Colloquium 2006

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  Comparison between Charge Ordered States in Organic 1D and 2D Quarter-filled Conductors
  
  
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  Dr. Pierre Monceau
  ( Centre de Recherches sur les Tres Basses Temperatures, CNRS, Grenoble, France )
  
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  Recent theoretical evaluations have predicted that a charge ordered (CO) insulating state is a possible ground state of organic compounds. Indeed, the quasi 1D conductor (DI-DCNQI)2Ag was the first compound for which a CO state was reported. CO phase transitions were also discovered by dielectric spectroscopy in the series of the 1D Fabre salts of formula (TMTTF)2X in which charge disproportionation on TMTTF molecules occurs below a critical temperature T_CO. We will report on dielectric permittivity measurements performed on the 2D salts, alpha-(BEDT-TTF)2I3 and beta-(BEDT-TTF)2RbZn(SCN)4, on which CO has been detected by NMR. The behavior is relatively similar for both salts: a smooth monotonic increase of the dielectric permittivity, epsilon, from room temperature with a more sharp growth near T_CO = 190K for beta-(BEDT-TTF)2RbZn(SCN)4 (with a noticeable hysteresis due to the first order character of the CO phase transition) and T_CO = 135K for alpha-(BEDT-TTF)2I3, a jump-like of epsilon' just below T_CO down to a small magnitude below the accuracy of the experimental set-up. These results are at variance with those in (TMTTF)2AsF6 for which epsilon shows a divergence in a narrow temperature range near T_CO well described by a Curie law with a slope at TT_CO, a typical feature of a second-order phase transition characteristic of a ferroelectric state. Our measurements have to be examined with respect to recent reports of inhomogeneous charge distribution above T_CO detected by an anomalous broadening of 13C-NMR spectrum. We will present a phenomenological explanation of the difference between the dielectric response in these 1D and 2D organic conductors, taking into account the structure change at T_CO in the 2D compounds, the difference in interchain and electron-phonon interactions, and finally the possibility of displacement of anion chains.
  
  
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