Landau-Zener spectroscopy of bulk glasses

Within the thesis at hand, for the first time a measurement technique was developed and established to perform the Landau-Zener spectroscopy of atomic tunneling systems (TSs) in bulk glasses at very low temperatures. Superconducting bridge-type micro-resonators with the sample as substrate were developed that allow a microwave drive under dynamic bias conditions. Beyond dielectric equilibrium measurements, extensively conducted in the past, the novel setup allows to perform measurements in non-equilibrium (Landau-Zener dynamics), which enables a comprehensive study of TSs in bulk glasses with a single device. As sample the borosilicate glass N-BK7 was used which was well-characterized in previous studies and is therefore well-suited for the validation of the setup. Non-equilibrium loss-measurements revealed an average dipole moment of p = 1.5D for the sample, which yields a TS density of P0 = 6.46 × 10^45 1/(Jm^3). A relaxation from the non-equilibrium loss back to the steady-state compatible with a one-phonon process was observed. In two-tone spectroscopy measurements a selective saturation of TSs could be demonstrated and detected through a modified dielectric function. Therewith, a lower limit for the average TS dephasing rate of τ2 ≳ 10 μs was determined. In combination with implemented detailed numerical simulations, the Landau-Zener spectroscopy of bulk glasses provides a new way of systematic TS characterization in amorphous materials at very low temperatures.