A metal-to-insulator

The highly conductive class of soHds based on TTF—TCNQ have less than complete charge transfer (- 0.6 electrons/unit for TTF—TCNQ) and display metallic behavior above a certain temperature. However, these soHds undergo a metal-to-insulator transition and behave as organic semiconductors at lower temperatures. The change from a metallic to semiconducting state in these chain-like one-dimensional (ID) systems is a result of a Peieds instabihty. Although for tme one-dimensional systems this transition should take place at 0 Kelvin, interchain interactions lead to effective non-ID behavior and inhibit the onset of the transition (6). 

SmRu4Pj2 undergoes a metal to insulator transition in conjunction with antiferromagnetic ordering at 16 K (Sekine et al., 1998). 

The Au films formed at the interface show interesting electrical transport properties that are dependent on the reaction temperature (see Figure 4).24 Four-probe electrical resistance measurements on the nanocrystalline films show a metal to insulator transition, metallic behavior being shown by the films formed at high temperatures (>45 °C). The films formed at lower temperatures (<45 °C) show insulating behavior. 

Morin F., Oxides which show a metal to insulator transition at the Neel temperature. Phys. Rev. Lett. 3 (1959) pp. 34 - 36. 

A metallic state predicted by one-electron band theory is not stable when its Fermi surface is nested, and becomes susceptible to a metal-to-insulator transition under a suitable perturbation. We now examine the nature of the nonmetallic states that are derived from a normal metallic state upon mixing its occupied and unoccupied band levels. For simplicity, consider the 2D representation of the nested Fermi surface shown in (100), where the vector q is one of many possible nesting vectors. The occupied and unoccupied wave vectors are denoted by k and k, respectively. Each unit cell will be assumed to contain one AO x Suppose we choose the k and k values to satisfy the relationship... 

In discussing metal-to-insulator and metal-to-supercon-ductor transitions, it is convenient to describe the insulating and the superconducting states as a consequence of a perturbation on the metallic state. In the following, we first examine why Fermi surface nesting is likely to induce a metal-to-insulator transition from the viewpoint of band... 

It is quite clear that one wants to avoid a metal-to-insulator transition if one wants to ultimately stabilize an organic superconductor. Nevertheless, most organic conductors do undergo a Peierls transition. The temperature dependence of the electrical conductivity of TTF-TCNQ (Fig. 9) is prototypical for the class of organic metals [1,2]. 

Mor et al. (2000, 2001, 2003) studied the hydrogen indueed eleetrieal, optieal and struetural properties of praseodymium (28-170 nm) films with and with out Pd over layer. The films were deposited by thermal evaporation onto Corning 7059 substrates. A metal to insulator transition was observed in the uncapped Pr films (28-90 nm) during in situ hydrogen loading... 

H NMR relaxation data and low- and high-resolution C NMR studies on TTF[Ni(dmit)2]2 have been reported [24]. The results confirm the occurrence of both HOMO and LUMO conduction bands. The CDWs above 160 K are associated with the LUMO bands. Thermopower studies show there is an inversion in the sign of the carriers, holes becoming the dominant carriers [25]. Usually the onset of a CDW state induces a metal-to-insulator transition in a molecular lowdimensional conductor. This is not observed in TTF[Ni(dmit)2]2, which remains metallic at ambient pressure down to low temperature. This is explained by the multi-Fermi surface system of the Ni(dmit)2 stacks consisting of HOMO and LUMO bands. The CDW instability could open a gap in only some of these bands, leaving the other bands unaffected, so some metallic bands will remain. 

Some needle-shaped crystals of a -TTF[Pd(dmit)2]2 [lc,ld,27] are also isostructural with TTF[Ni(dmit)2]2-This phase also undergoes a metal-to-insulator transition at around 220 K. However, this transition is completely reversible and shows no hysteresis [27]. The effect of high pressure is the suppression of this metal-to-insulator transition. The superconducting transition is observed at 5.93 K and 24 kbar [Id]. A gradual localization of the carriers is observed at low pressure, which can be related to a charge density wave condensation evidenced by X-ray diffuse scattering experiments [23]. a -TTF[Pd(dmit)2]2 is the first example in which competition between CDW and superconductivity is observed. 

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