Insight into the ground state properties of condensed matter is provided by density functional theory. With ab-initio calculations we achieve an accurate and realistic description of advanced materials. Properties like magnetic structure, elastic constants or response to electric fields can be calculated on a quantum-mechanical basis. Using what is considered to be the most precise electronic structure method in solid state physics, we concentrate on magnetic properties of (quasi) low-dimensional systems like surfaces or interfaces. On the right you see defect states formed near the fermi level of SrTiO₃, when a SrO row is missing

More information on the FLAPW method and code development can be found on our http://www.flapw.de/ pages.

Noncollinear magnetism

Contrary to ferro- or antiferromagnetic structures, in a noncollinear magnetic arrangement the direction of the magnetization may change from atom to atom by arbitrary angles. Due to topological frustration such structures can be formed on triangular lattices. The image shows a periodic Néel state. Here co-planar spins form 120° angles between nearest neighbors. A unit cell shown in red contains 3 atoms. The total magnetization M in the unit cell is zero. In this picture, the spins are oriented in the surface plane. The orientation of the spin with respect to the lattice is determined by the spin-orbit interaction.

(P. Kurz, G. Bihlmayer, S.Blügel).

Spin-orbit coupling and magnetocrystalline anisotropy

Magnetic recording would not be possible without a relativistic effect called spin-orbit coupling. It determines the orientation of the spins with respect to the crystal lattice, or the magnetocrystalline anisotropy. The strength of this effect determines the storage density on an magnetic storage device. Therefore, finding new materials with large magnetocrystalline anisotropies is of substantial importance and the theoretical understanding of this material-dependence is achieved by extensive ab-initio calculations. The picture on the right shows a Pt(111) step edge decorated with Co atoms. The formed Co wires are an example of a low-dimensional magnetic system that has an highly anisotopic out-of-plane magnetization.

(X. Nie, A. Shick, G. Bihlmayer, S.Blügel).

Electric fields on surfaces

In many experimental techniques (like in scanning tunneling microscopy) an electric field acts on a surface. How the electronic structure of a surface or interface (e.g. in a tunneljunction) is changed by this field can be studied on a ab-inito (quantum mechanical) basis. The figure shows the screening charge induced in the a Xe covered Ag(001) surface when a field is applied. We can see that the screening still occurs at the metal surface and not where the adsorbate sits and that the adsorbate atom becomes polarized.

((S. Clarke, M. Weinert, G. Bihlmayer, S.Blügel).