TMTSF insulator

We saw in Section 1.1 for N2 that electron localization implies insulating ground states and that localization can be reduced by applying external pressure. When reducing temperature from RT down to about 20 K, (TMTTF)2X and (TMTSF)2X... 

Examples of these two behaviours are provided by (TMTSF)2Re04 and (TMTSF)2C104. In (TMTSF)2Re04 the anions order at c. 180 K with a reduced wave vector (, and undergo a sharp metal-insulator transition with Ea = 0.16 eV (Jacobsen et al, 1982). The anion ordering transition of (TMTSF)2C104 occurs at 24 K with = (0, 0). This qr value implies that the... 

The neutral insulator TMTSF, which shows field-effect conduction with /Th — 0.2 cm s (Nam et al, 2003), when transformed into a Bechgaard salt also becomes superconducting, but at lower temperatures. In this case the perfect segregation of organic and inorganic molecular planes leads to confined electronic systems, which in the normal state are quasi ID. Organic superconductors based on the BEDT-TTF molecule represent the case of pure 2D electronic systems. 

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]. 

As far as all selenium molecular compounds are concerned, the shallow minimum of the resistivity is no longer observed, but different behaviors are observed for the representative compounds (TMTSF)2PF6 and (TMTSF)2C104. The conductivity of the former compound reaches 106 (ft cm)-1 at 12 K and then vanishes abruptly as the system undergoes a metal-insulator transition [15] toward an antiferromagnetic (SDW) ground state, as indicated by NMR [53] and susceptibility measurements [54]. Furthermore, 13C NMR has proved the incommensurate nature of this SDW ground state [52]. It is the presence of this additional periodicity in... 

The critical divergence of Eq. (20) is followed accurately for the onset of SDW ground states arising either from a quasi-one-dimensional metal as for (TMTSF)2PF6 or from a one-dimensional Mott-Hubbard paramagnetic insulator [i.e., (TMTTF)2Br at ambient pressure or (TMTTF)2PF6 under 13 kbar] [57] (Fig. 11). 

This short contact, which is the shortest Se-0 distance observed in any (TMTSF)2X salt, arises because below the metal-insulator transition at 137 K (47) the anion is shifted off the center of symmetry resulting in a crystallographic disorder (48). 

The common pressure dependence of the (TMTSF)2X salts is illustrated in Fig. 20. Below the critical pressure Pc the electronic ground state is an insulator of either the spin density wave type or the anion-assisted Peierls type. Above Pc the ground state is superconducting. The critical pressure Pc varies monotonically with the anion volume discussed above but the transition temperature at ambient pressure and the type of low-pressure ground state depend specifically on the... 

An unusual pressure-dependent resistivity has been reported for (TMTSF)2P02F2 (47). This compound has a metal-insulator transition at 135 K at ambient pressure, which is quite high considering that the anion volume of P02F2 is very close to that of C104. Furthermore, pressure has a relatively small effect, and even at pressures of 14.5 kbar... 

Many of the Bechgaard salts show at ambient pressure a metal-insulator transition, Tmi, around a few tens of K as can be seen from the resistivity behavior shown for several (TMTSF)2X salts in Fig. 2.5 [35]. This transition is... 

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