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Charge and anion ordering in (TMTTF) 2 X quasi-one-dimensional conductors

Charge and anion ordering in (TMTTF) 2 X quasi-one-dimensional conductors. Pierre Monceau Centre de Recherche sur les Très basses Températures, Grenoble, France F. Ya Nad Institut of Radio-Engineering and Electronics, Moscow, Russia J. M. Fabre

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Charge and anion ordering in (TMTTF) 2 X quasi-one-dimensional conductors

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  1. Charge and anion ordering in (TMTTF)2X quasi-one-dimensional conductors Pierre Monceau Centre de Recherche sur les Très basses Températures, Grenoble, France F. Ya Nad Institut of Radio-Engineering and Electronics, Moscow, Russia J. M. Fabre Laboratoire de Chimie Organique, Montpellier, France MT. Nakamura and K. Furukawa Institute for Molecular Science, Okazaki, Japan In collaboration with S. Brazovski (LPTMS, Orsay)

  2. Structure of (TM)2X Bechgaard-Fabre salts Transfer integral parallel to the stacks is one order of magnitude larger than in either of the transverse direction Slightly dimerized zig-zag stacks of donors Stacks delimit cavities filled by monovalent anions X The degree of dimerization increases from the « Se » to the « S » donors J.L. Galigne et al. Acta Cristallog, 1979 K. Bechgaard et al. Sid St. Comm., 1980 Symmetry of anions Centrosymmetric anions (CSA) spherical: Br- octahedral: PF6-, AsF6-,SbF6- Non-centrosymmetric anions (NCSA) tetrahedral: ClO4-, BF4-,ReO4- linear: SCN-

  3. Half-filled band Mott insulators Mott insulator in the case of half-filled band, due to strong interactions between electrons with strong U between neighoring sites Insulating state in TMTTF compounds due to electron-electron interactions A M-I transition is seen in resistivity at T≈100-200K If U is large, it is more favourable to localize the particules on the lattice sites to minimize the repulsion and the system is an insulator The localized state of (TMTTF)2X compounds was believed for a long time to be due to the Mott insulating state At low temperatures there is a wide variety of ground states occuring below 20K

  4. 1/4 filled band From the extended Hubbard model, the case without dimerization was first obtained by Mila and Zotos (Europhys. Lett. 1993) using the numerical Lanczos exact diagonalisation. The insulating state is stabilized in the large U/t and V/t which is due to the Wigner crystal-type CO state. The mean field approximation of the 1D Hubbard model show that when V exceeds a critical value, Vc, , charge disproportionation occurs among sites with alternating « charge rich » and « charge poor » sites (Seo and Fukuyama 1997).Besides, AF ordering between spins moments on the charge rich sites appear at low T With D ≠ 0 numerical calculations on the plane U and V for a fixed D where the metallic phase at D = 0 is replaced by the Mott insulating phase , and a phase with Wigner crystal-type CD is still present in the large U and V region U/t CO insulator Phase diagram from numerical calculations of the 1D extended Hubbard model on the plane of U and V, for a fixed value of the dimerization gap D Mott insulator V/t Shibata et al. 2001; Tsuchiizu et al. 2001

  5. Quarter-filled systems with an uniform stacking Possibility of a Mott insulator in a quarter-filled uniform system • (EDT-TTF-CONMe2)2AsF6 K. Heuzé et al. Adv. Mater. (2003) • (DMtTTF)2X with X= ClO4, ReO4C. Coulon ISCOM (2003); S. Sylvain et al. (ISCOM 2003) • Electronic localisation at 150K with appearance of a short range incommensurate • modulation with q= (-0.58, 0, 0.275) • DI-DCNQI)2Ag Wigner crystal type of charge ordering • K.Hiraki and K. Kanoda, Phys. Rev. Lett. 80 (1998) 4737

  6. (DI-DCNQI)2Ag Resistivity Magnetic susceptibility C13-NMR spectra K.Hiraki and K. Kanoda, Phys. Rev. B54 (1996) 17276 Temperature dependence of the NMR Knight shifts which measure the relative populations of electron in the molecular species K.Hiraki and K. Kanoda, Phys Rev. Lett. 80 (1998) 4737

  7. In 1D organic charge transfer salts, the ground state is governed by various interactions: electron-electron, electron-phonon, magnetic and interaction with anions For non interacting (or weakly interacting) electrons, a 2kF CDW can develop due to the Peierls instability with q = 2kF In the case where electron-electron interactions are dominant, a modulation at 4kF can develop (generalization of the classical Wigner lattice) The formation of a 4kF CDW depends of the magnitudes of on-site U and net neighboring site V relative to the mean kinetic energy determined by the width of the energy band W = 4t (t: transfer integral) In the case of (TMTTF)2X salts W ≈ 0.5 eV U = 4-5 eV V = 2-3 eV  U/W and V/W much larger than 1 (F. Castet, A. Fritsch and L. Ducasse 1996) Calculations based on the extended Hubbard model indicate the possibility of charge disproportionation or charge ordering (CO) in (TMTTF)2X salts

