Rigidity Percolation and Low-Energy Excitations in Amorphous As x Se 1- x

We report on a systematic study of the low-temperature specific heat C (0.1 K ⩽ T ⩽ 3K) of As x Se 1- x glasses over a wide range of concentrations x . The linear contribution to C which is attributed to two-level tunnelling states (TLS) shows a pronounced dependence on the average coordination number m = x + 2 with a minimum in the vicinity of the threshold of rigidity percolation m c = 2.4, and a maximum near m ≈ 2.7. This hints at two microscopically distinct types of TLS. The vibrational contribution to C shows a similar x dependence, suggesting a strong correlation between TLS and vibrational excitations.