# SOME PROPERTIES OF (TMTSF)2FS03 AT LOW TEMPERATURE UNDER PRESSURE

D Mailly, K Bechgaard, F Creuzet, T Takahashi, M Ribault, Denis Jerome

► To cite this version:

D Mailly, K Bechgaard, F Creuzet, T Takahashi, M Ribault, et al., SOME PROPERTIES OF (TMTSF)2FS03 AT LOW TEMPERATURE UNDER PRESSURE. Journal de Physique Colloques, 1983, 44, 10.1051/jphyscol/1983130. hal-05000823

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

Distributed under a Creative Commons Attribution 4.0 International License
SOME PROPERTIES OF $(TMTSF)_2FSO_3$ AT LOW TEMPERATURE UNDER PRESSURE

D. Mailly, K. Bechgaard, F. Creuzet, T. Takahashi, M. Ribault and
D. Jérôme

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

**Résumé** - Pour des pressions supérieures ou égales à 7kbar, le composé $(TMTSF)_2FSO_3$ présente une chute de résistivité à T = 3K. A T = 0,1K le champ magnétique critique est de 55kOe dans la direction c'' et bien supérieur à 60 kOe dans la direction b''. Nous n'avons pas observé de variation d'aimantation associée à cette transition. Les caractéristiques RMN de ce composé sont très différentes des autres composés de cette famille.

**Abstract** - We have observed a drop of resistivity in $(TMTSF)_2FSO_3$ under pressures higher than 7kbar at T=3K. At T = 0.1 K the critical magnetic field is 55 kOe along the c'' direction, and much higher than 60 kOe along the b'' direction. There is no variation of the magnetization associated with the transition. The NMR characteristics are very different from those of other $(TMTSF)_2X$ compounds.

INTRODUCTION

The $(TMTSF)_2X$ family exhibits a large variety of phase transitions. In the salts where the anion possesses inversion symmetry ($PF_6$, $AsF_6$, $SbF_6$, $TaF_6$, etc), the metal to insulator transition observed upon cooling is known to be a SDW transition/1,2/. Under pressure the magnetic transition is depressed and superconductivity appears. When the anion is non-centrosymmetric the behaviour is more complicated. The $ReO_4$ salt exhibits an anion ordering transition at 180K giving a CDW-type insulator/3/. Under pressure the $ReO_4$ salt remains metallic to low temperature and a SC transition appears. The $C_6O_4$ salt is so far the only compound which exhibits a SC transition at ambient pressure /4,5,6/. Recently it was shown that this transition can be suppressed when the sample is cooled rapidly through the temperature (24K) where anion ordering occurs /7/. The $FSO_3$ salt has virtually an identical unit cell as that of $C_6O_4$ except for the parameter a which is slightly reduced. Furthermore $FSO_3$ has a (small) permanent electric dipole moment /8/. Recently an important drop of resistivity below 3.5K was taken to be indicative of S.C. in this material at pressures above 4.5 kbar /9/.

EXPERIMENTAL

Resistivity and DC susceptibility measurements were performed in a CuBe clamp. The pressure medium was isopentane. The clamp was fixed to the mixing chamber of a dilution refrigerator. The resistivity measurements were performed using a four probe technique. Two samples were mounted in the clamp : one with the c'' direction vertical, the other with the b'' direction vertical. A superconducting coil in the helium bath gave a vertical magnetic field up to 60 kOe. DC susceptibility measurements were performed using a SQUID technique. AC-susceptibility was obtained with a compensated copper coil, holding 10 samples and a niobium reference (5% of the total volume); the samples were pressurized in a steel pressure cell and cooled to 1.2 K in a He$^4$-cryostat. $^{1}$H-NMR relaxation times ($T_1$ and $T_2$) on a single crystal was measured
by the usual pulsed technique at a pressure of 10 kbar. The external field of
10.6 kOe was applied in the b" direction.

RESULTS

Fig. 1 shows the temperature dependence of the resistivity at 7 kbar. The resistivity appears metallic down to T=100 K, then increase rapidly to reach a maximum at 20 K. The resistivity then decreases slowly and at T = 3K it drops. The ratio of resistivity at T=4K and T=50 mK is 2.6 for #1 and 1.7 for #2. Fig.2 shows the temperature dependence of the resistivity at 9 kbar. The curve is qualitatively the same as that under 7 kbar, except that the upturn of resistivity is less important. The low temperature transition occurs at the same temperature as at 7 kbar. The ratio of resistivity at T=4K and T = 50 mK is 3.7. At 11 kbar (Fig. 3) the upturn of resistivity is very attenuated and the 3K transition is roughly the same as for the other pressures. The ratio of resistivity at 4K and 50 mK is 4.4 for #1 and 3.2 for #2.

