(1986) Mebert, Joachim; Koblinger, Otto; Döttinger, Siegfried; Eisenmenger, Wolfgang
In this work we show that in the presence of a strong phonon scattering background absorption structures can only be well resolved by reducing sample thickness to the phonon mean free path. This mean free path can be determined by analyzing the pulse shape of 285 GHz phonons. By reducing sample thickness to the appropriate value of 0,3mm the 21,2 cm -1 crystalline field transition in CaF 2:Er 3+ could be evaluated with the very high resolution of 5 GHz. In experiments performed on a 1mm thick LaF 3 Er 3+ sample we observed an absoption line at 14,2 cm -1 not visible in FIR absorption measurements.
Summarizing, we observed strongly enhanced phonon transport from a real solid into 4 He starting at about 85 GHz which is independent of the He gas pressure up to liquid helium. With the isotope 3 He the onset frequency for enhanced phonon transport shifts to 100 GHz. The threshold frequencies are almost independent of the He temperature in the range between 0.5 K and 2.3 K. Only slight shifts to lower frequencies with decreasing He temperature have been observed. No significant influence of the generator material (Sn, Al, SiO) or the generator preparation on the onset frequency has been observed. Moreover at low temperatures and thin He films we observed an additional structure which might be interpreted as film thickness resonances in the He film layer on the generator.
