Title: Semimetallic and charge-ordered $α$-(BEDT-TTF)$_2$I$_3$: on the role of disorder in dc transport and dielectric properties,
Abstract: $\alpha$-(BEDT-TTF)$_2$I$_3$ is a prominent example of charge ordering among
organic conductors. In this work we explore the details of transport within the
charge-ordered as well as semimetallic phase at ambient pressure. In the
high-temperature semimetallic phase, the mobilities and concentrations of both
electrons and holes conspire in such a way to create an almost
temperature-independent conductivity as well as a low Hall effect. We explain
these phenomena as a consequence of a predominantly inter-pocket scattering
which equalizes mobilities of the two types of charge carriers. At low
temperatures, within the insulating charge-ordered phase two channels of
conduction can be discerned: a temperature-dependent activation which follows
the mean-field behavior, and a nearest-neighbor hopping contribution. Together
with negative magnetoresistance, the latter relies on the presence of disorder.
The charge-ordered phase also features a prominent dielectric peak which bears
a similarity to relaxor ferroelectrics. Its dispersion is determined by
free-electron screening and pushed by disorder well below the transition
temperature. The source of this disorder can be found in the anion layers which
randomly perturb BEDT-TTF molecules through hydrogen bonds.