The physics of strongly correlated electron systems
Strong correlations between electrons are behind outstanding physical properties like unconventional superconductivity and new quantum phases due to strong spin-orbit coupling. Many of these phenomena are observed in iridium, praseodymium, and uranium compounds and alloys. They are referred to as instabilities of the electronic configuration in the ground state. However, the physics of these materials is so rich and complex that it cannot be understood within conventional theories for metals and insulators. Within the COEL (Numerical study of dynamics and magnetic properties of strongly correlated electron systems) project, physicists went beyond the conventional descriptions of correlated electron states to understand their exotic properties. The COEL team applied a quantum Monte Carlo method to the interacting resonant level model – one of the standard test beds for such studies – to calculate physical quantities such as electric resistivity at finite temperatures. Based on these calculations, a new scenario with strong charge fluctuations was proposed to explain unusual quasiparticle states in samarium compounds. For the formation of unusual heavy fermion states in uranium compounds, COEL scientists needed a starting point beyond the ordinary Kondo effect. They found that the inclusion of a potential scattering term in addition to a ferromagnetic exchange interaction of localised spin and conduction electrons resulted in a decrease in resistivity with lowering temperature. In the course of the project, the scientists also studied spin relaxation induced by spin-orbit coupling in metals and semiconductors. The two most important mechanisms, the so-called Elliott-Yafet and the D'yahkonov-Perel mechanisms, were unified by means of a pseudopotential model developed for semiconductors with zinc-blende crystal structure. Lastly, a theory was formulated to enable the determination of electron-nuclear hyperfine coupling from experimental data. This hyperfine is due to the magnetic interaction of the nucleus with electrons through mechanisms such as Fermi contact and core polarisation. It plays a fundamental role in spin transport exploited in spintronics. Advances in the understanding of strongly correlated electron systems through efficient Monte Carlo simulation techniques combined with many-body system physics will have an impact beyond modern electronics. The COEL project's findings can be leveraged through engineering to improve the performance of solar cells, batteries and fuel cells that require understanding of electrochemistry at atomic level.
Keywords
Strongly correlated electron systems, unconventional superconductivity, quantum Monte Carlo, spintronics