Fyzikální ústav Akademie věd ČR

We contributed to the resolution of a long-standing controversy about the magnetic character of the equilibrium state of delta plutonium. While most of theoretical treatments predict magnetic ordering, experiment indicates a nonmagnetic ground state. We used an around-the-mean-field version of the LDA+U extension of the density functional and obtained the expected nonmagnetic ground state reproducing well experimental values of the equilibrium lattice constant, bulk modulus, and also photoemission spectra of Pu, Am, and their alloys.

Thermodynamic properties of a wide class of ordered and disordered magnetic alloys were investiagated by using a classical (random) Heisenberg model the parameters of which were determined by ab-initio electronic-structure calculations. Thermodynamic and magnetic characteristics of the system were determined by numerical statistical methods (Monte Carlo, local RPA). We succeeded in including effects of randomness into the determination of magnetic properties important for diluted systems displaying magnetic percolation effects.

We resolved the problems of a low doping efficiency of Mn and of the magnetization deficit in dilute magnetic semiconductors (Ga,Mn)As by predicting that Mn atoms partly occupy interstitial positions. Both charge and magnetic compensations result from the dynamical equilibrium of interstitial and substitutional Mn. By means of ab-initio calculations we also found a relation between the lattice constant and the degree of the compensation and obtained a comprehensive picture of Mn incorporation into (Ga,Mn)As.

We discovered a mechanism leading to the electron-induced ferromagnetism in various tetrahedral semiconductors doped with Mn. This opens the way to dilute magnetic semiconductors with n-type conductivity. Based on ab-initio calculations, we showed that this should be realized e.g. in Li(Zn,Mn)As. We found that Mn can substitute for Zn in a wide concentration range and that the concentration of carriers can be controlled by the number of excess Li atoms in interstitial positions.

We found a relation between hardness of covalent and ionic crystals and the microscopic structure of core electron levels. We succeeded to express this macroscopic quantity rigorously in terms of binding forces in the solid. We used an all-electron pseudopotential method to determine the hardness theoretically for a wide class of materials and re-examined its phenomenological definition.

We extended the weak-coupling diagrammatic expansion to the critical region of the metal-insulator transition in impurity models. We used the renormalization-group reasoning and simplified the parquet equations for the electron-electron and electron-hole irreducible vertices to a soluble form. We reproduced the universal features of the exact Kondo asymptotics and demonstrated reliability of the simplified parquet approximation in the strong-coupling regime.