Low T Study of PdH _x System by Torsional Oscillator Technique: x Dependent Responses

Abstract

Hydrogen atoms dissolve in Pd at densities up to one H atom per Pd, which provides higher atomic H density than in solid H₂. They are known to have large diffusion coefficient due to quantum tunneling. Torsional oscillator (TO) technique is employed to investigate the phases of H in Pd, which is known to show phase boundaries at the lowest T among metal-hydrogen (MH) systems. Specific heat measurements have been performed for PdH _x with x up to high H concentration specimens as well as studies of the resistivity study to establish the unique x−T phase diagram. We have, in addition, been performing TO experiments, in order to study the effect of atomic H intrusion and the dynamics in the PdH(D) _x system. The TO is a well-established, powerful instrument method to investigate superfluidity and quantum vortices of liquid He, especially in thin films, as well as dislocation dynamics in solids. In our TO experiments on PdH _x specimens have shown a resonance frequency shift for PdH _x , with 0.16≤x≤0.75 at the lowest T’s, which can be largely explained by the lattice deformation by H intrusion. This x dependent contribution has a smaller, additional change depending on x and T above ∼50 K. We will show the details of experimental data and discuss the correspondence to the phonon modes change and further possibility of studying occurrence of quantum phenomena for the hydrogen system.