arXiv:cond-mat/0004106v1 [cond-mat.str-el] 7 Apr 2000 n(DI-DCNQI) in antifer- an [3]. doping the insulator by even romagnetic state superconducting possibly a of inconceivable, creation that fundamen- seem ways can in otherwise structures physics might the low-temperature fea- of the important influence details high-T An tally the the that [2]. as is manganites ture the such properties and oxides the [1] has cuprates analyzing transition-metal structures in doped spin theme of dominant and a charge become inhomogenous to lead a bv h Otasto eprtr T temperature transition CO in- fluctuations the from CO above evidence for far pressure- is measurements by there transport since states And dependent ground system. be of the can range scale tuning particularly a new is the across and of examined influence [9,10], the by scale because explained energy significant be can new compounds a of intermedi- including class are this an in systems of phase phase the observation ate the which The to below (CO). line line), charge-ordered transition blue new (bold, require a diagram which of observations incorporation describe we the insu- Below, antiferromagnetic su- an [8]. a lator to of next existence of the phase ratio Note perconducting the scales. controlling energy 1), parameter competing two the (Fig. is diagram pressure using phase where summarized temperature/pressure been physi- single have of a exhibit diversity they remarkable properties For the cal [7]. years, salts twenty isostruc- transfer charge of nearly TMTSF family and the TMTTF is tural these among as Prototypical conductors organic well. some in occur inhomogeneities cate ain 71,4.Na oteS rudsaei high- a is state ground SP the to corre- Near on-site [7,13,14]. and com- lations dimensionality the tunable of from effects naturally and bined follows (AF), (SC)) antiferromagnetic ground superconducting observed (SP), of sequence (Spin-Peierls The states transition. CO [11,12]. the pressures ing of into range far a relevant over and are normal the the drive which actions bevtoso charge-ordering of Observations can correlations electronic that evidence Recent is edsusFg hl xlctyexclud- explicitly while 1 Fig. discuss we First .S Chow S. D. htcag/atc opigi o ekt rv h transi 76.60.-k the eviden 71.45.Lr, drive 71.30.+h, of ma to 71.20.Rv, lack weak associated #s the too PACS an is and involved coupling of charge/lattice are absence that freedom The of degrees charge densities. electron unequal fnce neuvln niomns rmlcls eo a Below temperat molecules. ambient or T environments, At temperature equivalent in salts. nuclei charge-transfer of of family TMTTF n (TMTTF) and sn n-adtodmninlNRsetocp ple to applied spectroscopy NMR two-dimensional and one- Using hreodrn nteTTFfml fmlclrconductors molecular of family TMTTF the in ordering Charge 2 g[]ad(BEDT-TTF) and [4] Ag )Dprmn fCeityadBohmsr CA o Angel Los UCLA, Biochemistry and Chemistry of Department 3) )Dprmn fPyisadAtooy CA o nee,C Angeles, Los UCLA, Astronomy, and Physics of Department 1) )Dprmn fPyisTcnclUiest fBudapest, of University Technical Physics of Department 2) 1 .Zamborszky F. , co 2 PF h pcr r xlie yasmn hr r w inequiv two are there assuming by explained are spectra the , 6 edmntaeteeitneo nitreit charge-or intermediate an of existence the demonstrate we , 2 .Alavi B. , 2 co 56 indi- [5,6] X h inter- the , 1 .J Tantillo J. D. , c 1 (TMTTF) ymtypaewt hregp(∆ gap charge a with phase a symmetry is transi- line discontinuous SC=, dashed be may the marks and tions. hashed transitions, The phase crossover. are lines solid ∆ AF=antiferromagnetic, h ehar at (TMTSF) salts Bechgaard the oteS rudsaei ihycnutn normal conducting highly a is Emery, state ground state. SC the to ln ocnrligtertoo h ieiaingpto gap dimerization the equiv- of is ratio pressure the con- of controlling the application to is, restores alent That and charges state. pressure the ducting applied deconfines as do, potential, would dimerization the in crease hntopril mlp rcse rdc charge a produce ∆ processes gap weakly Umklapp as two-particle considered distances, then are intermolecular alternating stacks with counte- molecular chains the of coupled the layers up If by lining separated rions. are then that and layers stacking into molecules, stacks by TMTTF formed planar are composite compounds the the The a to crossing diagram. led phase proposal without their occur and could boundary, insulat- phase behavior from metallic crossover to this ing which by mechanism simple ρ I.1 eprtr s rsuepaedarmfor diagram phase pressure vs. Temperature 1. FIG.

