In the archetypal strongly correlated electron superconductor ${\mathrm{CeCu}}_2{\mathrm{Si}}_2$ and its Ge-substituted alloys ${\mathrm{CeCu}}_2({\mathrm{Si}}_{1-x}{\mathrm{Ge}}_x{)}_2$ two quantum phase transitions—one magnetic and one of so far unknown origin—can be crossed as a function of pressure. We examine the associated anomalous normal state by detailed measurements of the low temperature resistivity ($\rho$) power-law exponent $\alpha$. At the lower critical point (at $p_{c1}$, $1\le \alpha \le 1.5$) $\alpha$ depends strongly on Ge concentration $x$ and thereby on disorder level, consistent with a Hlubina-Rice-Rosch scenario of critical scattering off antiferromagnetic fluctuations. By contrast, $\alpha$ is independent of $x$ at the upper quantum phase transition (at $p_{c2}$, $\alpha \simeq 1$), suggesting critical scattering from local or $q=0$ modes, in agreement with a density- or valence-fluctuation approach.

• Received 23 June 2005

DOI:https://doi.org/10.1103/PhysRevLett.96.047008