A MAGNETIC STATE IN THE ORGANIC
SUPERCONDUCTOR (TMTSF)2ClO4: COOLING
RATE AND STRONG MAGNETIC FIELD EFFECTS

Toshihiro Takahashi, Denis Jerome, Klaus Bechgaard

► To cite this version:

Toshihiro Takahashi, Denis Jerome, Klaus Bechgaard. A MAGNETIC STATE IN THE ORGANIC SUPERCONDUCTOR (TMTSF)2ClO4: COOLING RATE AND STRONG MAGNETIC FIELD EFFECTS. Journal de Physique Colloques, 1983, 44, 10.1051/jphyscol/1983006. hal-05000763

HAL Id: hal-05000763

https://hal.science/hal-05000763v1

Submitted on 21 Mar 2025

**HAL** is a multi-disciplinary open access
archive for the deposit and dissemination of sci-
entific research documents, whether they are pub-
lished or not. The documents may come from
teaching and research institutions in France or
abroad, or from public or private research centers.

L'archive ouverte pluridisciplinaire HAL, est
destinée au dépôt et à la diffusion de documents
scientifiques de niveau recherche, publiés ou non,
émanant des établissements d'enseignement et de
recherche français ou étrangers, des laboratoires
publics ou privés.

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

Distributed under a Creative Commons Attribution 4.0 International License
# A MAGNETIC STATE IN THE ORGANIC SUPERCONDUCTOR (TMTSF)₂ClO₄ : COOLING RATE AND STRONG MAGNETIC FIELD EFFECTS

T. Takahashi⁺, D. Jérôme and K. Bechgaard⁺⁺

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

**Résumé** - Des mesures de RMN sur les noyaux ¹H et ⁷⁷Se d'un monocristal du supraconducteur organique (TMTSF)₂ClO₄ révèlent qu'un état magnétique peut y être stabilisé de deux façons différentes. L'une est l'introduction d'un désordre des anions par un refroidissement rapide du cristal (état Q) : un taux de l'ordre de 2 K/sec conduit à une transition magnétique à environ 3,7 K, presque indépendante du champ magnétique. D'autre part, si l'échantillon est refroidi lentement (taux de 0,2 K/min, état R) un fort champ induit un état magnétique au-dessous de 2,15 K à 74 kOe. Cet état correspond à la phase semi-métallique observée sous champ par des mesures de chaleur spécifique et d'effet de Shubnikov-de Haas.

**Abstract** - ¹H and ⁷⁷Se - NMR on an oriented single crystal of the organic superconductor (TMTSF)₂ClO₄ reveal that a magnetic state is stabilized by two different ways. One is the introduction of anion disorder by rapid cooling of the crystal (Q-state) : a cooling rate of about 2 K/sec gives an almost field-independent magnetic transition at about 3.7 K. In a slowly cooled sample (cooling rate ≈ 0.2 K/min, R-state), a strong magnetic field induces a magnetic state below 2.15 K at 74 kOe. This state corresponds to the field induced semi-metallic phase observed by specific heat and Shubnikov-de Haas measurements.

## INTRODUCTION

An important feature of the family of quasi-one-dimensional conductors (TMTSF)₂X is the competition between two types of critical behavior at low temperature ; spin density waves (SDW) and superconductivity (SC)(1). A good example is the crossover at low temperature in the salts with centrosymmetric anions (X-PF₆, AsF₆, SbF₆, TaF₆, NbF₆, etc..) from a magnetic SDW state at ambient pressure to a 3D-ordered SC state above a critical pressure of about 8 kbar. (See a typical phase diagram for this family (1)).

The experimental situation for (TMTSF)₂ClO₄ is more complex. It is so far the only material that exhibits 3D-ordered superconductivity already at ambient pressure (2,3). Under certain experimental conditions, an upturn in resistivity (4,5) a decrease of the EPR intensity (6) and evidence for antiferromagnetic resonance (7) indicate an onset of a SDW state and possible coexistence of SDW and SC slightly above the SC transition. Moreover, specific heat and Shubnikov-de Haas (9) measurements reveal that the application of a strong magnetic field of about 60 kOe induces another type of phase of semimetallic nature. A similar high-field phase transition was observed in (TMTSF)₂PF₆ under pressure by ¹H-NMR relaxation (10) and Shubnikov-de Haas (11) measurements.

