Non-Fermi liquid states in the pressurized $CeCu_2(Si_{1-x}Ge_x)_2$ system: two critical points

TL;DR

This study investigates the non-Fermi liquid behavior in CeCu$_2$(Si$_{1-x}$Ge$_x$)$_2$ under pressure, revealing two distinct quantum critical points with different scattering mechanisms affecting the electronic states.

Contribution

It identifies and characterizes two quantum critical points in CeCu$_2$(Si$_{1-x}$Ge$_x$)$_2$, highlighting different scattering processes at each point through resistivity measurements.

Findings

At the lower critical point, resistivity exponent depends on Ge concentration and disorder.

At the upper critical point, resistivity exponent is independent of Ge concentration.

Different scattering mechanisms dominate at the 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 ($\rho$) power law exponent $\alpha$. At the lower critical point (at $p_{c1}$, $1\leq\alpha\leq 1.5$) $\alpha$ 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/valence fluctuation approach.