  Disordered non-interacting systems are classified into ten symmetry classes,
with the unitary class being the most fundamental. The three and four
dimensional unitary universality classes are attracting renewed interest
because of their relation to three dimensional Weyl semi-metals and four
dimensional topological insulators. Determining the critical exponent of the
correlation/localistion length for the Anderson transition in these classes is
important both theoretically and experimentally. Using the transfer matrix
technique, we report numerical estimations of the critical exponent in a U(1)
model in three and four dimensions.
