arXiv subjects

Peter A. van Aken

Publications and source records attributed to Peter A. van Aken.

At least 19 recordsLinked to original sources

arXiv2026

Hair is a functionally graded composite, not a uniform fiber

Hair provides mammals with diverse benefits, including protection, thermoregulation, and enhanced sensory perception. Unlike tendons and teeth, which are biomineralized, hair is hypothesized to accomplish its structure-function relationship purely through keratin, a fibrous protein that provides structural integrity. Recent research showed that mechanical properties can vary substantially both within and across hair types: the whiskers of Asian elephant (Elephas maximus) exhibit a two-order-of-magnitude material stiffness reduction from base to tip, whereas elephant body hairs are nearly homogenous. Here, we demonstrate that three hierarchical structures vary significantly along the body hairs and whiskers of domestic cat (Felis catus): the layered outer keratin wall, 250-nm-diameter melanosome-like granules in the cortex, and calcium enrichment of these granules. As occasionally described for human hair, the oblong granules are arranged in longitudinal channels, potentially reinforcing the cortex; their prevalence correlates with local mechanical properties along the hair's length. Prolonged chemical treatment of body hairs removes calcium from the granules while breaking down the outer cuticle and internal cortex, hardening bases and splitting tips. Though previously assumed uniform, morphology, composition, and elemental enrichment can change along hairs, producing composite structures with functional gradients.

physics.bio-ph↗

arXiv2026

Predicting Interface Structure using the Minima Hopping Method with a Machine Learning Interatomic Potential

Predicting atomic-scale interfacial structures remains a central challenge in materials science due to their structural complexity and the difficulty of direct comparison between computational and experimental results. In this study, we present an efficient approach for interface structure prediction that integrates the Minima Hopping Method (MHM) with the state-of-the-art machine learning interatomic potential (MLIP), Allegro. We demonstrate that the MHM-Allegro approach provides a robust and computationally efficient route for predicting interfacial structures in the benchmark system SrTiO3 Sigma 3 (112)[110] tilt grain boundaries (GBs), consistently identifying the lowest-energy configurations across different stoichiometries. Furthermore, we introduce a strategy for constructing defect-representative training datasets without explicitly including defective configurations, achieving excellent extrapolative performance in interface predictions. The predictive capability is further validated through direct comparison with experimental observations of the SrTiO3 Sigma 5 (310)[001] GB, where the predicted atomic configurations show strong agreement with experimental measurements. This work represents a significant step toward bridging the gap between ab initio predictions and experimentally observed interfacial structures.

cond-mat.mtrl-sci↗

arXiv2025

Comparative Raman study of Ruddlesden-Popper nickelates and the monolayer-trilayer polymorph

Ruddlesden-Popper (RP) nickelates have attracted intense interest following the discovery of superconductivity in several members of the series, including bilayer (BL) La$_3$Ni$_2$O$_7$, trilayer (TL) La$_4$Ni$_3$O$_{10}$, and structural polymorphs composed of monolayer-bilayer or monolayer-trilayer (ML-TL) units. However, an inherent propensity of the RP series to form intergrown phases during single-crystal synthesis, together with spatial variations in oxygen stoichiometry, has complicated the determination of their intrinsic material properties. As a consequence, conflicting reports have emerged on both their electronic phase transitions and lattice dynamics. In this work, we perform a comparative study of the phononic and electronic Raman responses of high-quality ML-TL single crystals and contrast them with those of other RP nickelates, using samples with optimized oxygen content. We establish several Raman spectral features that enable unambiguous phase identification across the series. Moreover, we uncover characteristics in the phononic and electronic Raman response of ML-TL that are not reflected in the pure ML and TL compounds. We attribute these differences to a distinctive electronic structure arising from self-doping and confinement effects induced by the ML unit within the ML-TL lattice architecture.

cond-mat.str-el↗

arXiv2025

Relating the dynamics of photo de-mixing in mixed bromide-iodide perovskites to ionic and electronic transport

