Optical properties metal-insulator transition

The origin of these transformations is very difficult to investigate. Yet it appears that the optical study should be very helpful for this purpose. An analysis of T dependence of the phase phonon absorptions at 317 and 253 cm -1 show that the 54 K metal-insulator transition is driven by the Peierls distortion on the TCNQ sublattice, whereas the distortion on the TTF chains increases markedly around 49-K phase transition [100]. It is a typical example of a close relationship between the optical properties of organic conductors and a molecular mechanism of the phenomena that occur in the material. 

Figures 13 and 14 show the crystal structure and the temperature dependence of electrical conductivity measured along the one-dimensional axis, fc-axis, of TTF-TCNQ [53]. The conductivity increases with decreasing temperature down to about 60 K below which the conductivity is characterized by thermally activated nature. The metallic properties are ascertained by much experimental evidence such as optical reflectivity, spin-magnetic susceptibility, and thermopower [54]. In the insulating state similar measurements also suggest the presence of a band gap at the Fermi level. These measurements suggest the metal-insulator transition to oceur at 53 K.

In Section 2.3 the structural and optical properties of neutral and cationic Na clusters at r = 0 K as functions of size are presented and compared with experimental data recorded at low temperature. The temperature-dependent line-broadening will be illustrated by the example of Na9, since in this case a comparison with experimental data at different temperatures is particularly instructive. In Section 2.4 the results of ab initio molecular dynamics (AIMD) studies on Li9 will serve to show different temperature behavior of distinct types of structures as well as their isomerization mechanisms. The study of possible metal-insulator transitions and segregation into metallic and ionic parts in finite systems carried out on prototypes of nonstoichiometric alkali halide and alkali hydride clusters with single and multiple excess electrons is presented in Section 2.5. A comparison of structural and optical characteristics of Na F and lAnUm (n > m) series allows us to illustrate the influence of different bonding properties. 

The first depletion spectra obtained for neutral sodium clusters N = 2-40 were characterized by structureless broad features containing one or two bands. The results were interpreted in terms of collective resonances of valence electrons (plasmons) for all clusters larger than tetramers [2, 52-55]. The analogies between findings for metallic clusters and observations of giant dipole resonances in nuclei have attracted a large attention. Therefore the methods employed in nuclear physics, such as different versions of RPA in connection with the jellium model, have also been applied for studying the optical properties of small clusters. Another aspect was the onset of conductivity in metal-insulator transitions. 

Nonstoichiometric alkali halide clusters X Y with X = Na, Li, K and Y = Cl, F containing single and multiple excess electrons have been extensively studied experimentally " and theoretically " as prototypes of possible metal-insulator transitions and segregation into metallic and ionic parts in finite systems. Hydrogenation of lithium clusters has also been investigated but considerably less than alkali halides (cf. Ref. 36 and references therein). Of course the ground state properties such as ionization potentials (IPs) were first available for both halides and hydrides. In the case of the optical probes the visible region was until recently experimentally more easily accessible than the infrared and therefore the data were incomplete. 

In situations where the performance is dependent on the uniqueness of the crystal structure of the material, such as the metal-Insulator transition in vanadium oxide, acousto-optic and acoustic-electric responses of AIN, ZnO, PZT, BaTiOg and SrTi03, superconducting properties of copper oxide based perovskites, A-15 silicides, and NbN, wide band gap and dopability of SiC, transistor action in Si/NiSi2(CoSi2)/Sl epitaxial layers etc., it is important to optimize the deposition conditions for the growth of a %[Pg.395]

The charge transport and optical properties of the [Si(Pc)0]-(tos)y)n materials as y=0 -+ 0.67 are reminiscent of the [Si(Pc)0]-(BF4)y)n system, but with some noteworthy differences. Again there is an insulator-to-metal transition in the thermoelectric power near y 0.15-0.20. Beyond this doping stoichiometry, the tosylates also show a continuous evolution through a metallic phase with decreasing band-filling. However, the transition seems somewhat smoother than in the BF4 system for y)>0.40, possibly a consequence of a more disordered tosylate crystal structure. Both [Si(Pc)0]-(tos)y)n optical reflectance spectra and four-probe conductivities are also consistent with a transition to a metal at y 0.15-0.20. Repeated electrochemical cycling leads to considerably more decomposition than in the tetrafluoroborate system. 

Temperature changes do not appreciably affect ultraviolet optical properties of both metals and insulators, although at low temperatures absorption bands associated with excitons and electron band transitions are usually sharper, and frequencies of peak absorption may shift slightly. In the soft x-ray region, transitions of core electrons buried in the interior of atoms hardly notice temperature changes. 

Although this theory explains theoretically the experimental observations in the case of ReOj, TiO, and VO, it fails to verify the conductivity characteristics of transition metal oxides such as TiO, VO, MnO, and NiO. Band theory explains the metallic characteristics but fails to account for the electrical properties of insulators or semiconductors and metal-nonmetal transitions because of neglect of electronic correlation inherent in the one-electron approach to the problem. Although there is no universal model for description of the conductivity, magnetic and optical properties of a wide range of materials (e.g., simple and complex oxides, sulfides, phosphides), several models have been proposed (for details, see Refs. 447-453). Of these, a generally accepted one is that described by Goodenough (451). 

Let us mention the following general rule optical transitions between states which may be described by wavefunctions localized over distances of the order of the lattice constant are changed relatively little by disorder. We shall see several examples of this rule. This may also explain why the optical properties of amorphous insulators often differ little from those of crystalline insulators. On the other hand, if the bonds are very delocalized (i.e. valence electron wavefunctions spread over long distances) the changes are also small. Indeed, the optical properties of liquid metals differ little from those of crystalline metals. Semiconductors with covalent bonding are the materials most sensitive to disorder. Their transport properties are more drastically changed than their optical properties. 

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