ELSEVIER Physica B 201 (1994) 490-492

PHYSICA[

Effects of superlattice on a novel magnetic oscillatory phenomenon in low-dimensional organic

conductors (TMTSF)2X

H. Shinagawa a'*, S. Kagoshima a, T. Osada b, N. Miura c

a Department of Pure and Applied Sciences, University of Tokyo, 3-8-1 Komaba, Meguro-ku, Tokyo 153, Japan ~ Research Center for Advanced Sc&nce and Technology, University of Tokyo, 4-6-1 Komaba, Meguro-ku, Tokyo 153, Japan

Institute for Solid State Physics, University of Tokyo, 7-22-1 Roppongi, Minato-ku, Tokyo 106, Japan

Abstract

We discuss on a novel type of magnetic oscillatory phenomenon, the so-called 'rapid oscillation', observed in quasi-one-dimensional organic conductors (TMTSF)2X. To clarify its origin, we made magnetotransport studies on (TMTSF)2C104 at low temperatures under high pressures. We found that the rapid oscillation is dramatically suppressed above a threshold pressure of about 5 kbar. It is known that this compound has a superlattice at low temperatures, and it disappears under high pressures. Therefore, the present results strongly suggest that the presence of the superlattice is responsible for the rapid oscillation in (TMTSF)2CIO4. These results are well understood by the model we proposed previously.

Organic charge transfer salts (TMTSF)zX [1], where TMTSF denotes tetramethyltetraselenafulvalene and X = PF6, C104, etc., 'the Bechgaard salts', have been widely studied because they are not only the first organic superconductors but also showed rich physical proper- ties such as the field-induced spin-density-wave (FISDW) transition under high magnetic fields. In these com- pounds, a novel type of magnetic oscillatory phenom- enon, the so-called 'the rapid oscillation', has been observed [2 7]. They appear in both transport and ther- modynamic quantities. They are observed both in nor- mal metallic, spin density wave (SDW) and in FISDW state. They are periodic against inverse of magnetic fields with the typical frequency of 250 T. Although this phe- nomenon is apparently similar to the conventional Shubnikov-de Haas or de Haas-van-Alphen effects, one

* Corresponding author.

cannot ascribe its origin to them since these compounds have only a pair of sheet-like Fermi surfaces. In this paper, we will discuss on the origin of this phenomenon based on our experimental results.

(TMTSF)zX has very anisotropic electronic structure. Their typical ratio of transfer integrals is ta:tb:tc = 300:30: 1. In most cases, we can neglect tc, so that the system is regarded as a highly anisotropic two-dimen- sional or quasi-one-dimensional [1]. Therefore, they have a pair of warped sheet-like Fermi surfaces as schematically shown in Fig. l(a). Although compounds belonging to (TMTSF)2X seem to have a common elec- tronic structure, they show the various properties de- pending on the variety of anion X and applied pressures. In some compounds the orientation of anion X is or- dered periodically at low temperatures forming a super- lattice. For example, in the case of (TMTSF)2CIO4, the anion ClOg orders below 24 K with the superlattice wave vector q = (0, 1/2, 0). This superlattice introduces

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H. Shinagawa et al. / Physica B 201 (1994) 490-492 491

/

i

-k F k F I I

k,

;. , / [ L/ ;

-iq. k~

l q

Fig. 1. Schematic Fermi surfaces of (TMTSF)2X: (a) with no superlattice; (b) with the superlattice of q = (0, 1/2, 0) , the case of (TMTSF)2CIO4.

a weak periodic potential in their electronic structure and modifies their original Fermi surface by opening gaps. In this case, the Fermi surface is shown in Fig. l(b). How- ever, it has been believed that their effects are not impor- tant in high magnetic fields because of the effect of magnetic breakdown.

Yan et al. [8] first claimed that this superlattice was responsible for the rapid oscillation. They pointed out that the coherent magnetic breakdown causes the oscilla- tory effect in the same way as Stark's quantum inter- ference I-9] in ultra-pure magnesium. The oscillatory frequency expected by their model is in good agreement with the observed one.

Independent of this, we made a model calculation of the electric conductivity [10] for the case with the super- lattice q = (0, 1/2, 0) by fully quantal method in the framework of the relaxation-time approximation. We get the result that the conductivity along the b-axis is oscilla- tory and its frequency is same as that mentioned above.

To clarify the relationship between the superlattice and the rapid oscillation, we made magnetotransport mea- surements in (TMTSF)2CIO4 under high pressures. It has been reported in this compound that the superlattice disappears above a certain threshold pressure 1-11]. So, according to our model, it is expected that the rapid oscillation should disappear above a certain pressure because of the disappearance of the superlattice.

Samples were mounted in a four-probe configuration using gold wires and gold paint to measure the resistance along the b-axis. Pressures up to 8 kbar were applied using a miniature clamp cell of Be-Cu. A 4He cryostat was used with a 12 T-superconducting magnet. Low tem- perature pressures which were different from room tem- perature ones were calibrated in advance by using a manganin resistor. The sample was cooled at the rate of 0.01 K/rain near the anion ordering transition temper-

ature (24 K) to achieve the very relaxed state. During cooling process, the resistance was monitored to check the sample condition. A series of measurements for sev- eral different pressures were done on the same sample. Special cares were taken to avoid any damages on the sample and to keep the same cooling process for each measurement.

