Fyzikální ústav Akademie věd ČR

One of the basic features of any material is its ability to conduct an electrical current. Although the electrical conductivity varies with temperature, for most materials the division between metals and insulators is given by their chemical composition and crystallographic structure. There are, however, compounds which can be converted from a metal to an insulator and vice versa by a small variation of external parameters such as pressure or temperature. In the group of Jan Kuneš from the Institute of Physics theoretical description of such phenomena is developed. They recently published two papers on this topic in Physical Review Letters.

In the first one Kuneš with Vlastimil Křápek studied a model, which behaves as a non-magnetic insulator at low temperatures but exhibits a strong magnetic response upon heating together with crossing over to a conducting state. Such a behavior observed in LaCoO₃ has been attracting attention of physicists for decades, but is still not completely understood. The calculations on supercomputer dorje in the Institute of Physics showed, that elevated temperature leads to excitation of some atoms to a metastable magnetic state. With sufficient number of such excited atoms the material becomes metallic. Kuneš and Křápek also showed that the excited atoms form a long range periodic pattern under specific conditions.

In the second paper, Kuneš with his colleagues from the University of Tokyo and Oak Ridge National Laboratory studied the behavior of oxide Sr₂IrO₄, which also exhibits the metal-insulator transition connected with the change of magnetic properties. Common to both these phenomena is a correlated behavior of electrons. In normal metals or insulators a electron feels only the averaged repulsion from the other electrons. Like a man crossing a crowded square is well aware of the crowd, but not inspecting each individual. On the other hand, electrons in the strongly correlated materials interact with their nearest neighbors and adjust to them instantly. Similarly to changing stations in a crowded gym where one has to wait for the new station to be freed before starting a new exercise. In normal metals the magnetic moments carried by almost free electrons cancel out. In the correlated materials the electrons are kept apart by their strong mutual repulsion and the uncompensated moments show up in their sizable magnetic response. The low electrical conductivity of such materials is a result of electrons blocking each others motion. A common feature is strongly correlated materials is their strong response small external perturbations such temperature, pressure or a magnetic field. This makes them very attractive for potential technological applications.

Although the electron-electron repulsion is described by a simple formula, mathematical description of a correlated motion of many electrons is an immensely difficult problem and we only have better or worse approximations. Still the solution of the relevant equations requires supercomputers with hundreds (such as dorje) to hundreds of thousands processors. The numerical simulations of correlated materials is a young field of the 21st century.

Real materials are often simplified to lattice models. The nodes correspond to atoms between which the electrons can hop. In the studied model of LaCoO₃ the atoms exists either in non-magnetic (circles) state with compensated magnetic moments or in a magnetic state where the electronic moments add up. At low temperatures the non-magnetic states dominate. Upon heating the magnetic atoms form an ordered pattern. Further heating above certain temperature leads to melting of this pattern. The calculations show that the high-temperature state with large number of randomly distributed magnetic atoms can support electrical current. The plot on the right shows a calculated magnetic susceptibility, which reflects how strongly the system responds to an external magnetic field.