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

PINNING OF CHARGE DENSITY WAVES IN IRRADIATED TTF-TCNQ

S. BOUFFARD

Section d'Etude des Solides Irradiés
Centre d'Etudes Nucléaires de Fontenay-aux-Roses (92260) - France

R. CHIPAUX and D. JEROME

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

K. BECHGAARD

H.C. Oersted Institute - Universitetsparken 5,
DK-2100, Copenhagen, Denmark

(Received on Sept. 26, 1980, Revised version on Nov. 13, 1980, Accepted on Nov. 19, 1980 by A. Blandin)

A significant fraction of the metallic conductivity of TTF-TCNQ is due to fluctuating collective mode. We study the effets of irradiation inducéd defects on the both components of the conductivity : the fluctuating collective and the single-particle conductivities. We show that the blocking effect of single-electrons is a typical 1-D effect and is effective for concentrations of about 1%; and the pinning of charge density waves is a 3-D effect and occurs with concentrations of an order of magnitude less.

The possibility of electron transport in a collective way in tetrathiafulvalene-tetra-cyanquinodimethane (TTF-TCNQ) has strongly motivated the development of the research effort. The existence of a significant collective contribution to the conduction has been verified through the possibility of achieving commensurability under pressure [1, 2]. The phase diagram of TTF-TCNQ obtained from transport property measurements [3, 4] shows that above 15 kbar only one first order phase transition remains in a 4 kbar range centered at 19 kbar. At 19 kbar, the transition temperature (T~C) reaches 74 K. The peaking and 1st order character of the transition suggest a commensurability between the charge density wave (C.D.W.) and the underlying lattice due to a 2/3 charge transfer [3]. The longitudinal conductivity versus pressure curves exhibit a large drop in the metallic state (near the Peierls transition temperature) in the same pressure range [2]. This loss has been suggested as evidence that a significant fraction of the metallic conductivity in uncommensurate TTF-TCNQ is due to a fluctuating C.D.W. collective transport mechanism. The commensurability (x3) provides a strong pinning for the sliding C.D.W. The relative contribution of the single particle (σ~F~ SP) and the collective (σ~F~ F) conductivities have been estimated [2,3] from the behaviour of the transverse conduction; in the uncommensurate state, these two contributions are about similar at 150 K, but around 19 kbar, the single-particle conduction dominates in the total conductivity. This commensurability phenomenon was also recently observed in TSF-TCNQ near 7 kbar [5].

However, the enhancement of the maximum of

conductivity near 12 kbar is somewhat sample dependent [2]. One may argue that is a consequence of the fluctuation conductivity mechanism being quite sensitive to impurities or lattice defects [6-10]. This effect was expected to become important at low temperature when the longitudinal coherence length becomes comparable to the distance between impurities. Until now, this C.D.W. pinning has been studied in the inorganic low-dimensional conductor NbSe~3~ by conductivity frequency dependence and non-linear transport [11].

The most convenient way to introduce a controlled disorder in organic compound is the irradiation, and more particulary X-ray irradiation. Although the nature of defects is still unknown, the effects of irradiation induced defects are presently well understood [12-14].

We report an investigation of the pinning of the C.D.W. by X-ray irradiation induced defects in TTF-TCNQ. The room temperature irradiation was performed at Fontenay-aux-Roses by a conventional X-ray generator using a copper tube. The maximum dose (~ 2.10~5~ kRöntgens) corresponds to a 4 day irradiation. Figure 1 shows the dose dependence of the longitudinal conductivity for two samples. The decrease of conductivity is approximatively 75 (Ω.cm)~1~, but there is a small scatter in the results between different samples. The pressure experiments were carried out in the same manner as in reference 2. The pressure dependence of irradiated crystals at different temperatures is shown on figure 2. The conductivity of all the samples has been normalized to 400 (Ω.cm)~1~ under ambient condition and before irradiation. The dotted lines indicate the conductivity of pure
<figure><img src="image_2.png" /><figcaption>Figure 1 :</figcaption></figure>

Longitudinal conductivity as a function of the irradiation dose (two samples) at room temperature. The upper scale indicates the calculated defect concentration.

