OBSERVATION OF DE HAAS-SHUBNIKOV OSCILLATIONS IN AN ORGANIC METAL, HMTSF-TCNQ*

M. Miljak,† A. Andrieux, R.H. Friend,‡ G. Malfait and D. Jérôme

Laboratoire de Physique des Solides, associé au CNRS, Université Paris-Sud, 91405 Orsay, France
and
K. Bechgaard

H.C. Oersted Institute, Universitetsparken 5, DK 2100, Copenhagen, Denmark

(Received 16 March 1978 by N.I. Meyer)

We report the first observation of magnetic quantum oscillations in an organic conductor, the de Haas-Shubnikov effect in HMTSF-TCNQ, under pressure in fields up to 75 kG. We find Fermi surface cross-sections in the *ab* plane of 7.2 × 10¹² cm⁻² at 10 kbar and 1.5 × 10¹³ cm⁻² at 9 kbar, and a cyclotron mass of 1/20 *m*ₑ at 10 kbar. These values confirm the semimetallic low temperature state of this material.

ONE OF THE GOALS of the present effort on the organic metals has been to find systems which remain metallic down to zero temperature. In all compounds that have been made to date, metallic conduction results from π electron overlap along stacks of planar molecules, so that conduction is largely confined to the direction of the stack and the metal tends to be sufficiently one-dimensional to undergo a Peierls distortion to an insulating state at low temperatures. This is the case for most of the family of charge transfer salts based on TTF-TCNQ [1].

One approach to prevent this happening has been to find systems where stronger interactions between stacks distort the Fermi surface far enough from the ideal one dimensional planar surface to prevent adequate "nesting" for the Peierls distortion to take place. The most promising of the TTF-TCNQ family salts is HMTSF-TCNQ [2], in which N-Se contact between adjacent stacks is strong [3], so that, for example, the conductivity anisotropy between the stacking axis (b) and the direction of the N-Se contact (a) is small (~30) [4, 5]. The stacking axis conductivity increases with falling temperature, rising by a factor of ~3 from its room temperature value by 30 K, below which it then decreases gently, falling to a little below its room temperature value at 4 K [2]. Although there is as yet no low temperature diffraction evidence for a Peierls distortion, 2k<sub>F</sub> diffuse X-ray scattering is observed at room temperature [6], and anomalies in specific heat and susceptibility indicate that there is a phase transition at 32 K [7]. This suggests that the low temperature fall in

conductivity arises from the partial destruction of the Fermi surface below the transition, rather than from a temperature dependent impurity resistivity [2].

Under pressure the low temperature decrease in conductivity is suppressed above 4 kbar [8]. At the distortion temperature, HMTSF-TCNQ is well into the three dimensional regime, where electron wave functions are coherent both parallel and perpendicular to the stacks, and the removal of the conductivity decrease has been interpreted as the removal of a Peierls transition as a result of an increase in the transverse transfer integral, *t*<sub>⊥</sub> with pressure, in a system where even at ambient pressure, *t*<sub>⊥</sub> has already reduced the Peierls transition from its *t*<sub>⊥</sub> = 0 value [9].

The band structure of HMTSF-TCNQ has been modelled by Weger [10]. The net effect of the various matrix elements for the intermolecular transverse overlap is to change the one-dimensional Fermi surface into a three-dimensional, semimetallic character Fermi surface. Ellipsoids or cylinders of electrons and holes directed along the *c* axis are formed, and the density of states at the Fermi level, *N*(*E*<sub>*F*</sub>) is reduced from the value for a strictly one-dimensional conductor. Only when thermal and lifetime broadening are less than the transverse overlap integral will the semimetallic Fermi surface show up. This condition is achieved in HMTSF-TCNQ below 100 K, where there is found to be a large Hall effect [4] and magnetoresistance [4, 8], and a Landau-Peierls diamagnetic contribution to the susceptibility [11].

Previous high pressure magnetoresistance measurements up to 30 kG [4, 8] indicated the likelihood of the observation of quantum oscillations at higher fields, and in this letter we report such experiments.

