Abstract
Light particles in metals are studied for low temperatures where the nonadiabatic coupling to conduction electrons strongly affects the tunneling between adjacent interstitial sites. Special attention is given to two-level systems formed by trapped hydrogen in niobium. The basic principles governing the tunneling dynamics of such systems are explained and they are applied to determine the dynamic structure factor. The theoretical findings are compared with neutron spectroscopic measurements. These experiments demonstrate a transition from low-temperature coherent tunneling with a well-defined tunneling frequency to hopping with an incoherent tunneling rate at elevated temperatures. The agreement between theory and experiment over a large range of temperature shows the dominant effect of conduction electrons on the motion of light interstitials in metals at low temperatures.
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