Journal ArticleWe report the temperature-dependent x-ray diffuse scattering of (NMP),(Phen)(1-x-)(TCNQ) as a function of conduction-electron density (x). With decreasing x three different unique electronic instability regimes are successively observed on the TCNQ chains: (i) the 2kF instability for (2/3<_x<_1, (ii)both 2kF and 4 kF instabilities  for 0.57(<_x<_2/3, and (iii) the 4kF instability alone for x close to 0.5. These results provide direct evidence for the critical role of interchain screening in determining the magnitude of the effective Coulomb interaction

Miller, Joel Steven

Pouget, J. P.

English

The University of Utah: J. Willard Marriott Digital Library

VOLUME 47, NUMBER 10 PHYSICAL REVIEW LETTERS 7 SEPTEMBER 1981 X-Ray Observation of Crossover of 2kp to 4kp Scattering in (N -Methylphenazinium) x (Phenazine)t_x (Tetracyanoquinodimethane) [(NMP)x (Phen)t_x(TCNQ)], O.5=e;x=e;1.0 A. J. Epstein Xerox Webster Research Center, Webster, New York 14580 and J. S. Miller Occidental Research Corporation, Irvine, California 92713 and J. P. Pouget and R. Comes Laboratoire de Physique des Solides, Associe au Centre National de la Recherche ScientiJique, Universite Paris-Sud, 91405 Orsay, France (Received 6 Apri11981) We report the temperature-dependent x-ray diffuse scattering of (NMP)X<Phen) I-x -(TCNQ) as a function of conduction-electron density (xL With decreasing x three different unique electronic instability regimes are successively observed on the TCNQ chains: (i) the 2kF instability for i ~x ~ I, (ii) both 2kF and 4kF instabilities for 0.57 ~ x ~ i, and (iii) the 4kF instability alone for x close to 0.5. These results provide direct evidence for the critical role of interchain screening in determining the magnitude of the effective Coulomb interaction. PACS numbers: 71.30.+h, 61.10.-i, 72.15. Nj Considerable progress has been made in the understanding of one-dimensional (lD) metals in the past several years. l The occurrence of a Peierls transition in tetrathiafulvalene tetracy-anoquinodimethane and related compounds, first deduced from the observation of low-temperature incommensurable modulated structures, was con-firmed by the ID character of the precursor 2k F scattering2 (twice the Fermi wave vector). Sub-sequent observation3 of ID x-ray diffuse scatter-ing at the wave vector 4k F, in only a few materi-als,4 led to a continuing controversy5-7 concern-ing the detailed cause of this anomaly, although it had already been considered in early theoreti-cal papers. 8 It has been proposed that the 4k F scattering occurs when the on-site electron-electron Coulomb repulsion, U, is greater than the bandwidth, W. The factors which determine the relative magnitude of U and W have, until now, remained unresolved. 9 The 4kF and 2kF scatterings have often been associated with the independent behavior of the donor and acceptor stacks in two-chain materials such as tetrathia-fulvalene tetracyanoquinodimethane, but it is conceivable that both anomalies arise on one chain. We have performed experiments to in-vestigate the roles of the value of kF and disorder in these systems and determine their effects on 4kF scattering. In this Letter we present the results of the x-ray diffuse scattering photographic investigation of the temperature-dependent dynamical proper-ties of a series of isomorphous compounds of varying Fermi wave vector. Several unique fea-tures concerning the electronic instabilities of ID conductors not observed in any of the pre-viously studied systems are reported. As the conduction-electron density is reduced, a cross-over from a two-chain conductor to a one-con-ducting-chain system is