<footnote>⁺Permanent address : Department of Physics, Gakushuin University, 1-5-1 Mejiro, Toshima-ku, Tokyo 171, Japan</footnote>

<footnote>⁺⁺Permanent address : H.C. Oersted Institute, Universitetsparken 5, DK-2100 Copenhagen, Denmark</footnote>
We report the results of ¹H and ⁷⁷Se-NMR measurements on single crystals of (TMTSF)₂ClO₄. They clearly show that (1) a magnetic phase transition is induced by rapid cooling of the sample (Q-state) (12) and that (2) the field-induced semimetallic phase in a slowly cooled sample (R-state) is again magnetic.

<figure><img src="image_2.png" /><figcaption>Fig. 1 - Temperature dependence of the ⁷⁷Se-NMR signal intensity and linewidth in the Q-state at a field of 55 kOe parallel to the c*-axis. The straight dashed line indicates Curie law behaviour.</figcaption></figure>

<figure><img src="image_3.png" /><figcaption>Fig. 2 - ⁷⁷Se spin lattice relaxation rate versus temperature. Insert shows the reduction of signal intensity measured in the same run. The line is a guide to the eye.</figcaption></figure>

## COOLING RATE EFFECT

Rapid cooling of the sample was found to stabilize a magnetic state below ~4 K.

Fig. 1 displays the temperature dependence of the ⁷⁷Se-NMR intensity after rapid cooling below 40 K at a rate of 25 K/min. (The external field is 53 kOe). At temperatures well above 5 K, the signal intensity follows a Curie law. Below 5 K, a rapid reduction of the intensity is observed. Since the ⁷⁷Se nucleus has I=1/2, there is no doubt that the reduction of the signal intensity is caused by the inhomogeneous broadening (or large shift) of the resonance due to an onset of a strong local field of hyperfine origin. This is a clear indication of magnetic transition.

The ⁷⁷Se spin-lattice relaxation rate has a peak in the same temperature region as the intensity drop, as shown in fig. 2. The enhancement of the relaxation rate is due to slow fluctuations (in time) of the local field just around the magnetic transition.

The transition temperature of this magnetic transition depends strongly on the cooling rate. Fig. 3 shows different runs on the same crystal with different heat treatments in a field of 65 kOe. The most rapid cooling rate in the present work is about 2 K/sec. This gives the highest transition temperature of about 3.7 K.
When the sample is cooled slowly enough (cooling rate ~ 0.2 K/min), the magnetic transition is suppressed at least to below 1.3 K (our experimental limit). We call these two extreme cases the Quenched-state (Q-state) and the Relaxed-state (R-state), respectively. While there are many 'intermediate-states', we concentrate only on the well-characterized states, hereafter.

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

Fig. 3 - Heat cycle effect on the magnetic transition in the same sample. The normalized signal intensity multiplied by temperature is plotted versus temperature (below 4.2 K). The straight dashed line indicates Curie law behavior.

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

Fig. 4 - ¹H spin lattice relaxation rate versus temperature in the Q-state (○) and in the R-state (●). Annealing at 10-12 K for 3 hours reduces the transition temperature by 0.6 K (△).

Note that the heat cycle effects are fairly well reproducible.

The remarkable difference in the low temperature behaviour between the R- and Q-state is observed also at much lower fields by ¹H-NMR. Fig. 4 shows the temperature dependence of the ¹H relaxation rate for the two states at 10.6 kOe. In the Q-state, a sharp peak indicates again a magnetic transition at 3.6 K. This temperature is almost the same as that observed by ⁷⁷Se-NMR at higher fields. The R-state, on the contrary, has no anomaly of this type.

The present results demonstrate that the magnetic (SDW) state can be stabilized only when the sample is cooled sufficiently fast (at least in low fields). This is consistent with the results of measurements of resistivity and electron paramagnetic resonance under careful temperature control (13). In the Q-state, a strong upturn in resistivity and a rapid drop of EPR intensity were observed, while metallic resistivity and Pauli-like susceptibility is observed in the R-state.

