JOURNAL DE PHYSIQUE Colloque C3, supplément au n°6, Tome 44, juin 1983 page C3-1083 INFLUENCE OF THE ANION DISORDER ON THE LOW TEMPERATURE BEHAVIOUR OF THE ORGANIC SUPERCONDUCTOR (TMTSF) 2 C10 4 . + ++ S. Tomic , D. Jérôme, P. Monod and K. Bechgaard Laboratoire de Physique des Solides, Bât. 510, Université Paris-Sud, 91405 Orsay Cedex, France Résumé - Nous montrons que le comportement à basse température et que l'état fondamental de (TMTSF)2C104 sont très sensibles au degré de désordre des anions qui peut être contrôlé par le taux de refroidissement. Abstract - We show that the low température behaviour and the ground state in (TMTSF)2C1Û4 are very sensitive t o a degree of the anion disorder which can be controlled by a cooling rate. After two years of investigation (TMTSF)2C104, so far the only organic superconduc- tor at ambient pressure ( 1 ) , i s still i n a focus of interest. The results of spéci- fie heat measurements ( 2 ) , NMR ( 3 ) , E P R a n d r e s i s t i v i t y measurements (4) clearly show the important influence of cooling rate on the low température (LT) behaviour. The X-ray work of J . P . Pouget et al. (5) definitely showed the existence o f a phase transition a t 24 K where new spots with a wave vector (0, 1/2, 0) appear. This was interpretated as arising from ordering of the anion lattice. Here we présent new EPR and resistivity results, which confirm our previous work and demonstrate in more détail the important effects of the anion lattice in influencing the ground state of this System. RESULTS Results of the electrical resistivity and EPR annealing measurements are given in Fig. 1, Fig. 2 and Fig. 3. The results show that slow cooling below 30 K (rate ^ 20 K/h) allows a f u i ! anion ordering (A0) transformation at about 22 K. This leads to a LT relaxed (R) state with enhanced conductivity and a finite Pauli-like susceptibility above 3.5 K. In this state the System becomes superconducting below about T s c % 1.3 K. Rapid cooling below 30 K (rate ^ 2 x 10" K/h) on the contrary leads t o a L T quenched (0) state with increased resistivity and vanishing EPR suscep- tibility below % 5 K ( 4 ) . This state is identified as a SDW state (6) and, as M. Ribault recently showed (7) superconductivity is suppressed to at least below 0.1 K. Intermediate cooling rates lead t o a wide range of mixed (M) states ( F i g . 4 ) in which the System before becotning superconducting (T s c < 1.3 K) displays semi- conducting (SDW) behaviour. DISCUSSION It is somewhat surprising that frozen in disorder (the CIO4 anions occupy two équi- valent positions statistically in the Q-state) favours t h e SDW instability which itself is periodic. We can however describe the System by two LT order parameters. Therefore we introduce i|) SC = A s c ei 'f 'sc and tp SDW = AgDW e^SDW where A and <)> a r e the amplitude and the phase of the respective order parameter. For the sake of simplicity we assume that the R-state is fully described by only ty sc and the Q-state Permanent address : Institute of Physics of the University, P.O.B. 304, 41001 Zagreb, Yugoslavia. Permanent address : H.C. Oersted Institute, Universitetsparken 5, DK-2100 Copenhagen, Denmark. Article published online by EDP Sciences and available at http://dx.doi.org/10.1051/jphyscol/1983144