Effect of hydrostatic pressure on irreversible thermal transformations   in a polymer glass at low temperatures
  Irreversible broadening of spectral holes in chlorin-doped polystyrene glass was studied for the first time in the temperature cycling experiments under high pressure (by raising the temperature from 5 K to various magnitudes up to 18 K and turning back to 5 K at several fixed pressures between 0 and 5 kbar). At all pressures the increment in the hole width observed after completing a temperature cycle exhibits a slightly superlinear (proportional to T to the power 3/2) dependence on the cycling temperature. The magnitude of this increment is essentially reduced under high pressure (e.g., at 4.9 kbar it makes up less than 2/3 of its initial value obtained at ambient pressure). The residual broadening of holes is interpreted as a result of irreversible thermally induced spectral diffusion arising from interaction of the electronic transition in a dopant molecule with two-level systems (TLSs) which perform thermally activated overbarrier jumps between two possible states of a TLS. The pressure effects are treated theoretically within the scope of the soft anharmonic potential model with asymmetric distribution of the cubic anharmonicity parameter. It is shown that more abundant are TLSs with such double-well potentials where the minimum, corresponding to a larger glass volume, is placed at a higher energy than the minimum, corresponding to a smaller volume. In this case, applied pressure reduces the number of almost symmetric TLSs (having very small energy difference between the two minima), which provide the largest contribution to the residual broadening of spectral holes after a temperature cycle. Our earlier results on isothermal hole burning at various fixed pressures [V. Hizhnyakov et al., Phys. Rev. B 62, 11296 (2000)] also qualitatively fit into this picture. 
J. Takahashi, A. Suisalu, An. Kuznetsov, A. Laisaar, V. Hizhnyakov, J.   Kikas