arXiv subjects

Nicolas M. Kawahala

Publications and source records attributed to Nicolas M. Kawahala.

6 recordsLinked to original sources

arXiv2026

Terahertz Lattice Imprint of a Hidden Multipolar Ordering Transition in Gd$_2$Ru$_2$O$_7$

Magnetic order carried by higher-rank multipoles produces no net moment and couples only weakly to conventional dipolar probes. Recently, the lattice has been proposed as an alternative detection channel for the associated ordering transitions. Here we report magneto-terahertz time-domain spectroscopy of the pyrochlore ruthenate Gd$_2$Ru$_2$O$_7$, in which a single spectrum resolves an exchange-split Gd$^{3+}$ transition and an infrared-active optical phonon. The temperature and field dependence of the magnetic mode give two independent determinations of the internal exchange field at the rare-earth site, which yield a magnitude of 5-6 T and a free-ion $g$-factor. The optical phonon, sensitive to spin-disorder scattering rather than to the static exchange field, exhibits a sharp anomaly in frequency and lifetime at 10 K that shifts to 30 K under 7 T, which we identify as an ordering transition of the Gd sublattice hidden from bulk thermodynamic probes. These results identify spin-phonon coupling as a route to rare-earth ordering transitions in multipolar magnets, one that is applicable to polycrystalline samples.

cond-mat.str-el↗

arXiv2026

Magnetic permeability and zone-folded phonons in layered NiPS$_3$

Nickel phosphorus trisulfide (NiPS$_3$) stands out as a Mott insulator exhibiting XY-type antiferromagnetic order. In this work, we investigate the electrodynamic response of single-crystalline NiPS$_3$ using Terahertz Time-Domain Spectroscopy (THz-TDS) as a function of temperature. In cases where the magnetic permeability cannot be approximated to unity, conventional THz transmission analysis is often restricted to the complex refractive index, hindering the distinction between magnetic and dielectric contributions. To overcome this limitation, we demonstrate an extraction methodology capable of decoupling the magnetic permeability from the electric permittivity. Assuming that the dielectric contribution remains invariant below the N'eel temperature (TN), we used data from the paramagnetic phase to isolate the intrinsic magnetic component at 13 K. The angular and thermal dependence of the spectra revealed a magnon mode near 1 THz (for 0$^\circ$ orientation) and, distinctively, a zone-folded phonon activated by symmetry breaking (at 90$^\circ$). The isolated magnetic permeability was successfully modeled using Drude-Lorentz oscillators, enabling the extraction of fundamental spin dynamics parameters in this material.

cond-mat.mtrl-sci↗

arXiv2025

Optimizing Aperture Geometry in THz-TDS for Accurate Spectroscopy of Quantum Materials

Terahertz time-domain spectroscopy (THz-TDS) provides a powerful platform for investigating low-energy excitations in quantum materials. Because these materials are often limited in size, experimental setups typically rely on tightly focused beams and metallic holders with small apertures. In this work, we perform a systematic study of how aperture geometry influences THz signal transmission in a standard free-space configuration. By analyzing time- and frequency-domain data for circular apertures of varying diameters and thicknesses, we quantify the spatial and spectral filtering effects imposed by aperture size. We show that small apertures progressively attenuate low-frequency components of the transmitted signal, while higher-frequency content remains comparatively unaffected. These effects become especially significant for apertures smaller than typical THz beam waists, resulting in amplitude suppression and phase distortions that compromise the accuracy of frequency-domain analysis and optical parameter retrieval. To validate these observations, additional measurements were performed on a representative quantum material, confirming the practical relevance of the identified aperture effects. The transmitted intensity as a function of aperture diameter also provides a straightforward method for estimating the beam waist at the focus. In contrast, standard aperture thicknesses do not introduce measurable distortions, confirming the adequacy of treating thick, non-resonant apertures as dielectric slabs. These findings establish practical guidelines for aperture selection in THz-TDS and underscore the importance of preserving low-frequency response for reliable characterization of quantum materials.

physics.optics↗

arXiv2025

Windowing in terahertz time-domain spectroscopy: resolving resonances in thin-film samples

Terahertz time-domain spectroscopy (THz-TDS) has become a powerful tool for investigating the optical properties of thin films, offering direct access to the complex permittivity in the terahertz range. However, in transmission-based measurements of thin films on thick substrates, multiple reflections and limited time windows can introduce artifacts that obscure resonant features such as phonon modes. Time-domain windowing remains one of the most widely adopted strategies to mitigate these effects, yet systematic guidelines on its application remain scarce. In this work, we organize a practical routine for extracting the complex permittivity from THz-TDS data, focusing on when and how to apply time-domain windowing. The routine incorporates decision points for truncation versus smooth apodization, and emphasizes tailoring the window length and shape to specific signal conditions. We demonstrate the approach using representative measurements on PbTe thin films, highlighting cases in which truncation suffices, where apodization is essential, and how different window functions and lengths influence the resulting spectra. We also propose simple metrics to assess signal continuity and guide window selection. Although other analysis techniques exist, including parametric spectral estimation, this study focuses on formalizing windowing-based processing into an accessible experimental workflow. Our results show that the choice of window parameters can significantly affect the accuracy of extracted material parameters, particularly for sharply resonant systems. This work provides an accessible framework for improving spectral fidelity in THz-TDS of thin-film samples.

physics.optics↗

arXiv2025

Metallicity-driven polar transitions in topological epilayers

Polar metals, materials that exhibit both electric polarization and high conductivity, can also host topological phases. Because free carriers strongly suppress distortive polar order and change the Fermi level, controlling charge dynamics is crucial for simultaneously tuning ferroelectric and topological phases in the same material. Here, we explore the experimental conditions that enable access to these phases in bismuth-doped Pb$_{1-x}$Sn$_x$Te epilayers. For samples in the topological phase at $x = 0.5$, we use terahertz time-domain spectroscopy to evaluate their complex permittivity as a function of temperature. We observe a non-monotonic variation in carrier concentration with bismuth doping, indicating a change in carrier type. By tracking the transverse optical phonon mode, we identify a ferroelectric phase transition when distortive polar order emerges below a critical temperature that depends on carrier concentration. We show that bismuth doping controls the metallicity-dependent order parameters in the softening and hardening phases. Our work demonstrates a tunable platform for engineering exotic states of matter that integrate metallicity, ferroelectricity and topology.

cond-mat.mtrl-sci↗

arXiv2024

Shaping terahertz waves using anisotropic shear modes in a van der Waals mineral

Naturally occurring van der Waals (vdW) materials are currently attracting significant interest due to their potential as low-cost sources of two-dimensional materials. Valuable information on vdW materials' interlayer interactions is present in their low-frequency rigid-layer phonon spectra, which are experimentally accessible by terahertz spectroscopy techniques. In this work, we have used polarization-sensitive terahertz time-domain spectroscopy to investigate a bulk sample of the naturally abundant, large bandgap vdW mineral clinochlore. We observed a strong and sharp anisotropic resonance in the complex refractive index spectrum near 1.13 THz, consistent with our density functional theory predictions for shear modes. Polarimetry analysis revealed that the shear phonon anisotropy reshapes the polarization state of transmitted THz waves, inducing Faraday rotation and ellipticity. Furthermore, we used Jones formalism to discuss clinochlore phononic symmetries and describe our observations in terms of its eigenstates of polarization. These results highlight the potential of exploring vdW minerals as central building blocks for vdW heterostructures with compelling technological applications.

cond-mat.mes-hall↗