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LoCoTED

LoCoTED - 04/09/2015

En réponse aux défis sociétaux liés à l’énergie, la grappe de projets ‘LOCOTED’ ambitionne de développer une solution innovante basée sur la thermoélectricité afin d’augmenter le rendement énergétique global d’installations industrielles. Concrètement, LOCOTED vise à identifier de nouveaux composés thermoélectriques performants et à les intégrer sous forme de revêtement en couches minces au sein d’un nouveau type de convertisseur thermoélectrique à faible coût afin de récupérer l’énergie thermique dégradée ou diluée rejetée dans les cheminées d’installations industrielles et à la convertir en électricité.

Projet (budget ULiège : 463 548,18€) supporté par Le Fonds Européen de Développement Régional (FEDER :185 419,17 €, 40%)   et la Wallonie (278 128,90 €, 60%).

Low-dimensional transport in bulk semiconductors!

Low-dimensional transport in bulk semiconductors! - 07/05/2015

In a recent Letter, we highlight how to achieve low-dimensional transport in bulk semiconductors taking advantage of the highly-directional character of some orbitals like the d-states. The method is demonstrated in the class of  Fe2YZ Heusler compounds, yielding isotropic thermoelectric  power factors 4-5 times larger than in classical thermoelectrics at room temperature. Our concept is general and rationalizes the search for alternative compounds exhibiting similar properties. 

Low-dimensional transport and large thermoelectric power factors in bulk semi-conductors by band engineering of highly-directional electronic states. D. I. Bilc, G. Hautier, D. Waroquiers, G.-M. Rignanese & Ph. Ghosez, Phys. Rev. Lett. 114, 136601 (2015).

Unexpected ferromagnetism and ferroelectricity in layered perovskites

Unexpected ferromagnetism and ferroelectricity in layered perovskites - 25/03/2015

Ferromagnetism, whereby electron spins collectively align throughout the crystal, is a fairly rare property, especially in insulators. On the other hand ferroelectricity, whereby electric dipoles collectively align and can be reversed with an external electric field, requires the system to be insulating to allow for the existence of a macroscopic polarization. This apparent dichotomy has hindered the concept of next generation memory devices based on simultaneously ferroelectric and ferromagnetic (multiferroic) materials.

In a recent work published in Nature Comunications, we set out to design new materials which could display the elusive ferromagnetic and ferroelectric ground state, and help keep the multiferroic-memory dream alive. Using fully predictive quantum mechanical calculations, we focused on transition-metal layered perovskites, which are known to sometimes exhibit a type of unconventional ferroelectricity, called (hybrid) improper ferroelectricity.

As expected, we found the improper ferroelectric phase in the layered perovskites, albeit with an unusually large electric polarization. However, what caught us by surprise was that the ground state d-electron spin ordering was not the expected antiferromagnetic state, as usually dictated by superexchange interactions in insulators, but the rare aligned ferromagnetic one. The ferromagnetic phase even appeared to be universally favored across a wide range of chemistries. Furthermore the origin behind the ferromagnetic ordering was very odd.

The inter-site spin alignment was argued to be favored on the basis of intra-site Hund’s rules. This unusual scenario is allowed due to an intricate charge and orbital ordering of the d electrons in these layered perovskites.  The orbital ordering was in turn found to be created via certain combinations of (non-polar) distortions to the lattice – the same non-polar distortions which also enable the improper ferroelectricity.

The discovery of this complex interplay between the various degrees of freedom (spin, charge, orbital and lattice) not only provides a new mechanism to achieve ferromagnetism, but could also be used to design new multiferroic systems with optimized device properties.

Ferromagnetism induced by entangled charge and orbital orderings in ferroelectric titanate perovskites. N.C. Bristowe, J. Varignon, D. Fontaine, E. Bousquet and Ph. Ghosez, Nature Communications 6, 6677 (2015).  - OPEN ACCESS -

http://www.nature.com/ncomms/2015/150325/ncomms7677/full/ncomms7677.html