Tunable low-temperature dissipation scenarios in palladium nanomechanical resonators

We study dissipation in palladium (Pd) nanomechanical resonators at low temperatures in the linear response regime. Metallic resonators have shown characteristic features of dissipation due to tunneling two-level systems (TLS). The system described here offers a unique tunability of the dissipation scenario by adsorbing hydrogen (H2), which induces a compressive stress. The intrinsic stress is expected to alter TLS behavior. We find a sublinear ~T0.4 dependence of dissipation in a limited temperature regime. As seen in TLS dissipation scenarios, we find a logarithmic increase of frequency from the lowest temperatures till a characteristic temperature Tco is reached. In samples without H2,Tco~1K was seen, whereas with H2 it is clearly reduced to ~700mK. Based on standard TLS phenomena, we attribute this to enhanced phonon-TLS coupling in samples with compressive strain. We also find that with H2 there is a saturation in low-temperature dissipation, which may possibly be due to super-radiant interaction between TLS and phonons. We discuss the data in the scope of TLS phenomena and similar data for other systems.