Ytterbium dirhodium disilicide
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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 2mK. 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.