Electron-electron interactions in the fulvalene family of organic metals

Solid State Communications,Vol. 23, pp. 471—475, 1977.

Pergamon Press.

Printed in Great Britain.

ELECTRON—ELECTRON INTERACTIONS IN THE FULVALENE FAMILY OF ORGANIC METALS Y. Tomkiewicz, B. Welber and P.E. Seiden IBM Thomas J. Watson Research Center, Yorktown Heights, NY 10598, U.S.A. and R. Schumaker IBM Research Laboratory, San Jose, CA 95193, U.S.A. (Received 18 March 1977 by G. Bums) Magnetic susceptibility as well as optical reflectivity data in the metallic regime of the conductivity of HMTTF—TCNQ are presented. It is shown that: (1) The spin susceptibility of the donor stack is temperature independent in the temperature range of the metallic conductivity 50< T< 300 K. (2) By comparing the magnitude of this susceptibility with the value estimated from plasma frequency measurements based on the model of non-interacting electrons, we conclude that the donor stack susceptibifity is not enhanced. (3) A comparison of data on the three compounds (HMTTF, TTF, TSeF)—TCNQ leads to the conclusion that the donor stack susceptibility is enhanced only in TTF—TCNQ. RECENTLY the existence of intense diffuse X-ray scattering [1, 2] in TTF—TCNQ (tetrathiafulvalenium tetracyanoquinodimethanide) at q = 0.41b* in addition to the q = 0.295b* scattering [3, 4] was interpreted independently [5—7]by different authors as an mdication of the existence of significant onsite Coulomb correlations. TTF—TCNQ is comprised of two different kinds of conducting stacks the TCNQ stack and the TFF stack. The bandwidth (B) associated with each kind of is potentially different [8—101and the Coulomb repulsion (U) can also vary due to different charge density distributions on the TTF and TCNQ molecules. Therefore it is worthwhile to evaluate which stack is responsible for the correlated behavior observed for TTF—TCNQ. The technique on which this evaluation is based involves [6] a comparison between the experimentally obtained magnetic susceptibilities for the individual stacks and the predicted values based on the model of non-interacting electrons. If the experimentally obtained susceptibility is found to be enhanced in comparison to the predicted value, it may be the result of significant onsite or longer range Coulomb interactions. The magnetic susceptibility is particularly useful in terms of separating the relative importance of correlations Ofl different stacks in view of the fact that it can be decornposed into donor and acceptor stack contributions [11]. The use of the enhanced susceptibility as a fingerprint of strong Coulomb interactions is in practice quite —

471

complicated because of the following reasons: (1) In all the known organic metals, with the exception of TTF—TCNQ, the magnetic susceptibility has a strong temperature dependence [12] in the whole temperature range of the metallic conductivity. This temperature dependence is not that expected from a narrow quasi one dimensional band [13]. The origin of this temperature dependence is presently controversial. Possible candidates include fluctuations of a large coherence length [14] or special band structure effects [15]. Therefore if the value of the susceptibility is smaller or equal to the estimated Pauli susceptibility (based on plasma frequency measurements) it is not clear whether the enhancement is offset by susceptibility reducing effects due to the mechanism resulting in the observed temperature dependence. The temperature independent susceptibility measured in UF—TCNQ [12, 16] in the temperature range 270 < T <410K made this coinpound a very appropriate candidate for the enhancement test. (2) Even for UF—TCNQ, in order to determine whether the susceptibility is enhanced one has to corn pare the measured value with an estimated Pauli suscep. tibility value. Obtaining the Pauli value involves an assumption about the band structure. Most commonly used is the tight binding approximation, with a given value of bandwidth and filling. The filling can be deter. mined from diffuse X-ray measurements [3,4] and because of the stoichiometric nature of these compounds

472

ELECTRON—ELECTRON INTERACTIONS IN THE FULVALENE FAMILY 2.50

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Vol. 23, No.7

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Fig. 2. Reflectivity of HMTTF—TCNQ. The points are experimental and the line is a fit to Drude theory.

