Forum DQMP

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Date(s) - 22/11/2016
13 h 00 min - 14 h 00 min

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Tuesday, November 22nd 2016 – 13h00
Auditoire Stückelberg
Ecole de physique

Coffee and tea will be available from
12h50 at the entrance of the Auditoire

Mechanism of Metal-Insulator Transition in Rare-Earth Nickelates
Oleg Peil (group of Prof. Georges)
Rare-earth (RE) nickelates, RNiO3, represent a class of materials with a rather unusual type of the metalinsulator transition (MIT) with a complex magnetic order in the ground state. Although the search for a
nickelate-based superconductor has thus far been futile RE nickelates still remain prospective compounds for material design, thanks to high tunability of their transport and magnetic properties. This large
sensitivity to external forces is related to a strong coupling of electronic and lattice degrees of freedom.
In this presentation I am going to present a conceptually simple theoretical framework which captures
many observable features of this class of compounds, including, most importantly, the concomitant
metal-to-insulator and structural transition. The MIT and the nature of the insulating state turn out to be
related to a peculiar mechanism whereby coupling of orbital degrees of freedom to the lattice plays an
essential role.

Orbital order and Crystal Structures in LaVO3 Thin Films under
Epitaxial Strain
Hugo Meley (group of Prof. Triscone)
Interplay between spin, charge, orbital and lattice degrees of freedom is extremely strong and at the origin of nu –
merous phenomena in complex oxides [1]. A remarkable case of lattice-orbit coupling is the Jahn-Teller (JT) effect.
The bulk 3d2 LaVO3 showcases a very unusual phase diagram [2]. At room temperature, the three crystal field orbitals remain split in energy due to GdFeO3-type distortions that already impose a C-type orbital ordering, but still,
orbitals fluctuate [3]. Surprisingly, at 140K, a spontaneous G-type orbital ordering induced by a strong enough JT
distortion removes orbital fluctuations.
We have explored different effects of biaxial strain in epitaxial thin films of LaVO 3. X-ray diffraction reveals that the
layers accommodate the strain imposed by the substrate assuming different patterns of octahedral tilts and rota –
tions. These structural changes have been predicted by ab−initio theory to alter the crystal field symmetry and to
lead to an orbital reconstruction of the t2g V states. We used temperature dependent X-ray diffraction, muon spectroscopy and optical conductivity to investigate the film structure under different strain states as well as the orbital
and magnetic order.
[1] D. I. Khomskii, Transition metal compounds (Cambridge University Press, 2014).
[2] Y. Ren et al., Nature (London) 396, 441 (1998); Phys. Rev.B 67, 014107 (2003).
[3] M. De Raychaudhury, E. Pavarini, and O. Andersen,Phys. Rev. Lett. 99, 126402 (2007)

Forum Committee : M.Gibert, A.Tamai, N.Ubrig (14.11.2016)

Réalisation : Sur Mesure concept