Peierls-type metal-insulator transition

After publication of the X-ray study, the charge transfer was obtained from the reciprocal-space position of the satellite reflections, which occur in the diffraction pattern at temperatures below the Peierls-type metal-insulator transition at 53 K (Pouget et al. 1976). Assuming that the gap in the band structure occurs at twice the Fermi wavevector, that is, at 2kF, the position of the satellite reflections corresponds to a charge transfer of 0.59 e, in excellent agreement with the direct integration. The agreement confirms the assumption that the gap in the band structure occurs at 2kF. 

The trisulphides (and triselenides) of Ti, Zr, Hf, Nb and Ta crystallize in onedimensional structures formed by MSg trigonal prisms that share opposite faces. Metal atoms in these sulphides are formally in the quadrivalent state, and part of the sulphur exists as molecular anions, M S2 S . TaSj shows a metal-insulator transition of the Peierls type at low temperatures (Section 4.9). NbSj adopts a Peierls distorted insulating structure suggesting the possibility of a transformation to a metallic phase at high temperatures, but does not transform completely to the undistorted structure. Electronic properties and structural transitions of these sulphides have been reviewed (Rouxel et al, 1982 Meerschaut, 1982 Rouxel, 1992). 

This type of metal-insulator transition is considered to be nothing but the Peierls transition that had been proposed by R.E. Peierls to explain the electronic properties of bismuth [58], He pointed out that a one-dimensional metal is unstable at low temperatures to undergo a metal-insulator transition accompanied by charge-density waves (CDW). A precursor phenomenon of the Peierls... 

All of the physical measurments point to the equivalence of all the platinum atoms (in a noninteger oxidation state) in a chain. The results of the numerous measurements on K2Pt(CN)4Bro.3(H20)s, demonstrates this system to be a one-dimensional metal undergoing a metal-insulator transition as the temperature is lowered. The far infrared and optical measurements show that the electronic excitation spectrum is not that of a simple one-dimensional metal but has a complex behavior at low frequencies. The available data from many diverse types of experiments have been analyzed in terms of numerous models. This system is currently best characterized in terms of a one-dimensional metal undergoing a Peierls transition to a semiconductor at low temperatures, with evidence for the presence of a pinned charge density wave. Further careful measurements of the partially oxidized tetracyanoplatinates are necessary to fully understand the applicability of various one-dimensional models to this class of materials. 

BEDT-TTF)4M(CN)4, M=Pt, Ni (6,7) In these compounds [6], the "ET" molecules form tetramerized slipped stacks (Fig. lb), already observed in other P-type "ET" salts, which present a tendency to a 2-D electronic structure. These salts exhibit also the two characteristic electronic (CT) absorption bands (Fig. 2) associated with strong vibronic IR modes, which are temperature-dependent [15]. Indeed, we have shown that there is a variation of the electron-molecular vibration (e-mv) coupling effect, associated with a "Peierls-like" phase transition around 200 K for both salts. These results are confirmed by the investigation of the electrical and the magnetic properties of the Ni(CN)4 salt, which indicate a metal-insulator transition in the same temperature range [9]. Nevertheless, low-temperature structural investigations are needed to fully characterize this phase transition. 

Fig. 6.16. Probing the transient electronic structure of TbTes in the course of its ultrafast insulator-to-metal transition induced by femtosecond-laser excitation (a) Time- and angle-resolved photoemission spectroscopy. A TbTcs sample was excited by an IR pulse (hi Pump = I.SeV, about 50 fs duration) and probed after a time delay At with a UV pulse (hi/pump = 6 eV, about 90 fs duration). The photoelectron intensity and kinetic energy E ,i were measured as a function of the emission angles (a, 9). (b) Insulator-to-metal transition Above the critical temperature Tc (or 100fsafterlaserexcitation)the band gap of the CDW phase closes, (c) "Snapshots" of the electronic band structure E(k)in TbTej fordifferenttimedelaysAt.Afterlaserexcitation, the gap has closed and the band dispersion near the Eermi level, Ep, changed after a time delay of 100 fs. Such a delayed collapse of the band gap is characteristic of the "Peierls type" mechanism (see text).

Structural correlations of Tc with chemical formula or structure type are limited. For the / -(ET)2X salts with linear anions there is a linear dependence of Tc on anion length (but this correlation fails for very long anions, as other phases form) [33]. The (TMTSF)2X salts with tetrahedral anions X show a linear dependence of the Peierls metal-to-insulator phase transition temperature with tetrahedral anion radius [33]. 

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