Photoinduced semiconductor-metal phase transition in a peierls system
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C PROPERTIES OF SOLIDS
Photoinduced Semiconductor–Metal Phase Transition in a Peierls System A. L. Semenov Ulyanovsk State University, ul. L’va Tolstogo 42, Ul’yanovsk, 432970 Russia e-mail: [email protected] Received June 7, 2006
Abstract—The dynamical equation for the order parameter of the metal–semiconductor phase transition, as well as the kinetic equation for the density of nonequilibrium electron–hole pairs of a Peierls system in a light field, has been derived. An expression for the time τ of the nonthermal photoinduced semiconductor–metal phase transition has been obtained from these equations for the case of an ultrashort light pulse. It has been shown that, to initiate the phase transition, the energy density W of the light pulse must be higher than the critical value Wc . The Wc , τ, and optical absorption coefficient γ0 that are calculated in the framework of the proposed model are in agreement with the experimental data (Wc ≈ 12 mJ/cm2 , τ ≈ 75 fs, and γ0 ≈ 105 cm–1) on the irradiation of a vanadium dioxide film by a laser pulse with a duration of τp ≈ 15 fs, a photon energy of ω0 = 1.6 eV, and an energy density of W = 50 mJ/cm2 . PACS numbers: 71.45.Lr, 78.20.-e DOI: 10.1134/S1063776107010086
1. INTRODUCTION A number of materials whose electronic properties can be described in the Peierls model and various its modifications are known in experiments. Platinum complexes with mixed valence [1], blue bronzes K0.3MoO3 and Rb0.3MoO3 [2, 3], salts TCNQ [1], TaS3 [2, 3], VO2 [4, 5], TaSe3 , NbS3 , NbSe3 , (NbSe4)10/3I, (NbSe4)2I, etc. [3] are most studied among these materials. The electronic spectrum of these compounds at the Fermi level has a quasi-one-dimensional band formed when the wavefunctions of the d and f electrons of the neighboring atoms arranged in the form of parallel chains overlap. The Peierls model considered in this work is a chain of atoms each containing one outer electron. In the high-temperature metallic phase, the atoms are equidistantly located in the chain, whereas the conduction band of the electronic spectrum is half filled with electrons. When temperature reaches the critical value T0 upon cooling, the metallic phase becomes unstable and the metal–semiconductor phase transition occurs. This transition is characterized by the formation of a gap in the electronic spectrum of the system at the Fermi level and by pairwise approach of atoms along the atomic chain [1]. In this case, the atomic chain itself is deformed [6]. The thermodynamically equilibrium metal–semiconductor (or semiconductor–metal) phase transition in the Peierls system can be initiated by pressure (both uniaxial and uniform) [4, 7], deviation from the ideality of a crystal lattice (in particular, when doping with sub-
stitutional impurities) [1, 4, 5, 8–10], constant electric field [11, 12], adsorption of molecules from the gas phase, etc. [12–14]. Near the critical point T0 in a Peierls-material film, a one-dimensional spatially periodic heterophase structure of alternating metal and semiconducting phases can be forme
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