Since then several new superconductors of this class, most containing uranium rather than cerium, have been identified. The charge carriers in these compounds have huge effective masses, typically several hundred times the mass of a bare electron, as seen in their enormous low-temperature heat capacity. Power laws in the low-temperature heat capacity, NMR relaxation time, ultrasound attenuation, and other properties suggest nodes in the superconducting energy gap. The material has several phase transitions as a function of temperature and pressure. In the lower-temperature phase, muon spin resonance suggests the presence of local magnetic fields, which may be evidence for an order parameter which violates time reversal symmetry.
Possible explanations are order paramter mixing and an inhomogeneous superconducting state.
We are using two types of measurements. First, we are studying the behavior of magnetic vortices. Several samples have been irradiated with columnar defects, which in high-temperature superconductors act as particularly strong pinning centers. We have shown with heat capacity measurements that the defects reduce the transition temperatures in our heavy fermion samples by comparable amounts to previous work in high-temperature and other superconductors. However, they appear not to increase the pinning in our samples.