Neutron Scattering Study on Low-Energy Excitations in Vapor-Deposited Molecular Glasses

点击翻译按钮获取中文摘要

We have developed a novel cryostat designed for the neutron scattering experiments on vapor-deposited glassy samples. By using this cryostat, glassy propylene of 0.15mm thickness was prepared at 20 K taking 5h. Neutron scattering spectra of this sample was measured with LAM-40 spectrometer (omega<10 meV,DELTAomega=0.2 meV) at High Energy Accelerator Research Organization (KEK). A broad inelastic peak, which is called a boson peak, was found at around 3 meV in dynamic structure factor S (Q,omega). The low energy part of this peak was reduced by annealing the sample at around the glass transition temperature (55 K). The phonon density of states calculated from S (Q,omega) was reproduced well by the soft potential model. The present results indicate that the low-energy excitation of molecular glasses is associated with the local potential softened by disorder and/or strain in the structure of glass.The neutron scattering spectra of vapor-deposited glassy propylene and benzene was measured by LAM-80ET spectrometer (omega<1.5 meV,DELTAomega=15 mueV). The elastic peak intensity was measured in the temperature range 16-70K to investigate the temperature dependence of the Debye-Waller factor. The logarithmic peak intensity, proportional to the mean square displacement<u^2>, of the glassy sample was more than two times larger than that of the crystalline sample. This result indicates that the vibrations of the glass is much softer than that of the crystal.