Bojana Korin-Hamzić

Electrodynamic response of the charge ordering phase: Dielectric and optical studies ofα-(BEDT-TTF)2I3

We report on the anisotropic response, the charge and lattice dynamics of normal and charge-ordered phases with horizontal stripes in single crystals of the organic conductor alpha-(BEDT-TTF)2I3 determined by dc resistivity, dielectric and optical spectroscopy. An overdamped Drude response and a small conductivity anisotropy observed in optics is consistent with a weakly temperature dependent dc conductivity and anisotropy at high temperatures. The splitting of the molecular vibrations nu27(Bu) evidences the abrupt onset of static charge order below TCO=136 K. The drop of optical conductivity measured within the ab plane of the crystal is characterized by an isotropic gap that opens of approximately 75 meV with several phonons becoming pronounced below. Conversely, the dc conductivity anisotropy rises steeply, attaining at 50 K a value 25 times larger than at high temperatures. The dielectric response within this plane reveals two broad relaxation modes of strength Deltaepsilon_LD ~= 5000 and Deltaepsilon_SD ~= 400, centered at 1 kHz < f_LD < 100 MHz and f_SD ~= 1 MHz. The anisotropy of the large-mode (LD) mean relaxation time closely follows the temperature behavior of the respective dc conductivity ratio. We argue that this phason-like excitation is best described as a long-wavelength excitation of a 2kF bond-charge density wave expected theoretically for layered quarter-filled electronic systems with horizontal stripes. Conversely, based on the theoretically expected ferroelectric-like nature of the charge-ordered phase, we associate the small-mode (SD) relaxation with the motion of domain-wall pairs, created at the interface between two types of domains, along the a and b axes. We also consider other possible theoretical interpretations and discuss their limitations.

Suppression of the charge-density-wave state in Sr14Cu24O41 by calcium doping

The charge response in the spin chain and/or ladder compound Sr14-xCaxCu24O41 is characterized by dc resistivity, low-frequency dielectric spectroscopy and optical spectroscopy. We identify a phase transition below which a charge-density wave (CDW) develops in the ladder arrays. Calcium doping suppresses this phase with the transition temperature decreasing from 210 K for x=0 to 10 K for x=9, and the CDW gap from 130 meV down to 3 meV, respectively. This suppression is due to the worsened nesting originating from the increase of the interladder tight-binding hopping integrals, as well as from disorder introduced at the Sr sites. These results altogether speak in favor of two-dimensional superconductivity under pressure.

Charge-density wave formation in Sr14-xCaxCu24O41

The electrodynamic response of the spin-ladder compound Sr

Anisotropic charge dynamics in the quantum spin-liquid candidate kappa-(BEDT-TTF)2Cu2(CN)3

_{14-x}

Collective excitations in the charge-ordered phase of α-(BEDT-TTF)2I3.

Ca

Nonlinear conductivity in the ground state of α-(BEDT-TTF)2KHg(SCN)4

_x

The angular-dependent magnetoresistance in α-(BEDT-TTF)2KHg(SCN)4

Cu

Unconventional Charge-Density Wave in the Organic Conductor α-(BEDT-TTF)2KHg(SCN)4

_{24}

Anion effects on the electronic structure and electrodynamic properties of the Mott insulator kappa

O

-(BEDT-TT

_{41}