Speaker
Dr
Lars Nordström
(Uppsala University)
Description
In most magnetic phase transitions the focus is on the
lowest multipole, the dipole which is directly related to
the magnetic moment. We will in this work argue that in
some cases higher order multipoles are driving the
transition and the dipole is a secondary effect.
The concept of spherical tensors or multipoles of an
open atomic shell is first reviewed and discussed.Some
of these multipoles play an important rule in e.g. x-ray
circular dichroism measurements, where with the use of
the famous sum rules by Carra et al. the spin and orbital
magnetic moments can be deduced.
Then we will describe how such multipoles can be
calculated in general in both the ground state as well as
excited states in terms of density functional methods
including a local correlation term, as in e.g. the so-called
LDA+U or LDA+DMFT methods. It will be demonstrated
how these multipoles can contribute significantly to the
exchange and correlation energies of transition metal
systems.
Especially, we will discuss in some depth materials
where these multipoles act as the main order
parameter, sometimes referred to as an "hidden order".
Especially, results for two cases will be presented the
magnetic/superconducting iron-pnictide LaOFeAs and the
heavy fermion compound URu2Si2.
Primary author
Dr
Lars Nordström
(Uppsala University)