We now address the origin of the drastic cooling rate effects. Crucial information was obtained from diffuse X-ray investigations, which show a structural phase transition at 24 K (with precursors below 30 K), giving rise to a superstructure with a periodicity of (a, 2b, c) (14). It is suggested that this is arising from an order-disorder transition in the anion ($ClO_4^−$) lattice. Because the $ClO_4^−$ ion has
tetrahedral symmetry, there are two equivalent orientations at the anion site in space group P1. At high temperature, $ClO_4^-$ ions occupy them randomly. Below 24 K, the anions order "ferroelectrically" with no change in the periodicity along the a-axis. Thus, it is quite reasonable to suppose that the random orientation of anions at high temperature is frozen in the Q-state and that the disorder potential due to the anions is important for the stabilization of the SDW's on the molecular

<figure><img src="image_6.png" /><figcaption>Fig. 5 - $^{77}$Se-NMR signal intensity, linewidth and spin-lattice relaxation rate versus temperature at a field of 73.9 kOe parallel to the c*-axis (R-state)</figcaption></figure>

chains. In fact, an entropy analysis from the specific heat measurements (15) has shown an excess entropy at the low temperature limit, indicating lattice disorder in the Quenched state.

## MAGNETIC FIELD EFFECT

We now turn to the R-state obtained by slow cooling. As stated earlier, there is no magnetic anomaly in the R-state above 1.3 K in low fields. However, there exists another phase boundary in the high magnetic field region. This boundary was first derived as a threshold field of Shubnikov-de Haas oscillations (9). Furthermore, specific heat measurements at 63 kOe (8,16) have demonstrated a phase transition at 1.4 K and it was suggested that the high field phase is semimetallic. A similar high field phase in $(TMTSF)_2PF_6$ was also observed at $P > 6.5$ kbar by $^{1}$H-NMR relaxation (10) and Shubnikov-de Haas (11) measurements. In $(TMTSF)_2PF_6$, a sharp peak of $^{1}$H-relaxation at the boundary (10) suggests that this high field phase is again magnetic. In order to verify this, a $^{77}$Se-NMR study at 74 kOe was carried out.

Fig. 5 displays the temperature dependence of the $^{77}$Se-NMR intensity, linewidth and spin lattice relaxation rate. The signal intensity at high temperatures follows a
Curie law and at 2.15 K the signal suddenly disappears. A rapid broadening of the resonance line is observed in the very narrow transition region (κ 0.2 K). Moreover, the spin-lattice relaxation rate is enhanced just above the transition temperature. For the same reason as already discussed above, these results are direct evidence of a magnetic phase transition induced by the strong field. The transition temperature (2.15 K at 74 kOe) agrees well with an extrapolation of the phase boundary derived from the threshold field of Shubnikov-de Haas oscillations (9).

<figure><img src="image_7.png" /><figcaption>Fig. 6 - Proposed phase diagram (T versus H//c*) (a) for the Q-state and (b) for the R state. The results of specific heat measurement (▼, Ref. (8)) and the onset of Shubnikov-de Haas oscillations (▲, Ref. (19)) and (▼, Ref. (18)). These results are included together with the present NMR results (X and O).</figcaption></figure>

We suggest that SDW ordering leads to a nesting of the Fermi surface which changes its topology from quasi-1D open orbits to compensated small closed orbits just across the boundary. This seems to be a reasonable explanation of the onset of the quantum oscillations (9,17) and the low density of states observed by specific heat measurements (8,16).

## DISCUSSION

The present experimental results are summarized in the two phase diagrams (temperature versus magnetic field along the c*-axis) for the Q-state and the R-state, as shown in fig. 6.

In the Q-state, a metallic-to-magnetic (SDW) phase transition appears at about 3.7 K. The transition temperature is almost field-independent up to 74 kOe. The stabilization of the magnetic phase is very likely related to the introduction of anion disorder by the quenching process. Recent resistivity measurements (18) has shown that there is no 3D-ordered SC phase in the Q-state at least above 0.1 K.

In the R-state, the field induced phase was found to be a magnetic (SDW) state. According to the low temperature measurements of magnetoresistance (19,20), this phase boundary may intersect the field axis at about 43 kOe.