Hydrostatic pressure up to 15 kbar was provided by a Be-Cu double stage intensifying cell, similar to that described by Delaplace *et al*. [12]. Isopentane was used

<footnote>* Work supported in part by D.G.R.S.T. Contract No. 75-7-0820.</footnote>

<footnote>† Permanent address: Institute of Physics of the University, P.O. Box 304, Zagreb, Yugoslavia.</footnote>

<footnote>‡ Present address: Cavendish Laboratory, Cambridge, England.</footnote>
<figure><img src="image_1.png" /></figure>

Fig. 1. $(1/R)(dR/dH)$ for HMTSF–TCNQ at 10 kbar with $H \parallel c$ and $j \parallel b$, after subtraction of a quadratic background magnetoresistance. Data are shown for temperature of 1.2, 2.4 and 4.2 K, and are offset by 0.5, 1.0 and $1.5 \times 10^{-4}$ respectively. R was taken as the resistance in zero field at 4.2 K.

<figure><img src="image_2.png" /></figure>

Fig. 2. $(1/R)(dR/dH)$ for two samples of HMTSF–TCNQ at 9 kbar and 1.2 K with $H \parallel c$, $j \parallel b$, after subtraction of a quadratic background magnetoresistance. Data are offset by $1 \times 10^{-5}$ and $-1 \times 10^{-5}$ for the upper and lower curves.

as the pressure medium, and good pressure homogeneity was ensured by freezing the isopentane at constant pressure. Magnetic fields up to 75 kG were provided by a superconducting solenoid around the pressure system. The high conductivity axis resistivity of single crystals of HMTSF–TCNQ was measured using a low frequency (70 Hz) alternating current with phase lock voltage

<figure><img src="image_3.png" /></figure>

Fig. 3. $1/H$ for maxima (integer) and minima (half integer) in $dR/dH$ plotted against quantum number for HMTSF–TCNQ at 9 and 10 kbar. Data at 9 kbar are taken from the lower of the two curves shown in Fig. 2.

detection. The standard 4 in-line contact geometry was used. The resistivity was measured as a function of field and data points recorded using a data acquisition system, typically at 100 G intervals. $dR/dH$ was calculated by a least squares fit to $R(H)$, typically over 1 kG. For all experiments the magnetic field was directed along $c$, the direction of the long axes of the ellipsoids or cylinders of Fermi surface in the model of Weger [10].

Oscillations in magnetoresistance were most clearly visible around 10 kbar, and Fig. 1 shows $(1/R)(dR/dH)$ plotted against $1/H$ for this pressure, after subtraction of a non oscillatory, quadratic background. At 1.2 K, $\Delta R/R$ was 400% for a field of 75 kG. Data are shown for temperatures of 1.2, 2.4 and 4.2 K, and the amplitude of the oscillations can be seen to be reduced at the higher temperatures. Figure 2 shows data for two samples in the same orientation at 9 kbar. $\Delta R/R$ was 250% at 75 kG for both. The amplitude of the oscillations was substantially lower than that shown in Fig. 1, probably as a result of lower sample purity, and in general we have found the amplitude of the oscillations to be very sensitive to sample quality, as judged from the temperature dependence of the resistivity measured when cooling.

We consider the periodicity in $1/H$ and the temperature dependence to be strong evidence for the quantum origin of these magnetoresistance oscillations. Maxima (integer) and minima (half integer) in $dR/dH$ are labelled according to quantum number in Fig. 3. From these plots we determine frequencies of 156 and 75 kG for the orbits at 9 and 10 kbar respectively. The field
dependence of the oscillation amplitude is not simple, but from measurement of the temperature dependence of the amplitude of the oscillations between $n = 3/2$ and $5/2$ we estimate the cyclotron mass to be ~1/20 electron masses at 10 kbar.

The mere observation of quantum oscillations in the magnetoresistance indicates that the low temperature, high pressure state of HMTSF-TCNQ is genuinely metallic, with long carrier relaxation times ($\hbar/\tau < \hbar\omega_c$ for $H > 30$ kG), and this compound can be prepared in a state of high purity. This behaviour contrasts with other organic systems which show moderately high conductivities at low temperatures, but in which disorder, either intrinsic or as a result of radiation damage, plays an important role in maintaining the conductivity by preventing the formation of a long range ordered Peierls distortion. Hence the dependence of the low temperature conductivity of TTT $I_x$ on iodine concentration, $x^{13}$, and the large increase in the low temperature conductivity of TSF-TCNQ with neutron bombardment [14].

