arXiv subjects

Ipek Efe

Publications and source records attributed to Ipek Efe.

7 recordsLinked to original sources

arXiv2026

Soft-mode nonlinearities away from ferroelectric phase transition

The interplay between ionic and electronic subsystems dictates the behavior of structural phase transitions in polar dielectrics, a coupling mediated by soft optical phonon modes. In incipient ferroelectrics such as SrTiO$_3$ (STO), strong local-field effects can drive the lattice into a non-perturbative regime near the phase boundary. However, disentangling the distinct contributions of local fields from those of spontaneous macroscopic polarization remains an experimental challenge. Here, we isolate these mechanisms by probing paraelectric STO deep within its symmetric phase, where macroscopic spontaneous polarization is suppressed. Linear terahertz (THz) spectroscopy reveals that the soft mode exhibits a hybrid character, predominantly driven by electronic polarizability. Utilizing two-dimensional THz spectroscopy, we map the underlying nonlinear signals, demonstrating that the system persists in a perturbative regime characterized by robust local-field coherence. By implementing a microscopic model of coupled electronic and lattice degrees of freedom mediated by local fields, we qualitatively reproduce these multidimensional coherent signatures. Our findings highlight that while local fields are necessary to initiate non-perturbative lattice dynamics, they are insufficient on their own. This reveals that spontaneous polarization plays a deterministic role in dictating soft-mode nonlinearities in strongly correlated polar dielectrics.

cond-mat.mtrl-sci↗

arXiv2026

Polarizing ultrathin ferroelectric BaTiO3 films through interfacial layer polarization

An important requirement for the integration of ferroelectric thin films into devices is deterministic control of the polarization state in films of only a few unit cells in thickness. Here, we utilize the charged atomic planes of (001)-oriented SmNiO$_3$ (SNO) buffer layers as a polarizing template to stabilize the polarization in ferroelectric BaTiO$_3$ (BTO) model system thin films. We show that an upwards (downwards) oriented polarization is achieved by selection of the [SmO]$^+$ ([NiO$_2$]$^-$) buffer termination. Most importantly, the charged atomic planes of SNO suppress the depolarizing-field-induced critical thickness in BTO, and we record the emergence of a net polarization in our BTO films from the first unit cell deposited. Our experiments, guided by density-functional-theory (DFT) calculations, further highlight the impact of charged defects on the polarizing effectiveness of the SNO buffer. Specifically, oxygen vacancies counteract the polarizing field of the negatively charged, [NiO$_2$]$^-$-terminated surface of the SNO buffer. Our findings provide important insights into the interplay of defect chemistry and polarizing interfaces to stabilize ferroelectric polarization down to the single-unit-cell limit.

cond-mat.mtrl-sci↗

arXiv2026

Manipulating ferroelectricity without electrical bias: A perspective

Ferroelectric materials are established candidates for beyond complementary metal-oxide-semiconductor technology, owing to their non-volatile spontaneous electrical polarization. The recent boom in electric dipole texture engineering and manipulation in such materials has revealed exciting routes for controlling ferroelectric polarization, offering alternatives to the classical, sometimes challenging, application of electrical fields. In this short perspective, we shed light on electrode-free external stimuli enabling control over polar states in thin films. We bring awareness to the polarizing role of chemically-engineered surface contributions and provide insights into the combination of chemical substitution and mechanical pressure, complementing the polar state tuning capabilities readily enabled by flexoelectricity. Finally, we describe recent developments in the optical modulation of polarization. Thus, our perspective aims to stimulate the advancement of alternative means to act on polarization states and facilitate the development of ferroelectric-based applications.

cond-mat.mtrl-sci↗

arXiv2025

Nanoscale electrostatic control in ferroelectric thin films through lattice chemistry

