TRANSPORT PROPERTIES UNDER PRESSURE OF
THE ORGANIC CONDUCTORS (TMTTF)2X; X =
PF6, C104, Br

F Creuzet, S S P Parkin, Denis Jerome, K Bechgaard, J M Fabre

► To cite this version:

F Creuzet, S S P Parkin, Denis Jerome, K Bechgaard, J M Fabre. TRANSPORT PROPERTIES UNDER PRESSURE OF THE ORGANIC CONDUCTORS (TMTTF)2X; X = PF6, C104, Br. Journal de Physique Colloques, 1983, 44, 10.1051/jphyscol/1983148. hal-05000789

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Distributed under a Creative Commons Attribution 4.0 International License
TRANSPORT PROPERTIES UNDER PRESSURE OF THE ORGANIC CONDUCTORS

$(TMTTF)_2X; X = PF_6, ClO_4, Br$

F. Creuzet, S.S.P. Parkin, D. Jérôme, K. Bechgaard and J.M. Fabre*

Laboratoire de Physique des Solides, Bât. 510, Université de Paris-Sud,
91405 Orsay, France

*Laboratoire de Chimie Organique Structurale, U.S.T.L., 34060 Montpellier,
France

**Résumé** - D'après les mesures de conductivité, la famille $(TMTTF)_2X$ présente des similarités avec la famille homologue sélenié. Cependant, la pression critique, correspondant à la suppression de l'état fondamental isolant, 25kbar pour le composé bromé, est plus élevée, peut-être en relation avec la plus forte dimérisation des molécules organiques le long de la chaîne. Nous avons étudié la possibilité d'existence d'un état supraconducteur dans $(TMTTF)_2Br$ par des mesures de conductivité, de susceptibilité alternative, et de magnétorésistance. Bien que l'existence d'une transition supraconductrice autour de 3K ne soit pas complètement démontrée, les fluctuations supraconductrices apparaissent importantes à basse température.

**Abstract** - According to conductivity studies, the $(TMTTF)_2X$ series shows some similarities with the selenium series. However, the critical pressure, corresponding to the suppression of the insulating ground state, 25kbar for the bromide salt, is much higher and may be related to the stronger dimerization of the organic molecules along the stack. We have examined the possibility of superconductivity in $(TMTTF)_2Br$ by conductivity, AC susceptibility and magnetoresistance experiments. Although the existence of a superconducting transition around 3K is not yet fully demonstrated, superconducting short range order may be important at low temperature.

## INTRODUCTION

The discovery of superconductivity in several members of the organic linear chain conductors $(TMTTF)_2X$ (1) has stimulated interest in this family. However it is not well understood which features make this family so particular, and especially why it is different from the isospectral sulphur family $(TMTTF)_2X$. At ambient pressure (2), the electronic properties are apparently quite different : at $T = 300K$ the DC-conductivity is one order of magnitude lower (except for $X = Br$) and a minimum of the resistance appears between 100-250K, with a weakly activated behaviour at lower temperature.

The determination of the temperature-pressure phase diagram of three $(TMTTF)_2X$ compounds may elucidate these differences, which are possibly related to the stronger dimerization of the organic molecules. Furthermore, a highly conducting metallic state can be stabilized under pressure in $(TMTTF)_2Br$.
## EXPERIMENTAL RESULTS

### Resistivity versus temperature measurements

#### (TMTTF)₂PF₆

For a pressure of 15kbar, the maximum in conductivity seen at ambient pressure near 230K is suppressed, and metallic behaviour is observed in the high temperature region down to 19K, the temperature at which the compound becomes an insulator. The phase transition temperature (defined as that temperature where $\frac{d(\text{Log}\rho)}{d(1/T)}$ has a maximum) can be calculated and the T-P phase diagram is constructed. In the insulating phase, an energy gap is defined, decreasing from 56K at 15kbar to 25K at 28kbar (Fig. 1).

<figure><img src="image_2.png" /><figcaption>Fig. 1 - Temperature - Pressure phase diagram of (TMTTF)₂PF₆</figcaption></figure>

Although the resistance ratio R(300K)/Rmin is only 5 at the highest pressure measured, we note a change in slope of the resistance curve near 120K, almost independent of pressure, which could be due to some lattice transformation.

#### (TMTTF)₂ClO₄

The results, for several pressures, are summarized in figure 2. At low pressure (P<22 kbar), the resistance increases as temperature is decreased, with no sign of a minimum. At higher pressure, a metal-insulator transition appears at low temperature and the T-P phase diagram is easily evaluated. The low resistance ratio and the lack of minimum in resistivity are probably due to a poor quality of the samples and/or the effect of pressurization.

Note a shoulder in the resistance curve at 6kbar near 50K. This

is certainly related to the anion ordering observed at 1bar near 70K (3), but the effect is small, and does not drive the transition.

#### (TMTTF)₂Br

The previous results on (TMTTF)₂PF₆ and (TMTTF)₂ClO₄ are similar to those for (TMTSF)₂X, but the critical pressure (corresponding to the suppression of the insulating phase) is much higher, estimated around 40kbar. From the ambient pressure results, (TMTTF)₂Br is more metallic and the critical pressure is expected to be lower; we find Pc = 24kbar.
<figure><img src="image_3.png" /><figcaption>Fig. 2 - Resistance curves for (TMTTF)₂ClO₄ at various pressures</figcaption></figure>

The phase boundary (Fig. 3) is quite similar to that of (TMTTF)₂X; in the metallic phase, the conductivity reaches more than 10⁶ (Ωcm)⁻¹, with a resistance ratio of the order of 400, indicating the same features as for the selenium compounds near the critical pressure. We are sure that there is no minimum of resistivity above the transition, at least for pressure greater than 3kbar.

