Ultrasonic Study of Hydrogen-Metal and Other Condensed Matter Systems

9501550休闲超声波方法将被用来研究氢同位素在稀土金属中的运动，可能还有其他金属。这些研究的目的将是阐明这些材料中氢的不寻常动力学。实验将在含氢和/或氢的钪、钚和钇单晶上进行。实验结果将被用来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置的最低振动能级之间的运动可视为一个二能级系统(TLS)。通常的理论假设TLS与环境的弱耦合，因此TLS的量子相干性不会被破坏。最近人们意识到，通常的理论受到严格限制，因为在许多情况下，TLS与金属中的电子或任何材料中较高温度下的声子的耦合，破坏了量子相干性。现代的耗散量子隧穿理论并不局限于与电子或声子的弱耦合。新的超声波技术将被应用于其他几个问题。共振超声光谱学将被用来测量经历相变的材料的全套弹性常数。随着10kbar压力室的建立，将在压力下测量小样品的全套弹性常数。100 khz至100 mhz(超声波)频率范围的声波将被用来研究氢离子在几种稀土金属中的运动。氢在这些金属中具有很高的流动性，通过在宿主金属晶格提供的间隙位置之间跳跃来移动。氢的运动受氢离子通过热振动和主体金属的传导电子与金属原子相互作用的影响。氢气的运动会影响超声波的速度和能量损失，而这些都是被测量的量。低温氢的运动不能用经典物理来理解，但需要量子力学的描述。氢通过量子力学隧穿在附近的间隙位置之间移动，隧穿粒子受到晶格热振动和传导电子相互作用的强烈影响。由此产生的运动被称为耗散量子隧道。耗散量子隧穿理论能否定量描述氢同位素在金属中的低温运动是一个悬而未决的问题，这也是本研究要回答的问题。这项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受压力的材料的弹性性质。在高压下同时测量全套弹性常数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. ***