Characterization of tunneling systems in molecular versus polymer glasses by high-pressure photon echo spectroscopy

摘要

Intrinsic differences between tunneling two-level systems (TLSs) in molecular versus polymeric glasses are revealed by studying the effect of compression on TLS dynamics.Photon echo studies under variable low-temperature (1.1-2.3 K) and high-pressure (0-30 kbar) conditions have been performed to contrast the effect of compression on molecular [2-methyl-tetrahydrofuran (2MTHF)] versus polymer [Polymethylmethacrylate (PMMA)] glasses.The pressure-induced reduction in the magnitude of the optical dephasing rate of rhodamine 640 in a molecular glass (2MTHF) is found to be comparable to the volume decrease of the glass (e.g.,approx 20% at 30 kbar),indicating that TLSs in 2MTHF are associated with void space or low-density regions of the glass.In contrast,the relative pressure insensitivity observed for organic polymer glasses (PMMA) supports the idea that these TLSs are associated with side chain defects.The power-law exponent for the temperature-dependent dephasing in 2MTHF also decreased significantly at high pressure,suggesting a change in the form of the TLS density of states upon compression of the molecular glass.