Heavy Fermions: Theoretical Aspects
“Heavy fermion” systems belong to the class of “intermediate valent systems”, in which the occupancy of the interior f-shell (4f- in rare earth, 5f in actinide compounds) is close to an instability. For simplicity the discussion will be restricted to Ce compounds (e. g. CeAl3), where the concept of “valence fluctuations” presents itself in its simplest way: In the ground state the occupancy of the f-shell may fluctuate between 1 f (one f electron present, the dominant configuration, having a probability of order 0.9), and 0 f (no f electron). A large variety of anomalies in thermal, lattice, magnetic and electrical behaviour are observed in these systems. The experimentally determined entropy at low temperatures , which is of order kBln2 per Ce around 10K (representing the entropy of a magnetic doublet) for CeAl3, similar to CeAl2 and Ce3Al11, drops to zero when the temperature is lowered in a continuous and smooth way, whereas in the companion compounds CeAl2 and Ce3Al11 most of the entropy is lost rapidly due to magnetic phase transitions. Whereas in the latter the magnetic (“spin”) degrees of freedom are frozen out in phase transitions, in the heavy fermion compound CeAl3 these degrees of freedom are frozen out in a continuous way, certainly an unusual and unexpected behaviour when first discovered. Recent reports about magnetic order with an extremely small ordered moment (0.01μB) at low T does not “explain” this behaviour, the main question remains why the dominant part of the entropy is not lost in a phase transition, the extremely small fraction, corresponding to the 0.01μB ordered moment, which actually might participate in long range order being of secondary importance only.
KeywordsConduction Electron Spin Orbit Coupling Heavy Fermion Charge Fluctuation Kondo Effect
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