In Fig. 2, we present the resistances of (TMTSF)2C104 at 4.2 K against magnetic field under several different pressures. In the present pressure range, the FISDW transition does not occur, and the sample remains metallic up to 12 T. The rapid oscillation is clearly observed at high magnetic field region under lower pressures. It seems that the oscillation is suppressed with increasing pressure.

To compare the amplitude of the oscillation at differ- ent pressures, we normalized the oscillatory part to the non-oscillating background. In Fig. 3, we present pressure dependence of the amplitude of the oscillation. One finds a threshold pressure at about 5 kbar above which the rapid oscillation is dramatically suppressed. It is noticed that the measurement at 1 kbar was performed at the end of our series of measurements. Therefore, the suppression of the oscillation under high pressures is intrinsic but not caused by extrinsic damages on the sample.

Temperature dependence of resistances around the anion ordering temperature is presented in the inset of Fig. 2. A shoulder-like structure on 3 kbar curve at about 24 K corresponds to the anion ordering transition. On the other hand, no clear structure is found on 6 kbar curve. This suggests that the anion ordering is suppressed by applied high pressures as has been reported by Kang et al. [11].

Thus, our experimental results strongly suggest that the presence of the anion ordering superlattice is respon- sible for the rapid oscillation in (TMTSF)2C104.

492 H. Shinagawa et al./ Physica B 201 (1994) 491-492

t , -

t . . . . . . ~" t 3kbar

51~bar

2 O 25

6kbai

5 10 15

Magnetic Field (T)

Fig. 2. Magnetoresistances of (TMTSF)2CIO4 under different pressures. The inset, temperature dependence of the sample resistance near 24 K.

O

<

J I L i i , g

o - - - o - . ~ 1: v _ _

\

0 5

/ 10

Pressure (kbar)

Fig. 3. Amplitude of the rapid oscillation against applied pressures. The inset, the oscillatory part normalized to non- oscillating background.

Then, does our model calculation completely describe this oscillatory phenomenon? According to our model the rapid oscillation should be observed in the resistance along the b-(interchain, secondary conducting)axis but not along the a-(the most conducting)axis. This is a check point of our model. Unfortunately, no sufficient experi- mental study has been done on this aspect. In our present experiments, measurements were performed along the b-axis. We have not detected any significant aniso- tropy in the rapid oscillation up to now. However, one should still keep the possible anisotropic property of this oscillatory phenomenon. We point out that the large anisotropy in the electric conductivity of the sample

causes a difficulty in precise measurements of the aniso- tropy. That is, the resistance along the a-axis may be easily masked by the b-axis component due to small inhomogeneity in the sample.

It is also interesting whether the thermodynamic quantities such as the magnetization oscillates in this system. While the rapid oscillation in magnetization is found in FISDW state of (TMTSF)2CIO4 [7], it is still unclear whether it is present also in the normal metallic state. In our model, the oscillation in thermodynamic quantities is not predicted in the normal state.

In summary, we experimentally showed that the pres- ence of the anion ordering superlattice was responsible for the rapid oscillation in normal metallic state of (TMTSF)2C104. The rapid oscillation is suppressed above the threshold pressure at which the anion ordering superlattice vanishes. The mechanism of the oscillation caused by the superlattice is well understood by our model. However, we need some more studies on the anisotropy in resistance and on the magnetization to verify our model.

We are grateful to Mr. T. Kouno for useful suggestion on experiments under high pressures. One of the authors, T. Osada acknowledges the Yamada Science Foundation for the financial support.

References

[1] T. Ishiguro and K. Yamaji, Organic Superconductors (Springer, Berlin, 1990), and references therein.

[21 H. Bando, K. Oshima, M. Suzuki, H. Kobayashi and G. Saito, J. Phys. Soc. Japan 51 (1982) 2711.

I-3] P.M. Chaikin, M. Choi, J.F. Kwak, J.S. Brooks, K.P. Martin, M.J. Naughton, E.M. Engler and R.L. Greene, Phys. Rev. Lett. 51 (1983) 2333.

14] H. Schwenk, S.S.P. Parkin, R. Schumaker, R.L. Green and D. Schweitzer, Phys. Rev. Lett. 56 (1986) 667.

[5] J.P. Ulmet, P. Auban, A. Khmou, S. Askenazy and A. Moradpour, J. Physique Lett. 46 (1985) 535.

[6] W. Kang, S.T. Hannahs, L.Y. Chiang, R. Upasani and P.M. Chaikin, Phys. Rev. Lett. 65 (1990) 2812.

1-7] X. Yan, M.J. Naughton, R.V. Chamberlin, S.Y. Hsu, L.Y. Chiang, J.S. Brooks and P.M. Chaikin, Phys. Rev. B 36 (1987) 1799.

[8] X. Yan, M.J. Naughton, R.V. Chamberlin, L.Y. Chiang, S.T. Hsu and P.M. Chaikin, Synth. Met. 27 (1988) B145.

[9] R.W. Stark and C.B. Friedberg, J. Low Temp. Phys. 14 (1974) 111.

1-10] T. Osada, H. Shinagawa, S. Kagoshima and N. Miura, Synth. Met. 55-57 (1993) 1795.

[11] W. Kang, S.T. Hannahs and P.M. Chaikin, Phys. Rev. Lett. 70 (1993) 3091.