Phonon scattering and internal friction in dielectric and metallic films at low temperatures
Phonon scattering and internal friction in dielectric and metallic films at low temperatures
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低温下电介质和金属薄膜中的声子散射和内摩擦
DOI：
10.1103/physrevb.63.125421
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发表时间：
2000
影响因子：
3.7
通讯作者：
R. Pohl
中科院分区：
文献类型：
--
作者：
P. Vu;Xiao Liu;R. Pohl
我们测量了高纯硅片抛光表面上电子束非晶硅、分子束外延（MBE）硅、电子束多晶CaF_{2}、$和MBE ${\mathrm {CaF}}_{2}、$介质薄膜以及电子束铝、溅射铝合金5056、电子束Ti和电子束Cu。使用Monte Carlo模拟分析的导电测量，我们已经确定了薄膜内的声子平均自由程，并发现所有这些都要短得多，甚至比在典型的散装非晶固体，没有例外。我们还测量了这些薄膜的内部摩擦低于10 K，并发现它们的内部摩擦在低温下是惊人的接近非晶固体，无论是在大小和温度的独立性，与MBE硅和合金Al 5056，其内部摩擦甚至比非晶固体小得多的例外。利用隧穿模型的内耗结果表明，在这些薄膜中的声子散射应该比实际观察到的要小得多。因此，我们得出结论，热传导测量不支持这些薄膜的晶格振动是玻璃状的这一观点，正如早先仅根据低温内摩擦测量对薄金属薄膜所推测的那样[Phys. Rev. B $59，$11 767（1999）]。最值得注意的是，分子束外延硅薄膜具有和其他薄膜一样强的热声子散射，而理想薄膜的内耗可以忽略不计。在这种薄膜中引起热声子强散射的无序是完全未知的。
We have measured the heat conduction between 0.05 and 1.0 K of high-purity silicon wafers carrying on their polished faces thin dielectric films of e-beam amorphous Si, molecular beam epitaxial (MBE) Si, e-beam polycrystalline ${\mathrm{CaF}}_{2},$ and MBE ${\mathrm{CaF}}_{2},$ and polycrystalline thin metallic films of e-beam Al, sputtered alloy Al 5056, e-beam Ti, and e-beam Cu. Using a Monte Carlo simulation to analyze the conduction measurements, we have determined the phonon mean free path within the films, and found all of them to be much shorter even than in typical bulk amorphous solids, with no exceptions. We have also measured the internal friction of these films below 10 K, and found their internal friction at low temperatures to be strikingly close to that of amorphous solids, both in magnitude and temperature independence, with the exception of the MBE Si and alloy Al 5056, whose internal friction is even much smaller than that of amorphous solids. Using the tunneling model the internal friction results indicate the phonon scattering in these thin films ought to be much smaller than is actually observed. Thus we conclude that heat conduction measurements do not support the picture that the lattice vibrations of these films are glasslike, as had been surmised earlier for thin metallic films, on the basis of low-temperature internal friction measurements alone [Phys. Rev. B $59,$ 11 767 (1999)]. At the least, the films must contain additional scattering centers which lead to the very small phonon mean free path. Most remarkably, the MBE Si shows the same strong scattering of thermal phonons as do other films, while having the negligible internal friction expected for a perfect film. The disorder causing the strong scattering of the thermal phonons in this film is completely unknown.