1 / 39

4k F CDW induced by long range Coulomb interactions

4k F CDW induced by long range Coulomb interactions. F. Ya Nad Institut of Radio-Engineering and Electronics, Moscow Pierre Monceau Centre de Recherche sur les Très basses Températures, Grenoble M. Nagasawa Dept. Natural Science and Material Science, Tokyo Denki Univ.

sheena
Download Presentation

4k F CDW induced by long range Coulomb interactions

An Image/Link below is provided (as is) to download presentation Download Policy: Content on the Website is provided to you AS IS for your information and personal use and may not be sold / licensed / shared on other websites without getting consent from its author. Content is provided to you AS IS for your information and personal use only. Download presentation by click this link. While downloading, if for some reason you are not able to download a presentation, the publisher may have deleted the file from their server. During download, if you can't get a presentation, the file might be deleted by the publisher.

E N D

Presentation Transcript

  1. 4kF CDW induced by long range Coulomb interactions F. Ya Nad Institut of Radio-Engineering and Electronics, Moscow Pierre Monceau Centre de Recherche sur les Très basses Températures, Grenoble M. Nagasawa Dept. Natural Science and Material Science, Tokyo Denki Univ. T. Nakamura and K. Furukawa Institute for Molecular Science, Okazaki, Japan • In collaboration with: S. Brazovski (LPTMS, Orsay), J.M. Fabre (Montpellier)

  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. 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

  4. 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

  5. CO at ECRYS 2005 • -M. Dumm Electomagnetic response • M. Nagasawa Non linear transport in Br salt • K. Kanoda (DI-DCNQI)2X • T. Ito X-ray study of charge and anion orderings • S. Mazumdar CO in 1/4 filled band charge transfer solids • -S. Brazovski • T. Takahashi CO, everywhere • -S. Brown NMR

  6. Unified Phase Diagram Sequence of electronic phases of (TMTSF)2X and (TMTTF)2X, (belonging to the same family) with pressure as the relevant parameter which allows to move a given compound throughout a unified (T-P) phase diagram Missing: -Anion ordering -structureless transition in (TMTTF)2SbF6 and (TMTTF)2ReO4 Phase diagram description following sequences of electron states determined by changes in anions, superstructures,…. S. Brazovski and V. Yakovenko 1986) C. Bourbonnais and D. Jérome, Advances in Synthetic Metals (1999

  7. AC conductivity of (TMTTF)2AsF6

  8. First characterizations of (TMTTF)2X salts Normalized resistivity of (TMTTF)2SbF6 (….), (TMTTF)2AsF6 (- - -) and (TMTTF)2PF6 (------) 4kFcharge localization  maximum of  at T IR vibronic intensity measurements indicate localized electrons on molecules which imply the presence of a gap of charge (2) on the stacks, whose effect becomes relevant below T ~ / No effect on the magnetic susceptibility at T  spin-charge separation R. Laversanne et al. J. Physique Lett. (1984) Magnetic susceptibility of (TMTTF)2AsF6 Electric resistivity of (TMTTF)2PF6 P. Vaca and C. Coulon, Phase Transitions (1991)

  9. Charge Gap 1- T T metallic type of conductivity 2- T maximum of G AsF6 - 230K SCN - 265K 3- T≤ T conductivity decreases

  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 Low magnetic state (AF) 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 Transition into a magnetic order state at TMO

  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. Vibrational modes in (TMTTF)2X ag stretching C=C mode: infrared active though electron-molecular (emv) coupling Neutral TMTTF: 3 = 1639 cm-1Meneghetti et al. TMTTF+: 3 = 1567 cm-1 J.Chem. Phys. (1984) TMTTF0.5+: 3 = 1603cm-1 Resonance frequency of emv coupled mode is a function of the charge (0.5 ±) on the molecule Temperature dependenceof the 3 mode AsF6 T=20K charge disproportionation: +0.63 and +0.37 (2=0.26) T=10K ------ : +0.60 and +0.40 (2=0.20) PF6 T=20K ------ : +0.56 and +0.44 (2=0.12) M. Dumm et al. ISCOM 2003

  15. 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

  16. 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 CSA: orentional disorder is weak because the great symmetry of CSA NCSA: possibility of formation and stabilisation of a CO state depends on the degree of disorder induced by the orientational disordered NCSA ReO4: anion has an intermediate degree of charge symmetry(tetrahedron) orientation disorder does not prevent the formation of a CO state SCN: strong non-symmetrical charge distribution leading to an inner disordered electric potential, that impeds the formation of a CO state

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

  18. Relaxation rate

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

  20. 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

  21. 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

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

  23. Effect of pressure

  24. (TMTTF)2SbF6 under pressure 0.5 GPa 0.16 GPa 1 bar Temperature dependence of the conductance of (TMTTF)2SbF6 at 1kHz

  25. Nagasawa et al., in press

  26. Dielectric permittivity of (TMTTF)2SbF6 100 kHz 3 Mhz Pressure = 0.38 GPa Ambient pressure at 100kHZ, 300 kHz, 1 MHz, 3 MHz

  27. TCO versus pressure for (TMTTF)2SbF6 V/t decreases with pressure  decrease of dimerization  Shift of TCO at lower T 1 MHz 100 kHz 10 kHz

  28. Effet of deuteration

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

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

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

  32. 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.)

  33. 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)

  34. 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- a new line in the unified phase diagram 5- huge effect of deuteration

  35. 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

  36. Quarter-filled systems without an uniform stacking • role of the 4kF umklapp scattering in the Mott-Hubbard localization • 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

  37. (DI-DCNQI)2Ag Resistivity Magnetic susceptibility K.Hiraki and K. Kanoda, Phys. Rev. B54 (1996) 17276 X-ray structural study of 4kF Wigner crystal Y. Nogami et al. ECRYS 1999 The calculated4kF intensity with the modulation of the molecular valence D-0.25D-0.75 is two orders of magnitude too weak Need to use variations in the intramolecular bond lengths C13-NMR spectra 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

  38. Dielectric permittivity of (DI-DCNQI)2Ag Resistivity Dielectric constant  100kHz 1MHz 5MHz The dielectric permittivity increases sharply below 220K, temperature at which a new charge ordered state with charge disproportionation occurs (NMR and x-rays). The increase of the permittivity reflects the polarizibility of the growing (soft) 4kF superstructure

  39. The main effect of pressure and deuteration may be explained within the common extended Hubbard model Pressure -the distance R between molecules decreases V determined by long range interactions is proportional to 1/R t determined by orbital overlap is proportional to 1/expR At ambient pressure , V/t is large enough for the formation of the CO state V/t decreases  decrease of charge diproportionation  shift of the maximum of ’  shift of TCO at low T Deuteration Deuteration may induce a negative pressure effect  a growth of the distance R between molecules Then V/t will increase  the increase of TCO That is in agreement with the growth of R in the anion sequence Br, PF6, AsF6, SbF6, Preliminary X-ray studies show that the the dimer length between TMTTF molecules is T dependent And it grows near TCO At first sight, it appears that the increase of TCO in deuterated samples can be associated with the growth of V/t with the growth of the ratio V/t due to the increase of the distance R A more deep theoretical analysis is clearly needed taking into account the change of the dimeriztion Along the chain axis.

More Related