  We report field-orientation specific heat studies of the pressure-induced
heavy fermion superconductor CeRhIn5. Theses experiments provide the
momentum-dependent superconducting gap function for the first time in any
pressure-induced superconductor. In the coexisting phase of superconductivity
and antiferromagnetism, field rotation within the Ce-In plane reveals four-fold
modulation in the density of states, which favors a d-wave order parameter and
constrains a theory of the interplay between superconductivity and magnetism.
