engleski

Charge Orderings in Strongly Correlated Systems

At the focus of this thesis are systems with strong correlations and reduced dimensionality: the inorganic quasi-1D chain compound BaVS3 with itinerant and localized charges, spin chain and ladder composite (La, Y, Sr, Ca)14Cu24O41, and the quasi-2D organic conductor alpha-(BEDT-TTF)2I3 with the so-called horizontal stripe charge order. Samples have been characterized by transport and optical methods used in solid state and low-temperature physics: anisotropy of dc conductivity, nonlinear conductivity and low-frequency dielectric response as well as terahertz and infrared spectroscopy. In the orbitally ordered phase of BaVS3 evidence is presented of collective excitations below the Peierls-like metal-insulator transition. Further, in (La, Y)y(Sr, Ca)14-yCu24O41 a crossover in charge transport is demonstrated at y ~= 2 from a one-dimensional hopping along the chain subsystem to a quasi-two-dimensional charge conduction in the ladder planes. It is found that an interdependent formation of long-range orders takes place in the two subsystems: an antiferromagnetic dimer order in chains and a charge-density wave in ladders. Lastly, electrodynamic response in the organic conductor alpha-(BEDT-TTF)2I3 has been investigated. Optical spectra show a sudden appearance of long-range charge order below the phase transition. Rather than a fully localized Wigner crystal, our results favor an interesting interpretation of horizontal stripes as a cooperative bond-charge density wave with a ferroelectric-like nature.

charge ordering; charge-density wave; orbital ordering; phase transition; strong correlations; conductivity; anisotropy; dielectric response; infrared spectroscopy

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