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1. Infrared-active phonon modes and static dielectric constants in α -(Al _x Ga _{1− x} ) ₂ O ₃ (0.18  ≤ x  ≤ 0.54) alloys

   https://doi.org/10.1063/5.0085958  

   Stokey, Megan; Gramer, Teresa; Korlacki, Rafał; Knight, Sean; Richter, Steffen; Jinno, Riena; Cho, Yongjin; Xing, Huili Grace; Jena, Debdeep; Hilfiker, Matthew; et al (March 2022, Applied Physics Letters)

   We determine the composition dependence of the transverse and longitudinal optical infrared-active phonon modes in rhombohedral α-(Al_xGa_1−x)₂O₃alloys by far-infrared and infrared generalized spectroscopic ellipsometry. Single-crystalline high quality undoped thin-films grown on m-plane oriented α-Al₂O₃substrates with x =  0.18, 0.37, and 0.54 were investigated. A single mode behavior is observed for all phonon modes, i.e., their frequencies shift gradually between the equivalent phonon modes of the isostructural binary parent compounds. We also provide physical model line shape functions for the anisotropic dielectric functions. We use the anisotropic high-frequency dielectric constants for polarizations parallel and perpendicular to the lattice c axis measured recently by Hilfiker et al. [Appl. Phys. Lett. 119, 092103 (2021)], and we determine the anisotropic static dielectric constants using the Lyddane–Sachs–Teller relation. The static dielectric constants can be approximated by linear relationships between those of α-Ga₂O₃and α-Al₂O₃. The optical phonon modes and static dielectric constants will become useful for device design and free charge carrier characterization using optical techniques. 

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2. Parallel pumping of magnons in inhomogeneous spin textures probed through NV spin relaxometry

   https://doi.org/10.1063/5.0192063  

   Trimble, J.; Gould, B.; Heremans, F_J; Zhang, S_S-L; Awschalom, D_D; Berezovsky, J. (February 2024, Journal of Applied Physics)

   We combine micromagnetic simulations and nitrogen-vacancy (NV) defect center spin relaxometry measurements to study magnon modes in inhomogeneous spin textures. A thin, micrometer-scale ferromagnetic disk is magnetized in a vortex state in which the magnetization curls around a central core. Micromagnetic simulations show that at zero applied field, the magnetization dynamics of the disk consist of a low frequency gyrotropic mode and higher frequency azimuthal magnon modes, all far detuned from the NV spin transition frequencies. An in-plane static magnetic field breaks the azimuthal symmetry of the vortex state, resulting in the magnon modes transforming in frequency and spatial profile as the field increases. Experimentally, we probe the dynamics of vortex magnetization as a function of applied in-plane static field and ac driving frequency by optically monitoring a nearby NV defect center spin. At certain values of the applied magnetic field, we observe enhanced spin relaxation when driving at twice the frequency of the NV ground state spin transition in optically detected magnetic resonance measurements. We attribute this effect to parallel pumping of a magnon mode in the disk producing magnons at half the excitation frequency. Micromagnetic simulations support this finding, showing spatial and spectral overlap of a confined magnon mode and an NV spin transition, with sufficient interaction strength to explain the observed signal. 

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3. Bounds on Substructure Dielectric Resonator Antennas Using Characteristic Modes

   https://doi.org/10.1109/AP-S/USNC-URSI47032.2022.9886490  

   Yang, Binbin; Adams, Jacob J. (July 2022, 2022 IEEE International Symposium on Antennas and Propagation and USNC-URSI Radio Science Meeting (AP-S/URSI))

   This paper presents a recent development on the fundamental limits of substructure dielectric resonator antennas (DRAs). Specifically, the substructure DRAs are found to have higher resonance frequencies than the superstructure DRA, and a bounding relation on Q factor is also obtained. Numerical examples based on a ring DRA and a cylindrical DRA is examined here to demonstrate the bounding relations. 

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4. High-frequency/high-field electron paramagnetic resonance generalized spectroscopic ellipsometry characterization of Cr3 + in β -Ga2O3

   https://doi.org/10.1063/5.0255802  

   Rindert, Viktor; Galazka, Zbigniew; Schubert, Mathias; Darakchieva, Vanya (February 2025, Applied Physics Letters)

   Electron paramagnetic resonance of Cr3+ ions in β-Ga2O3 is investigated using terahertz spectroscopic ellipsometry under magnetic field sweeping, a technique that enables the polarization resolving capabilities of ellipsometry for magnetic resonance measurements. We employed a single-crystal chromium-doped β-Ga2O3 sample, grown by the Czochralski method, and performed ellipsometry measurements at magnetic field strengths ranging from 2 to 8 T, at frequencies from 82 to 125 and 190 to 230 GHz, and at a temperature of 15 K. Analysis of the frequency-field diagrams derived from all Mueller matrix elements allowed us to differentiate between the effects of electron spin Zeeman splitting and zero-field splitting and to accurately determine the anisotropic Zeeman splitting g-tensor and the zero-field splitting parameters. Our results confirm that Cr3+ ions predominantly substitute into octahedral gallium sites. Line shape analysis of Mueller matrix element spectra using the Bloch–Brillouin model provides the spin volume concentration of Cr3+ sites, showing very good agreement with results from chemical analysis by inductively coupled plasma-optical emission spectroscopy and suggesting minimal occupation of sites with inactive electron paramagnetic resonance. This study enhances our understanding of the magnetic and electronic properties of chromium-doped β-Ga2O3 and demonstrates the effectiveness of high-frequency/high-field electron paramagnetic resonance generalized spectroscopic ellipsometry for characterizing defects in ultrawide-bandgap semiconductors. 

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5. Identifying fracture-controlled resonance modes for structural health monitoring: insights from Hunter Canyon Arch (Utah, USA)

   https://doi.org/10.5194/esurf-13-81-2025  

   Grechi, Guglielmo; Moore, Jeffrey R; McCreary, Molly E; Jensen, Erin K; Martino, Salvatore (January 2025, Earth Surface Dynamics)

   Abstract. Progressive fracturing contributes to structural degradation of natural rock arches and other freestanding rock landforms. However, methods to detect structural changes arising from fracturing are limited, particularly at sites with difficult access and high cultural value, where non-invasive approaches are essential. This study aims to determine how fractures affect the dynamic properties of rock arches, focusing on resonance modes as indicators of structural health conditions. We hypothesize that damage resulting from fracture propagation may influence specific resonance modes that can be identified through ambient vibration modal analysis. We characterized the dynamic properties (i.e., resonance frequencies, damping ratios, and mode shapes) of Hunter Canyon Arch, Utah (USA), using spectral and cross-correlation analyses of data generated from an array of nodal geophones. Results revealed properties of nine resonance modes with frequencies between 1 and 12 Hz. Experimental data were then compared to numerical models with homogeneous and heterogeneous compositions, the latter implementing weak mechanical zones in areas of mapped fractures. All numerical solutions replicated the first two resonance modes of the arch, indicating these modes are insensitive to structural complexity derived from fractures. Meanwhile, heterogenous models with discrete fracture zones succeeded in matching the frequency and shape of one additional higher mode, indicating this mode is sensitive to the presence of fractures and thus most likely to respond to structural change from fracture propagation. An evolutionary crack damage model was then applied to simulate fracture propagation, confirming that only this higher mode is sensitive to structural damage resulting from fracture growth. While examination of fundamental modes is common practice in structural health monitoring studies, our results suggest that analysis of higher-order resonance modes can be more informative for characterizing fracture-driven structural damage. 

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