We have only seen one oscillation frequency; presumably this corresponds to the Fermi surface extremum of the most mobile carriers, which are

electron-like as determined from the sign of the Hall coefficient [4]. The small measured Fermi surface cross-section ($7.2 \times 10^{12} \text{ cm}^{-2}$ at 10 kbar and $1.49 \times 10^{13} \text{ cm}^{-2}$ at 9 kbar) and the small cyclotron mass (~1/20 $m_e$ at 10 kbar) lend support to the semimetallic band structure model [10], in which typical cross sectional areas in the ab plane are ~3 $\times 10^{12} \text{ cm}^{-2}$. At 10 kbar and in a field of 30 kG, three quantum levels, separated from each other by $\hbar\omega_c$, lie below the Fermi level (Fig. 3). We thus estimate the Fermi energy for the carriers we have observed to be ~20 meV. Again this is broadly consistent with the band structure picture.

The rapid reduction in Fermi surface cross-section between 9 and 10 kbar indicates a further explanation, besides the increase in $t_\perp$ with pressure, for the suppression of the ambient pressure phase transition with pressure; the fall in size of the Fermi surface, and hence $N(E_F)$ will be effective in preventing a Peierls distortion. The low value of $N(E_F)$ under pressure is consistent with the failure of previous attempts to find superconductivity down to 50 mK [15].

Further experiments to follow the orientation and pressure dependence of the quantum oscillations are in progress.

## REFERENCES

1. *Chemistry and Physics of One Dimensional Metals* (Edited by KELLER H.J.). Plenum Press, New York (1977).

2. BLOCH A.N., COWAN D.O., BECHGAARD K., PYLE R.E., BANKS R.H. & POEHLER T.O., *Phys. Rev. Lett.* **34**, 1561 (1975).

3. PHILLIPS T.E., KISTENMACHER T.J., BLOCH A.N. & COWAN D.O., *J. C. S. Chem. Commun.* **334** (1976); The correct labelling of the stacking axis is (c) and of the direction of the N-Se contact (b). Here we have used (b) and (a) respectively, referring to the more familiar structure of the parent TTF-TCNQ system.

4. COOPER J.R., WEGER M., DELPLANQUE G., JÉROME D. & BECHGAARD K., *J. Phys. Lett.* **37**, 349 (1976).

5. COOPER J.R., JÉROME D., ETEMAD S. & ENGLER E.M., *Solid State Commun.* **22**, 257 (1977).

6. WEYL C., ENGLER E.M., BECHGAARD K., JEHANNO G. & ETEMAD S., *Solid State Commun.* **19**, 925 (1976).

7. BLOCH A.N., *Lecture Notes in Physics* **65**, 317, Springer Verlag (1977).

8. COOPER J.R., WEGER M., JÉROME D., LE FUR D., BECHGAARD K., BLOCH A.N. & COWAN D.O., *Solid State Commun.* **19**, 749 (1976).

9. FRIEND R.H., JÉROME D., FABRE J.M., GIRAL L. & BECHGAARD K., *J. Phys. C11*, 263 (1978).

10. WEGER M., *Solid State Commun.* **19**, 1149 (1976).

11. SODA G., JÉROME D., WEGER M., BECHGAARD K. & PEDERSEN E., *Solid State Commun.* **20** (1976).

12. DELAPLACE R., MALFAIT G. & JÉROME D., *Rev. Phys. Appl.* **11**, 327 (1976).

13. KAMINSKII V.F., KHIDEKEL M.L., LYUBOSKII R.B., SHCHEGOLEV I.F., SHIBAEVA R.P., YAGUBSKII E.B., ZVARYKINA A.V. & ZVERERA G.L., *Phys. Status Solidi (a)* **44**, 77 (1977); See also the effect of thermal annealing on TTT $I_{1.6}$, MIHALY G., JANOSSY A. & GRUNER G., *Solid State Commun.* **22**, 771 (1977); A positive magnetoresistance (45% at 1.43 K in 55 kG has been reported in TTT$_2$I$_3$, KARIMOV S., ZVEREVA G.I. & YAGUBSKII E.B., *JETP Lett.* **25**, 235 (1977).

14. ZUPPIROLI L. & BOUFFARD S. (unpublished).

15. JÉROME D., RIBAULT M. & BECHGAARD K. (unpublished).