Electron-phonon interactions and two-phonon modes associated with charge density wave in single crystalline1T−VSe2

Abstract

The charge density wave (CDW) is a unique phenomenon mostly realized in two-dimensional (2D) metallic layered transition metal dichalcogenides. We report on the observation of commensurate CDW (C-CDW) and incommensurate (I-CDW) transition in single crystal 1T-VSe 2 using the spectroscopy technique in the temperature range of 50-120 K. The room temperature Raman spectra showed a sharp A 1g mode ∼206 cm −1 along with two additional modes associated to E g ∼ 257 cm −1 symmetry and two-phonon (2 ph) ∼ 332 cm −1 interactions. The onset temperature of I-CDW and C-CDW is estimated from resistance measurements supported by magnetic measurements. Remarkably, at the onset of I-CDW ∼ 115 K, a significant enhancement in the intensity of the weak E g mode is observed along with emergence of a doubly degenerate E g (2) mode ∼144 cm −1 , while below 70 K, the emergence of the A 1g mode ∼170 cm −1 signifies the onset of C-CDW. The observation of a sudden rise in the intensity of the E g mode ∼257 cm −1 mode, below I-CDW, showed the involvement of electron-phonon coupling in 1T-VSe 2 .

Structural and Transport Properties of 1T-VSe2 Single Crystal Under High Pressures

“…Figure 1B displays the XRD pattern for powdered 1T-VSe 2 single crystals. All peaks can be well indexed by the P-3m1 space group, and the fitting yields the cell parameters a b 3.36 Å, c 6.09 Å, and V 59.54 Å 3 , in excellent accordance with the previous reports (van Bruggen and Haas, 1976;Pandey and Soni, 2020). A typical single-crystal diffraction pattern for 1T-VSe 2 is shown in the inset of Figure 1B.…”

“…The in-plane resistance R as a function of temperature at ambient pressure is given in Figure 1D. The R-T curve shows an overall metallic behavior but with a clear hump feature at around 105 K related to the CDW transition, in agreement with the previous studies (van Bruggen and Haas, 1976;Pandey and Soni, 2020). The CDW transition temperature T CDW is defined as the temperature at which the dR/dT is minimum (inset of Figure 1D).…”

“…The adjacent Se-V-Se sandwiches are held together by van der Waals interactions, and each V atom is surrounding by the nearest six Se atoms, constituting VSe 6 octahedron Figure 1A (van Bruggen and Haas, 1976). It exhibits an incommensurate CDW with 4a × 4a × 3.18c periodic lattice distortion upon cooling to around 110 K, followed by a second transition to a commensurate CDW state at ∼ 80 K (Thompson and Silbernagel, 1979;Tsutsumi, 1982;Eaglesham et al, 1986;Terashima et al, 2003;Pandey and Soni, 2020). The mechanism of the 3D CDW was initially explained by Fermi-surface nesting resulting from the warped 3D electron pocket centered around the M point at the edge of the Brillouin zone (Terashima et al, 2003;Sato et al, 2004;Strocov et al, 2012).…”

“…The mechanism of the 3D CDW was initially explained by Fermi-surface nesting resulting from the warped 3D electron pocket centered around the M point at the edge of the Brillouin zone (Terashima et al, 2003;Sato et al, 2004;Strocov et al, 2012). Recently, electron-phonon coupling was also found to contribute to the formation of the CDW (Pandey and Soni, 2020;Si et al, 2020). Previous studies showed that the valence band structure of 1T-VSe 2 can be transformed into two-dimensional (2D) character by alkali metal intercalation, which was attributed to charge transfer and decoupling of the layers induced by the intercalated alkali ions (Starnberg et al, 1993;Brauer et al, 1998).…”

van der Waals driven anharmonic melting of the 3D charge density wave in VSe2

“…3 f) and, therefore, fluctuation corrections are unnecessary to invoke the low T CDW of VSe 2 as compared to its 1 T counterparts. The critical role of the EPI has been recently suggested by Raman scattering 35 and DFT calculations 36 . Indeed, revisited ARPES experiments 37 in NbSe 2 revealed a pronounced dispersion along k z discarding the nesting-driven CDW formation and leaving EPI as the major contributor 20 .…”