Ferromagnetism of LaCoO$_3$ films

We study ferromagnetic ordering and microscopic inhomogeneity in tensile strained LaCoO_33 using numerical simulations. We argue that both phenomena originate from effective superexchange interactions between atoms in the high-spin (HS) state mediated by the intermediate-spin excitations. We derive a model of the HS excitation as a bare atomic state dressed by electron and electron-hole fluctuations on the neighbor atoms. We construct a series of approximations to account for electron correlation effects responsible for HS fluctuations and magnetic exchange. The obtained amplitudes and directional dependence of magnetic couplings between the “dressed” HS states show a qualitative agreement with experimental observations and provide a new physical picture of LaCoO_33 films.

Kinetics of the persistent photocurrent in a-Si:H

Although the persistent photocurrent (PPC) in amorphous hydrogenated amorphous silicon (a-Si:H) is known to be dominated by dispersive recombination kinetics which produce the power low decay, [Formula: see text], where [Formula: see text] is the dispersion parameter, the reason for the occurrence of the dispersive reaction is not clear. We discuss the origin of the dispersive nature in the PPC in a-Si:H. It is shown that band edge modulation due to microscopic inhomogeneity may play an important role in the PPC dynamics.

Modeling and designing micro- and nano-structured metamaterials: Towards the application of exotic behaviors

The lack of suitable computational methods has significantly restricted the creativity of engineers in designing the materials to be used in technological applications. When one wants exact analytical solutions for a given physical system, then usually drastic and restrictive simplifying assumptions are needed. In particular, homogeneity of physical and geometrical properties at lower length-scale is the standard assumption in continuum mechanics. On the other hand, it is well-known since the pioneering work of Gabrio Piola, and then re-established in the works by Mindlin, Toupin, Green, Adkins and Germain, that it is possible to synthetically describe microscopic inhomogeneity by means of field theories incorporating additional kinematical fields. The characteristic length-scale affecting macro-behavior can even be of the order of nanometers, in which case the intuition due to Richard Feynman about the importance of quantum effects at macro-scale could open the path to technological advancements. In the present paper we review some of the literature in the field and try to indicate some research perspectives that seem to us potentially ground-breaking. In particular, following the suggestion of Professor dell’Isola, we briefly describe his concept of pantographic lattices and sheets whose importance in nano-technology could be relevant.