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alpha-(BEDT-TTF)2I3: Complex electrodynamic response of the charge-ordered phase

Much experimental and theoretical attention has been attracted by organic systems with reduced dimensionality and strong Coulomb interactions, and deservedly so due to their novel broken-symmetry phases and corresponding excitations. Here we take a detailed look at the electrodynamics of one of the most prominent charge-ordered systems, the quasi-2D conductor α (BEDT-TTF)2I3. A semimetal at high temperatures, at 136 K this particular system transitions into an insulating, diamagnetic ground state. Within the insulating phase a long-range commensurate ordering appears in the BEDT-TTF molecular planes, the so-called "horizontal stripe" charge order. [1, 2, 3] We characterize the charge response of the low-temperature phase using dc resistivity, dielectric and optical spectroscopy in different crystallographic directions within the BEDT-TTF layer. [4, 5] Interestingly, two dielectric relaxation modes appear in the kHz-MHz range. The large mode features an anisotropic phason-like behavior, while the small mode presents a soliton-like characteristic. The observed type of excitations agrees with the most relevant physical picture of this charge order as a cooperative bond-charge density wave with ferroelectric-like features. [6] On the other hand, puzzling phenomena including negative differential resistance and voltage oscillations have been reported under application of high electric fields. [7] Our carefully designed electric-field-dependent measurements of conductivity anisotropy within the molecular plane qualitatively confirm these findings ; additionally, they reveal novel intriguing behaviors. [8]. [1] Y. Takano et al., J. Phys. Chem. Solids 62, 393 (2001). [2] Organic Conductors, Special Topics Section, J. Phys. Soc. Jpn. 75, No. 5, (2006). [3] T. Kakiuchi et al., J. Phys. Soc. Jpn. 76, 113702 (2007). [4] T. Ivek et al., Phys. Rev. Lett. 104, 206406 (2010) [5] T. Ivek et al., Phys.Rev.B 83, 165128, (2011). [6] R. T. Clay et al., J. Phys. Soc. Jpn. 71, 1816 (2002). [7] K.Tamura et al., J. Appl.Phys.107, 103716(1-5), (2010). [8] T. Ivek et al., Phys. Rev. B 86, 245125 (2012).

charge ordering; dielectric response; nonlinear conductivity; high electric fields; pulse electric field; organic conductor; domain wall motion; avalanche

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