arXiv subjects

Ellen M. Kiens

Publications and source records attributed to Ellen M. Kiens.

6 recordsLinked to original sources

arXiv2026

The Propagating Nature of Single- and Multimagnons in LaCrO$_3$

The dispersion of an excitation is commonly used to infer its microscopic character: dispersive modes are associated with propagation, whereas nondispersive features are often interpreted as localized. This distinction becomes less straightforward for composite excitations, whose spectral maxima can remain nearly dispersionless even when they are built from propagating quasiparticles. Here we use momentum- and temperature-dependent resonant inelastic X-ray scattering (RIXS) to reveal the origin of multimagnon excitations in antiferromagnetic LaCrO$_3$. Whereas the single magnon disperses strongly, the two- and three-magnon features remain nearly flat. On approaching the Neel temperature, thermal population of the magnon band produces an anti-Stokes branch and opens an additional three-magnon channel involving the creation of two magnons and annihilation of one. Its energy and momentum dependence are reproduced by a thermally weighted multimagnon joint density of states calculations. These results establish thermal population as a means of exposing the propagating constituents hidden within nondispersive composite excitations, and position RIXS as a probe of multimagnon dynamics and, ultimately, magnon-magnon interactions.

cond-mat.str-el↗

arXiv2026

Synthesis of epitaxial TaO$_2$ thin films on Al$_2$O$_3$ by suboxide molecular-beam epitaxy and thermal laser epitaxy

Tantalum dioxide (TaO2) is a metastable tantalum compound. Here, we report the epitaxial stabilization of TaO2 on Al2O3 (1-102) (r-plane sapphire) substrates using suboxide molecular-beam epitaxy (MBE) and thermal laser epitaxy (TLE), demonstrating single-oriented, monodomain growth of anisotropically strained thin films. Microstructural investigation is performed using synchrotron X-ray diffraction and scanning transmission electron microscopy. The tetravalent oxidation state of tantalum is confirmed using X-ray absorption and photoemission spectroscopy as well as electron energy-loss spectroscopy. Optical properties are investigated via spectroscopic ellipsometry and reveal a 0.3 eV Mott gap of the tantalum 5d electrons. Density-functional theory and group theoretical arguments are used to evaluate the limited stability of the rutile phase and reveal the potential to unlock a hidden metal-insulator transition concomitant with a structural phase transition to a distorted rutile phase, akin to NbO2. Our work expands the understanding of tantalum oxides and paves the way for their integration into next-generation electronic and photonic devices.

cond-mat.mtrl-sci↗

arXiv2025

Interfacial Control of Orbital Occupancy and Spin State in LaCoO$_3$

Transition metal oxides exhibit a wide range of tunable electronic properties arising from the complex interplay of charge, spin, and lattice degrees of freedom, governed by their $d$ orbital configurations, making them particularly interesting for oxide electronics and (electro)catalysis. Perovskite oxide heterointerfaces offer a promising route to engineer these orbital states. In this work, we tune the Co $3d$ orbital occupancy in LaCoO$_3$ from a partial $d^7$ to a partial $d^5$ state through interfacial engineering with LaTiO$_3$, LaMnO$_3$, LaAlO$_3$ and LaNiO$_3$. Using X-ray absorption spectroscopy combined with charge transfer multiplet calculations, we identify differences in the Co valence and spin state for the series of oxide heterostructures. LaTiO$_3$ and LaMnO$_3$ interfaces result in interfacial charge transfer towards LaCoO$_3$, resulting in a partial $d^7$ orbital occupancy, while a LaNiO$_3$ interface results in a partial Co $d^5$ occupancy. Strikingly, a LaAlO$_3$ spacer layer between LaNiO$_3$ and LaCoO$_3$ results in a Co $d^6$ low spin state. These results indicate that the Co spin state, like the valence state, is governed by the interfacial environment. High-resolution scanning transmission electron microscopy imaging reveals a clear connection between strain and spin configuration, emphasizing the importance of structural control at oxide interfaces. Overall, this work demonstrates that interfacial engineering simultaneously governs orbital occupancy and spin state in correlated oxides, advancing spin-engineering strategies in correlated oxides and offering new insights for the rational design of functional oxide heterostructures.

cond-mat.mtrl-sci↗

arXiv2025

Polarity and anti-distortive polarons in WO3 through epitaxial shear strain

Bestowing CMOS-compatible binary oxides with additional functionalities is a powerful strategy toward the realization of oxide electronics. Ideal candidates are thin films which display a strong sensitivity to strain, chemical doping or nanoscale confinement. Among these, crystalline tungsten trioxide WO3 exhibits exceptional structural flexibility, enabling a wide range of functionalities. Here, we reveal the emergence of a previously unreported polar phase in epitaxial WO3 thin films. We accomplish this by imposing epitaxial shear strain, which stabilizes a low-symmetry triclinic structure that persists up to large film thicknesses and elevated temperatures. At the atomic scale, a change in the oxygen octahedral tilt pattern facilitates this symmetry lowering into a polar phase, which manifests as a periodic in-plane polarized stripe domain configuration with needle-like bifurcations at the microscale. The stripe domain walls further exhibit a strongly enhanced electrical conductivity in conjunction with a pronounced reduction of a distortive structural mode, providing the first experimental evidence for the formation of anti-distortive polarons recently predicted in WO3.

cond-mat.mtrl-sci↗

arXiv2025

Guidelines for the optimization of hafnia-based ferroelectrics through superlattice engineering

Hafnia-based ferroelectrics are revolutionizing the data storage industry and the field of ferroelectrics, with improved materials and devices being reported monthly. However, full understanding and control has not been reached yet and the ideal material still needs to be found. Here we report ferroelectric hafnia-zirconia superlattices made out of zirconium-substituted hafnia (Hf$_{1-x}$Zr$_x$O$_2$) sublayers of varying stoichiometries alternating with pure ZrO$_2$ sublayers. It is observed that the ZrO$_2$ layers in these superlattices act as a booster for the total remnant polarization (P$_r$). By combining the benefits of the ZrO$_2$ layers and the added interfaces, which help prevent breakdown, we fabricate superlattices with a total 87.5% ZrO$_2$ content, exhibiting record polarizations with a 2P$_r$ value of 84 $μ$C/cm$^2$ that can be cycled 10$^9$ times, while maintaining a 2P$_r$ > 20 $μ$C/cm$^2$. Next to these attractive properties, substitution of HfO$_2$ by the much more abundant ZrO$_2$ offers a significant step towards the sustainable application of these devices.

cond-mat.mtrl-sci↗

arXiv2023

A complementary experimental study of epitaxial La0.67Sr0.33MnO3 to identify morphological and chemical disorder

Gaining insight into the characteristics of epitaxial complex oxide films is essential to control the behavior of devices and catalytic processes. It is known that substrate induced strain, doping, and layer growth can affect the electronic and magnetic properties of the film's bulk. In this study, we demonstrate a clear distinction between the bulk and surface of thin films of La0.67Sr0.33MnO3 in terms of chemical composition, electronic disorder, and surface morphology. We employed a combined experimental approach of X-ray based characterization methods and scanning probe microscopy. X-ray diffraction and resonant X-ray reflectivity revealed surface non-stochiometry in the strontium and lanthanum, as well as an accumulation of oxygen vacancies. Scanning tunneling microscopy showed a staggered growth surface morphology accompanied by an electronic phase separation (EPS) related to this non-stochiometry. The EPS is likely responsible for the temperature-dependent resistivity transition and is a cause of a proposed mixed-phase ferromagnetic and paramagnetic state near room temperature in these thin films.

cond-mat.mtrl-sci↗