Nanoscale electrostatic control of oxide interfaces enables physical phenomena and exotic functionalities beyond the realm of the bulk material. In technologically-relevant ferroelectric thin films, the interface-mediated polarization control is usually exerted by engineering the depolarizing field. Here, in contrast, we introduce polarizing surfaces and lattice chemistry engineering as an alternative strategy. Specifically, we engineer the electric-dipole ordering in ferroelectric oxide heterostructures by exploiting the charged sheets of the layered Aurivillius model system. By tracking in-situ the formation of the Aurivillius charged Bi$_{2}$O$_{2}$ sheets, we reveal their polarizing effect leading to the characteristic Aurivillius out-of-plane antipolar ordering. Next, we use the polarizing Bi$_{2}$O$_{2}$ stacking as a versatile electrostatic environment to create new electric dipole configurations. We insert multiferroic BiFeO$_3$ into the Aurivillius framework to stabilize a ferrielectric-like non-collinear electric-dipole order in the final heterostructure while maintaining the antiferromagnetic order of BiFeO$_3$. We thus demonstrate that engineering the lattice chemistry stabilizes unconventional ferroic orderings at the nanoscale, a strategy that may be expanded beyond the realm of electrically ordered materials.

cond-mat.mtrl-sci↗

arXiv2022

Magnetoelectric effect in hydrogen harvesting: magnetic field as a trigger of catalytic reactions

Magnetic fields have been regarded as an additional stimulus for electro- and photocatalytic reactions, but not as a direct trigger for catalytic processes. Multiferroic/magnetoelectric materials, whose electrical polarization and surface charges can be magnetically altered, are especially suitable for triggering and control of catalytic reactions solely with magnetic fields. Here, we demonstrate that magnetic fields can be employed as an independent input energy source for hydrogen harvesting by means of the magnetoelectric effect. Composite multiferroic CoFe2O4-BiFeO3 core-shell nanoparticles act as catalysts for the hydrogen evolution reaction (HER) that is triggered when an alternating magnetic field is applied to an aqueous dispersion of the magnetoelectric nanocatalysts. Based on density functional calculations, we propose that the hydrogen evolution is driven by changes in the ferroelectric polarization direction of BiFeO3 caused by the magnetoelectric coupling. We believe our findings will open new avenues towards magnetically induced renewable energy harvesting.

cond-mat.mtrl-sci↗

arXiv2021

Layer and spontaneous polarizations in perovskite oxides and their interplay in multiferroic bismuth ferrite

We review the concept of surface charge, first in the context of the polarization in ferroelectric materials, and second in the context of layers of charged ions in ionic insulators. While the former is traditionally discussed in the ferroelectrics community, and the latter in the surface science community, we remind the reader that the two descriptions are conveniently unified within the modern theory of polarization. In both cases, the surface charge leads to electrostatic instability - the so-called "polar catastrophe" - if it is not compensated, and we review the range of phenomena that arise as a result of different compensation mechanisms. We illustrate these concepts using the example of the prototypical multiferroic bismuth ferrite, BiFeO3, which is unusual in that its spontaneous ferroelectric polarization and its layer charges can be of the same magnitude. As a result, for certain combinations of polarization orientation and surface terminationits surface charge is self-compensating. We use density functional calculations of BiFeO3 slabs and superlattices, analysis of high-resolution transmission electron micrographs as well as examples from the literature to explore the consequences of this peculiarity.

cond-mat.mtrl-sci↗

arXiv2020

On the happiness of ferroelectric surfaces and its role in water dissociation: the example of bismuth ferrite

We investigate, using density functional theory, how the interaction between the ferroelectric polarization and the chemical structure of the (001) surfaces of bismuth ferrite influences the surface properties and reactivity of this material. A precise understanding ofthe surface behavior of ferroelectrics is necessary for their use in surface science applications such as catalysis as well as for their incorporation in microelectronic devices. Using the (001) surface of bismuth ferrite as a model system we show that the most energetically favoured surface geometries are combinations of surface termination and polarization direction that lead to uncharged, stable surfaces. On the unfavorable charged surfaces, we explore the compensation mechanisms of surface charges provided by the introduction of point defects and adsorbates, such as water. Finally, we propose that the special surface properties of bismuth ferrite (001) could be used to produce an effective water splitting cycle through cyclic polarization switching.

cond-mat.mtrl-sci↗