Problems in the picture of quasi 1D organic conductors in light of observations of

1. Problems in the picture of quasi 1D organic conductors in light of observations of the ferroelectricity and the charge ordering. Serguei Brazovskii LPTMS - CNRS, Orsay, France Landau Institute, Moscow, Russia “In science of organic metals, the problems are never solved: They are forgotten in favour of new ones” Igor Shchegolev, 1982.

2. (TMTCF)2X Black and white: Anniversary picture 1999 SC- superconductivity AF- AFM = SDW SP- Spin-Peierls LL- Luttinger liquid MI- Mott insulator Red line TCO – 2000's revaluation: Structurless transitions (Coulon et al 1985) = Ferroelectricity (Monceau et al) = Charge disproportionation (Brown et al) 15 years long fate of structurless transitions : unexplained, unattended, abandoned. Recentinsightsto their nature: Huge anomaly of dielectric susceptibility (Grenoble group) Charge disproportionation from the NMR (UCLA group) The breakthrough (Grenoble-Moscow): Low frequency methods for the dielectric permittivity Ree=e′: designed for pinned CDWs, applied to SDWs and finally to structurless transitions.

3. Questions on charge ordering CO and ferroelectricity FE: Why the CO is so common ? Is it even universal ? Why the astonishing ferroelectricity is its so frequent form? Why anti-FE and more complex patterns on other occasions ? Is it a spontaneously created Mott-Hubbard state? Is it a Wigner crystal, if yes then of what: electrons or polarons? Is it a 4KF CDW? Driven by electrons and stabilized by the lattice? If yes, then is it the molecular stack or the anionic column? Role of anions? Is there a key to the FE/anti-FE choice? Other Damocles swards dangling since decades ? Examples: plasma frequency mystery, special structure for superconducting phase. Still waiting for experimental studies: solitons (holons, spinons) in transport, optics, tunneling, PES.

4. WHERE WE ARE? Route of interactions. Phenomenological Hamiltonian componenents: H~-1[v(x)2 + v-1(t )2]from electron liquid or 4KF CDW, basic + Tcos(φ) from tetramerization or SP, spontaneous, frequent + Ucos(2)from dimerization , build-in or spontaneous, typical + Vcos(3)from trimerisation, TTF-TCNQ under pressure, only + Wcos(4) build-in, from host lattice, common g≡ Kρ controls quantum fluctuations of the phase , it defines: survival, against renormalization 0, of nonlinearities ~U,V,W; 2. their spontaneous generation – known for T,U; 3. physics of solitons and the collective mode in optics, kinetics; 4. interchain coupling, hence the long range order.

5. <1: renormalized U≠0 - gap originated by the build-in dimerization - generic Mott-Hubbard state, any repulsion is sufficient. solitons=holons as free excitations. Certainly valid for TMTCF's. Non applicable to new nondimerized materials like DMtTTF, (EDT-TTF-CONMe)2AsF6(Batail et al) - still would be metalic, unless fall to the next regime: g<1/2=0.5: spontaneous dimerization U is formed, no need for bare U. proved to be valid by the CO observation in TMTTF's (FE response, NMR) and DMtTTF(X-ray, S.Ravy et al) Waiting for optical signature: collective mode drops below two-particle gap – - bound states of solitons are favored. <4/9 = 0.39 trimerization lock-in (TMTTF-TCNQ, Jerome 78)

6. Other speculations which would be applicable if there is no charge ordering – if it is possible at all: temperatures above the 4KF condensation in TTF-TCNQ, the TMTSF subfamily -- unless the CO/FE are present hiddenly. <1/4 = 0.25 effects of ¼ filling come to play (cf. MX3 CDWs). <√5-20.24 ultimate SDW instability, even incommensurate (pairs confinement via interchain hopping, Yakovenko & SB) -- not the case of TMTSF, need HMF support to make FISDW g0.23 guess from optical high w tails <2/9  0.22 spontaneous trimerization -- not observed, need precise pressure to pinpoint exactly 1/3 <3-√20.17 last feature of electrons FS disappear. -- guessed from ARPES on TMTSF but not seen on more correlated TTF-TCNQ =0.125 spontaneous ¼ filling in totally incommensurate chains -- the usual CO would has happened already. Resume: most of "Lattinger liquid" effects would come at <1/2 where the system is already unstable to the CO.

