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Charge Ordering in the TMTTF Family of Molecular Conductors

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Charge Ordering in the TMTTF Family of Molecular Conductors Charge ordering in the TMTTF family of molecular conductors D. S. Chow1, F. Zamborszky2, B. Alavi1, D. J. Tantillo3, A. Baur3, C. A. Merlic3, S. E. Brown1 1) Department of Physics and Astronomy, UCLA, Los Angeles, CA 90095-1547 USA 2) Department of Physics Technical University of Budapest, Budapest, Hungary 3) Department of Chemistry and Biochemistry UCLA, Los Angeles, CA 90095-1569 USA 13 Using one- and two-dimensional NMR spectroscopy applied to C spin-labeled (TMTTF)2AsF6 and (TMTTF)2PF6, we demonstrate the existence of an intermediate charge-ordered phase in the TMTTF family of charge-transfer salts. At ambient temperature, the spectra are characteristic of nuclei in equivalent environments, or molecules. Below a continuous charge-ordering transition temperature Tco, the spectra are explained by assuming there are two inequivalent molecules with unequal electron densities. The absence of an associated magnetic anomaly indicates only the charge degrees of freedom are involved and the lack of evidence for a structural anomaly suggests that charge/lattice coupling is too weak to drive the transition. PACS #s 71.20.Rv, 71.30.+h, 71.45.Lr, 76.60.-k Recent evidence that electronic correlations can lead to inhomogenous charge and spin structures has Transition lines become a dominant theme in analyzing the properties ∆ρ of doped transition-metal oxides such as the high-Tc 100 Crossover cuprates [1] and the manganites [2]. An important fea- ? ture is that the details of the structures can fundamen- CO tally influence the low-temperature physics in ways that 10 might otherwise seem inconceivable, possibly even the creation of a superconducting state by doping an antifer- AF temperature (K) temperature SP romagnetic insulator [3]. 1 Observations of charge-ordering in (DI-DCNQI)2Ag [4] and (BEDT-TTF)2X [5,6] indi- SC cate inhomogeneities occur in some organic conductors as well. Prototypical among these is the family of isostruc- 0 tural TMTTF and TMTSF charge transfer salts [7]. For pressure nearly twenty years, the remarkable diversity of physi- FIG. 1. Temperature vs. pressure phase diagram for the Bechgaard salts (TMTSF)2X, and the sulfur analogs cal properties they exhibit have been summarized using (TMTTF)2X. The symbols are as follows: SP=Spin-Peierls, a single temperature/pressure phase diagram (Fig. 1), AF=antiferromagnetic, SC=superconductivity, where pressure is the parameter controlling the ratio of ∆ρ=dimerization charge gap, and CO=charge-ordered. The two competing energy scales. Note the existence of a su- solid lines are phase transitions, and the dashed line is a perconducting phase next to an antiferromagnetic insu- crossover. The hashed marks may be discontinuous transi- lator [8]. Below, we describe observations which require tions. the incorporation of a new transition line to the phase diagram (bold, blue line), below which the systems are symmetry phase with a charge gap (∆ρ), whereas near charge-ordered (CO). The observation of an intermedi- to the SC ground state is a highly conducting normal ate phase in this class of compounds can be explained by state. Emery, et al. [15] were the first to point out a including a new energy scale [9,10], and is particularly simple mechanism by which this crossover from insulat- significant because the influence of the new scale can be ing to metallic behavior could occur without crossing a examined across a range of ground states by pressure- phase boundary, and their proposal led to the composite tuning the system. And since there is evidence from in- phase diagram. The compounds are formed by stacking dependent transport measurements for CO fluctuations the planar TMTTF molecules, and then lining up the arXiv:cond-mat/0004106v1 [cond-mat.str-el] 7 Apr 2000 far above the CO transition temperature Tco, the inter- stacks into layers that are separated by layers of counte- actions which drive the transition are relevant far into rions. If the molecular stacks are considered as weakly the normal phase and over a range of pressures [11,12]. coupled chains with alternating intermolecular distances, First we discuss Fig. 1 while explicitly exclud- then two-particle Umklapp processes produce a charge ing the CO transition. The sequence of observed ground gap ∆ρ. Either an increase in transverse hopping or a de- states (Spin-Peierls (SP), antiferromagnetic (AF), and crease in the dimerization potential, as applied pressure superconducting (SC)) follows naturally from the com- would do, deconfines the charges and restores the con- bined effects of tunable dimensionality and on-site corre- ducting state. That is, application of pressure is equiv- lations [7,13,14]. Near to the SP ground state is a high- alent to controlling the ratio of the dimerization gap to 1 the transverse overlap integral (∆ρ/t⊥), which in turn 1.2 determines the properties of the normal state. 