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Abstract The monoclinic 2M phase of tungsten disulfide (2M-WS₂) has emerged as a promising platform for topological superconductivity, combining topological surface states, anisotropic thermoelectric response, and superconductivity with a critical temperature of ∼8.8K. A detailed understanding of its electron–phonon coupling and phonon anharmonicity is, however, still lacking. Here, we investigate the phonon properties of few-layer 2M-WS₂as functions of laser power, temperature, polarization, and magnetic field using polarized Raman spectroscopy down to 12.5 K. We uncover an unusual, non-linear temperature dependence of the Raman shifts in selected modes, consistent with strong, mode-dependent electron–phonon interactions that dominate over phonon–phonon scattering at low temperatures. Angle-resolved polarized Raman measurements further reveal a pronounced change in the polarization behavior of the $A_g$ (4) mode at ∼316cm⁻¹upon cooling, indicating a modification of the underlying lattice and electronic anisotropy. In contrast, the applied magnetic field has a negligible influence on the Raman response over the measured temperature range. Together, these results advance the microscopic understanding of vibrational dynamics and electron–phonon coupling in few-layer 2M-WS₂and provide important insight into the lattice-electronic interplay relevant for its unconventional superconductivity and potential quantum and thermoelectric device applications. 

Abstract Optical phonon engineering through nonlinear effects has been utilized in ultrafast control of material properties. However, nonlinear optical phonons typically exhibit rapid decay due to strong mode-mode couplings, limiting their effectiveness in temperature or frequency sensitive applications. Here we report the observation of long-lived nonlinear optical phonons through the spontaneous formation of phonon frequency combs in the van der Waals material CrXTe₃(X=Ge, Si) using high-resolution Raman scattering. Unlike conventional optical phonons, the highestA_gmode in CrGeTe₃splits into equidistant, sharp peaks forming a frequency comb that persists for hundreds of oscillations and survives up to 200K. These modes correspond to localized oscillations of Ge₂Te₆clusters, isolated from Cr hexagons, behaving as independent quantum oscillators. Introducing a cubic nonlinear term to the harmonic oscillator model, we simulate the phonon time evolution and successfully replicate the observed comb structure. Similar frequency comb behavior is observed in CrSiTe₃, demonstrating the generalizability of this phenomenon. Our findings demonstrate that Raman scattering effectively probes high-frequency nonlinear phonon modes, offering insight into the generation of long-lived, tunable phonon frequency combs with potential applications in ultrafast material control and phonon-based technologies. 

In this study, we investigate optically active nanostructures on Bi2Sr2CaCu2O8+δ (BSCCO). The nanostructures are constructed from carbon nanocrystals formed using the electron beam of a scanning electron microscope to locally decompose residual hydrocarbons at the BSCCO surface. Atomic force microscopy is used to characterize the dimensions of these structures as a function of beam exposure time. This technique has been used to induce localized intercalation to form carbon nanoparticles up to tens of nanometers into a variety of 2D materials. Cross-sectional scanning transmission electron microscopy in BSCCO shows the presence of these carbon deposits on the surface, but there is no evidence they penetrate into the BSCCO substrate. Therefore, this technique is not suitable for localized intercalation in BSCCO, and it does have promise as a cost-effective approach to pattern nanometer scale etch stops.