ELECTRICAL CONDUCTIVITY IN HMTSF-TNAP
UNDER PRESSURE

Denis Jerome, Richard H Friend, B. Mogensen, K. Bechgaard, K. Carneiro

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Denis Jerome, Richard H Friend, B. Mogensen, K. Bechgaard, K. Carneiro. ELECTRICAL CONDUCTIVITY IN HMTSF-TNAP UNDER PRESSURE. Solid State Communications, 1978. hal-04996350

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ELECTRICAL CONDUCTIVITY IN HMTSF-TNAP UNDER PRESSURE*

B. Mogensen†, R.H. Friend‡ and D. Jérome

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

K. Bechgaard and K. Carneiro

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

*(Received 27 February 1978 by N.I. Meyer)*

The resistivity of the organic conductor HMTSF-TNAP has been measured at pressures up to 25 kbar and at temperatures down to 1.2 K. Under pressure the increase in the stacking axis resistivity of HMTSF-TNAP (Δ²,² Bi-(4,5-trimethylene-1,3-diselenole)-11,11',12,12'-tetracyano-2,6-naphtoquinodimethane) below 47 K is reduced, although the transition temperature *T*<sub>p</sub> falls at only 0.7 K kbar⁻¹. This weak pressure dependence compared to that of HMTSF-TCNQ correlates with the larger resistivity anisotropy in the TNAP salt.

ONE OF THE AIMS of the current effort on the organic metals has been to find systems which remain metallic down to *T* = 0. In all compounds as yet made, metallic conduction results from π electron overlap along stacks of planar molecules, conduction is largely confined to the direction of the stack and the metal tends to be sufficiently one-dimensional to undergo a Peierls distortion to become insulating at low temperatures. This is the case for most of the family of charge transfer salts based on TTF-TCNQ [1]. One approach to prevent this happening has been to find systems where stronger interactions between stacks distort the Fermi surface far enough from the ideal one dimensional planar surface, and prevent adequate Fermi surface nesting for the Peierls distortion to take place. This will be effective when *t*⊥/*t*∥ ~ (k*T*<sub>p</sub><sup>0</sup>/E<sub>F</sub>)<sup>1/2</sup>, where *t*⊥ and *t*∥ are the transverse and parallel transfer integrals, *T*<sub>p</sub><sup>0</sup> the mean field transition temperature for the system in the absence of *t*⊥, and *E*<sub>F</sub> the Fermi energy [2].

One of the most promising of the TTF-TCNQ family salts is HMTSF-TCNQ [3], in which N-Se contact between adjacent stacks is strong [4], so that, for example, the conductivity anisotropy between the stacking axis and the direction of the N-Se contact is rather

small (*σ*<sub>∥</sub>/*σ*<sub>⊥</sub> ≈ 30) [5, 6]. The stacking axis conductivity increases with falling temperature, rising by a factor of 3 from room temperature to 30 K. Below this temperature it then decreases gently, falling to below its room temperature value at 4 K [3]. Although there is no low temperature diffraction evidence for a Peierls distortion, anomalies in specific heat and susceptibility indicate that there is a phase transition at 32 K [7]. This suggests that the low temperature fall in conductivity arises from the partial destruction of the Fermi surface below the transition, possibly on only one type of stack, rather than from a temperature dependent impurity resistivity. On the other hand, the resistivity measurements at low temperatures show a strong sample dependence, which indicate that some uncontrolled impurities or defects do have an effect on this feature. Therefore, however, one cannot distinguish between the following two possibilities. *Either* the HMTSF-TCNQ intrinsically undergoes a Peierls transition, which is more or less smeared out by impurities, *or* the material is intrinsically a metal at low temperatures, where impurities cause the resistance to increase. It is important to keep this ambiguity in our present understanding of HMTSF-TCNQ in mind, since the situation may be very similar in HMTSF-TNAP.

