Lukas M. Eng

Multiferroic domain walls in functional oxides exhibit properties distinct from the bulk and are increasingly exploited as active elements in nanoelectronic and photonic devices. Deterministic tuning of domain populations has typically remained limited to local solutions, or removal with temperature. Here, the manipulation of ferroelastic domain structure, and removal of the twin walls between them in single‐crystal using in situ uniaxial strain is demonstrated. Combining atomic force microscopy, X‐ray diffraction, and Raman spectroscopy with first‐principles calculations, the microscopic evolution of the twin domain population is mapped through a strain‐driven transition from the rhombohedral ground state toward the predicted orthorhombic phase. Applied strains below produce pronounced surface flattening and large‐scale domain reorganization, establishing uniaxial strain as a technically accessible tuning parameter for ferroelastic domain engineering. These results open a route to active alterations of domain architectures in ‐based heterostructures, with implications for epitaxial film properties, nanoscale phonon‐polariton optics, and ultrafast lattice control.

The implementation of polaritonic materials into nanoscale devices requires selective tuning of parameters to realize desired spectral or thermal responses. One robust material, α‐MoO ₃ , an orthorhombic crystal boasting three distinct phonon dispersions, provides three polaritonic dispersions of hyperbolic phonon polaritons (HPhPs) across the mid‐infrared (MIR). Here, the tunability of both optical and thermal responses in isotopically enriched α‐MoO ₃ ( ⁹⁸ MoO _3, Mo ¹⁸ O ₃ , and ⁹⁸ Mo ¹⁸ O ₃ ) is explored. A uniform ∼5% spectral redshift from ¹⁸ O enrichment is observed in both Raman‐ and IR‐active TO phonons. Both the in‐ and out‐of‐plane thermal conductivities for the isotopic variations are reported. Ab initio calculations both replicate experimental findings and analyze the select‐mode three‐phonon scattering contributions. The HPhPs from each isotopic variation are probed with s‐SNOM, and we report an HPhP Q ‐factor maxima increase in ⁹⁸ Mo ¹⁸ O ₃ of ∼50% along the [100] in the RB ₂ and ∼100% along the [001] in the RB ₃ with respect to ⁹⁸ MoO ₃ . Observations in both real and Fourier space of higher‐order HPhP modes propagating in slabs of isotopically enriched α‐MoO ₃ without the use of a subdiffractional surface scatterer are presented here. This work establishes the dual‐element isotope enrichment of α‐MoO ₃ as an intrinsic strategy to design optical, thermal, and polaritonic properties.

Accurate knowledge of thermal transport in amorphous oxides is essential for effective thermal management in advanced semiconductor and cryogenic electronic devices. In this work, we investigate the through-plane thermal conductivity (κz) of amorphous HfO2/SiO2 nanolaminates fabricated by atomic layer deposition across the temperature range from 30 to 315 K. These multilayers serve as a model system for studying heat transport in amorphous thin films, where interfacial effects may significantly influence thermal conduction. Thermal conductivities were determined using the differential 3ω method and analyzed within the framework of the heat equation. A composite model combining the minimum thermal conductivity approach for the individual amorphous layers with the diffuse mismatch model for interfacial effects quantitatively reproduces the experimental data. The multilayers exhibit low thermal conductivities of 0.77±0.08 and 0.050±0.0015Wm−1K−1 at 300 and 30 K, respectively.

Probing novel properties, arising from twisted interfaces, has traditionally relied on the stacking of exfoliated two-dimensional materials and the spontaneous formation of van der Waals bonds. So far, investigations involving intimate covalent or ionic bonds have not been a focus. Yet, we show here that an established technique, involving thermocompressional wafer bonding, works well for creating twisted non-van der Waals interfaces. We have successfully bonded z-cut lithium niobate single crystals to create ferroelectric oxide interfaces with strong polar discontinuities and have mapped the associated emergent interfacial conductivity. In some instances, a dramatic change in microstructure occurs, involving local dipolar switching. A twist-induced collapse in the capability of the system to effec8tively screen interfacial bound charge is implied. Importantly, this only occurs around specific moiré twist angles with sparse coincident lattices and associated short-range aperiodicity. In quasicrystals, aperiodicity is known to induce pseudo-bandgaps and we suspect a similar phenomenon here.

Hyperbolic phonon polaritons ‐ hybridized modes arising from the ultrastrong coupling of infrared light to strongly anisotropic lattice vibrations in uniaxial or biaxial polar crystals ‐ enable to confine light to the nanoscale with low losses and high directionality. In even lower symmetry materials, such as monoclinic ‐Ga ₂ O ₃ (bGO), hyperbolic shear polaritons (HShPs) further enhance the directionality. Yet, HShPs are intrinsically supported only within narrow frequency ranges defined by the phonon frequencies of the host material. Here, we report spectral tuning of HShPs in bGO by isotopic substitution. Employing near‐field optical microscopy to image HShPs in ¹⁸ O bGO films homoepitaxially grown on a ¹⁶ O bGO substrate, we demonstrate a spectral redshift of 40 cm ⁻¹ for the ¹⁸ O bGO, compared to ¹⁶ O bGO. The technique allows for direct observation and a model‐free estimation of the spectral shift driven by isotopic substitution without the need for knowledge of the dielectric tensor. Complementary far‐field measurements and ab initio calculations ‐ in good agreement with the near‐field data ‐ confirm the effectiveness of this estimation. This multifaceted study demonstrates a significant isotopic substitution induced spectral tuning of HShPs into a previously inaccessible frequency range, creating new avenues for technological applications of such highly directional polaritons.

We demonstrate that two-dimensional talc, a naturally abundant phyllosilicate mineral, supports hyperbolic phonon-polaritons (HPhPs) across the mid- and far-infrared wavelength range.