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Abstract Magnetic transition metal chalcogenides form an emerging platform for exploring spin-orbit driven Berry phase phenomena owing to the nontrivial interplay between topology and magnetism. Here we show that the anomalous Hall effect in pristine Cr 2 Te 3 thin films manifests a unique temperature-dependent sign reversal at nonzero magnetization, resulting from the momentum-space Berry curvature as established by first-principles simulations. The sign change is strain tunable, enabled by the sharp and well-defined substrate/film interface in the quasi-two-dimensional Cr 2 Te 3 epitaxial films, revealed by scanning transmission electron microscopy and depth-sensitive polarized neutron reflectometry. This Berry phase effect further introduces hump-shaped Hall peaks in pristine Cr 2 Te 3 near the coercive field during the magnetization switching process, owing to the presence of strain-modulated magnetic layers/domains. The versatile interface tunability of Berry curvature in Cr 2 Te 3 thin films offers new opportunities for topological electronics. 

We conducted a tip-enhanced Raman scattering spectroscopy (TERS) and photoluminescence (PL) study of quasi-1D TaSe 3− δ nanoribbons exfoliated onto gold substrates. At a selenium deficiency of δ ∼ 0.25 (Se/Ta = 2.75), the nanoribbons exhibit a strong, broad PL peak centered around ∼920 nm (1.35 eV), suggesting their semiconducting behavior. Such nanoribbons revealed a strong TERS response under 785 nm (1.58 eV) laser excitation, allowing for their nanoscale spectroscopic imaging. Nanoribbons with a smaller selenium deficiency (Se/Ta = 2.85, δ ∼ 0.15) did not show any PL or TERS response. The confocal Raman spectra of these samples agree with the previously-reported spectra of metallic TaSe 3 . The differences in the optical response of the nanoribbons examined in this study suggest that even small variations in Se content can induce changes in electronic band structure, causing samples to exhibit either metallic or semiconducting character. The temperature-dependent electrical measurements of devices fabricated with both types of materials corroborate these observations. The density-functional-theory calculations revealed that substitution of an oxygen atom in a Se vacancy can result in band gap opening and thus enable the transition from a metal to a semiconductor. However, the predicted band gap is substantially smaller than that derived from the PL data. These results indicate that the properties of van der Waals materials can vary significantly depending on stoichiometry, defect types and concentration, and possibly environmental and substrate effects. In view of this finding, local probing of nanoribbon properties with TERS becomes essential to understanding such low-dimensional systems. 

We report the results of polarization‐dependent Raman spectroscopy of phonon states in single‐crystalline quasi‐one‐dimensional NbTe₄and TaTe₄van der Waals materials. The measurements were conducted in the wide temperature range from 80 to 560 K. Our results show that although both materials have identical crystal structures and symmetries, there is a drastic difference in the intensity of their Raman spectra. While TaTe₄exhibits well‐defined peaks through the examined wavenumber and temperature ranges, NbTe₄reveals extremely weak Raman signatures. The measured spectral positions of the phonon peaks agree with the phonon band structure calculated using the density‐functional theory. We offer possible reasons for the intensity differences between the two van der Waals materials. Our results provide insights into the phonon properties of NbTe₄and TaTe₄van der Waals materials and indicate the potential of Raman spectroscopy for studying charge‐density‐wave quantum condensate phases.

 

We report the polarization-dependent Raman spectra of exfoliated MoI3, a van der Waals material with a “true one-dimensional” crystal structure that can be exfoliated to individual atomic chains. The temperature evolution of several Raman features reveals an anomalous behavior suggesting a phase transition of magnetic origin. Theoretical considerations indicate that MoI3 is an easy-plane antiferromagnet with alternating spins along the dimerized chains and with inter-chain helical spin ordering. The calculated frequencies of phonons and magnons are consistent with the interpretation of the experimental Raman data. The obtained results shed light on the specifics of the phononic and magnonic states in MoI3 and provide a strong motivation for further study of this unique material with potential for future spintronic applications.