Supersolidity and the Supersolid to Normal Solid Transition

技术文摘。块状固体氦和多孔介质中固体氦类超流体行为的观察他的实验室已经被其他十几个实验室复制了。量子蒙特卡罗模拟表明，在完美晶体中不可能存在超流体，但位错线可能是超流体。剪切模量测量表明，固体氦中的超流动性或超固体性是相互连接的位错线的硬化网络的结果。然而，该模型无法解释多孔介质中的结果，因为很难想象几纳米孔径内的位错线。本项目将解决三个问题。一组实验将确定在有很少或没有位错线的完美晶体中是否有可能存在超固态。其次，将进行热容，导热系数测量，以确定多孔介质中的超固体性与散装固体中的超固体性有何不同。最后，扭振测量将用于确定超固体系统中量子相相干的空间范围。实验极具挑战性，为参与本项目的博士后、研究生和本科生提供了良好的训练基地。P.I.在培养学生在学术界、工业和政府实验室成功的科学事业方面有着非常强大和持续的记录。非技术摘要当液氦被冷却到-271摄氏度以下，或绝对零度以上2 K时，它进入零粘度的超流体状态，允许它相对于容器壁流动而没有摩擦。这意味着，如果一个容器来回振荡，里面的超流体将完全静止。和超导性一样，超流动性是量子力学在宏观尺度上的表现。2004年，私家侦探和他以前的一名学生发现了证据，证明在0.2K以下，一小部分（约1%）的固体氦样品在容器振荡时保持静止。这个实验已经在世界各地的十几个实验室重复进行，它表明固体中存在超流动性，更方便地称为超固体。这种物质新状态的发现在物理界内外引起了相当大的兴奋。这种现象的一个突出问题是，在一个完美的氦晶体中，超固态是否可能存在，或者它需要在晶体中存在诸如位错线之类的缺陷。项目中提出的实验就是为了解决这个问题，将由博士后、研究生和本科生共同进行。P.I.在培养学生在学术界、工业和政府实验室取得成功的科学事业方面有着非常强大和持续的记录。

Technical Abstract.The observation of superfluid-like behavior in bulk solid helium and solid helium confined in porous media in the P.I.'s laboratory has been replicated in a dozen other laboratories. Quantum Monte Carlo simulations suggest superfluidity is not possible in a perfect crystal but dislocation lines may be superfluid. Shear modulus measurements indicate that superfluidity in solid helium or supersolidity is the consequence of a stiffened network of interconnected dislocation lines. This model however, cannot explain the results in porous media since it is difficult to envision dislocation lines inside a pore of a few nm. Three questions will be addressed in this project. One set of experiments will determine if supersolidity is possible in a perfect crystal with little or no dislocation lines. Secondly, heat capacity, thermal conductivity measurements will be carried out to determine how supersolidity in porous media is different from that in bulk solid. Lastly torsional oscillator measurements will be made to determine the spatial extent of quantum phase coherence in a supersolid system. The experiments are exceptionally challenging hence providing an excellent training ground for the post-doctoral, graduate and undergraduate students in the project. The P.I. has a very strong and sustained record in training students for successful scientific careers in academia and in industrial and government laboratoriesNon-technical AbstractWhen liquid helium is cooled below -271 Celsius, or 2 K above absolute zero, it enters the superfluid state with zero viscosity allowing it to flow relative to the walls of a container with no friction. What this means is that if a container is being oscillated back and forth, the superfluid inside will sit perfectly still. Like superconductivity, superfluidity is a manifestation of quantum mechanics at macroscopic scales. In 2004, the P.I. and one of his former student found evidence that below 0.2K, a small fraction (on the order of 1%) of a solid helium sample would remain stationary when its container is being oscillated. This experiment, since replicated in a dozen laboratories worldwide, indicates the existence of superfluidity in a solid, more conveniently called supersolidity. The discovery of this new state of matter has created considerable excitement in the physics community and beyond. An outstanding question of the phenomenon is whether supersolidity is possible in a perfect helium crystal or it requires the presence of defects such as dislocation lines in the crystal. The experiments proposed in the project are designed to resolve this question and will be carried out by post-doctoral, graduate and undergraduate students. The P.I. has a very strong and sustained record in training students for successful scientific careers in academia, industrial and government laboratories.