The observation of reversible de-mixing phenomena in mixed-halide perovskites under illumination is one of the most challenging as well as intriguing aspects of this class of materials. On the one hand, it poses critical constraints to the compositional space that allows reliable design of absorbers for perovskite photovoltaics. On the other hand, it holds potential for the development of novel optoionic devices where an ionic response is triggered via optical stimuli. Funda-mental questions about the origin of such photo de-mixing process remain unanswered, both in terms of its mechanism as well as thermodynamic description. Here, we relate in-situ measurements of ionic and electronic transport of mixed bromide-iodide perovskite thin films performed during photo de-mixing with the evolution of their optical and morpho-logical properties. The results point to the definition of different stages of the de-mixing process which, based on micros-copy and spectroscopic measurements, we assign to regimes of spinodal decomposition and nucleation of quasi-equilibrium iodide- and bromide-rich phases. Combined with density functional theory calculations, we explore the role of dimensionality in the mechanism and reversibility of photo de-mixing and dark re-mixing processes, referring to elec-tronic and ionic contributions to the de-mixing driving force. Additionally, our data emphasizes the role of the surface, as significantly different de-mixing dynamics, in terms of extent and reversibility, are observed for films with or without encapsulation. Our comprehensive analysis of transport, phase and optical properties of mixed-halide perovskites pro-vides guidelines for future materials design as well as for the more general fundamental understanding of light-induced ionic phenomena.

cond-mat.mtrl-sci↗

arXiv2025

Anomalous Sodium Insertion in Highly Oriented Graphite: Thermodynamics, Kinetics and Evidence for Two-Sided Intercalation

The difficult intercalation of sodium (Na) into graphite is studied by systematic and long-time investigations (of up to 2 years) using highly oriented pyrolytic graphite (HOPG). By studying chemical insertion of solid, liquid and gaseous Na at low and high temperatures (LT, HT) as well as using electrochemical insertion at 25 degree Celsius into uncoated and coated HOPG, it became clear that insertion equilibrium requires HT. On decreasing chemical intercalation temperature from HT (500 degree Celsius) to LT (25 degree Celsius), thermodynamic control was found to change to diffusion control and finally to interfacial control. For the electrochemical insertion, coating (TiO2) proved advisable (to avoid co-intercalation) and efficient in reducing the interfacial resistance. Measured saturation values were found to be not higher than about 1 mol %. Towards room temperature higher equilibrium values cannot be excluded but would in view of the very low driving force kinetically be very difficult to reach. The reversible cell voltage of the saturated composition (versus alkali metal) is distinctly lower than for the analogous cells using lithium (Li) or potassium (K). Detailed transmission electron microscopy (TEM) studies reveal the unexpected fact that at HT Na predominantly enters HOPG in the form of two-sided intercalation sandwiching carbon layers (bilayers), while at LT more highly aggregated layers appear to a comparable degree, accompanied with the formation of higher-dimensional crystal imperfections. The reasons for this peculiar feature and the non-monotonic thermodynamics in the sequence Li-Na-K-Rb-Cs are discussed not only from an energetic but also from an entropic point of view.

cond-mat.mtrl-sci↗

arXiv2025

Direct imaging of disordered residual oxygen and its impact on electronic structure in an infinite-layer nickelate superlattice

Infinite layer nickelates have garnered significant attention due to their potential for high-temperature superconductivity. Despite extensive research, the interplay between oxygen stoichiometry and electronic properties in infinite layer nickelates remains inadequately understood. In this study, we employ advanced electron microscopy techniques and theoretical modeling to directly visualize the distribution of residual oxygen within an 8NdNiO$_2$/2SrTiO$_3$ superlattice, providing novel insights into its structural and electronic effects. Our multislice ptychography analysis reveals a disordered arrangement of apical oxygen atoms, even in regions with low residual oxygen occupancy, invisible in conventional projected images but discernible in depth-resolved phase contrast images. This disordered distribution suggests the formation of local domains with varying degrees of oxygenation, leading to significant structural distortions. Electron energy-loss spectroscopy reveals inhomogeneous hole doping, which may influence the occurrence of superconductivity. Complementary density functional theory calculations show how residual oxygen and associated structural distortions, such as pyramidal and octahedral configurations, alter the electronic structure. Although superconductivity was not observed in the studied superlattice, our findings highlight the critical influence of residual oxygen in shaping electronic phases and suggest that precise control of oxygen stoichiometry is essential in infinite layer nickelates.

cond-mat.mtrl-sci↗

arXiv2025

Unveiling the Interfacial Reconstruction Mechanism Enabling Stable Growth of the Delafossite PdCoO2 on Al2O3 and LaAlO3

