Tuning Intertwined Energy Scales in f-Electron Systems by Chemical Substitution

Materials that contain f-electron elements often exhibit complex phase diagrams with different phenomena including nematic electronic states, charge and spin instabilities, the breakdown of Fermi liquid behavior, and unconventional superconductivity. This diversity of behavior is related to the fine balance between several factors including the magnetic exchange, Kondo effect, crystal electric field splitting and strong spin-orbit coupling. As a result, many such systems exhibit intertwined order parameters that are controllable through pressure, magnetic field, and chemical substitution. Here, we report results from chemical substitution studies in three distinct Kondo lattice systems. In each case, we are able to suppress an ordered state towards zero temperature at a possible quantum phase transition and study the resulting behavior. For CeCu$_2$Si$_2$, Si $\rightarrow$ P chemical substitution compresses the unit cell volume while adding $s/p$ electrons. This encourages complex magnetism and drives the system away from a quantum critical point. These results are understood by considering that the electronic hybridization between the f- and conduction electrons in this system is controlled by nearly independent parameters of unit cell volume and s; p; d shell filling, which drive the system's behavior along different axes. For CePd$_2$P$_2$, Pd $\rightarrow$ Ni substitution suppresses the ferromagnetism towards a disordered ferromagnetic QCP at $x_{\rm{cr}}$ = 0.7, where the breakdown of Fermi liquid behavior is observed. We also find that for CePd$_2$P$_2$, a pressure of $P_{\rm{c}}$ = 12 GPa would likely be sufficient to access a quantum phase transition. These results provide a useful experimental testbed for the Belitz-Kirkpatrick-Vojta (BKV) theory. For UCr$_2$Si$_2$, Cr $\rightarrow$ Ru substitution results in filling of the $d$-shell without significantly changing the unit cell volume. This suppresses the antiferromagnetic order $T_{\rm{N}}$ ($T_{\rm{N}}$ $\approx$ 24 K for UCr$_2$Si$_2$) and the structural phase transition $T_{\rm{S}}$ ($T_{\rm{S}}$ $\approx$ 200 K for UCr$_2$Si$_2$) that are seen in the parent compound. $T_{\rm{N}}$ approaches zero temperature near $x_{\rm{c, N}}$ = 0.08 while $T_{\rm{S}}$ reaches a minimum value near $x_{\rm{c, S}}$ = 0.16, after which the structural phase transition disappears for larger $x$. Near this concentration there is evidence for the breakdown of Fermi liquid behavior in the transport and heat capacity measurements, suggesting that this may be a model system for studying a lattice instability at zero temperature, its relationship to a nearby antiferromagnetic quantum critical point, and the resulting impact on electronic properties and lattice modes in a strongly correlated electron metal.