# SHUBNIKOV-DEHAAS OSCILLATIONS IN TMTSF-DMTCNQ

S Bouffard, M Ribault, Denis Jerome, K Bechgaard

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S Bouffard, M Ribault, Denis Jerome, K Bechgaard. SHUBNIKOV-DEHAAS OSCILLATIONS IN TMTSF-DMTCNQ. Journal de Physique Colloques, 1983, 44, 10.1051/jphyscol/1983037. hal-05000810

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<figure><img src="image_1.png" /></figure>

Distributed under a Creative Commons Attribution 4.0 International License
SHUBNIKOV-DE HAAS OSCILLATIONS IN TMTSF-DMTCNQ

S. Bouffard*, M. Ribault**, D. Jérôme** and K. Bechgaard**

*Section d'Etude des Solides Irradiés, CEN, B.P. 6, 92260 Fontenay-aux-Roses, France*

**Laboratoire de Physique des Solides, Université de Paris-Sud, 91405 Orsay, France

**Résumé** - Les oscillations de Shubnikov-de Haas observées dans la phase métal- ligue de haute pression de TMTSF-DMTCNQ prouvent que la surface de Fermi est tridimensionnelle et de petite section. De plus, nous discutons l'importante conductivité à basse température de ce composé.

**Abstract** - The observation of Shubnikov-de Haas oscillations in the metallic, high pressure phase of TMTSF-DMTCNQ demonstrates the three-dimensional, closed nature of the Fermi surface. Moreover, we discuss the high conducti- vity of TMTSF-DMTCNQ at low temperature.

Some quasi-one-dimensional organic conductors remain metallic at low temperature, and the topology of their Fermi surface is a topic of current interest /1/.

In the present work we have determined the Fermi surface topology of TMTSF-DMTCNQ in the high pressure metallic state, by observing Shubnikov-de Haas (SdH) oscillations in magnetic fields transverse to the highly conducting (a) axis. A highlight of this work is the derivation, from the SdH measurements of a single particle conductivity which is several orders of magnitude smaller than the observed values. We propose an interpretation of this discrepancy in terms of a contribution of short range 1-D superconducting fluctuations.

I - EXPERIMENTAL

The magnetoresistance of high purity TMTSF-DMTCNQ single crystals was measured by a classical four contacts and low frequency method. The resistance was recorded versus magnetic field without field modulation. The samples were pressurized up to 12kbar in a copper-beryllium cell and cooled slowly in a dilution refrigerator. The magnetic field (up to 7 Tesla) was aligned either along the b or c direction of the crystals. The accuracy of the alignment was not better than 10 degrees.

Raw magnetoresistance data are displayed on figure 1, for temperatures from 155 mK to 3 K and with the magnetic field along the c-axis. The oscillations are most clearly observed below 1 K, but their amplitude never grows above a few percent of the non-oscillating background. Data qualitatively similar were obtained with the magnetic field along the b-direction. The quantum origin of the oscillations of figure 1 is clearly demonstrated in figure 2. The plots of the field values corres- ponding to the extrema of the oscillations versus integers and half-integers gives straight lines. The slopes of these straight lines allow a determination of the periods 0.086 T⁻¹ and 0.24 T⁻¹ for the oscillations with H along the c and b-axis respectively. The corresponding cross-sections 1.16.10¹⁷ m⁻² and 2.6.10¹⁷ m⁻² in the (a-b) and (a-c) planes respectively amount to less than 0.1 % of a Brillouin zone cross section.

The analysis of the temperature dependence of the amplitude at fixed field gives an estimat of the cyclotron mass whereas the field dependence at constant temperature leads to the relaxation time /2/. We obtain :

$$m_c = (0.56 \pm 0.05)m_o \text{ and } \tau = 3 \times 10^{-12} \text{s.}$$
<figure><img src="image_2.png" /><figcaption>Fig. 1 - Magnetoresistivity of TMTSF-DMTCNQ under 12 kbar at different temperature. The applied magnetic field is parallel to the c-axis.</figcaption></figure>

<figure><img src="image_3.png" /><figcaption>Fig. 2 - Successive integers and half-integers corresponding to maxima and minima of the oscillations versus $H^{-1}$. The upper curve is for $H // c$ and the lower for $H // b$.</figcaption></figure>

## II - INTERPRETATION

The data of the present work prove conclusively the existence of closed orbits on the Fermi surface of the high pressure metallic phase of TMTSF-DMTCNQ at low temperature. These closed orbits are provided by nearly ellipsoidal pockets of very small volume. If we assume parabolic bands and use the SdH oscillation results, we obtain the Fermi energy of a pocket $\epsilon_F \approx 25$ K /3/. Moreover, the volume enclosed within such small pockets is about $2 \times 10^{-5}$ times smaller than the volume enclosed by the original high temperature 1-D Fermi surface. Thus, the density of carriers seen by SdH effects at low temperature in the metallic phase is about $10^{-4}$ per formula-unit, i.e. $2 \times 10^{17} \text{ cm}^{-3}$ (assuming two pockets of such carriers per Brillouin zone).

We infer that the present SdH results can be interpreted within the framework of the model /4/ proposed for the interpretation of the galvanomagnetic data of HMTSF-TCNQ at low temperature /5/. In this model a 1-D to 3-D band structure transformation occuring gradually upon cooling to low temperature was suggested. Weak couplings between chains transform the original planar Fermi surface derived from non-interacting chains into a semimetallic three-dimensional Fermi surface consisting of small pockets.

However, this semimetallic picture which can explain the low temperature state of HMTSF-TCNQ ($\sigma \sim 10^3-10^4 (\Omega_{cm})^{-1}$) fails to give a reasonable estimate of the
extraordinary large value of the conductivity of TMTSF-DMTCNQ.

In order to explain this discrepancy, an extra conduction provided from short range 1-D superconducting pairing has been suggested /6/. This fluctuating channel $\sigma'$ acts in parallel with the single particle channel $\sigma_0$. The strong sensitivity of the $\sigma'$ channel to irradiation has allowed a qualitative evaluation of the ratio $\sigma'/\sigma_0$: at helium temperature $\sigma'/\sigma_0 \approx 10-20/7$. Further evidence of this extra contribution is given in figure 3 where the dose dependences of the 21 K resistivities at low and

<figure><img src="image_4.png" /></figure>

Fig. 3 - The resistivity at 21 K is plotted versus the defect concentration. The upper curve was measured in the Peierls state (low pressure) /8/, the lower curve was measured in the high pressure metallic state /7/.

high pressure have been reported. At low pressure, the initial decrease is related to the stabilization of the metallic state /8/, when at high pressure, the initial increase can be attributed to the strong sensitivity to the defects of the fluctuating channel. At higher dose, the two curves reach the same absolute values and have the same dose dependence.

### III - CONCLUSION

The observation of Shubnikov-de Haas oscillations in the highly conducting state under pressure of TMTSF-DMTCNQ demonstrates the semimetallic 3-D character of the Fermi surface pressure. However the low density of carriers fails to explain the experimental conductivity at low temperature. The coexistence of a fluctuating superconducting contribution, with a single-particle contribution is not contradictory with the present results.
References

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