observed, accompanied by a crossover from a 2kF to a 4kF regime, with the coexistence in a narrow density range of both 2kF and 4kF anomalies. These results confirm the central role of inter chain screening in deter-mining the magnitude of U /w. The 4kF anomaly is directly associated with U jw > 1 by comparison of properties of the systems exhibiting 4kF with those of known large U /W systems and by com-parison with other system properties, such as plasma frequency. As part of this detailed study, the degree of disorder on each chain was varied. The results demonstrate the weak effects of dis-order on neighboring chains as compared with disorder on the conducting chain. The systems studied are based upon (NMP)-(TCNQ), and are achieved by substituting neutral phenazine, pheno, for up to 50% of the asymmet-ric NMP. lO The pheno is of similar Size, shape, © 1981 The American Physical Society 741 VOLUME 47, NUMBER 10 PHYSICAL REVIEW LETTERS 7 SEPTEMBER 1981 and polarizability to NMP+ but is neutral, closed shell, and symmetric. Detailed analysis has shown that the overall (NMP)(TCNQ) crystal structilrell remains unchanged even with re-placement of NMP by up to 50% pheno. 10 Conse-quently, the number of conduction electrons per unit cell to be shared among the donors and TCNQ is equal to the fraction x of NMP in the. alloy (NMP)" (Phen)1_" (TCNQ).l0 Conductivity, a, studies have shown these materials to have a a(T) similar to that of (NMP)(TCNQ) with a maxi-mum in a(T) at a temperature lower than that of (NMP)(TCNQ).12,13 The experimental setup for the photographic study of diffuse x-ray scatter-ing was that previously used for other investiga-tions. 3,14 The two end pOints of the series, (NMP)(TCNQ) and (NMP)o.s(Phen)o.s(TCNQ), have very different behavior. For x = 1.0, two types of temperature-dependent lD scattering are observed14 : A first lD scattering is observed at room temperature at the wave vector q 2 = 0.33a *; it couples three-dimensionally below 200 K. Further diffuse scat-tering is observed below 70 K at half the previous wave vector (q1 = 0.l65a*). As expected for an in-trinsically disordered system, no long-range 3D ordering is observed down to 20 K. A tempera-ture-independent diffuse x-ray scattering was also observed14 for x = 1.0, ,and assigned to the (short-range) orientational order of the asym-metric and polar NMP molecules. Substitution of the symmetrical phenazine molecule for the NMP molecule reduces the dipolar disorder at the expense of a substitutional disorder. For the alloys of lowest phenazine concentration (x ~ 0.6), the disorder keeps qualitatively the same cor-relations, although there is an increase in the WA • (a) (b) x x ---+ x x 4!11 f T----~ • +-Ix Ix a*' x ' x -+ , , I I x , x, I I • . ----. ~ +-b* ---=--. FIG. 1. (a) Position of the superstructure reflections ~) in (NMP)o. 5 (Phen)0. 5(TCNQ) with respect to the (NMP) (TCNQ) Bragg spots (dots). (b) Suggested NMP and Phen ordering in real space that would lead to (a). 742 correlation length of methyl groups along the chain with decreasing x. For x - 0.50, a 3D long-range order appears between NMP and phenazine molecules as shown in Figs. l(a) and l(b), for the donor plane. Substitution of the phenazine for the NMP mole-cules also results in substantial changes in the temperature-dependent lD x-ray diffuse scatter-ing, removing ambiguities14 in the ql and qa as-signments. A typical x-ray pattern for the alloy (NMP)O.7m(Phen)O.214(TCNQ) at 25 K is shown in Fig. 2(a). It presents two interesting features: (a) The q1 scattering is not observed and (b) the q 2 scattering is shifted towards the layers of main Bragg spots [q2= (0.28± O.Ol)a* for x = 0.786 J. The vanishing of the q 1 scattering can be due to the additional disorder introduced on the donor stacks (their Fermi wave vector now is not defined). This assigns the q 1 scattering to the NMP chain, and consequently qa to the TCNQ chains. Taking into account the amount of avail-able electrons, the assignment of q a to the TCNQ chains leaves but two possibilities14 : q a = 2k F or qa= l-4k F • The shift of q2 from ta* in pure FIG. 2. X-ray diffuse scattering patterns of (NMP),,-(Phenit_" (TCNQ): (a) forx=0.786 at 25 K and showing a quasi-ID scattering at 2kF = 0.28a* (white arrows), (b) for x = 0.59 at 25 K and showing quasi-ID scattering at 2kF = 0.28a* (white arrows) and 4kF = 0.56a* (black arrows), and (c) for x = 0.50 at 200 K, and showing a temperature-dependent quasi-ID scattering at 4kF = 0.5a* (black arrows). The a* direction is horizontal on these patterns. The intenSity distribution of the pat-terns taken at low temperature reveals an additional short-range 3D coupling as expected for a Peierls distorsion. VOLUME 47, NUMBER 10 PHYSICAL REVIEW LETTERS 7 SEPTEMBER 1981 (NMP)(TCNQ) to a smaller value in the phenazine-doped material rules out the possibility q 2 = 1-4kF which implies an increase of q2' Thus q2 can be assigned to a 2k F scattering from the TCNQ chain. This determines a charge transfer of i-electron per molecule in pure (NMP)(TCNQ), a value which has also been recently obtained from vibrational spectroscopy measurements. 15 Hence, the position of the q2 scattering directly gives the charge transfer to the TCNQ chains (p = 2q 2) in the series of phenazine-doped samples, Fig. 3. Figure 2(b) shows another typical x-ray pat-tern for (NMP)O.59(Phen)O.41 (TCNQ) at 25 K. In addition to the q 2 scattering which is still ob-served atq2=(0.28±0.01)a* (white arrows), another quasi-1D scattering can now be observed at awavevectorq3=(0.56±0.01)a* (black ar-rows), Le., at 2q2 and therefore corresponding to q3= 4kF • Figure 2(b) further shows that the '4 ~ 0.9 ~!5 i3 a 0.8 W 0:: 0.7 ~ ~ 0.6 !;i iLl 0.5 0.4 0.3 I 0.2 0.1 O~~~~~~~~~~ __ ~ __ ~ 0.5 0.6 0.7 0.8 0.9 1.0 FRACTION NMP, x FIG. 3. Nu.mber of conduction electrons per TCNQ (triangles) and per donor molecule (dots) vs NMP con-centration x. The solid line gives the total conduction-electron concentration available. The experimental determinations of the number of conduction electrons per TCNQ deduced from the value of the wave vector in chain direction of the 2RF and/or 4kF scattering are given with their error bars; the number of conduction electrons per donor, which is obtained by subtraction from the total available electrons x, is given without error bars. In the upper part of the figure the concen-tration ranges where 2kF and/or 4kF are observed is indicated. intensity distributions in the two types of 1D scattering differ noticeably. Similarly, tem-perature-dependent q3 (4k F ) scattering, although weaker, can be observed in addition to qa (2k F) scattering in an alloy with x = 0.64; this last ob-servation seems to set x "" i- as the upper limit for the observation of 4kF scattering. With these results we have obtained a unique example in which both 2kF and 4kF instabilities are observed on the same kind of chains (TCNQ). For (NMP)o.5(Phen)o.5(TCNQ) the temperature-dependent q 3 scattering has shifted to q 3 = O. 5a *. In Fig. 2(c) (200 K) it corresponds to diffuse in-tensity lines (black arrows) which are superim-posed on the superstructure satellites; at lower temperatures the intensity of the q 3 scattering increases with coalescing into broad discrete spots (3D coupling). The other 1D scattering