Experimental Studies at Ultralow Temperatures

超低温实验研究

• 批准号: 0307382
• 负责人: Hans Bozler
• 金额: $ 46.5万
• 依托单位: University of Southern California
• 依托单位国家: 美国
• 项目类别: Continuing Grant
• 财政年份: 2003
• 资助国家: 美国
• 起止时间: 2003-07-01 至 2007-06-30
• 项目状态: 已结题
• 来源: https://www.nsf.gov/awardsearch/showAward?AWD_ID=0307382&HistoricalAwards=false
• 关键词: Experimental; Studies; Ultralow; Temperatures

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.***

本文主要研究低温量子流体和固体性质中的两个问题：He-3薄膜的二维基态和气凝胶中高度无序He-3的超流态。 这些问题是完全不同的，但共享超低温制冷技术和使用SQUID的宽带NMR。他们也直接处理这些系统中的相变性质。在He-3薄膜的情况下，二维He-3在非常低的场中的有序性质的检查将需要改进的衬底，并且直接解决在零场下有限温度相变存在的争议问题。使用SQUID核磁共振技术允许这些薄膜的研究在很宽的领域，包括零场limit.The第二个努力将限制识别的超流序参量在杂质的存在下，通过使用纵向共振，探针，这是必不可少的originalidentification的散装superfluid phase。在这种情况下，SQUID NMR技术将用于寻找预测的纵向NMR共振。这个实验将测试标准的图片，其中“脏”超流体被视为只是其“干净”对应物的抑制版本。下一代高性能SQUID系统的仪器设备超越了SQUID NMR和毫度低温技术的商业技术水平，为博士研究生的培训提供了一个很好的工具。和硕士水平，以及本科生。%该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极好的模型，这些物理行为在更常规的温度下具有模拟性。 He-3系统的优点是它们相对简单，使它们能够在理论上得到更大程度的理解，从而提供了理论和实验之间的联系，这是推进凝聚态系统理解所必需的。 第一个实验涉及到He-3薄膜的研究，这些薄膜的行为就像是二维的。 有一些众所周知的理论（在一组模型中，相互作用是各向同性的）说，这些薄膜不能在绝对零度以上磁性有序，因为它们是二维的。另一方面，非常小的各向异性畸变可能具有高度不成比例的影响，导致磁相变。这项研究的早期版本表明，情况可能是这样的，但是在完全理解这个系统之前，仍然有许多问题必须回答。第二个项目涉及液体He-3中超流体的性质。这种液体本质上是已知的最纯净的液体，但将其放入气凝胶中会引入无序（污垢），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 NMR来研究。这种NMR技术对于这些实验具有许多优点。 它也有潜在的应用范围广泛的非破坏性测试和医学成像和诊断。这些实验项目提供了一个很好的方式来培养研究生和本科生在各种各样的实验和分析技术。