Enhanced Sommerfeld coefficient near the quantum critical point in YbMn6Ge6−xSnx

We investigate the YbMn6Ge6−xSnx alloys (4.00 ≤ x ≤ 5.55) by means of specific heat measurements in the temperature range 1.9 to 200 K. For x near ∼ 4.0, the Sommerfeld coefficient γ is close to that of a Lu-based counterpart (γ ∼ 40 mJ.mole−1.K−2). The γ coefficient increases upon Sn for Ge substitution, enters the heavy fermion regime and peaks (γ ∼ 210 mJ.mole−1.K−2) in the vicinity of the Yb magnetic instability (xc ∼ 5.23), before diminishing upon further Sn content increase. The enhanced γ values near xc agree with the close proximity of a quantum critical point previously inferred from electronic spectroscopy experiments.