Non-Fermi liquid states in the pressurized $CeC{u}_2(S{i}_{1-x}G{e}_x{)}_2$ system: two critical points

Abstract

In the archetypal strongly correlated electron superconductor CeCu${}_2$Si${}_2$ and its Ge-substituted alloys CeCu${}_2$(Si${}_{1-x}$Ge${}_x$)${}_2$ two quantum phase transitions -- one magnetic and one of so far unknown origin -- can be crossed as a function of pressure \cite{Yuan 2003a}. We examine the associated anomalous normal state by detailed measurements of the low temperature resistivity ($\ddot{I}$) power law exponent $\hat{I}\pm$. At the lower critical point (at $p_{c1}$, $1\le \hat{I}\pm \le 1.5$) $\hat{I}\pm$ 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, $\hat{I}\pm$ is independent of $x$ at the upper quantum phase transition (at $p_{c2}$, $\hat{I}\pm \simeq 1$), suggesting critical scattering from local or Q=0 modes, in agreement with a density/valence fluctuation approach.

4 pages, including 4 figures. New results added. Significant changes on the text and Fig. 4