<figure><img src="image_3.png" /><figcaption>Figure 2 :</figcaption></figure>

Pressure dependence of the longitudinal conductivity at constant temperature for irradiated TTF-TCNQ. The various sign correspond to different samples. The stars indicate our results on pure samples, and the dotted lines are the data for the "a" sample in ref. 2.

crystal at 100 K, 150 K and 290 K (sample a in reference 2). All the samples we used in this experiment come from the same batch, and we can see that the behavior of the unirradiated sample (stars in figure 2 and 3) is quite similar to the best one in reference [2].

The principal feature emerging from these data is the small decrease of the conductivity occuring around 19 kbar by about 560 (Ω.cm)⁻¹ instead of 3200 (Ω.cm)⁻¹ for the unirradiated sample observed on the 100 K conductivity versus pressure curve. At low temperature, the irradiation effects are more efficient in the uncommensurate state (~ - 4500 (Ω.cm)⁻¹) than in the commensurate one (~ - 2000 (Ω.cm)⁻¹). A satura-

<figure><img src="image_4.png" /><figcaption>Figure 3 :</figcaption></figure>

Phase diagram of irradiated TTF-TCNQ derived from the maximum of δ(log ρ)/δ(1/T). The results on pure sample are described by the dotted line and by the stars (our results). The black dot shows the Tc for a 8 MeV deuteron irradiation at the same defect concentration (ref. 15).

tion of the conductivity can be observed above 20 kbar in all the temperature range. The phase transitions are broadened and shifted to lower temperatures by irradiation-induced defects [15, 16], thus the transition temperatures are no longer well defined. Nevertheless the phase diagram of irradiated samples deduced from the maximum of δ(log ρ)/δ(1/T) shows at first, below 17 kbar a shift of 8 K towards low temperature (figure 3), and no peaking around 19 kbar. Our results are coherent with the shift measured at the same defect concentration for a 8 MeV deuteron irradiation [15] (black circle in figure 3).

As far as the defect concentration calculation is concerned we have supposed that under ambient conditions, the fluctuating collective conductivity is negligible compared to the single particule one [3] (σ<sub>σ</sub> F << 250 (Ω.cm)<sup>-1</sup>). So we may use a simple and general model recently proposed for the conductivities of disordered quasi-one-dimensional conductors [12]. In this model, the anisotropy is directly related to the defect concentration and as in reference 13, we have fitted the anisotropy (k = σ<sub>σ</sub> / σ<sub>⊥</sub>) versus irradiation curve by the following law :

$$k(c,T) = \frac{k(0,T)}{c \frac{\ell}{b} k(0,T) + \exp\left(-\frac{\Delta E}{kT}\right)}; \quad \text{eq.(1)}$$

where - k(0,T) is the anisotropy of pure crystal

- c the molecular defects concentration related to the irradiation dose (Φ) by c = α Φ (at low dose)

- ΔE is the transverse hopping actication energy (ΔE = εc)

- ℓ/b is a geometrical factor.

In figure 4, the solid line represents the best
<figure><img src="image_5.png" /><figcaption>**Figure 4 :**
Dose dependence of the anisotropy ($\sigma_{\parallel}/\sigma_{\perp}$). The line shows the calculated variation from equation 1.</figcaption></figure>

fit, and we find for the parameters :

