  Heavy Fermion physics deals with the ground state formation and interactions
in f-electron materials where the electron effective masses are extremely
large, more than 100 times the rest mass of an electron. The details of how the
f-electrons correlate at low temperature to become so massive lacks a coherent
theory, partially because so few materials display this heavy behavior and thus
global trends remain unclear. UHg_{3} is now found experimentally to be a heavy
Fermion antiferromagnet, just as are all the other U_{x}M_{y} compounds with
the metal M being in column II B (filled d electron shells) in the periodic
table (Zn/Cd/Hg) and the spacing between Uranium ions being greater than the
Hill limit of 3.5 Angstroms. This result that, independent of the structure of
these U_{x}M_{y}, M=Zn/Cd/Hg, compounds and independent of the value of their
Uranium Uranium spacings (ranging from 4.39 to 6.56 Angstroms), all exhibit
heavy Fermion antiferromagnetism, is a clear narrowing of the parameters
important for understanding the formation of this ground state. The sequence of
antiferromagnetic transition temperatures, T_{Neel}, of 9.7 K, 5.0 K, and 2.6 K
for U_{x}M_{y} as the metal M varies down column II B (Zn/Cd/Hg) indicates an
interesting regularity for the antiferromagnetic coupling strength.