Under pressure the low temperature decrease in conductivity is suppressed above 4 kbar [8]. At the distortion temperature, HMTSF-TCNQ is well into the three dimensional regime, where electron wavefunctions are coherent both parallel and perpendicular to the stacks (hence the large magnetoresistance [8] and Hall effect [5]), and the removal of the conductivity decrease has been interpreted as the removal of the Peierls transition

<footnote>* Work supported in part by DGRST Contract No. 75-7-0820.</footnote>

<footnote>† Permanent address: H.C. Ørsted Institute, Universitetsparken 5, DK-2100 Copenhagen, Denmark.</footnote>

<footnote>‡ Permanent address: Cavendish Laboratory, Cambridge, England.</footnote>

<footnote>§ Laboratoire associé au CNRS.</footnote>
<table><caption>Table 1. Initial logarithmic pressure derivatives of the stacking conductivity at room temperature for a number of organic conductors</caption><thead><tr><th></th><th>∂σ(P)/[σ(P)∂P] (kbar<sup>-1</sup>)</th><th>Ref.</th></tr></thead><tbody><tr><td>HMTSF-TNAP</td><td>0.14 ± 0.02</td><td>This work</td></tr><tr><td>HMTSF-TCNQ</td><td>0.15 ± 0.05</td><td>[8]</td></tr><tr><td>HMTTF-TCNQ</td><td>0.25 ± 0.02</td><td>[9]</td></tr><tr><td>TSF-TCNQ</td><td>0.18 ± 0.02</td><td>[6]</td></tr><tr><td>TTF-TCNQ</td><td>0.28 ± 0.02</td><td>[6]</td></tr></tbody></table>

with an increase in $t_⊥$ with pressure in a system where, even at ambient pressure $t_⊥$ has already reduced $T_p$ from its $t_⊥ = 0$ value [9].

We present measurements of the conductivity under pressure of the charge transfer salt HMTSF-TNAP, which has recently been prepared and characterized [10]. Its electrical properties are similar to those of HMTSF-TCNQ, although the conductivity maximum at $T \approx 47$ K is approximately six times the room temperature value and the absolute conductivity at this temperature $\sigma_{max} \approx 12\,000 (\Omega \cdot cm)^{-1}$, is one of the highest conductivities reported for an organic metal. However, below this temperature there is a sharper transition, and at 4 K the conductivity is back to its room temperature value. HMTSF-TNAP has a different structure from HMTSF-TCNQ, with alternating sheets of donors and acceptors [11], in contrast to the alternating donor and acceptor molecules in both the a and c directions for HMTSF-TCNQ [4]. Although there is a very short N-Se contact in the HMTSF-TNAP structure, the conductivity anisotropy between the stacking axis and both a and c axes is approximately 100, implying weaker interstack interactions than in HMTSF-TCNQ. Since this salt shows such a high conductivity at 47 K it is of interest to see whether this metallic behaviour can be stabilized to all temperatures under pressure. In fact we find that the transition is relatively insensitive to pressure, and this we relate to the fact that the resistivity anisotropy is higher than in HMTSF-TCNQ.

Crystals used were from the same sample growth batch as those used by Bechgaard *et al.* [10]. Resistance measurements were made using standard low frequency a.c. techniques. Contacts were made with Dupont 4929

silver paint in hexyl acetate solvent, and both the 4 in-line contact and Montgomery [12] geometries were used. Pressure techniques used are described elsewhere [9]; helium gas was used as the pressure transmitting medium up to 10 kbar, and isopentane above this pressure.

Figure 1 shows the increase in conductivity along the stacking direction at room temperature with pressure, and we find the initial logarithmic pressure derivative $\partial\sigma(P)/[\sigma(P)\partial P]$ to be $(0.14 \pm 0.02) \text{ kbar}^{-1}$. In Table 1 this value is compared to that of other organic conductors. It appears that the selenium compounds in this respect are significantly less pressure dependent than their sulphur analogues, so that an average value for the logarithmic pressure derivative is 15% per kbar and 24% kbar for the two classes of materials respectively. We discuss differences between the sulphur and selenium materials later. There is some tendency towards a saturation in the conductivity at high pressure; again this is similar to the behaviour of TSF-TCNQ [13] and contrasts with TTF-TCNQ where the conductivity is still increasing linearly at 33 kbar [13].

Resistance vs temperature curves are shown in Fig. 2. The magnitude of the resistivity increase at low temperature is reduced under pressure even though the temperatures of minimum resistance falls only slowly. As reported by Bechgaard *et al.* [10], there is considerable variation in the low temperature resistivity between samples. The data shown in Fig. 2 at 1 bar, 4 kbar and 8 kbar are for the same sample, and does represent the trend in behaviour under pressure for all samples.