10 Here we demonstrate the existence of an inter- 5 mediate, charge-ordered phase in (TMTTF)2PF6 and 0 1.0 (TMTTF)2AsF6, and propose that off-site Coulomb in- -5 teractions are responsible. Strictly speaking, introducing -10 -15 a new energy scale modifies the physical properties exhib- b' c* ited by a particular compound, so the phase diagram of peak positions (kHz) -20 0.8 -100 -50 0 50 100 Fig. 1 is better described as a slice of a diagram with at angle (degree) least one additional axis. Several previously-unexplained observations can be understood by recognizing the exis- 106K tence of the CO transition. 0.6 13 Our conclusions are based on C NMR 103K spectroscopy from samples of (TMTTF)2PF6 and (TMTTF)2AsF6 that were grown using standard elec- absorption (a.u.) 100K 0.4 trolysis. Spin-labeled molecules were synthesized at UCLA [16] with the two 100% 13C-enriched carbon sites 95K forming the bridge of the TMTTF dimer molecule. All 90K of the NMR measurements were made in an external 0.2 field of B0=9.00T, corresponding to an NMR frequency 85K of 96.4MHz. 13 In Fig. 2, seven 1D C NMR spectra 79K 0.0 for (TMTTF)2AsF6 at representative temperatures are shown. At ambient temperature, each molecule is equiv- -40 -20 0 20 40 alent, but the two 13C nuclei in each molecule have in- frequency (kHz) 13 equivalent hyperfine coupling, giving rise to two spectral FIG. 2. C NMR spectra for (TMTTF)2AsF6 recorded at lines. The angular dependence of the spectral frequencies different temperatures. The inset shows the angular depen- appears in the inset; the broken lines are the hyperfine dence of the spectrum at T=300K. A solid arrow denotes the shifts and the addition of a nuclear dipolar coupling gives angle at which the spectra in the main part of the figure were recorded. The dashed arrow refers to the angle associated to the solid lines. The solid arrow is the angle at which the the data of Fig. 4. seven spectra were recorded. Upon cooling, the NMR spectrum remains un- changed down to T=105K, below which each of the two 5 peaks appear to split. From each molecule there is a sig- nal from the nucleus with a stronger hyperfine coupling 4 and a signal from the nucleus with a weaker hyperfine (TMTTF)2 PF6 coupling. The doubling comes about because of two dif- (TMTTF)2 AsF6 ferent molecular environments of roughly equal number, 3 one with slightly greater electron density and one with a reduced electron density. Following the effects of the 2 charge disproportionation to low temperature was diffi- splitting (kHz) cult, because the SP fluctuations lead to line broadening 1 and spectral overlap. However, we were able to use 2D J- resolved spectroscopic techniques to ”unfold” unresolved 0 signals from coupled nuclear spins. These measurements are discussed below. 0 50 100 150 200 250 300 The obvious choice for investigating the general- temperature(K) FIG. 3. Spectral splitting (∼charge disproportionation or- ity of the CO phenomenon is (TMTTF)2PF6, a system der parameter) vs. temperature as obtained from 1D and 2D with physical properties originally used to identify Fig.1 13C NMR spectroscopy for two TMTTF-based salts. as the appropriate phase diagram [7,13,14], and recently found to be superconducting at a pressure of P≈5.2 GPa 13 of a charge-ordering occurring at a higher temperature. [17]. In previous high-field C NMR spectroscopy on Even though the spectra were complicated by overlap, this compound, we had identified four inequivalent nu- 2D J-resolved experiments led to unambiguous identifi- clei in the domain-walls of the incommensurate SP phase, cation of a CO transition at approximately T=65K. The rather than the expected two [18]. The present results temperature dependence of the order parameter exhib- demonstrate that this is a consequence 2 ited in Fig.3 shows that the transition is continuous to within the experimental resolution. Our measurements 10 confirm the hypothesis put forward in recent reports of 0 (a) ac transport measurements, where a large and strongly T=115K frequency-dependent dielectric constant was attributed -10 to the response of a charge-ordered phase [12,19]. An important puzzle of the TMTTF salts is 10 solved by these experiments. It has been known for a (kHz) 1 0 (b) long time that properties of certain TMTTF salts, for - f 76K 2 example (TMTTF)2SbF6, did not fit into the generally -10 accepted model [20]. The temperature dependence of the resistivity ρ(T) for this material is metallic, that is, 10 dρ/dT>0 down to T=155K, where it appears that a con- (c) tinuous metal-insulator transition takes place [21]. It was 0 8K referred to as ”structureless” because no signature was relative shift f = -10 found in X-ray scattering studies.
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