[Nordita Program] The role of higher multipoles in magnetically ordered systems

by Lars Nordström (Uppsala University)

Wednesday 24 Feb 2010, 14:15 → 15:15 Europe/Stockholm

Nordita Seminar Room

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.