Figure 3 shows plots of $-\partial R/\partial T$ vs temperature for the data shown in Fig. 2. If a well-defined transition exists in this materials, the maximum in $-\partial R/\partial T$ is a convenient definition of the transition temperature as observed in resistivity experiments [14], and in the case of HMTSF-TNAP, $T_p$ defined in this way falls initially at $0.7 \text{ K kbar}^{-1}$.

In Fig. 4 the resistivity is plotted against $T$. The temperature dependence of the resistivity in the metallic regime at atmospheric pressure is approximately quadratic. Fitting to $\rho = \rho_0 + AT^{\lambda}$, Bechgaard *et al.* [10] find $\lambda = 2.3$; similar values of $\lambda$ are found for all charge-transfer salts at ambient pressure. Under pressure $\lambda$ is rapidly reduced, falling to 1.1 at 8 kbar, and to 1.0 at

<table><caption>Table 2. Anisotropy and rate of suppression of $T_p$ with pressure for the HM charge transfer salts. The anisotropy shown is always the smaller of the two $\sigma_‖/\sigma_⊥$ ratios. $\partial T_p/\partial P$ for HMTTF-TCNQ is for the upper transition. (a) Ref. [9], (b) Ref. [6], (c) Ref. [8]</caption><thead><tr><th></th><th>HMTSF-TNAP</th><th>HMTTF-TCNQ</th><th>HMTSF-TCNQ</th></tr></thead><tbody><tr><td>σ<sub>‖</sub>/σ<sub>⊥</sub></td><td>80</td><td>80<sup>a</sup></td><td>30<sup>b</sup></td></tr><tr><td>∂T<sub>p</sub>/∂P (K kbar<sup>-1</sup>)</td><td>-0.7</td><td>-1.5<sup>a</sup></td><td>-8<sup>c</sup></td></tr></tbody></table>
<figure><img src="image_3.png" /><figcaption>Fig. 1. Normalized conductivity vs pressure measured at room temperature for samples in the gas pressure system (Δ, □) and in the liquid pressure system (●, ▲, ■).</figcaption></figure>

25 kbar. Again, similar behaviour has been seen in TSF-TCNQ under pressure [13], whereas a higher power law persists even at high pressures for TTF-TCNQ [13] and HMTTF-TCNQ [9].

We have measured the resistivity anisotropies using the Montgomery [12] and Van der Pauw [15] geometries. At room temperature and ambient pressure we find $σ_{||}/σ_{⊥}$ equal to 80 ± 20 and 100 ± 30 for the two transverse directions, and the absolute value of $σ_{||}$ to be 1700 ± 400 (Ω-cm)$^{-1}$. These values of the anisotropies are in good agreement with Bechgaard et al. [10] although their value of $σ_{||}$, of 2400 ± 600 (Ω-cm)$^{-1}$, determined with the 4 in-line contact geometry, is a little higher than the value we determined. We have followed the smaller of the two anisotropies as a function of temperature and pressure. Up to 10 kbar $|∂(σ_{||}/σ_{⊥})/∂P| < 0.5$ kbar$^{-1}$ at room temperature. This very small change resembles the behaviour observed in TTF-TCNQ and TSF-TCNQ [6]. At 200 K there is 20% increase in $σ_{||}/σ_{⊥}$ from its value at 300 K. There is some indication of a significant increase at the phase transition, and that this increase is reduced under pressure, although these measurements were complicated by irreversible jumps in the resistivity curves with this contact geometry, probably due to the very small contacts lifting away from the crystal. This result is similar to that reported for HMTSF-TCNQ [5].

The relatively low rate of suppression of the low temperature resistance in HMTSF-TNAP with pressure compared to HMTSF-TCNQ can be understood in terms of the differences in $t_{⊥}$ between these materials.