$$\alpha = 10^{-6} \left( \frac{\text{percent}}{\text{kRöntgen}} \right)$$

and $\epsilon = 2.10^4$ K; thus the maximum dose corresponds to $2.10^{-3}$ molecular defects. Along a chain, one over 500 molecules only is affected by a defect, but in three dimensions the distance between damage molecules is about 4.5 parameters. A defect or an impurity in a quasi-one-dimensional conductor induces a spatial oscillation of the charge density around it (namely, Friedel oscillation [17]). The wavelength of the oscillation is $\pi/k_F$. Due to Coulombic interchain coupling a long-ranged distortion is also induced in neighbouring stacks [10]. In the strong pinning case, the C.D.W. phase is pinned to each of these Friedel oscillations, and when the inter-defects distance and the C.D.W. correlation length become comparable, we may expect a substantial reduction of the conductivity related to the friction less sliding of fluctuating C.D.W.'s. In our experiment, we are very close to this condition: the distance between damaged molecules is about 50 Å and at 100 K the correlation length is 60 Å [18]. Since the single-particle conductivity is only affected by the chain segmentation [13] we may expect that the decrease of the fluctuating collective conductivity is larger than the decrease of the single-particle conductivity for the present experimental situation. The coupling between irradiation induced defects and C.D.W.'s which is taken as a three-dimensional effect in the present work, must not be confused with the 3-D Coulomb coupling between C.D.W.'s of neighbouring chains which occurs only in the very vicinity of the phase transition in a pure TTF-TCNQ crystal.

It has been proposed [19] that in a quasi-one-dimensional conductor, the diffusive transverse conductivity is proportional to the longitudinal single-particle relaxation time ($\tau_{\parallel}^{SP}$). Consequently the transverse conductivity and the single-particle conductivity exhibit the same pressure and temperature dependence. We have not

<figure><img src="image_6.png" /><figcaption>**Figure 5 :**
Pressure dependence of the total longitudinal conductivity of irradiated TTF-TCNQ (continuous line) and of single-particle conductivity of pure samples, as derived from reference [3].</figcaption></figure>

measured the pressure dependence of the transverse conductivity of irradiated samples, but we may compare the longitudinal conductivity of damaged crystals to the single-particle one of unirradiated TTF-TCNQ. In figure 5 we have displayed the pressure dependence of the single-particle conductivity (derived from the behaviour of the transverse conductivity [3] and normalized to 400 (Q.cm)$^{-1}$ under ambient conditions) together with the total conductivity of irradiated samples. The above comparison of the measured and calculated curves shows that below 17 kbar, the single-particle conductivity P-dependence agrees well with the irradiated conductivity results (at 100 K) :

$$\begin{aligned} \partial(\ln \sigma_{\parallel}^{\text{SP}})/\partial P &= .21 \text{ kbar}^{-1} \\ \partial(\ln \sigma_{\parallel}^{\text{irr}})/\partial P &= .25 \text{ kbar}^{-1} \end{aligned}$$

These results show that the conductivity of fluctuating collective origin is much more sensitive to defects than the conduction of classical origin. The blocking effect of single electrons is a typical 1-D effect and becomes effective for concentrations of 1% [12]. On the contrary the pinning of C.D.W. by charged impurities is a 3-D effect and occurs with concentrations of an order of magnitude less. At temperature below the phase transition the C.D.W. are also pinned on the lattice and ordered in three dimensions. The radiation induced defects disturb the coherence needed for the achievement of the phase transition, consequently the phase transitions are smeared and shifted to lower temperatures. The analysis of the shape of the damage curves in the insulating state shows that the defect is surrounded by a transformed volume of about 120 molecules [16]. This number which characterises the pinning efficiency of the defects is in agreement with the result of our experiment.

We have demonstrated that for $2.10^{-3}$ molecular defect, practically the whole collective contribution is removed from the total conductivity. The three-dimensional effect of irra-
diation-induced defects provides a strong pinning of the sliding C.D.W.'s, while the single-particle conductivity which is only sensitive to the chain breaking due to on-chain defects remains hardly effected.

Acknowledgments: We wish to thank L. ZUPPIROLI and H.J. SCHULZ for useful discussions and A. ANDRIEUX and G. BENEDEK for technical assistance.

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