TMTSF resistivity

As will be discussed later (Section 1.5), molecules containing no metallic elements are able to combine and form materials exhibiting metallic character, e.g., HMTSF-TCNQ, TTF-TCNQ, etc., or even lose any electrical resistance below a given temperature and thus become superconductors, e.g., (TMTSF)2C104. Metal-free molecules can also, in the solid state, show magnetic order, such as / -NPNN and /7-NC-C6F4-CNSSN, where in the absence of -electrons the magnetic properties are related to unpaired -electrons. 

Figure 22 Logarithm of the normalized resistance log(R/R0) versus the absorbed ionizing dose for samples of the organic charge-transfer salt TMTSF-DMTCNQ 1, electron irradiations 2, x-ray irradiations. (Adapted from Ref. 89.)...

Figure 1 Log-log plots of longitudinal resistivity p versus temperature T for representative organic conductors TTF-TCNQ (from Ref. 14), HMTSF-TCNQ (from Ref. 15), (TMTSF)2PF6 (from Refs. 17 and 19), TMTSF2C104 (from Ref. 16), (TSeT)2Cl (from Ref. 18), and pL-ET2I3 (from Ref. 20).

Figure 20 Kohler s plots for the transverse resistivity (p,) of (TMTSF)2C104 at various temperatures between 1.8 and 21 K and fields of up to 6 T in the intermediate direction. (From Ref. 13.)...

Figure 21 Resistivity of (TMTSF)2PF6 for current flow in the high-conductivity direction (a) with various magnetic fields applied in the low-conductivity direction (c ). (From unpublished results of W. Kang, J. R. Cooper, D. J6rome, and K. Bechgaard.)...

Figure 23 Plots of VAp/p (i.e., cot) for the high resistivity (/ or c ) direction of (TMTSF)2C104 and (TMTSF)2PF6 in a fixed field of 5.4 T along the i or b direction.

Figure 26 Normalized longitudinal resistivity p, of a single crystal of (TMTSF)2C104 at 4.2 K versus concentration of irradiation-induced defects (mole %). Initially, linear behavior is observed, corresponding to Matthiessen s rule, followed by an exponential behavior corresponding to Eqs. (11) and (12) (see the text). (From Ref. 112.)...

Figure 27 Transverse resistivity p, versus T2 for (TMTSF)2C104 doped with small quantities of Re04. There is evidence for a T2 law and for Matthiessen s rule. The anomaly associated with ordering of the C104 anions at 24 K is also visible. (From Ref. 104.)...

Figure 6 (a) Resistivity [after Ref. 47] and (b) EPR spin susceptibility of (TMTTF)2X compounds [after Ref. 46] undergoing a one-dimensional Mott-Hubbard localization below 7p (c) electron spin susceptibility of (TMTSF)2PF6 [after Ref. 38] (d) plot of the l3C spin lattice relaxation rate versus 7xf(7) for three compounds in the TM2X series. The temperature below which the charges are localized is indicated by 7p. No localization is observed for Se compounds (dashed line) above the SDW or SC ordering. (From Ref. 41b.)... 

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... 

Figure 12 (a) Temperature dependence of the longitudinal resistivity of (TMTSF)2PF6 obtained with the clamped contact technique. (From W. Kang, private communication, and O. Traetteberg, Thesis, Univ. Orsay, 1993.) (b) The conductivity anisotropy is temperature independent as long as the transverse motion remains incoherent. Data for (TMTSF)2PF6 under 12 kbar. (After Ref. 6.)... 

The very large pressure coefficient of the susceptibility (Fig. 14a) and conductivity in the metallic regime (d In room temperature [6]) raises a serious problem for the comparison with theory, which usually computes constant-volume temperature dependences. Hence the temperature dependence at constant pressure that is observed in actual experiments must be transformed into constant-volume data since the change of volume (due to the thermal expansion) cannot be ignored between 300 and 50 K. No detailed determinations of the constant-volume resistivity have been performed so far. However, a crude estimate of the intrinsic temperature dependence can be performed using the thermal expansion and the pressure dependence of the a axis at various temperatures [59] (Fig. 14b). 