Delafossites, comprised of noble metal (A+) and strongly correlated sublayers (BO2-), form natural superlattices with highly anisotropic properties. These properties hold significant promise for various applications, but their exploitation hinges on the successful growth of high-quality thin films on suitable substrates. Unfortunately, the unique lattice geometry of delafossites presents a significant challenge to thin-film fabrication. Different delafossites grow differently, even when deposited on the same substrate, ranging from successful epitaxy to complete growth suppression. These variations often lack a clear correlation to obvious causes like lattice mismatch. Unidentified stabilization mechanisms appear to enable growth in certain cases, allowing these materials to form stable thin films or act as buffer layers for subsequent delafossite growth. This study employs advanced scanning transmission electron microscopy techniques to investigate the nucleation mechanism underlying the stable growth of PdCoO2 films on Al2O3 and LaAlO3 substrates, grown via molecular-beam epitaxy. Our findings reveal the presence of a secondary phase within the substrate surface that stabilizes the films. This mechanism deviates from the conventional understanding of strain relief mechanisms at oxide heterostructure interfaces and differs significantly from those observed for Cu-based delafossites.

cond-mat.mtrl-sci↗

arXiv2025

Magnetotransport Properties in Epitaxial Films of Metallic Delafossite PdCoO$_2$: Effects of Thickness and Width Variations in Hall Bar Devices

We report on a combined structural and magnetotransport study of Hall bar devices of various lateral dimensions patterned side-by-side on epitaxial PdCoO$_2$ thin films. We study the effects of both the thickness of the PdCoO$_2$ film and the width of the channel on the electronic transport and the magnetoresistance properties of the Hall bar devices. All the films with thicknesses down to 4.88 nm are epitaxially oriented, phase pure, and exhibit a metallic behavior. At room temperature, the Hall bar device with the channel width $\text{W}=2.5\, μ\text{m}$ exhibits a record resistivity value of $0.85\,μΩ$cm, while the value of $2.70\,μΩ$cm is obtained in a wider device with channel width $\text{W}=10\, μ\text{m}$. For the 4.88 nm thick sample, we find that while the density of the conduction electrons is comparable in both channels, the electrons move about twice as fast in the narrower channel. At low temperatures, for Hall bar devices of channel width $2.5\,μ\text{m}$ fabricated on epitaxial films of thicknesses 4.88 and 5.21 nm, the electron mobilities of $\approx$ 65 and 40 cm$^2$V$^{-1}$s$^{-1}$, respectively, are extracted. For thin-film Hall bar devices of width $10\,μ\text{m}$ fabricated on the same 4.88 and 5.21 nm thick samples, the mobility values of $\approx$ 32 and 18 cm$^2$V$^{-1}$s$^{-1}$ are obtained. The magnetoresistance characteristics of these PdCoO$_2$ films are observed to be temperature dependent and exhibit a dependency with the orientation of the applied magnetic field. When the applied field is oriented 90° away from the crystal $c$-axis, a persistent negative MR at all temperatures is observed; whereas when the field is parallel to the $c$-axis, the negative magnetoresistance is suppressed at temperatures above 150K.

cond-mat.mtrl-sci↗

arXiv2024

Nonlinear optical response and spontaneous polarization in layer-stacked gallenene using second harmonic generation

Gallenene is a promising low-dimensional material with a structure down to the thickness of a single atom, similar to graphene. However, van der Waals stacking of two-dimensional (2D) gallenene under confinement remain poorly understood. In this study, we present evidence of the formation of parallel-stacked hexagonal gallenene (a100) structures in liquid gallium. The present study demonstrates the AB stacking of 2D gallenene a100 crystals in liquid gallium sandwiched between two graphene layers, as observed through transmission electron microscopy. A nonlinear optical response of the confined hexagonal gallenene was investigated through second harmonic generation (SHG) microscopy. The SHG signal exhibits periodic peak intensity shifts upon angular rotation up to 90 degrees and intensity dampening at elevated temperatures. These findings offer insights on device applications of 2D gallenene.

cond-mat.mtrl-sci↗

arXiv2024

Imaging atomic-scale magnetism with energy-filtered differential phase contrast method

We propose differential phase contrast (DPC) imaging using energy-filtered electrons to image the magnetic properties of materials at the atomic scale. Compared to DPC measurements with elastic electrons, our simulations predict about two orders of magnitude higher relative magnetic signal intensities and sensitivity to all three vector components of magnetization.