Fig. 1. Spin susceptibility of HMTFF—TCNQ. it is then known for both kinds of stacks. An effective bandwidth can be determined by plasma frequency measurements. However, the plasma frequency effectively measures the sum of the bandwidths of both stacks, and since each stack can, in principle, have a different enhancement the solution is not necessarily completely determined, In the case of TTF—TCNQ it is not clear which kind of stack has a wider band. The calculated [8—11] values range from 0.2 to 0.7 eV for TTF while the values for the TCNQ stack range between 0.4—0.6 eV. However, even using the smaller values the susceptibilities of both stacks are found to be enhanced, when analyzed within the tight binding approximation. In this paper we present magnetic and optical data on other compounds such as HMTTF—TCNQ (HMTTF hexamethylene tetrathiofulvalene) and TSeF—TCNQ (TSeF tetraselenofulvalene). We conclude that while the TCNQ stack susceptibility is enhanced in all these compounds the donor stack enhancement is unique to TFF—TCNQ. Figure 1 shows the total spin susceptibility of HMTTF—TCNQ as a function of temperature, in the metallic region of the conductivity, and its decomposition into donor and acceptor stack contributions. Static susceptibility data were obtained by the Faraday Balance Technique and the absolute spin susceptibility was obtained by adding the diamagnetiQ contribution [121(2.24 x iO~emu/mole) of the core electrons to static susceptibility. The data appearing in Fig. 1 are the relative spin susceptibility data normalized to the absolute values obtained from static susceptibility results. The data were obtained by determining the area under the single EPR absorption line. We chose to —

—

determine the spin susceptibility from EPR because of the following two considerations: (1) The susceptibility determined by the spin resonance is a direct measurement of the spin suscepti~ biity itself. Thus, it is more sensitive than static measurements, particularly when the diamagnetic correction is comparable to the spin susceptibility as in the investigated compound. (2) In principal, both the g.value and the susceptibiity can be sample dependent due to varying chemical purity and crystalline perfection of the samples. Therefore, it is important to measure g and the susceptibility on the same sample. This requirement is readily met by the EPR technique. The total spin susceptibility can be decomposed into donor and acceptor contributions by the following technique. In the case of two non-interacting donor and acceptor bands with respective susceptibilities XA and XD, the g value of a single EPR absorption line of Lorentzian shape is related to the donor and acceptor stack g.values, gA and gD, by the following relationship: ~T)

=

xTn

=

XD(T) XA (T) XT(T) ~D + XT(T) xD(T) + xA(T).

(1)

Therefore knowledge of xT(T), gD ~ and g(T) enables one to obtain xD(T) and y.~(T). The details of this decompositionwill be reported elsewhere [17]. The remarkable feature of y~for HMTTF—.TCNQ in the temperature range of interest is its temperature independence. M mentioned earlier the temperature independence of the susceptibility is an important feature for obtaining information on whether it is enhanced by Coulomb interactions. The Pauli

Vol. 23, No.7

ELECTRON—ELECTRON INTERACTIONS IN THE FULVALENE FAMILY

473

is significantly different [18] from that on TTF, thus, we choose a different approach. Since the TCNQ stack has virtually the same stacking distance and angle of

susceptibility of a single stack in the tight binding approximation is 4N0p~

4N0~4 irB sin irp/2 = (2) where N0 is Avogadro’s number, 1.LB the Bohr magneton, B the bandwidth (= 4t where t is the transfer integral), and p is the charge transferred per molecule. We defme = B sin ,rp/2 since it is this quantity which enters into both the susceptibility and the plasma frequency. The total susceptibility is given by =

4N0~4u1 Xi’

=

X~+ X~=

IT

~

+

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(3)

tilt in both TT’F—TCNQ and HMTTF—TCNQ [19—21] the corresponding bandwidths on these stacks might be very similar in both compounds. The calculated bandwidth values for the TCNQ stack, combined with preliminary values of the charge transfer [22] in HMTTF—TCNQ yield EA values in the range 0.33— 0.49eV. Therefore, from equation (6) we can obtain ED values in the range 0.6—0.4 eV. Clearly ED as determined from XD is in this range. Therefore, the HMTTF stack susceptibility is not enhanced. Let us now consider the experimental acceptor stack susceptibility, also shown in Fig. 1. The suscepti-

