Anomalous Isotopic Effect of Tunneling States in NbTi-H/D

The thermal conductivity, heat capacity and heat release of NbTi, NbTi-H and NbTi-D were measured at low temperatures. All three systems reveal low temperature anomalies typical for structural glasses. It is shown that above a concentration of 2\% H or D the tunneling states of the NbTi matrix disappear. Therefore, for higher concentration it is a good system to proof how the change of the mass of tunneling atoms influences the low temperature anomalies. For the heat capacity we found the expected isotopic effect. However, the anomalous isotopic effect observed for the heat release data cannot be explained within the standard tunneling model. A surprising result is that tunneling systems with very high barrier heights, even the systems with the maximum barrier height, influence remarkable the heat capacity and the heat release data at low temperatures. As a possible origin, we consider large-scale fluctuations in thermal expansion which could generate anomalous two-level systems.