cond-mat.mtrl-sci↗

arXiv2024

Atomic-scale observation of geometric frustration in a fluorine-intercalated infinite layer nickelate superlattice

Anion doping offers immense potential for tailoring material properties, but achieving precise control over anion incorporation remains a challenge due to complex synthesis processes and limitations in local dopant detection. Here, we investigate the F-ion intercalation within an infinite layer NdNiO2+x/SrTiO3 superlattice film using a two-step synthesis approach. We employ advanced four-dimensional scanning transmission electron microscopy (4D-STEM) coupled with electron energy loss spectroscopy to map the F distribution and its impact on the atomic and electronic structure. Our observations reveal a striking geometric reconstruction of the infinite layer structure upon fluorination, resulting in a more distorted orthorhombic phase compared to the pristine perovskite. Notably, F-ion intercalation occurs primarily at the apical sites of the polyhedron, with some occupation of basal sites in localized regions. This process leads to the formation of two distinct domains within the nickelate layer, reflecting a competition between polyhedral distortion and geometric frustration-induced neodymium (Nd) displacement near domain interfaces. Interestingly, we observe an anomalous structural distortion where basal site anions are displaced in the same direction as Nd atoms, potentially linked to the partial basal site F-ion occupation. This coexistence of diverse structural distortions signifies a locally disordered F-ion distribution within the infinite layer structure with distinct F-ion configurations. These findings provide crucial insights into understanding and manipulating anion doping at the atomic level, paving the way for the development of novel materials with precisely controlled functionalities.

cond-mat.mtrl-sci↗

arXiv2024

Adsorption-controlled Growth of Homoepitaxial c-plane Sapphire Films

Sapphire is a technologically highly relevant material, but it poses many challenges to performing epitaxial thin-film deposition. We have identified and applied the conditions for adsorption-controlled homoepitaxial growth of c-plane sapphire. The films thus grown are atomically smooth, have a controlled termination, and are of outstanding crystallinity. Their chemical purity exceeds that of the substrates. The films exhibit exceptional optical properties such as a single-crystal-like bandgap and a low density of F+ centers.

cond-mat.mtrl-sci↗

arXiv2024

alpha-Ta Films on c-plane Sapphire with Enhanced Microstructure

Superconducting films of alpha-Ta are promising candidates for the fabrication of advanced superconducting qubits. However, alpha-Ta films suffer from many growth-induced structural inadequacies that negatively affect their performance. We have therefore explored a new synthesis method for alpha-Ta films which allows the growth of these films with unprecedented quality. Using this method, high quality alpha-Ta films are deposited at a comparably high substrate temperature of 1150 C. They are single-phase alpha-Ta and have a single out-of-plane (110) orientation. They consist of grains above 2 um that have one of three possible in-plane orientations. As shown by scanning transmission electron microscopy and electron energy loss studies the substrate-film interfaces are sharp with no observable intermixing. The obtained insights into the epitaxial growth of body-centered-cubic films on quasi-hexagonal substrates lay the basis for harnessing the high structural coherence of such films in various applications.

cond-mat.supr-con↗

arXiv2024

Unraveling nano-scale effects of topotactic reduction in LaNiO$_2$ crystals

Infinite-layer nickelates stand as a promising frontier in the exploration of unconventional superconductivity. Their synthesis through topotactic oxygen reduction from the parent perovskite phase remains a complex and elusive process. This study delves into the nano-scale effects of the topotactic lattice transformation within LaNiO$_2$ crystals. Leveraging high-resolution scanning transmission electron microscopy and spectroscopy, our investigations uncover a panorama of structural alterations, including grain boundaries and coherent twin boundaries, triggered by reduction-induced transformations. In addition, our analyses unveil the formation of an oxygen-rich disordered transition phase encircling impurities and pervading crystalline domains, and the internal strain is accommodated by grain boundary formation. By unraveling these nano-scale effects, our findings provide insights into the microscopic intricacies of the topotactic reduction process elucidating the transition from the perovskite to the infinite-layer phase within nickelate bulk crystals.

cond-mat.mtrl-sci↗

arXiv2024

Ultrafast phonon-mediated dephasing of color centers in hexagonal boron nitride probed by electron beams