For diffusive motion of carriers transverse to the stacking axis, Soda et al. [16] have shown

$$ \frac{\sigma_{\perp}}{\sigma_{\parallel}} = \left(\frac{t_{\perp}}{t_{\parallel}}\right)^2 \left(\frac{a}{b}\right)^2 $$

<figure><img src="image_4.png" /><figcaption>Fig. 2. Normalized resistivity vs temperature curves at various pressures. Runs at 1 bar, 4 kbar and 8 kbar were on the same sample.</figcaption></figure>
<figure><img src="image_5.png" /><figcaption>Fig. 3. $-\partial R/\partial T$ vs T for the data shown in Fig. 2. At 25 kbar the derivatives were too small to be significant.</figcaption></figure>

<figure><img src="image_6.png" /><figcaption>Fig. 4. The high temperature regime from a series of measurements plotted as resistivity vs T. Closed symbols indicate 4 in-line contact and open symbols the Montgomery geometry.</figcaption></figure>

where a and b are characteristic interchain and intra-chain distances. Thus the conductivity anisotropy can be directly related to the criterion [2] for $t_⊥$ to impair Fermi surface nesting sufficiently to reduce $T_p$,

$$ \left(\frac{t_{\perp}}{t_{\parallel}}\right)^2 > \frac{kT_p^0}{E_F}. $$

$E_F$ for the charge transfer salts is typically ~200 meV [17], and with $kT_p^0 \sim 2.5$ meV, little pressure dependences of $T_p$ is expected until $(t_‖/t_⊥)^2 < 80$. For $(a/b)^2 \sim 2$ this requires that $\sigma_‖/\sigma_⊥$ be less than 40 before

$T_p$ is appreciably affected by pressure induced increases in $t_⊥$. HMTSF-TCNQ does fulfil this criterion, whereas HMTSF-TNAP does not. Table 2 lists values of $\sigma_‖/\sigma_⊥$ and $\partial T_p/\partial P$ for both HMTSF compounds, and also for HMTTF-TCNQ, which above 3 kbar has only one transition [9] and shows similar conductivity behaviour to the HMTSF compounds. It is clear that the large values of $\sigma_‖/\sigma_⊥$ and small values of $\partial T_p/\partial P$ for HMTTF-TCNQ and HMTSF-TNAP are consistent with the anisotropy criterion derived above.

With the characterization of HMTSF-TNAP, this series of charge transfer salts is now large enough for trends in behaviour to become clear. We note the following systematic differences between the sulphur and selenium compounds: (a) higher stacking axis room temperature conductivities for the Se compounds, (b) a weaker pressure dependence of the room temperature conductivity, and a tendency to saturate at high pressures for the Se compounds, and (c) a tendency towards a linear temperature dependence of the resistivity of the Se compounds under pressure.

These trends are consistent with the electron-spin excitation scattering model recently proposed by Jérôme [18] for the metallic regime resistivity. In this model the inverse relaxation time,

$$ 1/\tau \sim \left( \frac{1}{1 - \text{Un}(E_F)F(2k_F, T)} \right)^2 $$

where U is the on-site Coulomb repulsion, $n(E_F)$ the density of states at the Fermi level and $F(2k_F, T)$ the Lindhardt function. $1/\tau$ is strongly enhanced when the denominator is close to zero, and most of the pressure and temperature dependence of the resistivity for TTF-TCNQ (where this criterion is most likely fulfilled [16]) derives from the effect of b axis dilatation
on $n(E_F)$ in the denominator. $U$ will be smaller for the $\pi$ orbitals derived from the Se-$4p$ orbitals than for those derived from the S-$3p$ orbitals, and the increased $t_\parallel$, resulting from greater overlap between the larger Se derived $\pi$ orbitals, will lower $n(E_F)$ in the Se compounds. (This is seen for example in the enhanced paramagnetic susceptibility of TTF-TCNQ and the smaller susceptibility of TSF-TCNQ [19].) Thus $1/\tau$ is less strongly enhanced in the Se materials, explaining the higher room temperature conductivities, the smaller pressure coefficients, and the weaker temperature dependence of the high pressure resistivity.

Whilst we are able to explain some of the trends in behaviour between the various charge transfer salts, the very short N-Se contact in HMTSF-TNAP of 3.04 Å (c.f. 3.10 Å in HMTSF-TCNQ [4]) is hard to understand in view of the small $t_\perp$ deduced from the resistivity anisotropy and the pressure dependence of $T_p$.

*Acknowledgements* — B. Mogensen's stay at Orsay was made possible by a travelling grant from the French government. We thank A. Andrieux and G. Malfait for their skilful technical assistance.

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