KNbO3 or BaMnO3 – whose ferroelectric instabilities rely upon strong dipole-dipole interactions – become paraelectric when electrostatic couplings are screened out. By contrast, when ferroelectricity has a different origin (chemical, geometric), the effect of metallization is negligible. In passing, note that this opens a way to designing ferroelectric metals with a potentially switchable polarization [7,8,9].

Focusing on the ferroelectrics that respond strongly to changes in screening, I will describe first-principles results predicting explicitly that the ensuing ferroelectric-paraelectric transformation can indeed be driven by photoexcitation [10]. I will also discuss experimental evidence for this effect [11,12], commenting on the comparison between the predicted and actually observed number of photoexcited electrons needed to trigger the transition. Finally, I will present first-principles results that illustrate one possible practical application of such a light-driven phase transformation, namely, the dynamic tuning of thermal conductivity in KNbO3 [13] and BaTiO3 [14], of potential application in novel concepts for phonon-based computing [15].

These works were done in collaboration with many colleagues, including C. Cazorla (Universitat Politècnica de Catalunya), R. Rurali (ICMAB-CSIC), H.J. Zhao (Jilin Univerisity), V. Fiorentini and A. Filippetti (Università di Cagliari), C. Paillard and L.

Bellaiche (University of Arkansas), P. Chen (Guangdong Technion-IIT), H.J. Xiang (Fudan University), Y. Yang (Nanjing University), A. Gruverman (University of Nebraska – Lincoln) and many others. Work in Luxembourg was funded by the Luxembourg National Research Fund (FNR), more recently through grant C21/MS/15799044/FERRODYNAMICS.

Bio: Born in Spain in 1974, Jorge Íñiguez-González studied Physics at the University of the Basque Country, where he obtained his Ph.D. in 2001. After two postdoctoral appointments in the USA, at Rutgers University and the NIST Center for Neutron Research, in 2005 he joined the Institute of Materials Science of Barcelona (ICMAB-CSIC) as a permanent researcher. Then, in 2015 he moved to the newly created Luxembourg Institute of Science and Technology (LIST), where he leads a research group focused on modeling of functional materials. Since 2018 he also holds an invited position as Affiliate Full Professor of Physics at the University of Luxembourg. Íñiguez-González’s research specializes in applying theoretical and computational methods to address problems in condensed matter physics and materials science. He is best-known for his works on ferroelectric and multiferroic phenomena as well as his original methodological contributions, e.g., for predictive dynamical simulations of nanostructured ferroelectrics. Recent highlights include the discovery of unprecedented electromechanical properties in novel fluorite ferroelectrics, the explanation of anomalous dielectric effects (negative capacitance and voltage amplification) in ferroelectric heterostructures, and the discovery of electric skyrmion bubbles. Since 2022 he is a Fellow of the American Physical Society “For ground-breaking contributions to the computational theory of ferroelectric and multiferroic materials.