Effect of transition-metal substitutions on competing electronic transitions in the heavy-electron compoundURu2Si2

The moderately heavy-electron compound ${\mathrm{URu}}_2$ ${\mathrm{Si}}_2$ is known to exhibit two electronic phase transitions at low temperatures, one to an antiferromagnetically ordered state at $T_N$=17.5 K followed by another to a superconducting state at $T_c$∼1.5 K. The shape of the specific-heat anomaly at $T_N$, which is reminiscent of a second-order BCS-type mean-field transition, suggests the formation of a spin- or charge-density wave opening a gap over part of the Fermi surface. The effect of chemical substitution of the transition metals M=Re, Tc, Os, Rh, and Ir for Ru in ${\mathrm{URu}}_2$ ${\mathrm{Si}}_2$ has been investigated by means of electrical resistivity, magnetic susceptibility, and specific-heat measurements in ${\mathrm{URu}}_{2\mathrm{-}\mathrm{x}}$ ${\mathrm{M}}_{\mathrm{x}}$ ${\mathrm{Si}}_2$ for x≤0.2. The anomaly associated with the 17.5 K transition involves a very small magnetic entropy and is smeared out by very small concentrations of the M substituent. An inverse correlation between $T_c$ and $T_N$ in the Rh- and Ir-doped materials for x<0.01 and in the Os-doped series for x<0.1 is consistent with the picture of two electronic transitions competing for states at the Fermi level.