In the same tight binding approximation the plasma frequency is given by biity is not Pauli like, however, it appears to be to[6]. flatten above anddetermine it is reasonable to 2N(E~+ EA) (4) starting electrons In susceptibility this case200K we can the upper assume that this is due to conduction = b limit of the apparent susceptibility bandwidth from the where b is the intermolecular distance along the stacking susceptibility at 300 K. The value obtained is EA < axis, N the density of conduction electrons (donor + 0.25 indicating that there is some enhancement on the acceptor) per unit volume. acceptor stack. Note also that this bandwidth is small The plasma frequency in the direction parallel to enough to ensure the validity of the consistency conthe conducting axis was determined by fitting the dition optical reflectivity to a Drude expression for the dielectric constant. BT(wp) B~(x)+ BA(x). (7) —~-

~‘

2

‘

w + zw/r’

(5)

Figure 2 shows the fit to the experimental data. The plasma frequency obtained is 1.25 ±0.1 eV, from which equation (4) gives ED + EA

temperature independent. It is clearly seen that in TFF—TCNQ both kinds of stacks exhibit enhanced

(6) susceptibilities. A third compound in this group is TSeF—TCNQ. The value of XD is 0.72 x iO~4emu/mole from which Table 1 also includes the pertinent data for this sysequation (2) gives ED = 0.56 eV. This value is a lower tern. Note, that the susceptibility of the donor stack limit since if the susceptibility were enhanced the true at 300 K is roughly the same as in HMTTF—TCNQ. bandwidth would be even larger, i.e. in principle it Secondly, if we again assume that the bandwidth of could be as large as 0.93 eV. Physically this would the TCNQ stack is roughly that found for TTF—TCNQ, mean that the plasma frequency would be due to the we get a theoretical value of EA (x) = 0.4. Adding this donor stack only and would result in an enhancement to the observed ED(x) = 0.6 gives a total ET 1.0 of 1.6, which is the maximum enhancement possible. which agrees with the value obtained from the plasma This assignment means that there would be no conduc- frequency implying that the donor stack susceptibility tivity on the acceptor stack, an assumption which is in this compound is unenhanced. However, this connot reasonable in the light of what is already known clusion relies on the assumption that the temperature about these systems. independent value of the susceptibility of TSeF—TCNQ We can further explore the question of donor is close in magnitude to the value measured [23] at stack enhancement by asking if the HMT TF stack can 300 K. have a larger effective bandwidth than 0.56 eV. Since What can be the source of the drastically different no band structure calculations have as yet been perenhancement observed for TTF and HMTTF? Assuming formed for HMTTF we would have to use the values that the cosine like dispersion is a good approximation obtained for TTF. However, preliminary calculations in both cases, the different enhancement implies a indicate that the charge density distribution on HMTTF different magnitude of U, since the bandwidth values =

0.93 eV.

In Table 1, we compare the enhancement of the susceptibility on the respective stacks in HMfl’F— TCNQ to that 1TF—TCNQ in the temperature regime in observed which theinmeasured susceptibility is

474

ELECTRON—ELECTRON INTERACTIONS IN THE FULVALENE FAMILY Co.pod

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Vol. 23, No.7

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The susceptibility is not yet flatat 300 K. Ref. 8. The two values for the TTF stack are calculated by respectively omitting and including d-electrons. Ref. 9. The TCNQ bandwidths are scaled for appropriate b-spacing using ref. 8. The value quoted for the plasma frequency of TTF-TCNQ is from the present work. Previous workers have also reported values for this parameter. Bright, A. A., Garito, A. F. and Heeger, A. J., Solid State Commun. 13, 943 (1973) and Fhje. Rev. BlO, 1328 (1974). Grant, P. M., Green, R. L. Wrighton, G. C. and Castro, G., Phyr. Rev. Leu. 31, 1311 (1973). 3C in Local susceptibilities TFF-TCNQ. These results have are alsoshown been obtained in parentheses. from Knight shift measurements on ‘ Rybaczewski, E. F., Smith, L. S. Garito, A. F., Heeger, A. I. and Silbernagel, B. G., Phys. Rev. B14, 2746 (1976).

are quite similar. There are two possible ways to explain this finding: (1) The magnitude of the bare U (the difference in energy between the repulsion of two electrons on the same site and their repulsion when they are on adjacent sites) varies because the charge density distribution on the HMTTF molecule is different than on the TTF molecule. As mentioned earlier preliminary calculations support this picture indicating [18] that the methylene groups pull the charges away from the center of the molecule. Even if the bare U is the same [24] for all the compounds the sereening on a given site by the highly polarizable counter ion should vary

depending on the crystal and chemical structure. This dependence can be understood in terms of LeBlanc’s suggestion [25] according to which the reduction of the bare U due to screening is the result of induced electric dipoles on the counter ions in response to the electric fields resulting from local charge fluctuations.

