SPIN DENSITY WAVE IN THE ORGANIC CONDUCTOR (TMTSF)₂NO₃: ANTIFERROMAGNETIC

CRITICAL EFFECTS AND NON-LINEAR ELECTRICAL TRANSPORT

S.TOMIC

Institute of Physics of the University, Zagreb (Yugoslavia) and Laboratoire de
Physique des Solides, Université de Paris-sud, Orsay (France)

D.JÉRÔME

Laboratoire de Physique des Solides, Université de Paris-sud, Orsay (France)

J.R.Cooper

Institute of Physics of the University, Zagreb (Yugoslavia)

K.BECHGAARD

H.C.Oersted Institute, DK-2100 Copenhagen (Denmark)

ABSTRACT

We have studied the low-temperature behaviour of (TMTSF)₂NO₃ at ambient pressure by means of magnetic and transport measurements. In particular, the critical behaviour of the electron spin resonance linewidth close to the antiferromagnetic transition was studied. The critical divergence obtained is discussed in the framework of a recent theory and compared with corresponding data for other organic compounds. Furthermore, we report experiments on electric field dependent transport in the SDW state. We discuss the results obtained and compare them with non-linear conduction in the charge-density-wave compounds.

INTRODUCTION

The organic conductors (TMTSF)₂X are single chain systems in which nominally quarter conduction band is created by a charge delocalization on the organic chain [1]. The spatial anisotropy of the overlapping molecular orbitals leads to an open Fermi surface and to strong anisotropy in the electronic properties. At low temperatures the ground states can be either insulating - due to the formation of a spin density wave (SDW) phase or due to anion ordering (AO), metallic or superconducting.

(TMTSF)₂NO₃ appears to be somewhat of a special case among the extensively studied members of the (TMTSF)₂X family [2]. At ambient pressure a metallic state persist to about 10K, where a phase transition to a semiconducting, SDW state takes place. Structural investigations show a superstructure (2a,b,c)
below 45K due to the anion ordering [3]. This transition is barely visible in the transport properties. Under pressure the SDW transition is suppressed, but superconductivity has not been observed up to 24 kbar [4].

The purpose of this paper is to present briefly a study of the ambient pressure, low-temperature behaviour of $(TMSF)_2NO_3$. A detailed account will be published elsewhere [5,6]. We have used the electron spin resonance (ESR) technique to characterize the magnetic transition and to investigate the critical behaviour of the ESR linewidth close to the antiferromagnetic (AF) transition $T_N$. Furthermore, we have used a short dc pulse technique to search for electric field dependent transport in the SDW state.

## EXPERIMENTAL RESULTS AND DISCUSSION

The results of ESR experiments are shown in Fig. 1.a,b. They are independent of the cooling rate below 70K. The vanishing ESR susceptibility and concomitant divergence of the ESR linewidth ($\Delta H$) is qualitatively the same behaviour observed at the AF transition in other $(TMSF)_2X$ salts ($X=PF_6^-$, $AsF_6^-$) [7]. A log-log plot of $\Delta H$ vs $(T-T_N)/T_N$ over one decade or so close to $T_N=8.3$K shows a critical behaviour of the form $\Delta H \sim (T-T_N/T_N)^{-\mu}$ with $\mu=0.5 \pm 0.1$ (Fig. 2).

<figure><img src="image_1.png" /><figcaption>Fig. 1. Normalized ESR susceptibility (a) and linewidth (b) versus temperature.</figcaption></figure>

This critical exponent differs considerably from the value of $1.5 \pm 0.1$ observed for three different types of organic conductors. The latter value is in excellent agreement with the predictions of the recent theory [8]. It is important to note that the basic idea of the theoretical analysis is that the
<figure><img src="image_2.png" /><figcaption>Fig. 2. Log-log plot of the ESR linewidth versus the critical temperature width $(T-T_N)/T_N$ with $T_N=8.3K$.</figcaption></figure>

critical profile of $\Delta H$ originates from the enhancement of the dipolar broadening by the critical fluctuations. However, in the $(TMSF)_2NO_3$ system, the anisotropy in the spin degrees of freedom might be quite different and the mutual relevance of the dipole-dipole interaction and the spin-orbit coupling might be changed. [5].

Transport measurements were performed on single crystals of varying lengths and with typical dimensions 2x0.5x0.03 mm. Gold pads were evaporated onto the samples and 17 µm thermally annealed gold wires were attached to the pads with silver paint in the four-probe configuration. Below $T_N$ a plot of the logarithm of the resistance vs inverse temperature yields a small activation energy $\Delta$ of about 8K indicating the semiconducting nature of the SDW ground state. To search for electric field dependent transport in the SDW state, we have used a short dc pulse technique together with a bridge circuit to substruct the linear component of the conductivity, similar to that used previously for TTF-TCNQ [9]. The typical pulse length was 40 µsec and the repetition time 23 msec. The main results are as follows.

1. There is a nonlinear conduction above a certain critical voltage which is correlated to the sample length giving $E_T \approx 40$ mV/cm (Fig. 3).

2. The magnitude of the nonlinear conductivity is related to the defect concentration measured by the resistivity ratio $R_{RT}/R_{min}$.
<figure><img src="image_3.png" /><figcaption>Fig. 3. Nonlinear conductivity $\sigma(E) - \sigma(E \to 0)/\sigma(E \to 0)$ versus logarithm of electric field (E) at 4.2K.</figcaption></figure>

III Nonlinear effects, namely $E_T$ and the fractional increase in conductivity above $E_T$, are temperature independent up to about 6K. At high temperatures (80-100K) the conductivity is field independent to within 1% for fields up to 0.7V/cm.

IV Since the threshold electric field of about 40 mV/cm provides much less energy ($\epsilon$) over an electron mean free path than $k_B T$ ($\epsilon/2\Delta \sim 0.001$), it follows that observed non-linearity cannot be attributed to hot electron effects or to the destruction of the condensed state by the electric field.

The above features clearly show that the nonlinear conductivity found in the $(TMSF)_2NO_3$ compound is an intrinsic property of the SDW ground state. Furthermore, our results are reminiscent of those in the charge-density-wave (CDW) systems, where the nonlinearities have been attributed to the sliding CDW depinned in high enough electric fields. We note also that qualitatively similar behaviour for the SDW is predicted independently in recent theories by Maki and Virosztek [10] and Tutto and Zawadowski [11]. Therefore, it is tempting to propose that a sliding spin-density-wave might be a plausible mechanism for the observed nonlinear effects in $(TMSF)_2NO_3$.

REFERENCES

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