Studies of Supersolidity in Solid 4He

* 技术摘要 * 最近，金恩松（Eunsong Kim）和陈摩西（Moses Chan）在低于200 mK的温度下发现了固体4 He中的“超固体”相，这使世纪前的一个预测得以实现。 最初对4 He晶体中超固态的建议是基于在足够低的温度下固体氦中可能发生玻色-爱因斯坦凝聚的可能性。关键是要确定超固态是否代表一种新型的超流体。为了澄清这一问题，将采取几种办法。首先，超固体的流体动力学将在非圆形几何结构中进行研究，其中基于普通超流体流体动力学的预测行为是明确的。第二，多频复合振荡器将被用来区分一个真正的超流体的影响所产生的异常，发现由日和Beamish，在低温性能的弹性剪切模量。第三，存在的热力学相变的超固态的问题将通过测量的预期热容异常与过渡利用超灵敏SQUID技术来解决。该项目预计将涉及一到两名本科生，他们应该从目前凝聚态研究前沿的快速发展领域中获益匪浅。最近在凝聚态物理学中最令人兴奋的发现之一是在仅比绝对零度高零点几度的温度下观察到所谓的“超固体”状态。 这种超固态具有一个显著的特性，即只要温度足够低，一小部分氦固体实际上可以作为无摩擦流通过自身。 超固体的这种性质是违反直觉的，与我们对固体的本能概念不一致。 超固体可能是超流体系统的另一个例子，取代了先前已知的超流体;超流体3 He和4 He，超导电子，以及在低密度捕获原子气体中观察到的超流体。 要解决的最重要的问题之一是，迄今为止观察到的超固体现象是否是固体氦中真正的超流状态，或者它是已知在与超固体现象相同的温度区域出现的固体的不寻常弹性性质的表现。这个研究项目将从几个方面解决这个问题。该方法提出的问题，超流体服从超流体流体力学和弹性的影响可以从那些超流体分离的多频实验？ 第二种方法将检查的性质的相变到超固态的超灵敏的热容量测量利用国家的最先进的超导技术。这些项目将涉及几名本科生，并将使他们在科学研究方面领先一步。

****Technical Abstract****The recent discovery of a "supersolid" phase in solid 4He at temperatures below 200mK by the Eunsong Kim and Moses Chan provides the realization of a prediction made more than a quarter of a century ago. The initial suggestion for a supersolid state in crystalline 4He is based on the possibility that Bose-Einstein condensation might occur in solid helium at sufficiently low temperatures. It is crucial to establish whether or not the supersolid state represents a new type of superfluid. In an attempt to clarify this issue several approaches will be pursued. First, the hydrodynamics of the supersolid will be examined in non-circular geometries where the predicted behavior based on ordinary superfluid hydrodynamics is clear. Second, multiple frequency compound oscillators will be used to distinguish a true superfluid from effects arising from the anomaly, discovered by Day and Beamish, in the low temperature properties of the elastic shear modulus. Third, the question of the existence of a thermodynamic phase transition to the supersolid state will be addressed through measurement of the expected heat capacity anomaly associated with the transition utilizing ultra sensitive SQUID technology. This project is expected to involve one to two undergraduate students who should profit greatly from working in a fast evolving area at the current forefront of condensed matter research.****Non-Technical Abstract****One of the most exciting recent discoveries in condensed matter physics has been the observation of the so-called "supersolid" state at temperatures only a few tenths of a degree above absolute zero. This supersolid state has the remarkable property that a fraction of a helium solid can actually pass through itself as a frictionless flow provide the temperature is low enough. This property of the supersolid is counterintuitive and is at odds with our instinctive concept of a solid. The supersolid may be another example of a superfluid system to take its place beside the previously know superfluids; superfluid 3He and 4He, superconducting electrons, and the superfluids observed in low density trapped atomic gases. One of the most important questions to be addressed is whether the supersolid phenomena thus far observed is actual a true superfluid state in solid helium or is it a manifestation of the unusual elastic properties of the solid know to appear in the same temperature region as the supersolid phenomenon. This research project will address this question on several fronts. The approach asks the question, does the superfluid obey superfluid hydrodynamics and can the effects of elasticity be separated from those of a superfluid by multi-frequency experiments? The second approach will examine the nature of the phase transition to the supersolid state by a ultra sensitive heat capacity measurement utilizing state-of-the-art superconducting technology. These projects will involve several undergraduate students and will give them a head start in scientific research.