# 1H-NMR RELAXATION STUDIES ON THE NATURE OF THE SEMICONDUCTING STATE OF (TMTTF)2Br AND (TMTTF)2PF6: MAGNETISM

T Takahashi, F Creuzet, Denis Jerome, J M Fabre

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T Takahashi, F Creuzet, Denis Jerome, J M Fabre. 1H-NMR RELAXATION STUDIES ON THE NATURE OF THE SEMICONDUCTING STATE OF (TMTTF)2Br AND (TMTTF)2PF6: MAGNETISM. Journal de Physique Colloques, 1983, 44, 10.1051/jphyscol/1983147. hal-05000746

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¹H-NMR RELAXATION STUDIES ON THE NATURE OF THE SEMICONDUCTING STATE OF (TMTTF)₂Br AND (TMTTF)₂PF6 : MAGNETISM

T. Takahashi, F. Creuzet, D. Jérome 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** - Les propriétés électroniques de (TMTTF)₂Br et de (TMTTF)₂PF6 ont été étudiées par des mesures de relaxation nucléaire du proton. Au vue des anomalies de T₁, T₂ et T₂', il apparaît que l'état semiconducteur existant à basse température dans (TMTTF)₂Br est un état d'onde de densité de spin à pression ambiante et à 13 kbar. Une transition de phase est aussi observée dans (TMTTF)₂PF6, mais dont la nature magnétique n'est pas aussi prononcée. Il semble probable que des modulations de spin et de réseau coexistent à basse température dans (TMTTF)₂PF6. Nous discutons le crossover possible entre les états "basse température" de la famille (TMTTF)₂X.

**Abstract** - The electronic properties of (TMTTF)₂Br and (TMTTF)₂PF6 have been studied by ¹H-NMR relaxation measurements. The low temperature semiconducting state in (TMTTF)₂Br is shown to be a SDW state at ambient pressure and at 13kbar, by the anomalies in T₁, T₂ and T₂'. A phase transition is also observed in (TMTTF)₂PF6 but the magnetic nature is not so pronounced. It seems likely that the spin and lattice modulations coexist in the low temperature phase of (TMTTF)₂PF6. We discuss the possible crossover in the low temperature state in (TMTTF)₂X family.

## INTRODUCTION

The discovery of superconductivity in the Bechgaard salts /1/ made this family of a great interest. An important common feature of all these compounds is the characterization of the insulating state appearing at low temperature (apart from the effects of anion ordering) as a spin density wave state (SDW) /2/. Recent conductivity measurements /3/ on the sulphured analog family (TMTTF)₂X show clearly that its electronic properties are very similar to those of (TMTSF)₂X, but with higher critical pressure. From this study, (TMTTF)₂Br seems to lie very near the (TMTSF)₂X family, and is a good candidate for the investigation of the nature of the insulating state. Moreover, paramagnetic susceptibility /4/ and X-ray diffuse scattering /5/ data have suggested that the insulating state in (TMTTF)₂PF6 is of another nature, with a 2kF lattice distortion. Thus, in order to determine what phase appears at low temperature in (TMTTF)₂X, we have performed ¹H-NMR measurements on these two compounds.

## EXPERIMENTAL RESULTS

The ¹H-relaxation times were measured on powdered samples, with a high power home-made pulsed NMR-spectrometer, at a resonance field of 10.6 kOe. The spin lattice relaxation time T₁ was determined by observing the recovery in time of the free induction signal after a saturation comb pulse with the usual spin-echo technique we measured the spin-spin relaxation time T₂ and the linewidth T₂⁻¹ (assuming a single exponential shape).
The figure 1 gives the results obtained at ambient pressure for (TMTTF)₂Br. A clear peak of the spin relaxation is observed near T = 14 K; around this temperature the decay is non-single exponential (probably coming from the random orientation of the samples) and we have plotted the initial and final slopes, joined by a dashed line. Moreover, a striking feature is given by the behaviour of T₂ and T₂*: while T₂* decreases by a factor three, T₂ increases by about 20% below the transition. These anomalies are a clear evidence of the existence of a SDW-state below 14 K: the inhomogeneous local field due to the spin density wave (evaluated as 40e at ¹H-sites) gives a broadening of the linewidth and a decoupling between nuclear spins. The measurements at a pressure of 13 kbar (figure 2) lead to the same conclusion of a magnetic transition, but at a higher temperature T = 16 K. The transition seems narrower, as seen by the width of the T₁⁻¹ peek and the rapid variation in T₂*. (Note the depression of the spin lattice relaxation rate in the region 18K < T < 30K due to the application of pressure).

