heavy fermion
/ HEV-ee FER-mee-on /
Imagine trying to run through waist-deep water. Your legs are unchanged, but you feel enormously heavier and slower because the water clings to and drags every motion. Now imagine the drag is so severe you feel a thousand times your real weight. Some electrons inside certain crystals feel exactly that — they move as if they were hundreds of times heavier than a normal electron.
These materials, often compounds containing cerium or uranium, hold rows of localized magnetic atoms sitting in a sea of mobile electrons. Through a collective version of the Kondo effect, each mobile electron gets so entangled with the magnetic atoms it must drag a heavy cloud of correlation everywhere it goes. The upshot is a quasiparticle with an enormous effective mass — hence 'heavy fermion,' since electrons belong to the family of particles called fermions.
It matters because these are some of the most strongly correlated metals known, and many of them slide into exotic superconductivity or unusual magnetism at low temperatures, making them prized laboratories for studying correlation. The honest caveat: the electrons are not literally more massive — their underlying mass is fixed. It is the dressed-up quasiparticle that responds as if heavy.
In the compound CeCu2Si2, the conduction electrons behave as quasiparticles roughly a thousand times heavier than a free electron — measured by how much heat the metal soaks up. This same material was the first 'unconventional' superconductor ever found, in 1979, surprising everyone with how strong correlation and superconductivity could coexist.
In CeCu2Si2 the electron quasiparticles act about a thousand times heavier than free electrons.
The 'mass' here is the effective mass — a measure of how reluctant the quasiparticle is to accelerate, inferred from heat capacity and other measurements. A huge effective mass signals fierce correlation, since it means the electron is heavily dressed by its interactions.