Defect centers in hexagonal boron nitride have been extensively studied as room temperature single photon sources. The electronic structure of these defects exhibits strong coupling to phonons, as evidenced by the observation of phonon sidebands in both photoluminescence and cathodoluminescence spectra. However, the dynamics of the electron phonon coupling as well as phonon mediated dephasing of the color centers in hexagonal boron nitride remain unexplored. Here, we apply a novel time resolved CL spectroscopy technique to explore the population decay to phonon states and the dephasing time T2 with sub femtosecond time resolution. We demonstrate an ultrafast dephasing time of only 200 fs and a radiative decay of about 585 fs at room temperature, in contrast with all optical time resolved photoluminescence techniques that report a decay of a few nanoseconds. This behavior is attributed to efficient electron-beam excitation of coherent phonon polaritons in hexagonal boron nitride, resulting in faster dephasing of electronic transitions. Our results demonstrate the capability of our sequential cathodoluminescence spectroscopy technique to probe the ultrafast dephasing time of single emitters in quantum materials with sub femtosecond time resolution, heralding access to quantum path interferences in single emitters coupled to their complex environment.

quant-ph↗

arXiv2024

Magnetotransport Properties of Epitaxial Films and Hall Bar Devices of the Correlated Layered Ruthenate Sr$_3$Ru$_2$O$_7$

For epitaxial Sr$_3$Ru$_2$O$_7$ films grown by pulsed laser deposition, we report a combined structural and magnetotransport study of thin films and Hall bar devices patterned side-by-side on the same film. Structural properties of these films are investigated using X-ray diffraction and high-resolution transmission electron microscopy, and confirm that these films are epitaxially oriented and nearly phase pure. For magnetic fields applied along the $c-$axis, a positive magnetoresistance of 10\% is measured for unpatterned Sr$_3$Ru$_2$O$_7$ films, whereas for patterned Hall bar devices of channel widths of $10$ and $5\, μ$m, magnetoresistance values of 40\% and 140\% are found, respectively. These films show switching behaviors from positive to negative magnetoresistance that are controlled by the direction of the applied magnetic field. The present results provide a promising route for achieving stable epitaxial synthesis of intermediate members of correlated layered strontium ruthenates, and for the exploration of device physics in thin films of these compounds.

physics.app-ph↗

arXiv2024

Interface Design Beyond Epitaxy: Oxide Heterostructures Comprising Symmetry-forbidden Interfaces

Epitaxial growth of thin-film heterostructures is generally considered the most successful procedure to obtain interfaces of excellent structural and electronic quality between three-dimensional materials. However, these interfaces can only join material systems with crystal lattices of matching symmetries and lattice constants. We present a novel category of interfaces, the fabrication of which is membrane-based and does not require epitaxial growth. These interfaces therefore overcome limitations imposed by epitaxy. Leveraging the additional degrees of freedom gained, we demonstrate atomically clean interfaces between three-fold symmetric sapphire and four-fold symmetric SrTiO3. Atomic-resolution imaging reveals structurally well-defined interfaces with a novel moiré-type reconstruction.

cond-mat.mtrl-sci↗

arXiv2023

Unconventional crystal structure of the high-pressure superconductor La$_3$Ni$_2$O$_7$

The discovery of high-temperature superconductivity in La$_3$Ni$_2$O$_7$ at pressures above 14 GPa has spurred extensive research efforts. Yet, fundamental aspects of the superconducting phase, including the possibility of a filamentary character, are currently subjects of controversial debates. Conversely, a crystal structure with NiO$_6$ octahedral bilayers stacked along the $c$-axis direction was consistently posited in initial studies on La$_3$Ni$_2$O$_7$. Here we reassess this structure in optical floating zone-grown La$_3$Ni$_2$O$_7$ single crystals that show signs of filamentary superconductivity. Employing scanning transmission electron microscopy and single-crystal x-ray diffraction under high pressures, we observe multiple crystallographic phases in these crystals, with the majority phase exhibiting alternating monolayers and trilayers of NiO$_6$ octahedra, signifying a profound deviation from the previously suggested bilayer structure. Using density functional theory, we disentangle the individual contributions of the monolayer and trilayer structural units to the electronic band structure of La$_3$Ni$_2$O$_7$, providing a firm basis for advanced theoretical modeling and future evaluations of the potential of the monolayer-trilayer structure for hosting superconductivity.

cond-mat.supr-con↗