Apparently there are two different mechanisms which can stabilize a magnetic state in (TMTSF)₂ClO₄: a random anion potential and a strong magnetic field. There is at present no theory which explains these effects well. However it is experimentally established that the introduction of a random potential suppresses the SC ordering, as demonstrated by irradiation effects (21) and investigations of alloy systems such as (TMTSF₁-xTMTTFx)₂X (22) and (TMTSF)₂(ClO₄)₁-x(ReO₄)x (23). The quasi-1D electron gas model suggests that the SDW and SC instabilities are competing at low temperatures in this family. If one of them (the SC instability in this case) is suppressed, the other instability (SDW) may be stabilized.
The magnetic field effects may also arise from the SDW-SC competition. The SC instability is sensitive to the field. The 3D-ordered SC state is suppressed above a critical field $H_{c2}$ (about 1 kOe in the $c$'-direction at absolute zero (16)). However the pseudo gap in the electronic density of states (24,25) the anomaly in thermal resistance (26) and the anomalous electronic specific heat (above $T_c$) (16) are all suppressed at much higher field than $H_{c2}$. These effects are attributed to the quasi-1D nature of the SC fluctuations which appears to have a large condensation energy of the order of the pseudo gap ~ 40 K (1). Within the framework of this picture, the stabilization of the SDW state at high fields may thus again be caused by the suppression of the competing SC instability.

REFERENCES

1) See a review by Jérôme D. and Schulz H.J., *Advances in Phys.* **31** (1982), 299.

2) Bechgaard K., Carneiro K., Olsen M., Rasmussen F.B. and Jacobsen C.S., *Phys. Rev. Lett.* **46** (1981), 852.

3) Schwenk H., Neumaier H., Andres K., Wudl F. and Aharon-Shalom E., *Mol. Cryst. Liq. Cryst.* **79** (1982), 277.

4) Parkin S.S.P., Ribault M., Jérôme D. and Bechgaard K., *J. Phys. C* **14** (1981), 5305.

5) Murata K., Ukachi T., Anzai H., Saito G., Kajimura K. and Ishiguro T., *J. Phys. Soc. Japan* **51** (1982), 1817.

6) Bechgaard K., Carneiro K. Rasmussen F.B., Olsen M., Rindorf G., Jacobsen C.S., Pedersen H. and Scott J.C. *J. Am. Chem. Soc.* **103** (1981), 2440.

7) Walsh Jr. W.H., Wudl F., Aharon-Shalom E., Rupp Jr. L.W., Vandenberg J.M., Andres K. and Torrance J.B., *Phys. Rev. Lett.* **49** (1982), 885.

8) Garoche P., Brusetti R., Jérôme D. and Bechgaard K., *J. Physique-Lett.* **43** (1982), L-147.

9) Kwak J.E., Schirber J.E., Greene R.L. and Engler E.M., *Mol. Cryst. Liq. Cryst.* **79** (1982), 111.

10) Azevedo L.J., Schirber J.E., Greene R.L. and Engler E.M., *Physica* **108B** (1981), 1183.

11) Kwak J.E., Schirber J.E., Greene R.L. and Engler E.M., *Phys. Rev. Lett.* **46** (1981), 1296.

12) Takahashi T., Jérôme D. and Bechgaard K., *J. Physique-Lett.* **43** (1982), L-565.

13) Tomic S., Jérôme D., Monod P. and Bechgaard K., *J. Physique-Lett.* **43** (1982), L-839.

14) Pouget J.P., Shirane G., Bechgaard K. and Fabre J.P., submitted to *Phys. Rev.*

15) Garoche P., Brusetti R. and Bechgaard K., *Phys. Rev. Lett.* **49** (1982), 1346.

16) Brusetti R., Garoche P. and Bechgaard K. to be published in *J. Phys. C* (1983).

17) Chaikin P.M., this conference.

18) Ribault M., this conference.

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

20) Kajimura K., Tokumoto H., Tokumoto M., Murata K., Ukachi T., Anzai H., Ishiguro T. and Saito G., *Solid State Commun.* **44** (1982), 1573.

21) Bouffard S., Ribault M., Brusetti R., Jérôme D. and Bechgaard K., *J. Phys. C*, **15** (1982), 2951.

22) Coulon C., Delhaes P., Amiell J., Manceau J.P., Fabre J.M. and Giral L., *J. Physique* **43** (1982), 1721.

23) Tomic S. et al., this conference.
24) Fournel A., More C., Roger G., Sorbier J.P., Delrieu J.M., Jérôme D., Ribault M., Bechgaard K., Fabre J.M. and Giral L., Mol. Cryst. Liq. Cryst. 79 (1982), 261.

25) Ng H.K., Timusk T., Delrieu J.M., Jérôme D., Bechgaard K. and Fabre J.M., J. Physique Lett. 43 (1982), L-513.

26) Djurek D., Prester M., Jérôme D. and Bechgaard K., J. Phys. C, 15 (1982), L-669.