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.^[1]