Acknowledgement

— The authors are grateful to H. Liienthal for the static susceptibility measurements,

and P.M. Grant and B.D. Silverman for helpful discussions.

REFERENCES

1. 2.

POUGET J.P., KHANNA S.K., DENOYER F., COMES R., GARITO A.F. & HEEGER A.J., Phys. Rev. Lett. 37, 437 (1976). KAGOSHIMA S., ISHIGURO T. & ANZAI H.,J. Phys. Soc. Japan 41 2061 (1976). ,

Vol. 23, No.7

ELECTRON—ELECTRON INTERACTIONS IN THE FULVALENE FAMILY

3.

DENOYER F., COMES R., GARITO A.F. & HEEGER AJ., Phys. Rev. Lett. 35,445(1975).

4.

KAGOSHIMA S., ANZAI H., KAJIMURA R. & ISHIGURO T.,J. Phys. Soc. Japan 39, 1143 (1975).

5.

TORRANCE J.B., MOOK H.A. & WATSON C.R., Phys. Rev. Lett. (in press).

6.

TORRANCE J.B., TOMKIEWICZ Y. & SILVERMAN B.D., Phys. Rev. B 15,4738(1977).

7. 8.

EMERY V.J., Phys. Rev. Lett. 37, 107 (1976). BERLINSKY A.J., CAROLAN J.F. & WElLER L., Solid State Commun. 15, 795 (1974).

9.

KARPFEN A., LADIK J., STOLLHOF G. & FULDE P., Chem. Phys. 8,215(1975).

475

10.

SALAHUB D.R., MESSMER R.P. & HERMAN F.,Phys. Rev. B13, 4252 (1976).

11.

TOMKIEWICZ Y., TARANKO A.R. & TORRANCE J.B.,Phys. Rev. Lett. 36, 751 (1976);Phys. Rev. B 15, 1017 (1977).

12.

See for example: SCOTT J.C., GARITO A.F. & HEEGER A.J., Phys. Rev. RiO, 3131(1974).

13.

SHIBA H. & PINCUS P.A.,Phys. Rev. B5, 1966 (1972).

14.

LEE P.A., RICE T.M. & ANDERSON P.W.,Phys. Rev. Lett. 31,462(1973).

15.

BERNSTEIN U., CHAIK1N P.M. & P1NCUS P.,Phys. Rev. Lett. 34,271(1975).

16.

TOMKIEWICZ Y., SCOTT B.A., TAO U. & TITLE R.S.,Phys. Rev. Lett. 32, 1363 (1974).

17. 18.

TOMKIEWICZ Y., TARANKO A.R. & SCHUMAKER R., Phys. Rev. B (in press). SILVERMAN B.D. & LAPLACA S.J. (to be published).

19.

GREENE R.L, MAYERLE J.J., SCHIJMAKER R., CASTRO G., CHAIKIN P.M., ETEMAD S. & LA PLACA S.J., Solid State Commun. (in press).

20.

KISTENMACHER T.J., PHILIPS T.E. & COWAN D.O.,Acta Cryst. B30, 763 (1974).

21. 22.

24.

SCHULZ A.J., STUCKY G.D., BLESSING R.H. & COPPENS P.,J. Amer. Chem. Soc. 98, 3194 (1976). The value of the charge transfer for HMTTF—TCNQ was estimated to be 0.65 by assuming that the ratio of charge transfer in going from the HMTSeF to the HMTTF compound would be proportional to the ratio of the TSeF and T1’F compound Diffuse X-ray scatteringexperiments have determined the charge transfer for the other three compounds but have not yet been done for HMTTF—TCNQ. In reference [18] the charge transfer for HMTTF—TCNQ is estimated to be 0.6 ±0.07 from bond length considerations. BIJRAVOV L.I., LYUBOVSKAYA R.N., LYUBOVSKY R.B. & KHIDEKEL M.L., J. Exp. Theor. Phys. 70, 1982 (1976). TORRANCE J.B., SCOTT B.A. & KAUFMAN F.B., Solid State Commun. 17, 369 (1975).

25.

LEBLANC O.H. Jr., J. Chem. Phys. 42,4307 (1965).

.

23.