Superconductivity in Zr$_{2}$(Co$_{1-x}$M$_{x})$ (M = Cu, Ga)

As revealed in the powder x-ray diffraction and crystallographic data, the body-centered tetragonal structure of the parent compound Zr$_{2}$Co is retained in both Zr$_{2}$(Co$_{1-x}$Cu$_{x})$ and Zr$_{2}$(Co$_{1-x}$Ga$_{x})$ systems with the solubility limit near $x$ = 0.3. The refined lattice parameters indicate that there is a movement for $c$ to decrease and $a$ to increase, due to the doping with Cu or Ga in the compound. Since the percentage change in lattice parameters $c$ and $a$ is comparable, a prominent peak in the unit cell volume $v$ versus $x$ curve therefore appears around$ x$ = 0.15 and $x$ = 0.2 for Zr$_{2}$(Co$_{1-x}$Cu$_{x})$ and Zr$_{2}$(Co$_{1-x}$Ga$_{x})$ systems, respectively. Magnetic and electrical measurements show that there is an explicit maximum T$_{c}$ close to $x$ = 0.05 for both systems. As compared with the Zr$_{2}$(Co$_{1-x}$Ni$_{x})$ system$^{1}$, it may imply that the superconducting transition temperature in Zr$_{2}$(Co$_{1-x}$Cu$_{x})$ and Zr$_{2}$(Co$_{1-x}$Ga$_{x})$ relate more to the spin density fluctuations than to the density of states at the Fermi level. $^{1}$M. Takekuni, H.Sugita and S. Wada, Phys. Rev. B \textbf{58}, 11698 (1998).