• This keynote presents the history, current status, and future prospects of the defect control in polar perovskites of BaTiO3 and Bi1/2Na1/2TiO3 and related polar perovskites. Polar materials include not only ferroelectrics but also ferrielectrics that were first discovered in high-quality Bi1/2Na1/2TiO3-based single crystals. This will also discuss the dielectric properties of ferrielectric ceramics which may surpass BaTiO3-based ceramics in terms of ultra-high permittivity under strong DC bias fields.

  • The ferroic community has seen a rise in interest regarding topological polarization textures, which have considerable potential for enhancing solitonic information technologies and driving new innovations in ferroelectrics. Recent key findings include the observation of non-Ising and chiral domain walls, as well as various patterns such as bubble domains and skyrmions. Current research emphasizes exotic topological structures that may reveal novel physical and functional properties. Concurrently, experimental techniques are being developed and refined to allow the detection of local symmetry violations and establish structure-property correlations down to the nanoscale. In this context, second-harmonic generation has emerged as effective approache for non-invasively probing the internal structure, chirality, and complex polarization textures. In this lecture, I will provide a comprehensive review of advancements in our understanding of the internal structure of domain walls in ferroelectrics, with a particular emphasis on local symmetry and properties, facilitated by the integration of machine learning algorithms into nonlinear optical microscopy.

  • This presentation will outline a general thermodynamic formulation and the corresponding phase-field implementation to model, understand and predict the formation of domains and their responses to mechanical and electric stimuli in the possible presence of atomic, ionic, and electronic defects.

  • Perovskite-type oxides are almost ubiquitous in the field of electroceramics. This is due to the broad range of properties that can result when choosing appropriate compositions: insulating behavior in dielectric SrTiO3 and ferroelectric Pb(Zr,Ti)O3 (PZT), high electronic conductivity in (La,Sr)MnO3 (LSM), high electrocatalytic activity in (La,Sr)CoO3 (LSC) or (La,Sr)FeO3 (LSF), etc. Not surprisingly, researchers often mainly focus on one class of perovskite oxides, with the aim of understanding and improving a specific property and its use in devices. However, despite their often strikingly different properties, all these perovskites share several strongly related phenomena, and on a fundamental level unifying descriptions are often possible. This is exemplified in the talk.

  • Piezoelectric thin films based on Al1-xScxN, PbZr1-xTixO3 and K1-xNaxNbO3 are being commercialized in piezoelectric microelectromechanical systems (MEMS) for resonators, low drive actuators, and the facile approach to making transducer arrays. As these materials are incorporated into devices, it is critically important that they operate reliably over the lifetime of the system over a wide range of DC and AC excitation conditions. Increasingly, wurtzite structured ferroelectrics such as Al1-xScxN, Al1-xBxN, and Zn1-xMgxO are also being explored for ferroelectric memory applications.  Here too, the current generation of materials is limited in terms of reliability in terms of cycling, as well as the propensity to high leakage currents in thickness-scaled devices.  This paper discusses the links between defect chemistry and the lifetime-limiting behaviors.  A combination of thermally stimulated depolarization current, deep level transient spectroscopy, impedance spectroscopy, highly accelerated lifetime testing, and transmission electron microscopy is typically needed to fully characterize the mechanism for failure.  In some cases, once the key mechanisms for failure are understood, significant increases in the lifetime and performance can be achieved. 

