Infrared and Raman Studies of Charge Ordering in Organic Conductors, BEDT-TTF Salts with Quarter-Filled Bands

Abstract

This paper reviews charge ordering in the organic conductors, β″-(BEDT-TTF) (TCNQ), θ-(BEDT-TTF) 2 X, and α-(BEDT-TTF) 2 X. Here, BEDT-TTF and TCNQ represent bis(ethylenedithio)tetrathiafulvalene and 7,7,8,8-tetracyanoquinodimethane, respectively. These compounds, all of which have a quarter-filled band, were evaluated using infrared and Raman spectroscopy in addition to optical conductivity measurements. It was found that β″-(BEDT-TTF)(TCNQ) changes continuously from a uniform metal to a chargeordered metal with increasing temperature. Although charge disproportionation was clearly observed, long-range charge order is not realized. Among six θ-type salts, four compounds with a narrow band show the metal-insulator transition. However, they maintain a large amplitude of charge order (Δρ~0.6) in both metallic and insulating phases. In the X = CsZn(SCN) 4 salt with intermediate bandwidth, the amplitude of charge order is very small (Δρ < 0.07) over the whole temperature range. However, fluctuation of charge order is indicated in the Raman spectrum and optical conductivity. No indication of the fluctuation of charge order is found in the wide band X = I 3 salt. In α-(BEDT-TTF) 2 I 3 the amplitude of charge order changes discontinuously from small amplitude at high temperature to large amplitude (Δρ max~0 .6) at low temperature. The long-range chargeordered state shows ferroelectric polarization with fast optical response. The fluctuation of multiple stripes occurs in the high-temperature metallic phase. Among α-(BEDT-TTF) 2 MHg(SCN) 4 (X = NH 4 , K, Rb, Tl), the fluctuation of charge order is indicated only in the X = NH 4 salt. α′-(BEDT-TTF) 2 IBr 2 shows successive phase transitions to the ferroelectric state keeping a large amplitude of charge order (Δρ max~0 .8) over the whole temperature range. It was found that the amplitude and fluctuation of charge order in these compounds is enhanced as the kinetic energy (bandwidth) decreases. OPEN ACCESSCrystals 2012, 2 1292

Charge-order fluctuations and superconductivity in two-dimensional organic metals

“…By increasing ∆ρ above zero all the four vibrations become coupled together and to the electronic system with the resulting non-linearities observed in the Figure. The red and blue curves now correspond to charge-poor and charge-rich molecules, respectively. Actually, as it has been already noted, 12 the aν 3 mode retains a linear behavior for small …”

Pressure-dependent optical investigations ofα−(BEDT-TTF)2I3: Tuning charge order and narrow gap towards a Dirac semimetal

“…Vibrational spectroscopy is probably the most suitable methods to determine the charge per molecule quantitatively and with high accuracy; [14][15][16] our present pressuredependent infrared experiments enable us to trace the charge disproportionation of α-(BEDT-TTF) 2 I 3 down to low temperatures as the transition is suppressed. On the one hand pressure enhances the coupling between the organic BEDT-TTF molecules and the I − 3 anion layer via the hydrogen bonds; on the other hand it also modifies the orbital overlap making electronic correlations less effective.…”

“…2(a). The appearance of a second Pake doublet in 13 C-NMR 7 , the splitting of charge sensitive modes in Raman- 8,16 and IR-spectroscopy, 9,19,30 and careful x-ray diffraction measurements 29 reveal a significant enhancement in the charge disproportionation. Strong optical nonlinearity and second harmonic generation evidence that the inversion symmetry between the molecules A and A ′ is broken; 31 the symmetry class of the crystal changes from P1 to P1 at low temperatures.…”

Electrically induced phase transition inα−(BEDT-TTF)2I3: Indications for Dirac-like hot charge carriers

“…From NMR 6,7 and optical spectroscopy, [8][9][10][11][12][13] as well as x-ray diffraction studies 14 it is well known that below T CO = 135 K charge disproportionation develops on the BEDT-TTF molecules. Since its discovery three decades ago, the intererst in the title compound never faded because it exhibits a rich temperature-pressure phase diagram, with a number of intriguing quantum phenomena ranging from electronic ferroelectricity [15][16][17] to superconductivity, 18,19 from nonlinear transport 20,21 to zero-gap semiconductivity 22,23 characterized by Dirac cones and massless Dirac fermions, [24][25][26][27] but also the appearance of persistent photoconduction, 28 photoinduced phase transition, [29][30][31] and nonlinear ultrafast optical response.…”