"Electronic ferroelectricity in carbon-based systems:from reality of organic conductors to promises of polymers." by Prof. Natasha Kirova.

ABSTRACT:

All families of carbon based materials – graphite, graphene, nanotubes, fullerenes and a vast multitude of conjugated polymers and organic crystals – are promising electronically and optically active substances. Recently a new perspective has been added to their researches:  an electronic ferroelectricity.

Ferroelectrics are exploited as active gate materials, in capacitors, electro-optical-acoustic modulators, in WiFi devices, in perspective RAM. Flexible lightweight carbon ferroelectrics are desired as ultrasound sensors in medical imaging. The super-polarisable state of “the ferroelectric Mott semiconductor” was discovered (Grenoble-Orsay-Moscow) in conducting organic stacks. Less conducting ferroelectrics complexes with neutral-ionic transitions (Tokyo-Tsukuba) became the objects of studies by advanced femto-second and tera-Hz optics (Tokyo). The principle novelty of organic ferroelectrics is that the polarization comes from electronic ordering rather than from ionic displacements.

The microscopic design of all new materials is based on coexisting symmetry lowering effects: the dimerization of bonds and the dimerization of sites. With this forecast we could indicate the class of conducting polymers where the ferroelectricity should be present. The theory predicted an existence of solitons (which might have been already observed in Utah experiments on Kyoto materials) with non-integer variable charges, which are the walls separating domains with opposite electric polarisations. Their physics will serve to relate transient ferroelectric processes and the visible range optics. The same symmetry principle applies also to zigzag edges of graphene nano-ribbons which can be tested for the ferroelectricity.

HOST:Professor Shinobu Hikami