Time-Resolved Picosecond Reflectivity Study of Laser-Excited Layered Compounds
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TIME-RESOLVED PICOSECOND REFLECTIVITY STUDY OF LASER-EXCITED LAYERED COMPOUNDS C.Y.HUANG*, A.M.MALVEZZI*, N. BLOEMBERGEN** AND F.J.DI SALVO, JR. * Los Alamos National Laboratory, Los Alamos, New Mexico USA **
Division of Applied Sciences, Harvard University, Cambridge, MA 02138, USA AT&T Bell Laboratories, Murray Hill, N.J. 07974, USA
ABSTRACT We have employed the pump-and-probe technique to perform picosecond time resolved measurements of the reflectivity changes in two archetypal layered compounds, IT-TiS 2 and lT-TiSe 2 probed at 1.064 gm after pumping by 20 ps, .532 gm laser pulses. At the threshold fluence, 40 mJ/cm2 , the reflectivity drops sharply, marking the occurrence of a phase transformation on the surface of the sample. Above threshold, the reflectivity reaches a value as low as -0.1 at high fluences, strongly suggesting that, like in graphite, the high temperature phase is not metallic. INTRODUCTION Very recently, the transient optical properties of crystalline graphite surfaces excited by visible picosecond laser pulses have been investigated with 20 - 30 ps time resolution 1,2 Reflectivity measurements at low excitation levels have revealed the presence of a fast decaying elecron-hole plasma evolving from the initial semimetallic configuration. More interestingly, at a well defined pump threshold value of -140 mJ/cm2 a phase transition occurs, which is characterized by lower reflectivity values. We have concluded that the high temperature phase is not metallic, having a lower value of the index of refraction, but, rather, intermediate between the one of graphite and that of diamond. This unexpected new phase is presumably caused by a strong perturbation of the spatial arrangement of carbon atoms in the graphite lattice, which consists of hexagonally arranged, loosely stacked atomic layers. The question thus arises whether the electronic properties of the high temperature phase originate from a particular initial structure. For this reason, it is of interest to investigate other archetypal layered compounds. In this paper, we have chosen two crystalline layered compounds, IT-TiS 2 and IT-TiSe 2 . The titanium atoms are octahedrally coordinated with chalcogen atoms forming a chalcogen-metal-chalcogen sandwich layer configuration. 3 In a plane, atoms are hexagonally packed. However, the coordination around the non-metal is quite lopsided, leading to the marked cleavage properties perpendicular to the hexagonal symmetry axis. The basic atomic structure of loosely coupled X - Ti - X (X = S, Se) atom sheet sandwiches makes the mechanical, thermal and electrical properties extremely anisotropic, like those of graphite. The lattice constants for TiS 2 4 are slightly smaller
Mat. Res. Soc. Symp. Proc. Vot. 74.
1987 Materials Research Society
270
than those of TiSe 2 . 5 It is now well established that the former is an extrinsic semiconductor with an indirect band gap of 0.2 0.3 eV, 6-9 but it is normally non-stoichiometric and thus exhibits metallic conductivity arising from an excess of Ti atoms. TiS
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