Ognjen Milat

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.

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

We have in detail characterized the anisotropic charge response of the dimer Mott insulator κ−(BEDT-TTF)2Cu2(CN)3 by dc conductivity, Hall effect,

The incommensurate structure of (Sr, Ca)14Cu24O41: a study by electron diffraction and high-resolution microscopy

The modulated structure in (Sr, Ca)14Cu24O41 has been studied using electron diffraction and high-resolution microscopy. The structure can be considered as consisting of two interpenetrating substructures. The first sheet-like substructure is shown to be hardly modulated while the second substructure, consisting of c-oriented chains, contains most of the modulation. High-resolution electron microscopy allows either separate imaging of the two substructures or identification of the misfit between them.

The modulated structure of Ca.85CuO2 as studied by means of electron diffraction and microscopy

The structure of the compound Ca.85CuO2, which was recently determined by neutron and X-ray powder diffractometry, was reexamined making use of electron diffraction and electron microscopy. The general features of the previously proposed model are confirmed. However, the modulated structure, for which only a tentative proposal was made in previous work, was found to be rather different. The calcium distribution along the “tunnels” formed within the CuO2 framework is responsible for the modulation. The average CaCa separation along these tunnels is found to be different from the CuCu separation along the same direction in the surrounding framework, five calcium spacings corresponding with six copper spacings. This relation explains the complicated stoichiometry. The non-uniform distribution of the calcium along the tunnels causes modulation waves along (011)o planes, or along the equivalent (011)o planes. The presence of these two symmetry related variants leads to modulation twins.

On the origin of the modulated structure in Ca0.85CuO2: An electron microscopy study

Abstract Different electron microscopy techniques were used to investigate the modulated structure of the compound Ca 0.85 CuO 2 . The orthorhombic basic structure agrees with recent literature based on neutron and X-ray diffraction evidence, but the modulated structure along (011) 0 or (011) 0 differs from it. The origin of the modulation is related to the calcium distribution within the CuO 2 framework. The Ca-Ca separation is different from the Cu-Cu separation in the surrounding network; five calcium spacings correspond with six copper spacings. This explains the deviation of stoichiometry from the CaCuO 2 composition.

Novel spin lattice in Cu3TeO6: an antiferromagnetic order and domain dynamics

We report on magnetic properties of cubic compound Cu3TeO6 studied by

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

ac

-(BEDT-TT

and

_2Ag2(CN)3

dc

Structural variants of Ca0.85CuO2(Ca5+xCu6O12)

susceptibility and neutron powder diffraction for the first time. A novel magnetic lattice, three dimensional spin web, composed of almost planar regular hexagons of Cu2+, S =1/2 spins, defines the properties of Cu3TeO6. The behavior of the magnetic susceptibility in the paramagnetic state at approximately 170 K is suggestive for a competition between local anisotropies of Cu2+ hexagons. The resulting frustration is weaker than the antiferromagnetic nearest-neighbor interaction which leads to a collinear (or slightly canted) spin arrangement (k=0,0,0) and formation of magnetic domains below TN=61 K.