Two important differences are found between the samples synthesized in Montpellier (indexed by F) and in Copenhagen (indexed by B). Firstly, the B-samples have better metallic properties, seen in the resistance ratio R(300K)/R(4K) and in the absolute value of the conductivity, which are both much greater than in F-samples. Secondly, the behaviour of the resistivity in the low temperature region (T<3K) is different; for the B-samples the resistivity saturates as tem-

perature is decreased, whereas R decreases monotonically for the F-samples, as shown in Figure 4 for the same pressure (P = 25kbar).

## Superconductivity in (TMTTF)₂Br?

The first results reported (4) indicated a superconducting transition near 3K at 24kbar in this compound, but some problems with residual resistivity made this result not so clear. In order to check if there is such a transition or not, we have performed AC-susceptibility measurements at 25kbar; no signal was observed down to 1.4K and thus we observe no bulk superconductivity in this range of temperature. We have measured the magnetoresistance (with magnetic field along c*) as shown in figure 5. At 10kbar, no effect is observed up to 80kG. By increasing pressure, the magnetoresistance appears gradually: the rate ($ρ(80kG) - ρ(0kG) / ρ(0kG)$) evaluated at T = 15K, is increasing

<figure><img src="image_4.png" /><figcaption>Fig. 3 - Phase diagram for (TMTTF)₂Br</figcaption></figure>
<figure><img src="image_5.png" /><figcaption>Fig. 4 - Comparison of behaviour of resistivity of F and B samples in $(TMTTF)_2Br$</figcaption></figure>

from 0.31 at 16kbar to 1.22 at 22kbar. These values, rather large for a metal, are in the same range than those observed in the $(TMTTF)_2X$ family (5). In line with the interpretation of the $(TMTTF)_2X$ series, we suggest that this could arise from the suppression of short range superconductive fluctuations.

## DISCUSSION

The $(TMTTF)_2X$ family has similar properties as $(TMTSF)_2X$, but higher pressure is needed to observe these effects, as demonstrated by the phase diagrams :

- in $(TMTTF)_2Br$ in the high pressure metallic regime, the resistivity follows a power law dependence, almost quadratic for the B-samples.

- the insulating phase in $(TMTTF)_2Br$ has been shown to be a spin density wave state at ambient pressure (6,7) and at 13kbar (7). But in $(TMTTF)_2PF_6$ (for which the critical pressure is very high) at ambient pressure, this state seems more complicated with both magnetic (7) and lattice distortion (8) effects.

- a large magnetoresistance is observed at low temperature for pressure below Pc.

An interpretation of these properties of the two families of compound was given by Emery et al (9) and Horovitz et al (10). Due to the special crystal structure of the compounds, Umklapp scattering can become important. Following Emery, this process is related to the dimerization of the organic molecules along the stack. According to the models, the Umklapp scattering process, described by the parameter g3 in the g-ology theory, stabilizes an insulating state (either spin density wave or spin-Peierls state) and has to be reduced to let superconductivity appear. If we assume that pressure reduces the dimerization, and consequently g3, we rationalize that the critical pressure is large for the strongly dimerized $(TMTTF)_2X$ family. Furthermore, as the pressure is increased, the g3-parameter becomes irrelevant and the competition between superconductive and insulating states is more and more efficient : the superconductive fluctuations may appear and with those the strong magnetoresistance. Finally, the nature of the insulating state in the more strongly dimerized compounds (like $(TMTTF)_2PF_6$ at P = 1bar) may be rationalized in terms of coexistence of a spin-Peierls state.

The data on $(TMTTF)_2ClO_4$ emphasize that anion ordering must be considered simultaneously with the molecular stack instabilities. The characteristic pressure needed to suppress anion ordering may be higher or lower than Pc, and depends on the material considered.

## CONCLUSION

The members of the $(TMTTF)_2X$ family behave similar to those of the
(TMTSF)<sub>2</sub>X series. Although this result is unexpected considering the ambient pressure data, it is not so surprising in view of the closely related crystal structure.

The main result is the stabilization of a metallic state in a sulphur compound, with a large resistance ratio (>400) and a very high conductivity (>10<sup>6</sup> (Ωcm)<sup>-1</sup>). The results also suggest that the critical pressure is related to the degree of dimerization of both families.

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REFERENCES

(1) see for example : JEROME D. and SCHULZ H.J., Advances in Physics, Vol. 31, N° 4, 299,490

(2) DELHAES P., COULON C., AMIELL J., FLANDROIS S., TOREILLES E., FABRE J.M. and GIRAL L., Mol. Cryst. Liq. Cryst. 50 (1979) 43.

(3) POUGET J.P., MORET R., COMES R., BECHGAARD K., FABRE J.M., GIRAL L., Mol. Cryst. Liq. Cryst. 79 (1982) 129.

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(4) PARKIN S.S.P., CREUZET F., RIBAULT M., JEROME D., BECHGAARD K., FABRE J.M., Mol. Cryst. Liq. Cryst. 79 (1982) 249.

(5) JACOBSEN C.S., MORTENSEN K., WEGER M., BECHGAARD K., S.S.C. 38 (1981) 423.

(6) PARKIN S.S.P., SCOTT J.C., TORRANCE J. and ENGLER E.M., Phys. Rev. B (submitted).

(7) CREUZET F., TAKAHASHI T., JEROME D. and FABRE J.M., J. de Physique Lettres 43 (1982) 755.

<figure><img src="image_8.png" /></figure>

(8) POUGET J.P., Chemica Scripta 17 (1981) 85.

(9) EMERY V.J., BRUINSMA R. and BARISIC S., Phys. Rev. Lett. 48 (1982) 1039.

(10) HOROVITZ B., GUTFREUND H., WEGER M., S.S.C. 39 (1981) 541.