  8. Models -Important role of long-range Coulomb interaction and charge- induced e-e correlations -Charge disproportination resulting from strong electronic interactions Seo and Fukuyama J.Phys. Soc. Japan, 66 (1997) 1249 lattice model in the Mean Field approximation at T=0 taking into account the onsite U and the nearest intersite V repulsion potentials. Charge disproportionation occurs at V>Vcr These states are accompanied by different types of spin of AFM spin arrangments. • Interplay between electronic correlations and lattice effectsin 1D quarter-filled bands • Ung et al. Phys. Rev. Lett. 73 (1994) 2603 • S. Mazumdar et al. Phys. Rev. B62 (2000) 13400 • -Anion potentials: Anions can play a dominant role if they are allowed to undergo small • displacements (along arbitrary directions) leading to local changes of the on-site electronic • energies • J. Riera and D. Poilblanc, Phys. Rev. B62 (2000) R16243 • J. Riera and D. Poilblanc, Phys. Rev. B63 (2001) 241102

  9. AC conductivity of (TMTTF)2AsF6

  10. « Structureless » transitions Thermopower of some (TMTTF)2X salts Dielectric constant of some (TMTTF)2X salts   H. Javadi, R. Laversanne and A.J. Epstein Phys. Rev. B37 (1988) 4280 C. Coulon, S.S.S. Parkin and R. Laversanne, Phys. Rev. B31 (1985) 3583 • -Anomaly in the dielectric constant at microwave frequencies • -a break in the T dependence of the thermopower increase • of the charge gap • no supersrtucture • no change in intensity of the main Bragg peaks • A phase transition with a purely electronic origin?

  11. Conductance of (TMTTF)2X salts Br PF6 SCN ReO4 AsF6 SbF6 Non-centrosymetric anions Centrosymetric anions SCN (linear): anion ordering at TAO=169K Superstructure with q= (0,1/2,1/2); CO=2000K ReO4 (tetrahedron) Structureless transition at TCO=227.5K; CO=1400K Anion ordering transition at TAO=154K; CO=2000K AsF6 and SbF6 (octahedron) Abrupt bend at TCO (100.6K for AsF6 and 154K for SbF6) Thermally activated decrease of conductivity with CO=400-500K

  12. Real part of dielectric constant of (TMTTF)2X salts AsF6 ’ = ImG/ SbF6 ReO4 PF6 1- For all anions: at T≈ T, there is no anomaly 2- for CSA and ReO4 anions, ’diverges at TCO. Huge magnitudes of ’ : 2.106 for AsF6, 5.105 for ReO4

  13. Charge disproportionation C13 NMR spectra for (TMTTF)2AsF6 NMR measurements in an external field of 9T (freq 96.4 MHz) Below TCO, doubling of the spectral line due to two inequivalent molecules with unequal electron densities Charge disproportionation : 3:1 from T1-1 measurements Spectral splitting (~charge disproportionation order parameter)versus temperature D.S. Chow et al. Phys. Rev. Lett. 85 (2000) 1698 At high temperatures the unit cell consists of two equivalent TMTTF molecules related by inversion about the counterion. The breaking of the inversion symmetry within the unit cell below TCO, and the spontaneous dipole moment associated with the charge imbalance on the two molecules yield the ferroelectric behaviour.

  14. Dielectric constant of (TMTTF)2X with NCSA anions ReO4 SCN ReO4 salt at TAO does not show any peak but, a jump-like decrease of the ’ magnitude SCN a) at T>TAO no anomalies characteristic of a CO transition b) at T≈TAO , q=(0,1/2,1/2) a small wide maximum two orders of magnitude smaller than in CSA

  15. Ferroelectric character The ferroelectric state is triggered by the uniform shift of anions yielding a macroscopic ferroelectric polarization which is gigantically amplified by the charge disproportionation on the molecular stacks ( S. Brazovski, ISCOM 2003, cond-mat: 0306006, 0401309) CSA and ReO4 salts show at TCO a second order phase transition described by the Curie law A ’ = ----------  T- TCO  1/ ’ (T) is close to be linear Ratio AL / AH (AL at TTCO AH at T>TCO) in CSA: AL / AH ≈ 2 in ReO4 AL / AH ≈ 1.5 PF6 SbF6 AsF6 ReO4

  16. Summary of data Also BF4: T = 215-230K; TCO=83K; TAO=39K

  17. Relaxation rate

  18. Imaginary part of the permittivity of(TMTTF)2AsF6 T <TCO T > TCO = 101 K

  19. Motion of domain walls Frequency of the maximum in ’’ the same at T=97 K and T=105 K (TCO = 101K) The slow relaxation processes involved in the shoulder of ’’ may correspond to the motion of the domain wall structure developped in the ferroelectric state Freezing of the ferroelectric domainstructure below 90K = TCO - 10K