dmrzto hregp n Ocag-ree.The CO=charge-ordered. and gap, charge =dimerization temperature (K) 100 10 3 1 ρ .Baur A. , tion. 0 ihra nraei rnvrehpigo de- a or hopping transverse in increase an Either . SP otnoscag-reigtransition charge-ordering continuous 2 efrasrcua nml suggests anomaly structural a for ce .Tesmosaea olw:SP=Spin-Peierls, follows: as are symbols The X. CO ntcaoayidctsol the only indicates anomaly gnetic r,tesetaaecharacteristic are spectra the ure, 13 pnlbld(TMTTF) spin-labeled C tal. et 3 .A Merlic A. C. , s A90516 USA 90095-1569 CA es, uaet Hungary Budapest, 09-57USA 90095-1547 A ∆ ρ AF 1]wr h rtt on u a out point to first the were [15] pressure ? ln oeue with molecules alent ee hs nthe in phase dered 2 3 ,adteslu analogs the and X, .E Brown E. S. , Transition lines Crossover SC 2 AsF ρ ,weesnear whereas ), 6 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. Also, the spin sus- ceptibility is transparent to the structureless transition. 10 Later, Coulon, et al. identified a feature in the ther- (d) mopower of (TMTTF)2AsF6 at T≈100K, and through 0 6K doping studies, they were able to establish that it was -10 the same type of transition [22]. Taken together with our observations, the implication is that the structureless -10 -5 0 5 10 transition is a CO transition, it appears to be continu- dipolar coupling f1 (kHz) ous, it is primarily the charge degrees of freedom which FIG. 4. 2D J-resolved 13C NMR spectra are involved, and it is ubiquitous to the TMTTF fam- from (TMTTF)2AsF6 at representative temperatures. a) T ily. The charge-ordering is probably the reason why the greater than the charge ordering transition temperature Tco. activation energy obtained from transport measurements b) T

3 sition, the two carbon nuclei are ”unlike” spins because (1997). of the different hyperfine shifts. The two coupled I=1/2 [10] S. Mazumdar, S. Ramasesha, R. Torsten Clay, D. K. spins form four non-degenerate energy states. When the Campbell, Phys. Rev. Lett. 82, 1522 (1999). [11] H. H. S. Javadi, R. Laversanne,A. J. Epstein, Phys. Rev. hyperfine shifts decrease for T

[1] J. Tranquada, B. Sternlieb, J. Axe, Y. Nakamura, S. Uchida, Nature 375, 561 (1995). [2] S. Mori, C. Chen, S.-W. Cheong, Nature 392, 473 (1998). [3] V. J. Emery, S. A. Kivelson, J. M. Tranquada,Proc. Natl. Acad. Sci. 96, 8814 (1999). [4] K. Hiraki, K. Kanoda, Phys. Rev. Lett. 80, 4737 (1998). [5] R. Chiba, H. Yamamoto, K. Hiraki, T. Takahashi, T. Nakamura, J. Phys. Chem. Solids (in press). [6] T. Takano, K. Hiraki, H. Yamamoto, T. Nakamura, T. Takahashi, J. Phys. Chem. Solids (in press). [7] C. Bourbonnais, D. J´erome, cond-mat/9903101. [8] D. Jerome, A. Mazaud, M. Ribault, K. Bechgaard, J. Physique Lett. 41, L95 (1980). [9] H. Seo, H. Fukuyama, J. Phys. Soc. Japan 66, 1249

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