Optical evidence for a change in the heavy electron Fermi surface at a magnetic quantum critical point of CeNi_1-xCo_xGe ₂

Optical properties of CeNi_1-xCo_xGe₂ (0 ≤ x ≤ 1) have been studied via infrared spectroscopy. Tuning the Co-doping concentration reveals clear demarcation in the optical properties at x = 0.3, where non-Fermi liquid behavior appears. For x > 0.3, a hump in the optical conductivity σ₁ is observed at about 0.2 eV, resulting from strong hybridization between conduction electrons and Ce 4f electronic states. For x ≤ 0.3, in contrast, no such hump is observed. The low frequency plasmon observed below the coherence temperature T* for x > 0.3 is also consistent with the existence of heavy quasiparticles. These dramatic changes in the optical response at x = 0.3 indicate that the heavy electron Fermi surface of CeNi_1-xCo_xGe₂ (x > 0.3) ceases to exist at this magnetic quantum critical point.