<figure><img src="image_2.png" /><figcaption>**Figure 2** : ¹H relaxation times at low temperatures in (TMTTF)₂Br at 13 kbar.</figcaption></figure>

<figure><img src="image_3.png" /><figcaption>**Figure 1** : ¹H relaxation times at low temperatures in (TMTTF)₂Br at 1 bar.</figcaption></figure>

In (TMTTF)₂PF6 at ambient pressure, the results (figure 3) seem quite different but also give a magnetic nature of the transition. The spin lattice relaxation rate reveals a weak step-like anomaly around T = 17K, with a non-single relaxation in the transition temperature region. T₂ and T₂* behave similarly as in (TMTTF)₂Br but the excess local field is one order of magnitude smaller (0.50e).

## DISCUSSION

The data described above show that the semiconducting phase in (TMTTF)₂Br is characterized as a SDW-state and strongly support the suggestion given by conductivity measurements under pressure, of the similarity between (TMTTF)₂X and (TMTSF)₂X. This point was confirmed by antiferromagnetic resonance measurements /6/, giving a Neel temperature Tn = 13+/-1K. From the magnitude of the
<figure><img src="image_4.png" /><figcaption>Figure 3 :<sup>1</sup>H relaxation times at low temperature in (TMTTF)<sub>2</sub>PF<sub>6</sub> at 1 bar.</figcaption></figure>

inhomogeneous local field, the amplitude of the spin density wave is of the order of 0.01 µg. We believe that (TMTTF)<sub>2</sub>Br has the same electronic properties as the selenide family, but with a critical pressure somewhat bigger. The highly conducting state found at 25 kbar /3/ may be related to the existence of superconductive fluctuations. It is very likely that the higher critical pressure is due to the stronger dimerization of the organic molecules along the stack, observed in the sulphur family at ambient pressure. As suggested by EMERY et al /7/, the nature of the low temperature state depends on the amplitude of the intra-chain dimerization, via Umklapp scattering for the electrons, which may be reduced by application of pressure.

The increase of the transition temperature with pressure is not well understood. Two possibilities may explain this behaviour. Firstly, the pressure may enhance the magnetic interchain coupling (at least at low pressure), leading to a more magnetic phase as seen experimentally. Secondly, the T-P phase diagram may be modified under magnetic field.

The description of the insulating state for (TMTTF)<sub>2</sub>PF<sub>6</sub> (the most strongly dimerized compound : /7/ and references therein) is more difficult. The observation of a 2k<sub>F</sub> lattice distortion by X-ray measurements must be reconciled with the NMR data. Following EMERY et al, one may suggest that the low temperature state is a spin-Peierls state as in their strongest dimerization limit. Here, we need some magnetic excitations, probably with a small gap from the ground state, in order to explain the observed magnetic behaviours. If the nature of the low temperature state depends on the amplitude of the dimerization, going from a spin-Peierls via SDW-state to superconducting state, as dimerization decreases, it seems reasonable to suppose that (TMTTF)<sub>2</sub>PF<sub>6</sub> at ambient pressure is near the crossover between the first two states.

## CONCLUSION

These results confirm that the electronic properties are quite similar in (TMTTF)<sub>2</sub>X and (TMTTF)<sub>2</sub>X families. As already mentioned/3,4/, the difference in the electronic properties of the compounds can be controlled by the application of high pressure or a slight chemical change. It seems that the nature of the insulating state in (TMTTF)<sub>2</sub>PF<sub>6</sub> is not a pure charge density wave state and its characterization may lead to an unified approach of the properties of the quasi unidimensional electron gas.

## REFERENCES

1) JEROME D., MAZAUD A., RIBAULT M., BECHGAARD K., J. Phys. Lettres 41 (1980) 95.

2) FRIEDEL J., JEROME D., to be published in Contemporary Physics (1982).

3) CREUZET F., PARKIN S.S.P., JEROME D., BECHGAARD K., FABRE J.M., this conference.

4) COULON C., DELHAES P., FLANDROIS S., LAGNIER R., BONJOUR E., FABRE J.M., to be published in J. Phys. (1982).

5) POUGET J.P., MORET R., COMES R., BECHGAARD K., FABRE J.M., GIRAL L., International Conference on Low Dimensional Conductors (Boulder).

6) PARKIN S.S.P., SCOTT J.C., TORRANCE J.B., ENGLER E.M., this conference.

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