  Following demands for materials with peculiar transport properties, e.g. in
magnetoelectronics or thermoelectrics, there is a need for materials modeling
at the quantum-mechanical level. We combine density-functional with various
scale-bridging tools to establish correlations between the macroscopic
properties and the atomic structure of materials. For examples, magnetic memory
devices exploiting the tunneling magneto-resistance (TMR) effect depend
crucially on the spin polarization of the electrodes. Heusler alloys, e.g.
Co2MnSi, if perfectly ordered, are ferromagnetic half-metals with (ideally)
100% spin polarization. Their performance as electrodes in TMR devices is
limited by atomic disorder and deviations from perfect stoichiometry, but also
by interface states at the tunneling barrier. We use ab initio thermodynamics
in conjunction with the cluster expansion technique to show that excess
manganese in the alloy and at the interface helps to preserve the desired
half-metallic property. As another example, nanostructured materials with a
reduced thermal conductivity but good electrical conductivity are sought for
applications in thermoelectrics. Semiconductor heterostructures with a regular
arrangement of nanoscale inclusions ('quantum dot superlattices') hold the
promise of a high thermoelectric figure of merit. Our theoretical analysis
reveals that an increased figure of merit is to be expected if the quantum dot
size, the superlattice period and the doping level are all suitably fine-tuned.
Such a superlattice thus constitutes a material whose transport properties are
controlled by geometrical features at the nanoscale.
