Quantum Magnetism Beyond Spin Up and Spin Down

秩序在复杂系统中自发产生，例如行星的运动和晶体的周期性结构。理解这种秩序是如何产生的是科学的核心追求。现在，这方面的大量工作集中在电子和原子在接近绝对零度的温度下的行为上，在这种温度下，弱相互作用可以占主导地位，新的物质状态出现。这些新的状态通常是奇异的和意想不到的，例如以粒子的特定排列为特征的磁性状态，或者由于流体中粒子运动的长距离同步性而流过微小收缩而没有阻力的超流体。这些现象最初是为了满足人类理解我们所生活的世界的基本兴趣而研究的，但它们往往会导致技术进步，如计算机硬盘或超导性。由这笔赠款资助的特定项目旨在提高我们对有序如何产生磁性的基本理解。在微观层面上，磁铁粘在冰箱上是因为磁铁中的大部分电子都在同一方向上旋转。为了理解磁性，我们需要了解电子为什么会定向，以及它们显示出什么类型的相关运动。因为这些现象涉及许多相互作用的粒子，所以到目前为止，我们还没有完全理解这个问题。这个小组正在开发新的方法来理解这样的问题。当今物理学研究的一个强大趋势是使用超冷原子作为固体中电子的模拟物，以研究磁性。原子气体比固体更容易研究，因为它们可以在没有任何杂质的情况下制备，并且原子-原子相互作用可以调整。在这项由该奖项资助的工作中，原子将被限制在一个波纹状的光笼中，类似于一个鸡蛋箱。 通过同时使用发射不同颜色光的多个激光器，“蛋箱”的几何细节可以平滑地变形为各种图案，改变原子所在的口袋的形状，甚至将几个口袋合并成一个更大的口袋。 这使得参与这项工作的科学家能够探索为理解真实的固体材料而发展的丰富理论，例如“费米-哈伯德模型”。“在原子气体中，人们可能会发现导致实际应用的新现象，这种对许多原子或电子相互作用集合行为的更好理解可能会给我们提供新的工具来设计具有所需特性的材料。该项目核心的特定系统是锶的费米同位素Sr-87，其核自旋为9/2。这种核自旋类似于传统磁性材料中的电子自旋，但一个电子有10种可能的自旋取向，而不是传统的2种。由于锶是一种闭壳层原子，Sr-87原子之间的相互作用与原子核的取向无关。这在系统的基态中产生了巨大的简并度，预计这将显示与核自旋的对齐和相关性相关的丰富的新现象。实验计划是研究大自旋气体在体系统中的热力学，并探测当单个原子被限制在由光驻波形成的光学晶格势的位置时核自旋的排列。

Order arises spontaneously in complex systems, such as the motions of the planets and the periodic structure of crystals. Understanding how such order arises is a central pursuit of science. A great deal of this effort now focuses on the behavior of electrons and atoms at temperatures near absolute zero, where weak interactions can dominate and new states of matter emerge. These new states are often exotic and unexpected, such as magnetic states that are characterized by specific alignments of particles, or super fluids that flow through tiny constrictions with no resistance because of a long-range synchronicity of particle motion within a fluid. Such phenomena are originally investigated for fundamental interest that satisfies a human drive to understand the world in which we live, but they often lead to technological advances such as computer hard-drives or superconductivity. The particular project funded by this grant aims to improve our fundamental understanding of how order arises to produce magnetism. At a microscopic level, a magnet sticks to a refrigerator because a significant fraction of the electrons in the magnet are spinning or rotating in the same direction. To understand magnetism, one needs to understand why the electrons orient themselves, and what types of correlated motions they display. Because these phenomena involve many interacting particles, a full understanding of this problem has eluded us so far. This group is developing new methods to understand problems like this. A powerful trend in physics research today is to use ultracold atoms as analogs for electrons in solids in order to study magnetism. Atomic gases are much easier to study than solids because they can be prepared without any impurities and the atom-atom interactions can be tuned, for example. In the work funded by this award, atoms will be confined in a corrugated cage of light, analogous to an egg crate. By simultaneously using multiple lasers which emit light of different colors, the geometric details of the "egg crate" can be smoothly morphed into a variety of patterns, changing the shapes of the pockets into which the atoms are nestled, and even merging several pockets into one larger pocket, for example. This allows the scientists involved in this work to explore the richness of theories that have been developed to understand real solid materials, such as the "Fermi-Hubbard model." It is possible that in atomic gases one might discover new phenomena that lead to practical applications, and this improved understanding of the behavior of interacting collections of many atoms or electrons may give us new tools for designing materials with desired properties.The specific system at the heart of this project is the fermionic isotope of strontium, Sr-87, which has a nuclear spin of 9/2. This nuclear spin provides the analog of the electron spin in conventional magnetic materials, but with ten possible spin orientations rather than the conventional two for an electron. Because strontium is a closed-shell atom, interactions between Sr-87 atoms are independent of the orientation of the nucleus. This creates a huge degeneracy in the ground state of the system, which is predicted to display a rich collection of new phenomena related to the alignment and correlation of nuclear spins. The experimental plan is to study the thermodynamics of a large-spin gas in a bulk system and to probe the alignment of nuclear spins when individual atoms are confined to sites of an optical-lattice potential formed by standing waves of light.