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的中子散射实验的新型低温恒温器。用该恒温器在20K下保温5h，可制备出厚度为0.15 mm的玻璃态丙烯。该样品的中子散射谱是在高能加速器研究机构(KEK)用LAM-40能谱仪(10 meV，DELTAomega=0.2 meV)测量的。在动态结构因子S(q，omega)的3 meV附近，发现了一个宽的非弹性峰，称为玻色子峰。通过在玻璃化转变温度(55K)附近对样品进行热处理，降低了该峰的低能部分。用软势模型很好地再现了由S(q，欧米伽)计算的声子态密度。结果表明，分子玻璃的低能激发与玻璃结构中无序和应变引起的局域势能软化有关。用LAM-80Et能谱仪测量了气相沉积的丙烯和苯的中子散射谱(omega&lt；1.5 meV，DELTAomega=15 mueV)。在16-70K温度范围内测量了弹性峰强度，研究了德拜-沃勒因子与温度的关系。玻璃样品的对数峰强度与均方位移成正比，是晶态样品的两倍多。这一结果表明，玻璃的振动比晶体的振动小得多。

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.