Jan Loos, an emeritus member of the Department of Magnetics and Superconductors at the Cukrovarnická site, passed away on 9 November 2025 at the age of 95.
After graduating from the Faculty of Mathematics and Physics at Charles University in 1953, he worked for five years as an assistant in physics at the Institute of Chemical Technology in Prague, and from 1959 onward he was an employee of the Institute of Physics of the Czech Academy of Sciences.
From his arrival at the Institute until the early 1970s, he worked on the theory of ferromagnetic resonance and relaxation, which he formulated using a quantum quasiparticle description. He was the first to use quantum kinetic equations to derive the instability threshold in ferromagnetic resonance. This method was later generalized and elaborated (by White, Sparks) for all nonlinear processes in the microwave region. His further work in this field concerned phenomena above the instability threshold, especially the influence of magnon interactions and strongly relaxing impurities. Part of his results was experimentally confirmed in collaboration with the “Institut für magnetische Werkstoffe” in Jena. His research on dynamic phenomena in rare-earth ferrimagnetic garnets was closely followed by work on the stability of their magnetic ordering at low temperatures and in magnetic fields. For YbIG, he analytically derived the stability boundaries of collinear and non-collinear configurations and the properties of field-induced spin-reorientation phase transitions. Parallel to his work at the Institute, he was also active in teaching. From 1965 he lectured for nine years at the Faculty of Mathematics and Physics of Charles University, teaching fourth-year courses on solid-state magnetism, and supervised several diploma theses by students from the Faculty of Nuclear Sciences and Physical Engineering, Czech Technical University, and from the Faculty of Mathematics and Physics, Charles University.
From the 1980s onward, Jan Loos focused on strongly correlated electron systems with strong electron–phonon coupling. Together with physicists from the University of Bayreuth (and later also from the University of Greifswald), he developed a general method for describing the formation of polaronic states in the Holstein model, applicable for arbitrary values of the model parameters and not restricted to low carrier concentrations.
The culmination of this work may be regarded as three papers in J. Phys. Condens. Matter, in which the analytical derivation of the polaron and phonon spectral functions and of the optical conductivity of polaronic charge carriers was published. These theoretical results were then used in detailed numerical calculations that showed how the optical conductivity changes with electron–phonon interaction strength from weak to strong coupling, where quasi-localized small polarons are formed.
Another of his research interests concerned systems in which the coupling between the spin and polaronic properties of charge carriers plays a key role. He demonstrated that such coupling can lead to a superconducting state of the “anisotropic s-wave” type. The interplay between spin and transport properties with polaron formation became the basis of a theory of colossal magnetoresistance in manganites, which he published in a series of papers in PRB, together with colleagues from the University of Bayreuth.
In the last period of his active scientific career, i.e., roughly a decade before 2015, Jan Loos studied the conductivity and thermoelectric properties of systems with molecular transport junctions. He developed an original theory based on the formalism of non-equilibrium Green's functions, which is not limited to the area of linear response. Numerical calculations performed by colleagues at the University of Greifswald showed that this theory is consistent with existing experimental results.
Those of us who had the opportunity to get to know Jan Loos better all admired his dedication and ability to select interesting physics problems and work on them theoretically with minimal approximations. We also appreciated how he was always willing to advise and help us, his younger colleagues.