qa assigned to 2k F for the lower phenazine concen-trations is no longer observable. A diffuse scat-tering at 0.5a* implies one conduction electron per repeat unit if it is associated with 2k F or 0.5 conduction electron per repeat unit if it is as-sociated with 4k F • From the chemical require-ment that there is a maximum total of 0.5 con..:. duction electron for both stacks together, q 3 must be associated with 4k F, corresponding to a full charge transfer (Fig. 3). In summary the x-ray diffuse scattering in-vestigation of (NMP)" (Phen)l_x (TCNQ) has re-vealed several unique features concerning the electronic instabilities in 1D conductors which were not observed in any of the previously studied systems: (a) In pure (NMP)(TCNQ) the donor and acceptor stacks have distinct 2k F instabilities at, respectively, q1 =ta* and q2=~a*, with i- elec-tron transferred to the TCNQ molecule. (b) The disorder introduced on the donor NMP stacks by the substitutional phenazine leads to a poorly defined Fermi wave vector, and consequently to the suppression of the corresponding 2k F scatter-ing (q 1)' (c) The progressive decrease of elec-tron density for increasing phenazine concentra-tions induces in the first stage, % <x < 1, a sim-ple shift of the corresponding 2k F wave vector from the acceptor TCNQ stack (decrease of q2)' (d) For lower electron densities, 0.57 -::; x -::; i, 2kF and 4kF scattering from instabilities occur-ring on the same TCNQ stacks can be observed. (e) At 50% phenazine concentration (x = 0.5) full charge transfer is found and the 4kF instability alone is observed on the TCNQ chains. In ad-dition, the diffuse x-ray scattering from the TCNQ stacks seems quite insensitive to the de-743 VOLUME 47, NUMBER 10 PHYSICAL REVIEW LETTERS 7 SEPTEMBER 1981 tailed order on the donor stacks. The simultaneous appearance of 4k F scatter-ing together with a nearly complete charge trans-fer occurs at about the same phenazine concen-tration as the anomalous red shift of the plasma frequency reported by Weinstein et aZ. 16 On this basis as well as magnetic susceptibility17 and electron-spin-resonance results/a they proposed that as x decreases below - ~ there is an empty-ing of the conduction band on the donor stack and a simultaneous shift from small U /W to large U / w behavior .19 As the bandwidth increases10 slightly in going from x = 1.0 to x = 0.5, the in-crease in U/W must be due to an increase in U. This increase in U coincides with the emptying of the planes of donor molecules of mobile con-duction electrons; these results directly demon-strate for the first time the critical role of inter-chain screening in determining the effective Cou-lomb repulSion and that the Coulomb repulsion is not determined solely by on-site interaction.2o It has been earlier noted19 that the (NMP)o,s-(Phen)o.s(TCNQ) is nearly indistinguishable in its transport and magnetic behavior from other one-quarter-filled-band TCNQ compounds such as (quinolinium)(TCNQ)2' The behavior of these systems has been interpreted21 in terms of U /W > 1, although this assignment has been called in-to question.22 The direct observation of 4k F scat-tering in the x = 0.5 system, as well as strong 4kF scattering and weak 2k F scattering in (quinolini-um)(TCNQ)2,23 now confirms that U > W. It has also been proposed that disorder-induced locali-zation is of paramount importance in the one-quarter-filled-band systems.24 The presence of indistinguishable transport and magnetic proper-ties for the crystallographic ally ordered (NMP)o.s(Phen)o.s(TCNQ) indicates the secondary role of off-chain disorder in inducing electron localization in these systems. Part of this work was performed by one of us (A.J.E.) as visiting scientist at Universite Paris-Sud. ISee, for example, The Physics and Chemistry of Low-Dime,nsional Solids, edited by L. Alcacer (D. Reid-el, Boston, 1980). 