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We comparably performed polarized Raman scattering studies of MoTe2 and WTe2. By rotating crystals to tune the angle between the principal axis of the crystals and the polarization of the incident/scattered light, we obtained the angle dependence of the intensities for all the observed modes, which is perfectly consistent with careful symmetry analysis. Combining these results with first-principles calculations, we clearly identified the observed phonon modes in the different phases of both crystals. Fifteen Raman-active phonon modes (10 Ag + 5 Bg) in the high-symmetry phase 1T'-MoTe2 (300 K) were well assigned, and all the symmetry-allowed Raman modes (11 A1 + 6 A2) in the low-symmetry phase Td-MoTe2 (10 K) and 12 Raman phonons (8 A1 + 4 A2) in Td-WTe2 were observed and identified. The present paper provides basic information about the lattice dynamics in transition-metal dichalcogenides and may shed some light on the understanding of the extremely large magnetoresistance in this class of materials. [ABSTRACT FROM AUTHOR]

We have performed polarized and temperature-dependent Raman scattering measurements on extremely large magnetoresistance compounds TaAs2 and NbAs2. In both crystals, all the Raman active modes, including six Ag modes and three Bg modes, are clearly observed and well assigned with the combination of symmetry analysis and first-principles calculations. The well-resolved periodic intensity modulations of the observed modes with rotating crystal orientations verify the symmetry of each assigned mode and are fitted to experimentally determine the elements of Raman tensor matrices. The broadening of two Ag modes seen in both compounds allows us to estimate the electron-phonon coupling constant, which suggests a relatively small electron-phonon coupling in the semimetals TaAs2 and NbAs2. The present study provides the fundamental lattice dynamics information on TaAs2 and NbAs2 and may shed light on the understanding of their extraordinary large magnetoresistance. [ABSTRACT FROM AUTHOR]

The recent renaissance of black phosphorus (BP) as a two-dimensional (2D) layered material has generated tremendous interest, but its unique structural characters underlying many of its outstanding properties still need elucidation. Here we report Raman measurements that reveal an ultralow-frequency collective compression mode (CCM) in BP, which is unprecedented among similar 2D layered materials. This novel CCM indicates an unusually strong interlayer coupling, and this result is quantitatively supported by a phonon frequency analysis and first-principles calculations. Moreover, the CCM and another branch of low-frequency Raman modes shift sensitively with changing number of layers, allowing an accurate determination of the thickness up to tens of atomic layers, which is considerably higher than previously achieved by using high-frequency Raman modes. These findings offer fundamental insights and practical tools for further exploration of BP as a highly promising new 2D semiconductor. [ABSTRACT FROM AUTHOR]

The novel ferroelectric-like structural transition observed in metallic LiOsO3 [Y. Shi et al., Nat. Mater. 12, 1024 (2013)], has invoked many theoretical and experimental interests. In this work, we have performed polarized and temperature-dependent Raman scattering measurements on high-quality single crystal LiOsO3 and identified Raman-active modes in both centrosymmetric phase (300 K, R3?c) and noncentrosymmetric phase (10 K, R3c). Only four phonon peaks are observed in the former phase, while there are 12 peaks in the latter phase because of the reduction of crystal symmetry. With the help of careful symmetry analysis and first-principles calculations, we can make a systematic assignment for the observed Raman modes in both phases. The significant changes in linewidth and the continuous evolution of Raman frequencies with temperatures were observed for the Eg modes around the transition temperature, which suggests that the ferroelectric-like structural transition is a continuous order-disorder transition. The result sheds light on the coexistence of ferroelectricity and metallicity in the compound. [ABSTRACT FROM AUTHOR]