  20. Relaxation rate in (TMTTF)2AsF6  ~ 1/1-T-TCO This divergence correponds to the theory of a classical ferroelectric and is due to the softening of the oscillatory mode responsible for the FE transition

  21. Relaxation rate in (TMTTF)2PF6 Diffuse phase transition in ferroelectrics with compositional heteroginity, so-called relaxator ferroelectrics

  22. Effet of deuteration

  23. Conductance of (TMTTF)2AsF6 hydrogenated SbF6 salt deuterated

  24. Real part of the dielectric permittivity 100kHz H H D D ----------- 16K -------- 13K

  25. Deuteration effect on (TMTTF)2Re04 D H Shift of TCO of 7K No effect on TAO TAO 1 MHz

  26. ESR Temperature dependence of the ESR linewidth, Hpp , of (TMTTF-h12)2AsF6 and (TMTTF-d12)2AsF6 Nakamura et al. ICSM 2004 and Furukawa et al. 2005

  27. Anion molecular dynamics from 19F NMR T1-1 peaks at T=135K and 210K related to rotation modes of SbF6 anions (TMTTF-h12)2SbF6: TCO=155K (TMTTF-d12)2SbF6: TCO=163K Spin-lattice relaxation rate, 19F T1-1 of (TMTTF-h12)2SbF6 and (TMTTF-d12)2SbF6 K. Furukawa, T. Hara and T. Nakamura (2005)

  28. A possible key parameter: the distance R between the TMTTF molecules Along the sequence Br, PF6, AsF6 and SbF6: Volume W and the length a of the unit cell increases Room conductivity decreases TCO increases R determines the overlap of orbitals of TMTTF molecules

  29. Deuteration Charge ordering is currently explained on the base of the extended Hubbard model taking into account Coulomb correlated electron interactions on TMTTF chains. The magnitude of these interactions is governed by the ration V/t with V= the potential of the next neighboring interaction and t=1/2(t1 +t2) is the effective intrachain transfer integral with t1 within dimers and t2 between dimers. Preliminary structural data indicate an increase of the distance between TMTTF Molecules along their molecular chains ( T. Nakamura et al.) It is suggested that the increase of TCO in deuterated samples is associated with the Growth of the ratio V/t (and consequently an increase of dimerization  an increase of TCO.)

  30. Anion ordering transitions CSA: 1- No indication of anion ordering and of formation of a superstructure with q  0 at T > TMO 2- This is due to the charge symmetry of these anions, with all orintations in the cavities equivalent 3- the « structureless » transition at TCO > TMO is associated with charge ordering on the TMTTF molecular chains NCSA: 1- large anisotropy of the charge distribution. Various orientations within the molecular cavities are not equivalent 2- the short contacts between anions and S atoms favors orientational anion ordering with formation of a superstructure with q  0 SCN superstructure with q = (0, 1/2, 1/2) antiferroelectric behaviour ReO4: superstructure with q = (1/2, 1/2, 1/2)

  31. Conclusions 1- Electron correlations due to long range Coulomb interactions on the molecular chains are the driven force of the formation of a charge ordered state , of the Wigner type in (TMTTF)2X conductors 2- Shift of the anions chains ( displacement transition) provides the stabilization of this CO state, which shows many features of a ferroelectric state 3- The form of charge symmetry of anions determine largely the possibility of stabilization of the CO state, the nature of the CO transition and the magnitude of the dielectric constant near TCO. 4- huge effect of deuteration

  32. References F. Nad, P. Monceau and J.M. Fabre J. Physique IV 9 (1999) 10-361 F. Nad, P. Monceau, C. Carcel and J.M. Fabre Phys. Rev. B62 (2000) 1753 F. Nad, P. Monceau and J.M. Fabre J. Phys.:Condens. Matter 12 (2000) L435 P.Monceau, F. Nad and S. Brazovski Phys. Rev.Lett. 86 (2001) 4080 F. Nad, P. Monceau, C. Carcel and J.M. Fabre J. Phys.:Condens. Matter 13 (2001) L717 F. Nad and P. Monceau J. Physique IV 12 (2002) Pr 9-133 F. Nad, P. Monceau, H. Hiraki and T. Takahashi J. Phys.:Condens. Matter 16 (2004) 1 F. Nad, P. Monceau, T. Nakamura and K. Furukawa submitted to J. Phys.:Condens. Matter M. Nagasawa, F. Nad, P. Monceau and J-M. Fabre submitted to Solid state Communications F. Nad, P. Monceau, L. Kaboub and J-M. Fabre submitted to Europhysics Letters

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