Remarkably robust and correlated coherence and antiferromagnetism in (Ce$_{1-x}$La$_x$)Cu$_2$Ge$_2$ single crystals

We present results of transport and thermodynamic measurements on La diluted Kondo lattice compound CeCu$_2$Ge$_2$. La-substitution suppresses $T_N$ in an almost linear fashion from $\sim$ 4 K, for $x$ = 0, to below 0.36 K, for $x>$ 0.8. Curiously, the system also shows low temperature coherent scattering below $T_{coh}$ up to $\sim$ 0.9 of La, indicating a small percolation limit $\sim$ 9$\%$ of Ce that separates a coherent state from a single-ion Kondo impurity state. $T_{coh}$($H$) was found to have different functional dependencies in coherent and single-ion regimes. Surprisingly, $(T_{coh})^2$ was found to be proportional to $T_N$ over wide range of $x$. For Ce concentrations, $y=1-x$, in the range 0.01 $\leq y \leq$ 0.08, $T_{min}$ in the resistivity data is proportional to $y_{Ce}^{1/5}$ and field-dependent thermopower shows features as expected for the single-ion Kondo impurity. This work was supported by the Department of Energy, Basic Energy Sciences under Contract No. DE-AC02-07CH11358 and the AFOSR-MURI grant No. FA9550-09-1-0603.