Cooling Rate and Electric Field Effects in
(TMTSF)2FSO3

S Tomic, Denis Jerome, K Bechgaard

► To cite this version:

S Tomic, Denis Jerome, K Bechgaard. Cooling Rate and Electric Field Effects in (TMTSF)2FSO3. Molecular Crystals and Liquid Crystals, 1985, 119 (1), pp.59 - 63. 10.1080/00268948508075134. hal-05010377

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## Cooling Rate and Electric Field Effects in (TMTSF)₂ FSO₃

S. Tomicª, D. Jéromeª & K. Bechgaardª

ª Laboratoire de Physique des Solides, Băt., 510, 91405, Orsay, France

Published online: 17 Oct 2011.

**To cite this article:** S. Tomic, D. Jérome & K. Bechgaard (1985) Cooling Rate and Electric Field Effects in (TMTSF)₂ FSO₃, Molecular Crystals and Liquid Crystals, 119:1, 59-63

**To link to this article:** http://dx.doi.org/10.1080/00268948508075134

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COOLING RATE AND ELECTRIC FIELD EFFECTS
in (TMTSF)₂FSO₃

S. Tomic*, D. Jérôme and K. Bechgaard +

Laboratoire de Physique des Solides, Bât. 510, 91405 Orsay, France.

Abstract

We have performed an experimental investigation of the
influence of the anion kinetics and anion-dipole order on
the ground state of (TMTSF)₂FSO₃ in the pressure range 1 bar
to 11 kbar.

The sensitivity to the anion ordering (AO) of the low temperature
(LT) behaviour and the ground state of (TMTSF)₂X (X non centro-
symmetric anion) compounds and alloys is now quite well established
experimentally ¹⁻². (TMTSF)₂FSO₃ and recently synthetized (TMTSF)₂
F₂PO₂ are the only ones whose anions carry a permanent dipole
moment ³⁻⁴. In the former salt the superconducting (SC) transition
is apparently not complete ⁵ and only a weak Meissner effect sugges-
ts that SC is well developped in only 2 % of the total sample
volume ⁶. In the latter one where a dipole moment is much stronger,
SC is not observed at all.

We have performed resistivity measurements at pressures 1 bar to
11 kbar. The experiments done at low pressures where a dc current
pulse was applied along the sample are described elsewhere ⁷. In
short, at LT, a dc current pulse can switch the material from a
semiconducting to a low resistance metastable state. The so obtai-
ned drop of the resistivity at 4.2 K can reach 10⁵ (in most cases
the ratio was ~ 10³). This state is stable in time and is destroyed

<footnote>* Permanent address : Institute of Physics of the University, P.O.B.
304, 41001 Zagreb, Yugoslavia.</footnote>

<footnote>+ Permanent address : H.C. Oersted Institute, Universitetsparken 5, DK-2100 Copenhagen, Denmark.</footnote>
<figure><img src="image_3.png" /></figure>

**FIGURE 1** - R vs T curve at 7 kbar on cooling. Open and solid circles for slow and rapid cooling, respectively.

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

**FIGURE 2** - R vs T curve at 7 kbar. Solid circles for rapid cooling and open triangles for slow warming.

<figure><img src="image_5.png" /></figure>

**FIGURE 3** - The onset of the "SC" transition for slow (open circles) and rapid (solid circles) cooled sample, at 7 kbar.

above 30 K where the material displays a transition to a high resistance stable state. Experiments to separate electric field effects from fast quenching of the sample are in progress, with the aim to understand better the origin of this striking result.

Here we present in greater details thermal treatment measurements at 7 kbar and

11 kbar performed on two samples of approximate dimensions 1.5 x 0.13 x 0.05 mm³. In each run samples were cooled from above 100 K down to 1.3 K and rewarmed up slowly before subsequent cooling. The main results of these experiments are as follows : (i) At
<figure><img src="image_6.png" /><figcaption>FIGURE 4 - R vs T curve at 11kbar.</figcaption></figure>

<figure><img src="image_7.png" /><figcaption>FIGURE 5 - The onset of the "SC" transition for slowly cooled sample at 11 kbar.</figcaption></figure>

7 kbar rapid cooling down to 4.2 K drives a system to a LT state with lower resistivity than in the slow cooling treatment. Below 4.2 K the resistivity decreases more rapidly and $T_{SC}$ is higher ($T_{SC}$ = 1.6 K and 1.3 K for Q and R samples, respectively). $T_{SC}$ is defined as the temperature where the resistivity drops to half of its value just above the transition. Furthermore, the increase of the resistivity under the magnetic field ($H \perp a$) is larger for Q samples than for R samples : 27 % and 13 % at 1.3 K under a field $H$ = 450 gauss, respectively. Note that $= 450$ gauss is the field sufficient to restore the normal state resistivity in (TMTSF)₂ReO₄². (ii) At 11 kbar a metallic behaviour is recovered and the value of the resistivity at 4.2 K is the same for R and Q samples. However, there are hysteresis effects in the T-region 35 K - 86 K where the resistivity is lower on slow cooling.

Moreover, the SC transition temperature is higher for R-samples (1.65 K) than for Q-ones (1.5 K). The increase of the resistivity under $H$ = 450 gauss is about 40 % at 1.3 K for R-samples in contrast to only 25 % for Q-samples. (iii) The temperature below which the resistivity starts to increase does not
depend on the pressure and is equal to the transition temperature at p = 1 bar, i.e. 86 K. Even at 11 kbar, there is a break in the slope of R(T) vs T at ~ 86 K. This is in contrast with (TMTSF)₂ReO₄ behaviour under pressure ⁸.

R. Lacoe et al. ³ have already investigated the phase diagram of (TMTSF)₂FSO₃ and have shown that it was very similar to that of the ReO₄ salt. Furthermore, they have found the SC transition temperature above 2 K and assumed that this increase in TSC is due to the increased electron-phonon interaction renormalized by the dipole moments. In our measurements the SC transition was never complete ⁵ and TSC attains similar values as in (TMTSF)₂ReO₄² (only onset ~ 2 K which is higher than in other (TMTSF)₂X compounds). Moreover, TSC is cooling rate dependent in the similar manner as in (TMTSF)₂ReO₄. However, the striking point is that the SC transition is cooling rate dependent once the metallic behaviour is restored. Hysteresis effects hint that the order parameter of the LT phase is still not fully developed at long range. All these features indicate that the necessary symmetry due to the anion-dipole order and compatible with SC is not established on the long range scale. This is, maybe, not so surprising because each of the two equivalent anion positions has now four subgroups depending on the position of the fluorine atom. In other words, the uniform potential along the conducting chains is created only if both anions and dipoles order together creating a superstructure such as (0,1/2,0). Therefore, it is tempting to assume that this order is not fully developed even at 11 kbar and consequently that residual dipole moment disorder prevents the establishment of bulk SC within the entire sample volume. More work is now in progress to understand better the unusual behaviour of (TMTSF)₂FSO₃ as far as superconductivity is concerned.

We thank V.J. Emery for useful discussions.
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