Superconductivity and magnetism in heavy-fermion UPd${}_2$(Al,Ga)${}_3$

We present bulk properties (resistivity, specific heat, and susceptibility) of the quasiternary system UPd${}_2$(Al${}_{1-x}$Ga${}_x)$${}_3$ and derive the superconducting and magnetic phase diagrams. For low Ga substitution $(x<~0.25)$ a complete suppression of superconductivity is found, while the magnetic properties are hardly affected. For larger $x$ the magnetic transition temperature $T_N$ gradually decreases, and the mass enhancement of the electrons increases, until at $x$=0.8–0.9 a crystallographic transition takes place from the PrNi${}_2$Al${}_3$ to the BaB${}_2$Pt${}_3$ lattice. At the structural transition $T_N$ discontinuously increases, while the electronic specific heat $\gamma$ grows smoothly through the transition. We discuss the relationship between the alloying parameter $x$ and the magnetic ordering and electronic hybridization, respectively. The strong suppression of the superconductivity in UPd${}_2$Al${}_3$ with Ga suggests an unconventional mechanism of superconductivity, most probably related to spin fluctuations mediating the pairing.