Figure 13 Temperature dependence of the resistivity of (TMTSF)2AsF6, log-log plot (a) and versus T2 (b) following the law p = p0 + AT2, where p0 = 0 for the nonalloyed sample.

Figure 14 (a) Pressure dependence of the spin susceptibility x (T,T)-l/2 from NMR data. (From Ref. 41b.) (b) Constant-pressure and constant-volume temperature dependences of the resistivity of (TMTSF)2AsF6 derived point by point from the constant-pressure data of Fig. 12. The lattice parameters are from Ref. 33 and the pressure coefficient of the conductivity from Ref. 57.

Figure 20 displays the superconducting transition of (TMTSF)2PF6 salts observed by resistivity data [6,9]. What is remarkable in Fig. 20 is the strong temperature dependence of p(7) above Tc, unlike the behavior of regular metals, for which the resistivity is limited at low temperature by temperature-independent elastic scattering. The behavior of the temperature dependent resistivity at low temperature in TM2X compounds has been ascribed to a precursor effect above Tc [6]. 

Fig. 2. Temperature dependence of the electrical resistivity of some organic conductors historical development. The dashed line shows (TMTSF)2PF6 at high pressure.

Fig. 18. The electrical resistivities of selected (TMTSF)2X salts, having anions of various geometries, at ambient pressure (redrawn from ref. 81).

The usual features of the electrical resistivities of (TMTSF)2X compounds shown in Fig. 18 are as follows. The room-temperature value... 

Finally, the resistivity characteristics of the ambient-pressure superconductor j8-(ET)2I3 resemble those of (TMTSF)2C104 a great deal except for the much higher Tc of the triiodide salt [its room temperature value is 0.03 Q cm (9, 105, 106, 114)]. 

The electrical resistivity in directions other than the molecular stacking axis has been measured in a few cases. In (TMTSF)2PF6 one finds pa pb. pc, 1 200(3000) 3 x 104(106), where numbers in parentheses refer to T = 20 K, i.e., just above the transition (88). Here b is a vector in the ab plane perpendicular to a, and c is the reciprocal lattice vector orthogonal to the same plane. The situation in (TMTSF)2PF6 is probably typical for the series, whereas in (ET)2ClO4(TCE)0 5 one finds PilP — 1-2 (86), which suggests a very two-dimensional electronic structure in agreement with other experimental results, as well as band structure calculations (89). 

Because of the softness of organic metals one expects them to show interesting behavior under applied pressures. This had been demonstrated earlier by Jerome and co-workers on several compounds and in the case of TMTSF-DMTCNQ (DMTCNQ = dimethyltetracyanoqui-nodimethane) a pressure of 10 kbar transforms it abruptly from a Peierls semiconductor with Tm = 50 K to a metal at all temperatures (91). When the temperature-dependent resistance of the (TMTSF)2X family became known, the very low transition temperatures in some of the compounds suggested that these salts would easily become metallic, and maybe even superconducting, under pressure. 

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... 

Fig. 2.5. Temperature dependence of the dc resistivity for various (TMTSF)2X salts in double-logarithmic scale. Prom [35]...

For the highly anisotropic organic superconductors, of course, the critical fields depend also on the field direction. Most experimental data exist for (TMTSF)2C104 since for this material measurements at ambient pressure are possible. Figure 2.10 shows the temperature dependence of Bc2 obtained from resistivity measurements [103]. The anisotropy is clearly seen. The extrapolated critical fields at zero temperature are B 2 2.8 T, B 2 2T, and... 

Fig. 3.7. Schematic FS of (TMTSF)2C104. Closed ( ) and open (1) orbits (dotted) for the field applied parallel to a. Paths 3 (long dashed) and 2 (short dashed) are the trajectories for angles where the field is near maxima of the resistivity data. Prom [271]...

As an example of rapid oscillations Fig. 4.1 shows the magnetic field dependence of the relative resistivity of (TMTSF)2N03 for different temperatures [92]. The periodicity of two sets of oscillations in 1/5 is clearly visible. However, the oscillations are largest for 4.2 K and are reduced both for higher and lower temperatures. A similar inconsistency with the predictions of (3.6)... 

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