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Coherent phonons in the Terahertz (THz) regime have gained attention as potential candidates for next-generation high-speed, low-energy information carriers in atomically thin phononic or phonon-integrated on-chip devices. Nevertheless, achieving efficient control of the phonon generation dynamics over THz coherent phonons continues to pose a considerable challenge. In this work, we explore THz coherent phonon generation in exfoliated van der Waals (vdW) flakes of WSe2 on Au (WSe2/Au) and Si (WSe2/Si) by using time-resolved pump–probe spectroscopy. The generation of THz coherent phonons was studied as a function of the WSe2 layer thickness and laser wavelength. Notably, a significant enhancement in THz coherent phonon generation was observed in the WSe2/Au structure, but only within a specific range of WSe2 thicknesses and laser wavelengths. The results from numerical simulations, which consider a self-hybridized optical cavity depending on WSe2 thickness and optical reflectance and Raman spectroscopy measurements, all align well with the time-domain observations of THz coherent phonon generation. We propose that the observed enhancement in THz coherent phonon generation is strongly influenced by light–matter interactions in the WSe2 cavity, a mechanism that may be applicable to a broader range of vdW materials. These findings offer promising insights for the development of THz phononic or phonon-integrated devices. 

The phase transitions in IrTe2 have been extensively studied but the symmetry at each phase is yet to be settled. Employing second harmonic generation (SHG) measurements over a temperature range of 4 –300 K, we probe the evolution of the symmetry of IrTe2. Our results indicate shifts in two distinct transition temperatures (Ts1 and Ts2 with Ts1 > Ts2) through thermal cycling, providing an explanation for the variations of reported values in literature. The SHG polarimetry measurements identify symmetries in different temperature ranges, confirming the trigonal symmetry above Ts1, the triclinic symmetry between Ts1 and Ts2, and the coexistence of multiple stripe phases below Ts2. The most striking feature is the reemergence of a trigonal phase as reflected by six-fold symmetry below ~ 10 K which is likely responsible for phenomena observed at low temperatures. 

The noncentrosymmetric Weyl semimetal PtBi2−x (t-PtBi2−x) exhibits various interesting technologically important physical properties. We report the experimental investigation of PtBi1.6 via second harmonic generation (SHG), single-crystal x-ray diffraction, magnetic susceptibility, and electrical resistivity measurements. While bulk structural, magnetic, and electrical properties show no phase transitions below room temperature, the temperature dependence of the SHG intensity reveals two anomalies: one at T ∗ ∼ 60 K and another at Tx ∼ 200 K. Quantitative analysis indicates that the SHG signal results from both the buckled Bi1 surface termination with the 3m symmetry and flat Bi2 surface termination with the m symmetry. However, the anomalies are mainly driven by Bi1 on the surface: (1) T ∗ marks the onset of surface states which is also manifested in the c-axis resistivity drop and (2) Tx corresponds to the lowest thermal contraction of the structure and enhanced magnetic susceptibility. This study demonstrates that SHG is a powerful technique for probing surface properties even for noncentrosymmetric materials. 

Inorganic lead-halide perovskite, cesium lead bromide (CsPbBr3), shows outstanding optoelectronic properties. Both solution- and melt-based methods have been proposed for CsPbBr3 crystal growth. The solution-based growth was done at low-temperature, whereas the melt-based growth was done at high-temperature. However, the comparison of optical, physical, and defect states using these two different growth conditions has been scarcely studied. Here, we have compared the thermal and optical properties of solution-grown and melt-grown single crystals of CsPbBr3. Positron Annihilation Lifetime Spectroscopy (PALS) analysis showed that melt-grown crystal has a relatively smaller number of defects than the chemical synthesis method. In addition, crystals grown using the chemical method showed a higher fluorescence lifetime than melt-grown CsPbBr3. 

A novel antiferromagnetic semiconductor, Eu 3 Sn 2 P 4 , has been discovered. Single crystals of Eu 3 Sn 2 P 4 were prepared using the Sn self-flux method. The crystal structure determined by single crystal X-ray diffraction shows that Eu 3 Sn 2 P 4 crystallizes in the orthorhombic structure with the space group Cmca (Pearson Symbol, oP 216). Six Sn–Sn dimers connected by P atoms form a Sn 12 P 24 crown-shaped cluster with a Eu atom located in the center. Magnetization measurements indicate that the system orders antiferromagnetically below a T N ∼14 K at a low field and undergoes a metamagnetic transition at a high field when T < T N . The effective magnetic moment is 7.41(3) μ B per Eu, corresponding to Eu 2+ . The electric resistivity reveals a non-monotonic temperature dependence with non-metallic behavior below ∼60 K, consistent with the band structure calculations. By fitting the data using the thermally activated resistivity formula, we estimate the energy gap to be ∼0.14 eV. Below T N , the resistivity tends to saturate, suggesting the reduction of charge-spin scattering.