Normal and superconducting state properties of U₆Fe at low temperatures and high magnetic fields

High purity U₆Fe polycrystals with a resistance ratio rrr=9 at T=4 K are found to exhibit a record onset temperature (T₀ > 4.0 K) to superconductivity for U materials. Our measured values of the electronic coefficient of heat capacity γ^*=(150±3) mJ/mol · K², the mean Debye temperature Θ(T→0)=166 K and the normalized jump in heat capacity at T_c, Δ/γT_c=2.3±0.1, are all in good agreement with previous work^† † L.E Delong et al.Physics.Rev.Lett.51(1983)312., and suggest that U₆Fe is a strong-coupled superconductor. Resistive upper critical field measurements on high purity samples revealed a pronounced S-shaped curvature of H_c2(T) and evidence for anisotropy, similar to effects seen in UPt₃^‡ † L.E Delong et al.Physics.Rev.Lett.51(1983)312., but not observed in lower purity U₆Fe samples^† † L.E Delong et al.Physics.Rev.Lett.51(1983)312.. We find H_c2(0) > 10T and transition widths ΔT_c∝H ^{1 2} at moderate fields, followed by an abrupt increase in ΔT_c for H>8T. Our H_c2 data cannot be explained by existing theoretical models. Preliminary heat capacity measurements in magnetic fields 0≤H≤10T suggest that γ^* may decrease by ≈10% at H=10 T. These results and electrical resistivity data will be discussed in terms of a potential charge density wave instability in U₆Fe at T{less-than or approximate}110 K.