A. Nittke et al., LOW-TEMPERATURE HEAT RELEASE, SOUND-VELOCITY AND ATTENUATION, SPECIFIC-HEAT AND THERMAL-CONDUCTIVITY IN POLYMERS, Journal of low temperature physics, 98(5-6), 1995, pp. 517-547
We have measured the long time (t = 5 to 200 h) heat release of Polyme
thylmethacrylate (PMMA) and Polystyrene (PS) at 70 mK less than or equ
al to T less than or equal to 300 mK. After cooling from om a ''chargi
ng'' temperature of 80 K the heat release in PMMA shows a t(-1) depend
ence in the measured rime and temperature ranges in agreement with the
tunneling model. In contrast, for PS we observe strong deviations fro
m a t(-1) dependence and a factor of ten smaller heat release than in
PMMA in apparent contradiction to specific heat and thermal conductivi
ty data for PS. To compare the heat release with other low temperature
properties and to verify the consistency of the tunneling model we ha
ve measured also the acoustical properties (sound velocity and attentu
ation), the specific heat and the thermal conductivity of PMMA and PS
in the temperature range 70 mK less than or equal to T less than or eq
ual to 100 mK, 70 mK less than or equal to T less than or equal to 200
mK and 0.3 K less than or equal to T less than or equal to 4 K, respe
ctively. We show that the anomalous time dependence of the heat releas
e of PS is due to the thermally activated relaxation of energy states
with excitation energies above 15 K.