* Corresponding author. Fax: #81-78-969-2154; e-mail: misochko@crl.go.jp. Physica B 263 — 264 (1999) 57 — 59 Phonons in V  O  above and below the Mott transition: a comparison of time- and frequency-domain spectroscopy results O.V. Misochko*, M. Tani, K. Sakai, K. Kisoda, S. Nakashima, V.N. Andreev, F.A. Chudnovsky Kansai Advanced Research Center, Communications Research Laboratory, Iwaoka, Kobe, Hyogo 651-2401, Japan Department of Applied Physics, Osaka University, Osaka 565, Japan Ioffe Physical-Technical Institute, 194021 St.-Petersburg, Russia Abstract We report an optical study of the Mott transition in V  O  carried out by time- and frequency-domain spectroscopies. We have observed time-resolved spectra through the Mott transition with oscillations in reflectivity associated with the fully symmetric phonon modes. Raman spectrum also shows an abrupt change of both the phonon and electronic excitations through the phase transition. We notice that although the phonon spectra obtained by the time- and frequency-domain spectroscopies are essentially similar, there are distinct differences in the phonon lineshape.  1999 Elsevier Science B.V. All rights reserved. Keywords: Phonons; Mott transition; Femtosecond spectroscopy; Raman scattering V  O  undergoes a metal-insulator phase transi- tion at T  +150 K. Through the transition, the triclinic crystal structure changes to monoclinic and the electrical resistivity jumps over seven or- ders of magnitude. The V  O  crystal is considered as a prototype for the physics of Mott—Hubbard transition [1] and its properties may be related to ground state properties of high-¹  superconduc- tors and of other strongly correlated systems. The studies of phonons in V  O  have been conducted exclusively by frequency-resolved spectroscopic methods [1,2,6] Such methods yield information on thermally excited phonons. Using femtosecond time-domain spectroscopy one observes phonon modes that are excited coherently rather than ther- mally. Most solids are equally easily studied in either time or frequency domain, but sometimes one domain is more convenient than the other. To make such a comparison for V  O  was one of the motivations for the present study. Optical measurements were performed on single crystals of V  O  . The crystals were annealed to provide stoichiometric composition, polished and mounted on a temperature-controlled stage of an optical cryostat. Excitation and detection of coher- ent phonons were carried out with a conventional pump-probe setup using mode locked Ti : sapphire 0921-4526/99/$ — see front matter  1999 Elsevier Science B.V. All rights reserved. PII: S 0 9 2 1 - 4 5 2 6 ( 9 8 ) 0 1 1 9 5 - 8