Terahertz Spectroscopy and DFT Analysis of Phonon Dynamics of the Layered Van der Waals Semiconductor Nb₃X₈ (X = Cl, I)

Abstract

We have conducted a terahertz spectroscopic study and a density functional theory analysis of the phonon dynamics of the layered van der Waals semiconductors Nb3Cl8 and Nb3I8. Several infrared-active phonon modes were observed in the terahertz region, and their frequencies were found to be in excellent agreement with our first-principles lattice dynamics calculations. For Nb3Cl8, the observed phonon spectra are consistent with a structural transition at 90 K from the high-temperature P3̅m1 phase to the low-temperature R3̅m phase. Also, our study confirmed that the structural and magnetic transitions were coupled in Nb3Cl8. For Nb3I8, which is nonmagnetic at and below room temperature, no significant temperature or magnetic field dependence was observed in the phonon spectra. Our study provides an intriguing connection between the structural properties and the paramagnetic–nonmagnetic transitions in Nb3Cl8 and Nb3I8.

Raman study of layered breathing kagome lattice semiconductor Nb3Cl8

“…The magnetic susceptibility of the material with respect to temperature in figure 1(d) shows hysteresis between the cooling and heating transitions, with the critical cooling temperature found to be at 90 K and the critical heating temperature found to be at 103 K, both corresponding to the transition from a paramagnetic state at room temperature to a nonmagnetic singlet state [5,6] at low temperatures. This magnetic transition has been linked to the structural phase transition which Nb 3 Cl 8 undergoes at low-temperatures in the literature, and therefore the presence of this magnetic transition in our data is the evidence that the structural transition has occurred [4][5][6][7].…”

“…In this case, there is a significant change in the type of Raman active phonons expected for the low-temperature phase (A g and B g ) as well as the Raman tensors governing the polarization selection rules [37][38][39]. However, other groups have reported that the low-temperature phase corresponds to the R 3m space group instead [5,7]. Since crystals with P 3m1 and R 3m symmetries share the same point group (D 3d ) and hence the same Raman tensors for the A g and E g modes, there would be no difference in the polarization-dependence of these modes.…”

“…We have also observed a shift in peak frequency of up to 5 cm −1 between 300 K and 7.6 K data for our measured peaks between 163 cm −1 and 350 cm −1 measured using a 532 nm laser (see supporting information figure S4). To investigate if the shift in frequency is abrupt as observed in [7], we present additional Raman peak position data at 140 K and 80 K for a 2 L sample (see supporting information figure S5). We note that the experiment was conducted using a different setup (Opticool) with distinct lasers and a spectrometer compared to the Raman experiment carried out at 300 K and 7.6 K (Montana C2).…”

“…This change in crystal symmetry is seen as the crystal changes from trigonal P 3m1 at high temperatures to a predominantly monoclinic C2/m at low temperatures [4]. However, other studies suggested that the low-temperature phase is R 3m symmetric instead [5,7]. Additionally, Nb 3 Cl 8 features favorable cleavage energies for the separation of layers from the bulk crystal, speaking to the feasibility of its exfoliation to thinner layers [8].…”