Charge ordering
Charge ordering (CO) is a (first- or second-order) phase transition occurring mostly in strongly correlated materials such as transition metal oxides or organic conductors.
Due to the strong interaction between electrons, charges are localized on different sites leading to a disproportionation and an ordered superlattice.
The charge order transition is accompanied by symmetry breaking and may lead to ferroelectricity.
This long range order phenomena was first discovered in magnetite (Fe3O4) by Verwey in 1939.
The charge ordered structure of magnetite was solved in 2011 by a group led by Paul Attfield with the results published in Nature.
[6] The extended one-dimensional Hubbard model delivers a good description of the charge order transition with the on-site and nearest neighbor Coulomb repulsion U and V. It emerged that V is a crucial parameter and important for developing the charge order state.
[7] The extended Hubbard model for a single chain including inter-site and on-site interaction V and U as well as the parameter
for a small dimerization which can be typically found in the (TMTTF)2X compounds is presented as follows:
can be set to zero Normally, the on-site Coulomb repulsion U stays unchanged only t and V can vary with pressure.
Organic conductors consist of donor and acceptor molecules building separated planar sheets or columns.
The carriers are delocalized throughout the crystal due to the overlap of the molecular orbitals being also reasonable for the high anisotropic conductivity.
They possess a huge variety of ground states, for instance, charge ordering, spin-Peierls, spin-density wave, antiferromagnetic state, superconductivity, charge-density wave to name only some of them.
[8][9] The model system of one-dimensional conductors is the Bechgaard-Fabre salts family, (TMTTF)2X and (TMTSF)2X, where in the latter one sulfur is substituted by selenium leading to a more metallic behavior over a wide temperature range and exhibiting no charge order.
While the TMTTF compounds depending on the counterions X show the conductivity of a semiconductor at room temperature and are expected to be more one-dimensional than (TMTSF)2X.
[11] In the middle of the eighties, a new "structureless transition" was discovered by Coulon et al.[12] conducting transport and thermopower measurements.
They observed a suddenly rise of the resistivity and the thermopower at TCO while x-ray measurements showed no evidence for a change in the crystal symmetry or a formation of a superstructure.
A dimensional crossover can be induced not only by applying pressure, but also be substituting the donor molecules by other ones.
From a historical point of view, the main aim was to synthesize an organic superconductor with a high TC.
The key to reach that aim was to increase the orbital overlap in two dimension.
With the BEDT-TTF and its huge π-electron system, a new family of quasi-two-dimensional organic conductors were created exhibiting also a great variety of the phase diagram and crystal structure arrangements.
Below 122 K, the combination of 2+ and 3+ species arrange themselves in a regular pattern, whereas above that transition temperature (also referred to as the Verwey temperature in this case) the thermal energy is large enough to destroy the order.
