EFFECT OF TRANSITION-METAL SUBSTITUTIONS ON COMPETING ELECTRONIC-TRANSITIONS IN THE HEAVY-ELECTRON COMPOUND URU2SI2

The moderately heavy-electron compound URu2Si2 is known to exhibit two electronic phase transitions at low temperatures, one to an antiferromagnetically ordered state at TN=17.5 K followed by another to a superconducting state at Tc1.5 K. The shape of the specific-heat anomaly at TN, 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 URu2Si2 has been investigated by means of electrical resistivity, magnetic susceptibility, and specific-heat measurements in URu2-xMxSi2 for x0.2. 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 Tc and TN 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. © 1990 The American Physical Society.