744 2See , for example, the review by R. Comes and G. Shirane, in Highly Conducting One Dimensional Solids, edited by J. T. Devreese (Plenum, New York, 1979) . 3J. P. Pouget, S. K. Khanna, F. Denoyer, R. Comes, A. F. Garito, and A. J. Heeger, Phys. Rev. Lett. 37, 437 (1976); S. Kagoshima, T. 1shiguro, and H. Anzai, J. Phys. Soc. Jpn. 41,2061 (1976). 4J. P. Pouget, R. Comes, and K. Bechgaard, in Ref. 1, p. 113. 5V. J. Emery, Phys. Rev. Lett. 37, 107 (1976); P. A. Lee, T. M. Rice, and R. A. Klemm, Phys. Rev. B 15, 2985 (1977); J. B. Torrance, Phys. Rev. B 17, 3099 (1978). 6J . Kondo and K. Yamaji, J. Phys. Soc. Jpn. 43, 424 (1977); J. Hubbard, Phys. Rev. B 17, 494 (1978). 7M. Weger, in R~f. 1, p.77. -8A . A. Ovchinikciv, Zh. Eksp. Teor. Fiz. 64, 342 (1973) [Sov. Phys. JETP 37, 176 (1973)]; J:-Bernasconi, M. J. Rice, W. R. Schneider, and S. Strassler, Phys. Rev. B 12, 1090 (1975). 9J. B.Torrance, Ann. N.Y. Acad. Sci. 313, 210 (1978). IOJ. S. Miller and A. J. Epstein, J. Am. Chern. Soc. 100, 1639 (1978). ---nc. J. Fritchie, Acta Crystallogr. 20, 892 (1966). 12A . J. Epstein and J. S. Miller, Solid State Commun. 27, 325 (1978). 13A. J. Epstein, E. M. Conwell, D. J. Snadman, and J. S. Miller, Solid State Commun. 23, 355 (1977). 14J • P. Pouget, S. Megtert, R. Comes, and A. J. Ep-stein, Phys. Rev. B 21, 486 (1980). 15H . Kuzmany and M. Elbert, Solid State Commun. 35, 597 (1980). 16B • A. Weinstein, M. L. Slade, A. J. Epstein, and J. S. Miller, Solid State Commun. 37, 643 (1981). 17A. J. Epstein and J. S. Miller, t-;;-be published. 18A. P. Troup, A. J. Epstein, and J. S. Miller, to be published. 19A. J. Epstein and J. S. Miller, in Ref. 1. 20Similar concepts are discussed in A. N. Bloch, to be published; H. Gutfreund and W.A. Little, in Highly Conducting One Dimensional Solids. edited by J. T. Devreese (Plenum, New York, 1979); J. B. Torrance, Acc. Chern. Res. 12, 79 (1979). 21p. M. Chaikin,J. F. Kwak, and A. J. Epstein, Phys. Rev. Lett. 42, 1178 (1979). 22L . 1. Buravov, D. N. Fedutin, and 1. F. Shchegolev, Zh. Eksp. Teor. Fiz. 59, 1125 (1971) [Sov. Phys. JETP 32, 612 (1971) 1. 23J . P. Pouget, to be published. 24G. Gruner, A. Janossy, K. Holczer, and G. Mihaly, in Quasi One -Dimensional Conductors II, Lecture Notes in Physics, Vol. 96, edited by S. Barisic et aJ. (Spring-er, Berlin, 1979), p.246. a , ~ , ". I ' FIG . 2 . X-ray d iffuse scattering patterns of (NMP):r-(Phe n)t _x(TCNQ): (a) forx = O.786 al25 K and showing a quas i- IO seaneri ng at 202 ,, = 0. 28a'' (white arrows), (h) for x = 0.59 at 25 K and s howing quas i-ID scattering at 2k F = 0. 28a '" (while arrows) and 4RF = O.56",* (black a lTows), and (el fo r x = 0 .50 at 200 K, and showing a te mperature-dependent quasi -ID scatte ring at 4RF = 0.5a'"' (b lack a rrow s) . T he a * di l'ect ion is horizontal on these patterns. T he intensity d is tribu tion of the pat-te rn s take n a t low tempe r atu re revea ls a n additional s ho l,t- r ange 3D coupli ng as expected fOl' a Pe ie rl s disl ors ion. 

X-ray observation of crossover of 2kF to 4kF scattering in (N-Methylphenazinium)x(Phenazine)1-x(Tetracyanoquinodimethane) [(NMP)x(Phen)1-x(TCNQ)], 0.5 (< _x <_ 1.0)

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X-ray observation of crossover of 2kF to 4kF scattering in (N-Methylphenazinium)x(Phenazine)1-x(Tetracyanoquinodimethane) [(NMP)x(Phen)1-x(TCNQ)], 0.5 (< _x <_ 1.0)

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