Experimental Studies at Ultralow Temperatures

本文主要研究了低温量子流体和固体性质中的两个问题：氦-3薄膜的二维基态和气凝胶中高度无序的氦-3的超流态。这些问题完全不同，但共享超低温制冷技术和使用squid进行宽带核磁共振。它们还直接处理这些系统中相变的性质。在氦-3薄膜的情况下，二维氦-3在极低场中的有序性质的检验将需要改进衬底，并直接解决零场存在有限温度相变的争议问题。使用SQUID核磁共振技术可以在包括零场限制在内的广泛领域研究这些薄膜。第二次努力将通过使用纵向共振来限制在杂质存在下超流体序参量的识别，纵向共振是一种在原始的体超流体相识别中必不可少的探针。在这种情况下，SQUID核磁共振技术将用于寻找预测的纵向核磁共振。这个实验将检验一种标准的观点，即“脏”超流体仅仅被视为“干净”超流体的抑制版本。下一代高性能SQUID系统的仪器设备超越了商业最先进的SQUID核磁共振技术和毫度低温技术，为培养博士和硕士研究生以及本科生提供了极好的工具。这个项目的重点是在接近绝对零度的温度下发生的有趣的基础物理学。He-3系统为广泛的物理行为提供了良好的模型，具有更传统的温度模拟。He-3系统的优势在于它们相对简单，这使得它们可以在更大程度上从理论上理解，从而提供了理论和实验之间的联系，这是推进对凝聚态系统的理解所需要的。第一个实验涉及研究表现为二维的He-3薄膜。有一些众所周知的理论（在一组相互作用是各向同性的模型中）认为，由于这些薄膜的二维性，它们在绝对零度以上的磁性上不能有序。另一方面，非常小的各向异性畸变可能会产生高度不成比例的影响，从而导致磁相变。这项研究的早期版本表明，情况可能是这样的，然而，在完全理解这个系统之前，仍有许多问题必须得到回答。第二个项目研究He-3液体中超流体的性质。这种液体本质上是已知最纯净的液体，但将其放入气凝胶中会产生引入无序（污垢）的效果，从而改变有序状态。无序介质中这种有序状态的性质将用SQUID核磁共振进行研究。这种核磁共振技术在这些实验中具有许多优点。它还具有广泛的潜在应用，如无损检测、医学成像和诊断。这些实验项目为培养研究生和本科生各种各样的实验和分析技术提供了一个很好的方法

This work focuses on two problems in the properties of low temperature quantum fluids and solids: the two dimensional ground state of thin films of He-3, and the superfluid state of highly disordered He-3 contained in aerogel. These problems are quite different, but share both the techniques of ultralow temperature refrigeration and the use of SQUIDs for wide band NMR. They also deal directly with thenature of phase transitions in these systems. In the case of He-3 films, the examination of the nature of ordering by two-dimensional He-3 in very low fields will demand an improved substrate, and directlyaddress the controversial issue of the existence of a finite temperature phase transition at zero field. Using SQUID NMR techniques allows the study of these films over a wide range of fields including the zero field limit.The second effort will constrain the identification of the superfluid order parameter in the presence of impurities by the use of longitudinal resonance, a probe which was essential in the originalidentification of the bulk superfluid phases. In this case, SQUID NMR techniques will be used to look for the predicted longitudinal NMR resonance. This experiment will test the standard picture where the "dirty" superfluids are seen as merely suppressed versions of their "clean" counterparts. Next-generation instrumentation of high performance SQUID systems surpassing commercial state of the art and techniques for SQUID NMR and the millidegree low temperature techniques produce an excellent vehicle for training graduate students at the Ph.D. and Master's level, as well as undergraduate students.%%%The project centers on interesting fundamental physics that occurs at temperatures close to absolute zero. He-3 systems provide excellent models for a wide range of physical behavior that has analogsat more conventional temperatures. The advantage of the He-3 systems is their relative simplicity allowing them to be understood theoretically to a much greater degree, and thus provide the connection between theory and experiment that is needed to advance the understanding of condensed matter systems. The first experiment involves the study of He-3 films that behave as though they are two-dimensional. There are well known theories (within a set of models where interactions are isotropic) that say these films cannot order magnetically above absolute zero because of their two-dimensionality. On the other hand, very small anisotropic distortions may have a highly disproportionate effect resulting in magnetic phase transitions. An earlier version of this study showed that this may be the case, however there remain many questions thatmust be answered before this system can be fully understood. The second project deals with the nature of the superfluid in liquid He-3. This liquid is intrinsically the purest liquid known, but placing it inside aerogel has the effect of introducing disorder (dirt) that modifies the ordered state. The nature of this ordered statewithin the disordered medium will be studied using SQUID NMR. This NMR technique has many advantages for these experiments. It also has potential applications as wide ranging as non-destructive testing andmedical imaging and diagnostics. These experimental projects provide an excellent way to train graduate and undergraduate students in a wide variety of experimental and analytical techniques.***