The first heavy fermion superconductor, CeCu2Si2, was discovered in 1979. 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. The transition temperatures are low, generally below 1K, but several unusual properties suggest unconventional pairing symmetry, as in the better-known high-temperature cuprate superconductors. Small-moment magnetic order (with magnetic moment less than 0.3 Bohr magnetons per uranium or cerium atom) can coexist with superconductivity. Power laws in the low-temperature heat capacity, NMR relaxation time, ultrasound attenuation, and other properties suggest nodes in the superconducting energy gap. Finally, two heavy fermion compounds, UPt3 and (U,Th)Be13, exhibit phase transitions within the superconducting state.

We are studying U_{0.97}Th_{0.03}Be{_13}. 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. The parent compound, UBe13, has an unusual critical field curve, with dB_{c2}/dT not monotonic. 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.

Our second line of work involves thermodynamic measurements under pressure. In addition to a standard hydrostatic pressure cell that reaches about 28 kbar, we have built a uniaxial pressure cell operated by a helium bellows, for changing pressure while at low temperature. We plan to investigate the thermodynamics of low-temperature pressure-driven transitions with this apparatus. One such transition occurs in U_{0.978}Th_{0.022}Be_{13}, with a change in heat capacity appearing at the same pressure that merges the two temperature transitions into one. We also will search for a second transition in pure UBe13, either a splitting of the single transition as a function of uniaxial pressure, or a low-temperature transition that moves up in temperature when pressure is applied.

This material is based upon work supported by the National Science Foundation under Grant No. 9733898. Any opinions, findings, and conclusions or recommendations expressed in this material are those of the author(s) and do not necessarily reflect the views of the National Science Foundation.