Ytterbium dirhodium disilicide (YbRh2Si2), also abbreviated YRS, is a heavy fermion solid state compound of ytterbium, rhodium and silicon.
It becomes superconducting when cooled to 2 mK.
Just above this temperature the heat capacity is extremely high, and the electrons behave as if they were 1,000,000 times heavier than they really are.
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