  • In this talk, I will share our work on the ionic electrochemical synapses, whose electronic conductivity we control deterministically by electrochemical insertion/extraction of dopant ions into/out of the channel layer. This work is motivated by the need to enable significant reductions in the energy consumption of computing, and is inspired by the ionic processes in the brain. Proton as the working ion in our research presents with very low energy consumption, on par with biological synapses in the brain. Our modeling results indicate the desirable material properties, such as ion conductivity and interface charge transfer kinetics, that we must achieve for fast (ns), low energy (< fJ) and low voltage (<1V) performance of these devices. Importantly, the target material is a mixed proton-electron conductor, whose electronic conductivity depends on the proton concentration through doping and phase change effects. The candidate materials are a spectrum of intercalation oxides as well as 2D van der Waals materials. We have assessed the electron polaron and proton mobilities in these systems, to understand the conductivity modulation mechanisms, and down-select most promising materials. In addition, the conductance change in these electrochemical devices depends non-linearly on the gate voltage, due to field-enhanced ion migration in the electrolyte, and charge transfer kinetics at the electrolyte-channel interface. We are leveraging these intrinsic nonlinearities to emulate bio-realistic learning rules deduced from neuroscience studies, such as spike timing dependence of plasticity and Hebbian learning rules. Our findings provide pathways towards brain-inspired hardware that has high yield and consistency and uses significantly lesser energy as compared to current computing architectures.

  • The climate and sustainability crisis is currently the greatest challenge to the survival of humanity. Alarmingly, catastrophic droughts, wildfires, floods, and storms are becoming increasingly common, signaling that several planetary boundaries have been breached, with little time left before the first tipping points are reached. Despite clear evidence from scientific research and everyday experiences, greenhouse gas emissions continue to rise exponentially due to barriers to change at political, societal, and individual levels. Given its systemic nature, the sustainability crisis cannot be addressed through technological progress alone but requires a complete paradigm shift. As scientists, we bear a special responsibility to lead the communication of the urgent need for action to both the public and decision-makers [1]. Recently, as a group of concerned researchers, we founded the International Alliance of Societies for a Sustainable Future (https://sfs-alliance.org). Motivated by the fragility of existing political networks, our vision is to leverage the robust and stable international scientific network to alert the global public about the sustainability crisis and recommend measures for socio-ecological transformation. This alliance is not limited to natural sciences and technology but spans all disciplines across borders and cultures [2].

    References
    [1] J. Rödel, A. Frisch, and D. Damjanovic, J. Mater. Sci. 58, 15577, https://link.springer.com/article/10.1007/s10853-023-09031-z (2023).  
    [2] J. Rödel, F. Faupel, and S. Klein, Nature Materials, https://www.nature.com/articles/s41563-024-02063-z (2024).

  • Atomic Force Microscopes (AFMs) have become a standard tool for high resolution surface mapping of a wide variety of nanoscale samples. The vast majority of existing AFMs make use of an optical beam detector (OBD) that measures the bending of the flexible cantilever beam. Despite its popularity, accurate and reproducible mechanical measurements using this detection approach remains extremely challenging. Specific barriers to widespread accurate AFM include (i) highly inconsistent sensitivity calibrations, (ii) measurement noise floors significantly higher than thermal motion of the cantilever probes and (iii) uncontrolled mixing of vertical and in-plane forces acting on the tip. Component mixing inevitably complicates attempts at accurate mechanical measurements and can lead to enormous, and often unacknowledged uncertainties. In this work, we build on earlier previous interferometric results to develop and demonstrate new workflows that allow the full three-dimensional nanoscale mechanical response of samples – limited by the fundamental thermal (Brownian) fluctuations of the cantilever with an accurate sensitivity calibrated by the wavelength of light. These workflows are based around a new quadrature phase differential interferometer (QPDI) that routinely achieves a detection noise down to ≈5 fm⁄√Hz on standard commercial cantilevers. The QPDI measurement remains linear and accurate for large deflections (>1 μm) down to sub-picometer thermal fluctuations. This improved low noise floor and accurate calibration reveals details and features that have been hidden from view using conventional OBD measurements. We demonstrate new workflows for a variety of materials including functional ferroelectrics, including high frequency (5G) filter design and manufacturing, beyond Moore’s law computing materials such as HfO and HZO and 2D van der Waals materials including twisted graphene and hBN and soft polymeric samples. We demonstrate significantly improved accurate force quantification of in-plane and vertical forces that are typically mixed in an uncontrolled manner with OBD.