Center of Solid State Physics| Contact Us | Staff | Publications |
Quantum Atomistic Solid State Theory (QUASST)
Acta Phys. Pol. B 31 (2000) 3079, arXiv:cond-mat/0010081,Acta Physica 2, 1 (2007)
According to QUASST, we point out the importance of the: crystal-field (CEF) and spin-orbit (SO) interactions.
In contrast to the other modern theoretical approaches for the magnetism of 3d-compounds, we do not simplify a treatment of 3d compounds by assumption that orbital moment for 3d compounds "is quenched".
We have performed and publicized calculations for the concrete 3d/4f/5f compounds and groups of compounds: (see publication list)
BASIC theoretical line
Atomic-scale magnetism and low-energy electronic structure are the fundamental ingredients for the physically adequate description of the magnetism and electronic structure of 3d/4f/5f compounds. We point out the importance of local effects, local symmetry, crystal-field and spin-orbit interactions.
Our approach we named: Quantum Atomistic Solid State Theory (QUASST). Its basic idea is that the physically-adequate description of a 3d/4f/5f solid we should start from analysis of single-ion properties of constituting atoms/ions, in particular of the atomic-like electronic structure of 3d/4f/5f shells under the influence of the crystal-field interactions and the intra-atomic spin-orbit interactions.
We claim that the many-electron crystal-field approach with many-electron states of the dn/fn configuration is superior to single-electron approaches popular at present in modern solid-state theories.
Our approach for CEF calculations basis on the methods invented by H. A. Bethe in 1929 ( Ann. Phys. Lpz. 3 (1929) 133 ) and developed by: E. U. Condon, G. H. Shortley, R. J. Elliot, K. W. H. Stevens, G. Racah'a and B. R. Judd.
We think that in a solid the intra-atomic correlations responsible for the formation of the atomic physics terms are stronger than crystal-field, spin-orbit and other solid-state interactions. Thus we think that the integrity of the atomic shell preserves when a 3d/4f/5f atom is put to a solid (not only when is put, but also when it becomes the full part of the crystal lattice) - the preservation of the atomic-like integrity explains the name "Atomistic" in QUASST. It should be noted that the electronic configuration of the 3d/4f/5f atom in a solid is defined during the formation of the compound depending on its partner(s), surrounding, stoichiometry and so on. For example, in LaCoO3 there are La3+, Co3+ and O2- ions. The six electrons of the Co3+ ion form strongly-correlated atomic-like 3d6 configuration. Its description we start from the study of its atomic terms under the influence of the crystal-field and spin-orbit coupling (Phys. Rev. B 67 (2003) 172401). The ESR experiment verifies positively our QUASST approach. Please visit our publication list. We thought that this approach is obvious. It turns out that the magnetic community does like this approach. Thus, we have undertaken the scientific dispute with Editors of Phys. Rev. Letters and Phys. Rev. B, with dr M. Blume and the American Physical Society (the scientific bet for 1 million dollars). They claim, for instance, that covalency effects are much more important than the spin-orbit coupling.
We claim:
1. the need for the "unquenching" of the orbital magnetic moment in the modern solid-state theory,
2. the need to take into account the spin-orbit coupling in description of 3d-atom containing compounds,
3. the need to take into account the very strong electron correlations in description of 4d/4f/5f compounds - QUASST recognizes the strong electron correlations to be of
predominantly of the intra-atomic origin - QUASST works in the very-strongly correlated limit
We assume that:
1. In 3d/4f/5f-metal and ions compounds exists discrete electronic structure. This discrete structure originates from the quasi-atomic states of the atom involved. This discrete electronic structure predominantly determines magnetic and electronic properties of the whole compound.
2. 3d and 4f(5f) electrons form a strongly-correlated electronic system.
3. This strongly-correlated system is described, following the atomic physics, by quantum numbers S, L of the whole shell ==> then we work in the (2S+1)(2L+1) space of the ground term.
4. In a solid the magnetic atom experiences charge interactions from the surrounding charge distribution – these interactions are known as crystal-field interactions.
5. Crystal-field interactions causes the fine (discrete) electronic structure of the atom in a solid, that largely determines properties of the whole compound.
6. This fine electronic structure is very sensitive to the local symmetry. The symmetry of the local surrounding is reflected in the symmetry of the crystal-field (CEF) Hamiltonian.
7. The magnetic ordering is reflected in the time-reversal symmetry breaking of the symmetry of the eigenfunctions of atomic-like states.
8. For correct calculations of fine electronic structure, the spin-orbit coupling must be taken into consideration.