13. 11. 2019

Andrii Sotnikov
(Akhiezer Institute for Theoretical Physics, NSC KIPT, Kharkov, Ukraine):
Ferromagnetism of LaCoO3 and orbital ordering of cold atoms in optical lattices

Abstract:

The first part of the talk is devoted to unconventional ferromagnetic (FM)
ordering in the insulating LaCoO3 under tensile strain [1]. We argue that
the FM exchange in this compound originates from the highly-fluctuative nature
of high-spin (HS) states, which can be viewed as bi-excitons of itinerant
intermediate-spin states [2,3]. To confirm that, we construct a series of
theoretical approximations to account for crucial electron correlation effects
responsible for HS fluctuations and magnetic exchange. The obtained amplitudes
and spatial characteristics of magnetic couplings between the "dressed" HS
states show a good agreement with experimental observations and provide
important details to the physical picture of LaCoO3.

In the second part, we focus on cold atoms in state-dependent optical
lattices [4]. These have provided an extended toolbox for simulations of not
only antiferromagnetism [5], but a number of long-range-ordered phases peculiar
to multiorbital solid-state materials. By means of dynamical mean-field theory
we show that the four-component mixture of cold Yb-173 atoms with realistic
lattice depths, interaction parameters, and measurements possibilities is
a good candidate, in particular, for studies of orbital ordering relevant to
LaMnO3 and KCuF3.

[1] J. Fujioka et al., Phys. Rev. Lett. 111, 027206 (2013).
[2] A. Sotnikov and J. Kuneš, Sci. Rep. 6, 30510 (2016).
[3] R.-P. Wang et al., Phys. Rev. B 90, 035149 (2018).
[4] F. Scazza et al., Nat. Phys. 10, 779 (2014).
[5] A. Mazurenko et al., Nature 545, 462-466 (2017).