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

我们研制了一种用于气相沉积玻璃样品中子散射实验的新型低温恒温器。利用该恒温器在20 K下用5 h制备了厚度为0.15mm的玻璃态丙烯。该样品的中子散射谱是在高能加速器研究组织（KEK）用LAM-40谱仪（Ω <10 meV，Δ Ω =0.2 meV）测量的。一个广泛的非弹性峰，这是所谓的玻色子峰，被发现在3 meV左右的动态结构因子S（Q，ω）。该峰的低能量部分通过在玻璃化转变温度（55 K）附近退火样品而减少。由S（Q，ω）计算的声子态密度与软势模型的结果吻合较好。用LAM-80 ET中子散射谱仪测量了气相沉积玻璃态丙烯和苯的中子散射谱（Ω <1.5meV，Δ Ω =15 μ eV）。在16- 70 K温度范围内测量了弹性峰强度，研究了Debye-Waller因子的温度依赖性。玻璃态样品的对数峰强度（与均方位移<u^2>成正比）比结晶态样品的对数峰强度大两倍以上。这一结果表明，玻璃的振动比晶体的振动软得多。

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.