Abstract
Quantum mechanical tunneling of protons is a universal phenomenon commonly seen in a wide variety of physical and biochemical processes1,2. Generally, the proton occupies a double-well potential surface forming hydrogen bond bridges of the form O-H������O, where the tunneling motion between minima is the rate limiting process for protonic transfer and migration. Proton tunneling in these systems strongly depends on the microscopic dynamics of the O-H stretching motion as well as vibrations of the surrounding environment, which alter the shape of the double-well potential3. In particular, crystalline oxides are ideal systems to investigate the energy dynamics of these omnipresent hydroxyl groups, since the associated stretch mode can be excited cleanly and is usually well separated in energy from the phonon bath. Here, we report experimental observation of proton tunneling in the perovskite oxide KTaO3. The tunneling rate is extracted from the vibrational lifetimes of the O-H and O-D stretch modes measured by pump-probe infrared spectroscopy. Both stretch modes are exceptionally long lived and exhibit a large ��reverse�� isotope effect4 due to the phonon-assisted tunneling process, which involves the O-Ta-O bending motion. Our findings suggest that the high protonic conductivity in perovskite oxides is aided by an exceptionally long lifetime of the proton transfer mode. The measured lifetimes presented here have great significance in elucidating the diffusivity of hydrogen in solids and will be a starting point for further investigation of the energy transfer channels of local vibrational modes.