Ultrasonic Study of Hydrogen-Metal and Other Condensed Matter Systems

超声波方法将用于研究稀土金属中氢同位素的运动，也可能用于研究其他金属。这些研究的目的将是阐明这些材料中氢的不寻常动力学。实验将在含有氢和/或氘的钪、镥和钇单晶上进行。实验结果将用于检验固体中量子扩散的现代理论。氢离子在金属中邻近间隙位的最低振动能级之间的运动可以看作是一个双能级系统（TLS）。通常的理论假设TLS与环境弱耦合，因此TLS的量子相干性不会被破坏。最近人们认识到，通常的理论受到了严重的限制，因为在许多情况下，TLS与金属中的电子或任何材料中高温下的声子的耦合会破坏量子相干性。耗散量子隧穿的现代理论并不局限于与电子或声子的弱耦合。新的超声技术将应用于其他几个问题。共振超声光谱学将用于测量相变材料的完整弹性常数。利用10kbar压力箱的构造，测量了小样品在压力作用下的完整弹性常数。100千赫到100兆赫的声波（超声波）将被用来研究几种稀土金属中氢离子的运动。氢在这些金属中是高度可移动的，通过在主金属晶格提供的间隙点之间跳跃来移动。氢离子与宿主金属原子的相互作用通过宿主金属的热振动和传导电子来影响氢的运动。氢的运动影响超声波的速度和能量损失，这是测量量。低温氢的运动不能用经典物理学来理解，而需要量子力学的描述。氢通过量子力学隧道在附近的间隙点之间移动，并且隧道粒子受到晶格热振动和传导电子相互作用的强烈影响。由此产生的运动称为耗散量子隧穿。耗散量子隧穿理论能否定量描述金属中氢同位素的低温运动是一个悬而未决的问题，这也是本研究拟回答的问题。工作的第二部分将涉及使用新的超声波技术来测量经历相变的材料和压力下的材料的弹性特性。同时测量高压下完整弹性常数集的能力将是独一无二的。＊＊＊

9501550 Leisure Ultrasonic methods will be used to study hydrogen isotope motion in the rare-earth metals, and possibly other metals as well. The objectives of these studies will be to elucidate the unusual dynamics of hydrogen in these materials. Experiments will be performed on scandium, lutetium, and yttrium single crystals containing hydrogen and/or deuterium. The experimental results will be used to test modern theories of quantum diffusion in solids. The movement of hydrogen ions between the lowest vibrational levels of nearby interstitial sites in a metal may be regarded as a two-level system (TLS). The usual theory assumes weak coupling of the TLS to the environment, so that the quantum coherence of the TLS is not destroyed. It has been realized recently that the usual theory is severely restricted because, in many cases the coupling of the TLS to electrons in metals, or to phonons at higher temperatures in any material, destroys the quantum coherence. Modern theories of dissipative quantum tunneling are not restricted to weak coupling to the electrons or phonons. Novel ultrasonic techniques will be applied to several other problems. Resonant ultrasound spectroscopy will be used to measure the complete set of elastic constants of materials undergoing phase transitions. With the construction of a 10 kbar pressure cell, the complete set of elastic constants of small samples will be measured under pressure. %%% Sound waves in the frequency range 100 kHz to 100 Mhz (ultrasound) will be used to study the motion of hydrogen ions in several rare- earth metals. Hydrogen is highly mobile in these metals and moves by hopping among the interstitial sites provided by the host metal lattice. The hydrogen motion is affected by the interactions of the hydrogen ions with the host metal atoms via tbe thermal vibrations and conduction electrons of th e host metal. Hydrogen motion affects the speed and energy loss of the ultrasonic waves which are the measured quantities. Low-temperature hydrogen motion cannot be understood using classical physics, but requires a quantum-mechanical description. The hydrogen moves between nearby interstitial sites by quantum-mechanical tunneling, and the tunneling particle is strongly affected by interactions with both the lattice thermal vibrations and conduction electrons. The resulting motion is called dissipative quantum tunneling. It is an open question as to whether the theory of dissipative quantum tunneling can quantitatively describe the low temperature motion of hydrogen isotopes in metals, and it is this question which the proposed study intends to answer. A second part of the work will involve the use of novel ultrasonic techniques to measure the elastic properties of materials undergoing a phase change and of materials under pressure. The ability to measure simultaneously the complete set of elastic constants at high pressure will be unique. ***