Various nuclear effects in atomic systems and in a particular type of solids, namely, in unconventional superconductors, are investigated. The first process considered, internal pair conversion in heavy ions, can play an important role in numerous scattering processes to be examined at existing or upcoming high-energy heavy-ion-accelerator facilities. The rate of nuclear excitation and thus the number of created pairs is found here to be strongly increased by ion planar channeling through a crystal. The time-reversed process of pair conversion, nuclear excitation by resonant positron annihilation, provides an alternative mechanism of positronmatter interaction and constitutes a state-selective way to excite nuclei which is complementary to photo- and Coulomb excitation. Furthermore, weak-interaction effects are examined in the context of parity violation in unconventional p-wave superconductors. We suggest schemes to effciently enhance the effect and to enable its future experimental study. The considered effects represent new phenomena at the interface of atomic and nuclear physics and quantum electrodynamics, and provide effective ways to investigate fundamental interactions.