Strongly Interacting Fermi Gases of Ultracold Atoms

我们的现代世界是由电子运行的——它们流经我们的智能手机、电脑和机器，执行无数的任务，从数据存储和计算到使用电磁铁时的重物搬运。令人惊讶的是，我们没有更好地了解电子是如何协同工作的。该奖项支持对一种新物质的研究，这种物质是一种由强相互作用的原子组成的超冷气体，其行为在很多方面都像电子。例如，就像金属在低温下变成“超导”并开始无阻力地导电一样，原子气体变成“超流体”，原子在没有摩擦的情况下流动。然而，如果按照金属中电子的密度进行缩放，由于原子间强烈的相互作用，超流体将在远远高于室温的原子气体中发生。就像电子、质子和中子一样，原子属于一种叫做费米子的粒子，它们不能共享同一种状态。这一要求使得计算极其困难，而实验对于了解费米子的行为是必不可少的。原子费米气体被限制在人造的光“盒子”中，将成为了解强相互作用费米子状态方程的原始平台，就像它们出现在现代材料中一样，例如高温超导体，也出现在中子星和核物质中。通过这些和其他的研究，我们可能会对如何实现室温超导有一个了解。这个项目也有可能观察到费米子物质的新状态，比如超固体——一种像晶体一样有序的超流体。这项研究将为研究生提供一个刺激的学习经历。原子的超冷费米气体代表了费米子物质的一种典型形式，其中粒子间相互作用、外部约束和自旋组成的所有细节都是精确已知的，并在实验者的控制之下。该项目采用了锂-6原子的费米气体，试图回答以下几个长期存在的问题：1)二维和三维系统的热力学；2)自旋不平衡下费米子超流体的命运；3)费米子超流体的非平衡动力学。费米气体将被限制在特定的势中，特别是均匀盒势和混合谐波盒势。创造一个均匀的费米气体将消除许多现有的实验限制，以获得准确的热力学信息。盒子势允许进入以前没有观察到的费米子物质的新相。二维的Berezinskii-Kosterlitz-Thouless超流体具有代数顺序，它将被非均匀陷阱所掩盖。存在自旋不平衡的均匀三维费米超流体应该自发地通过形成一列孤子来打破平移对称性，其中多余的费米子位于序参数的节点中。这描述了著名的拉金-奥夫钦尼科夫（LO）态，这是一种物质的超固体相，尽管经过了50年的研究，但仍未被最终观察到。在目前的工作中，将在存在自旋不平衡的情况下直接产生孤子列车，从而设计LO状态。

Our modern world is run by electrons--they flow through our smart phones, computers, and machines to carry out a myriad of tasks from data storage and calculations to heavy lifting when employed in electromagnets. It is surprising that we do not better understand how electrons work together. This award supports studies of a novel substance, an ultracold gas of strongly interacting atoms, which behaves in many ways like electrons. For example, just like a metal becomes "superconducting" at low temperatures and starts to conduct electricity without resistance, the atomic gas becomes "superfluid" and atoms flow without friction. However, scaled to the density of electrons in metals, superfluidity would occur in the atomic gas already far above room temperature, thanks to the strong interatomic interactions. Just like electrons, but also protons and neutrons, the atoms belong to the class of particles called fermions, which cannot share one and the same state. This requirement makes computations extremely difficult and experiments indispensable to learn about the behavior of fermions. Confined in an artificial "box" of light, the atomic Fermi gas will be a pristine platform to learn about the equation of state of strongly interacting fermions, as they occur in modern materials, for example high-temperature superconductors, but also in neutron stars and nuclear matter. With the help of these and other studies, we might be led to an understanding on how to realize room temperature superconductivity. The project also holds the potential for observing new states of fermionic matter such as a supersolid--a superfluid that is also ordered like a crystal. The research will present a stimulating learning experience for graduate students.Ultracold Fermi gases of atoms represent a paradigmatic form of fermionic matter, where all details of the interparticle interaction, the external confinement, and the spin composition are precisely known and under the control of the experimenter. This project employs a Fermi gas of Lithium-6 atoms to try to answer long-standing questions about 1) the thermodynamics of two- and three-dimensional systems, 2) the fate of fermionic superfluidity in the presence of spin imbalance and 3) non-equilibrium dynamics in fermionic superfluids. The Fermi gas will be confined in tailored potentials, in particular a homogeneous box potential and a hybrid harmonic-box potential. Creating a homogeneous Fermi gas will take away many of the existing experimental limitations in obtaining accurate thermodynamic information. The box potential allows accessing new phases of fermionic matter that have not been observed before. The Berezinskii-Kosterlitz-Thouless superfluid in two dimensions features algebraic order that would be masked in an inhomogeneous trap. A homogeneous 3D Fermi superfluid in the presence of spin imbalance should spontaneously break translational symmetry by forming a train of solitons, where excess fermions reside in the nodes of the order parameter. This describes the famous Larkin-Ovchinnikov (LO) state, a supersolid phase of matter that has not been conclusively observed despite five decades of research. In the present work, soliton trains will be directly created in the presence of spin imbalance, thereby engineering the LO state.