Multiphonon interactions and collective excitations in charge density wave of transition metal dichalcogenides

Charge density wave (CDW) is a periodic modulation of electronic charge leading to a reconstruction of the lattice, an emergence of zone folded mode along with collective excitations.1 Transition metal chalcogenides have shown a great potential to study the underlying physics of multi body interactions like electron-phonon (e-ph) as well as plasmons-phonon coupling,2,3 excitonic complexes4 and CDW instabilities.5,6 We are elaborating on the collective excitation and multiphonon interactions like amplitude, zone-folded and two phonon (2_ph) mode associated with CDW in 2H-TaS2 and 2H-NbSe2. The 2_ph is a unique excitation appears due to anharmonicity mediated longitudinal acoustic modes associated with the Kohn anomaly and provides a substantial evidences of e-ph coupling.7 The experimental observations of anomalous response of 2_ph mode across CDW is clarified with phonon self-energy formalism. Transport measurements shows that both materials exhibit incommensurate CDW (TI-CDW) at ~ 76 K (2H-TaS2) and ~ 30 K (2H-NbSe2). As revealed by the estimated e-ph coupling (l ~ 0.007 for 2H-NbSe2 and ~ 0.013 for 2H-TaS2) and anharmonicity constant (d ~ 6.92 for 2H-NbSe2 and ~ 2.723 for 2H-TaS2), we emphasize on higher e-ph coupling in 2H-TaS2 while larger anharmonicity 2H-NbSe2.