7. Generic origins Ckin,Cpot of basic parameters g,vr:Interactions among electrons or with phonons? potential parameter Cpot : 1+ e-e repulsion. kinetic parameter Ckin : 1+lattice adjoined mass gcontains a product of C's -- not distinguishable separately vrcontains a ratio of C's -- not distinguishable separately wp*contains only Ckin -- direct access to the joint lattice dynamics Coulomb hardening factor Cpot > 1 acts upon  and v[as measured in a CDW by Hennon, Pouget et al] but cancels in their product which gives plazma frequency p*

8. Outcome: Resolution of the dilemma, as old (Jacobsen et al <80) as recent (Digeorgi et al): divergence between values of plasma frequency ωp extracted from two frequency ranges: high - p0, and intermediate - p* Mass enhancement is not effective above phonon frequencies: Ckin is a function of w: Crossover: phonons' frequency ph Magnitude: Ckin-1~(D/wph)2 – like for CDWs (Lee, Rice, Anderson) Visualisation: CO state is a bilateral electron-lattice 4KF CDW = Wigner crystal of polarons (MX4 CDWs, electrons at He surface): selftrapped electrons gain effective mass from local deformations.

9. collective mode or exciton = two kinks bound state Eg=2 - pair of free kinks. Only that will contribute to photoconductivity Optical Conductivity, Dressel et al, PRL 96 optically active phonon of FE state • Illustrative interpretation of optics on TMTSF in terms of • firm expectations for CO/FE state in TMTTF's: g<½. • For the Mott state without CO, ½<g<1, • no collective mode peak below Eg – need photoconductivity ! • Call for experiments on low gap CO states like in (TMTTF)2Br • Recall a great experience of optics in conjugated polymers.

10. OPTICAL PERMITTIVITY ()IN FERROELECTRIC CASE. 1. Fano antiresonance at 0 2. combined electron-phonon resonance at 3. FE soft mode at -overdamped near TCO , at T<TCO the FE mode frequency follows the order parameter to become finally comparable with 0t. Mixed electron-phonon contribution at T>TCO : p* - renormalized metallic plasma frequency 0 - bare frequency of the molecular vibration associated to CD cr(T) - critical value of the optical gap t(T): spontaneous CO at t< cr -- at the criticality Z(TCO)=1

11. Optics: collective and mixed modes, solitons. Main expected features. I. In any case of the CD, for both FE or Anti-FE orders, we expect: Ia) Strongest absorption feature comes from the phase mode t, analogy of the exciton as the bound kink-antikink pair; Ib) Two-particle gap 2D (photoconductivity e.g.) lies higher, it is given by free pairs of p-solitons=kinks; Ic) Spectral region t<  <2D may support also quantum breathers – higher bound states of solitons. II. In case of the FE order we expect additionally or instead of I: IIa) Fano antiresonance at the bare a phonon mode coupled to the CD; IIb) Combined electron-phonon resonance at 0t > 0, t substitutes for Ia); IIc) FE soft mode at fe (it increases with the FE order parameter). TMTTFs: multiple phonon lines filling just the relevant region. This obstacle is not in vain -- another indication for the CD. Surprisingly (since 80’s, Jacobsen et al) high intensity of molecular vibrations -- Just due to the inversion symmetry lifting by the CD. TMTSF: weaker phonon lines -- fluctuational CD, pseudogap for electrons.

12. SUMMARY Range of the FE anomaly (TCO±30K) dominates the whole region below and even above these already high TCO – up to 230K. Even much higher are the conduction gaps -- up to 2000K. Remind the TTF-TCNQ with its ever present 4KF fluctuations. That high energy scale of a “Grand Unification” knows no differences of interchain couplings, anion orderings, ferro- and anti-ferroelectric types, Sulphur and Selenium subfamilies, old weakly dimerized compounds and new quarter-filled ones. The formation of the Electronic Crystal - however we call it: disproportionation, ordering, localization or Wigner crystallization of charges; 4KF density wave, etc. is the starting point and the frame to consider lower phases. Richness of symmetry-defined effects of the Charge Ordering, (Anti) Ferro-Electricity and various Anion Orderings allows for qualitative assignments and interpretations, particularly on routes of solitons.

13. Remaining main challenges: hidden existence of CD/FE in the metallic Se subfamily physics of solitons via conductivity, optics, NMR optical identification of gaps and soft modes ferroelectric hysteresis

14. () : Optical Conductivity Digeorgi group Dressel et al, PRL 96