The dependence of the low temperature
resistivity in magnetic field is very
anisotropic. Fig. 4 shows the typical
behaviour when the field is parallel to
c". At the lowest temperature and if we
take as Hc₂ the value at which R=0.9R(4K),
we find Hc₂ = 50 kOe. When the field is
parallel to b" (Fig.5) the field depen
dence is vanishingly small.

The measurement of the DC magnetization
performed on a single crystal at P=7.5kbar
and P = 10 kbar, with a field of 0.1 Oe
along the c" direction, gave neither
Meissner expulsion nor diamagnetic shiel-
ding signal down to 0.1 K. In comparison
with measurements on similar samples of
(TMTSF)₂ReO₄, effects stand less than
1/1000.

AC susceptibility measurement performed
on an assembly of 10 crystals at P=10kbar
do not show any diamagnetic signal down
to 1.2K within experimental limit of 1%
compared to the niobium sample.

The ¹H-NMR relaxation rate T⁻¹ is about
one order magnitude higher than that of
the perchlorate salt (small dots in Fig.6)
at T > 5K. Below 4K, T⁻¹ decreases rapidly

(more rapidly than the usual Korringa bebahiour). An increase of T⁻² (narrowing of the resonance line) was also detected below 3K.

<figure><img src="image_2.png" /><figcaption>Fig. 1 : Temperature dependence of the
resistivity at 7 kbar.</figcaption></figure>

<figure><img src="image_3.png" /><figcaption>Fig. 2 : Temperature dependence of the resistivity at 9 kbar.</figcaption></figure>

<figure><img src="image_4.png" /><figcaption>Fig. 3 : Temperature dependence of the
resistivity at 11 kbar.</figcaption></figure>
<figure><img src="image_5.png" /><figcaption>Fig.4 : Resistivity versus magnetic field with the field along c*. From the bottom up : T=0.1K; T=0.3K; T=0.5K; T=0.75K; T=1.3K; T=1.7K; T=2.1K; T=2.5K; T=2.7K; T=3.5K.</figcaption></figure>

<figure><img src="image_6.png" /><figcaption>Fig.5 : Resistivity versus magnetic field with the field along b* : from the bottom up : T=0.5K; T=1.0K; T=1.5K; T=2.0K; T=2.2K; T=3.5K.</figcaption></figure>

DISCUSSION

The increase of resistivity from 100K to 20K was described by R.C. Lacoë et al /9/, in terms of anions glass which modifies the mobility of the carriers. The same authors assumed that the low temperature transition is a SC transition. The major discrepancy between their experimental results and ours is the magnitude of the critical fields; they find at 6 kbar a perpendicular critical field of 4.4 kOe at T = 1.2K.

Our experimental results obtained of samples from two batches, consistently show the following properties :

- The "SC transition" is far from being complete.

- The transition is very broad.

- The critical field for the c* direction is of the order of the paramagnetic limit (assuming a S.C. origin of the transition).

- Along the b* direction there is no effect of the field up to 60 kOe.

- No Meissner effect or AC susceptibility signal was observed at T < 4 K.

- The NMR response indicates high susceptibility in this system above about 3.5 K. It is striking that the apperant magnetism ceases at the same temperature where the strong decrease of resistivity occurs.

<figure><img src="image_7.png" /><figcaption>Fig.6 : $^1H-T_1^{-1}$ versus temperature (the dots are those of $(TMTSF)_2C_6O_4$) and $^1H-T_2^{-1}$ versus temperature.</figcaption></figure>
These properties imply, that if the transition is SC in nature, it could only be a filamentary or granular type superconductivity. The absence of Meissner effect indicates that the grains or filaments have a size smaller than the penetration depth.

At present the difference between our results and those of Lacoë et al is somewhat confusing. It is tempting to ascribe this discrepancy to sample problems. We do however want to emphasize that our NMR results in the vicinity of the "S.C." transition indicate very different behaviour of $(TMTSF)_2FSO_3$ as compared with other $(TMTSF)_2X$ systems.

REFERENCES

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