Superfluidity in a Two-Dimensional Bose Gas with Tuneable Interactions

具有可调相互作用的二维玻色气体中的超流动性

• 批准号: EP/I010580/1
• 负责人: Zoran Hadzibabic
• 金额: $ 71.07万
• 依托单位: University of Cambridge
• 依托单位国家: 英国
• 项目类别: Research Grant
• 财政年份: 2011
• 资助国家: 英国
• 起止时间: 2011 至 无数据
• 项目状态: 已结题
• 来源: https://gtr.ukri.org/projects?ref=EP%2FI010580%2F1
• 关键词: Superfluidity; Two; Dimensional; Bose; Gas

When a gas of identical atoms about million time thinner than air is cooled down to extremely low temperatures, about a millionth of a degree above absolute zero, quantum mechanical effects become important. At these temperatures the gas can exhibit various exotic phases of matter, which could have practical applications in quantum computation and precision sensors. Further, these phases are in many ways universal - analogous phenomena occur in a range of other physical systems, including liquid helium, exotic solid state materials such as superconductors, and even neutron stars. The atomic gases are often much easier to manipulate in the laboratory than those other physical systems, allowing us to study fundamental many-body physics in a highly controlled environment. This could eventually also allow the design of better real materials for practical applications. For example materials which would be superconducting (i.e. transmit current without any losses) at room temperature would allow dramatic energy savings.Central to the understanding of the physics of ultracold gases are the concepts of Bose-Einstein condensation and superfluidity. The former refers to the accumulation of a large fraction of atoms in a single quantum mechanical state, as predicted by Einstein in 1925 and finally directly observed in an atomic gas in 1995. The latter refers to the fascinating ability of the gas to flow without any friction. The two concepts have been conceptually linked ever since the discovery of superfluidity in liquid helium in 1937, but they are nevertheless clearly distinct and their exact quantitative connection is often elusive. In particular, in many of the most interesting physical situations the superfluid and the condensed fraction of the gas can be very different. Such situations for example include systems in which the interactions between the particles are very strong, or gases which do not move in the standard three-dimensional world, but are confined to only two or one dimension. Two-dimensional physics is also believed to be at the heart of the still not understood phenomenon of high-temperature superconductivity.In this work we will address two essential outstanding issues in the field of ultracold atomic gases:First, we will develop a two-dimensional (2D) atomic gas in which the strength of interactions between the particles will be tunable by applying an external magnetic field. This will allow us to perform the first comprehensive study of the so-called Berezinskii-Kosterlitz-Thouless (BKT) superfluid transition. This transition is fascinating because contrary to the usual intuition in 2D superfluidity can occur in the absence of any Bose-Einstein condensation. The signatures of the BKT transition were first observed in liquid helium films, and its microscopic origin was most directly confirmed in an atomic gas, in our recent work. However many issues remain open, in particular because this transition is expected to fundamentally depend on the strength of interactions among the particles, and in no previous 2D experiment could this strength be controllably tuned.Second, based on our recent theoretical proposal, we will develop experimental methods for a direct measurement of the superfluid density of an atomic gas. The fundamental physics is often universal, but the experimental methods of different sub-fields are often very different. So in liquid helium the superfluid density is routinely measured, but the condensed density is difficult to extract. On the other hand, ultracold atomic gases are celebrated for the direct observation of Bose-Einstein condensation, but a direct measurement of their superfluid density remains elusive. Our work will open the possibility for the two quantities to be measured and directly compared in a variety of physical situations in the same experimental system.

当由相同原子组成的气体比空气薄约一百万倍时，被冷却到极低的温度（比绝对零高约百万分之一度）时，量子力学效应变得很重要。在这些温度下，气体可以表现出各种奇异的物质相，这可能在量子计算和精密传感器中具有实际应用。此外，这些相在很多方面都是普遍存在的——类似的现象也发生在一系列其他物理系统中，包括液氦、超导体等奇异固态材料，甚至中子星。原子气体通常比其他物理系统更容易在实验室中操纵，使我们能够在高度受控的环境中研究基础多体物理学。这最终还可以为实际应用设计更好的真实材料。例如，在室温下超导（即传输电流而没有任何损耗）的材料将能够显着节省能源。理解超冷气体物理学的核心是玻色-爱因斯坦凝聚和超流性的概念。前者指的是大部分原子在单一量子力学状态下的积累，正如爱因斯坦在 1925 年预测的那样，并最终于 1995 年在原子气体中直接观察到。后者指的是气体在没有任何摩擦的情况下流动的迷人能力。自从 1937 年发现液氦的超流动性以来，这两个概念在概念上就一直存在联系，但它们仍然是明显不同的，而且它们确切的定量联系往往难以捉摸。特别是，在许多最有趣的物理情况下，超流体和气体的冷凝部分可能非常不同。例如，这种情况包括粒子之间相互作用非常强的系统，或者在标准三维世界中不移动但仅限于二维或一维的气体。二维物理学也被认为是尚未被理解的高温超导现象的核心。在这项工作中，我们将解决超冷原子气体领域的两个重要的突出问题：首先，我们将开发一种二维（2D）原子气体，其中粒子之间相互作用的强度将通过施加外部磁场来调节。这将使我们能够对所谓的 Berezinskii-Kosterlitz-Thouless (BKT) 超流体转变进行首次全面研究。这种转变非常有趣，因为与二维超流性的通常直觉相反，在没有任何玻色-爱因斯坦凝聚的情况下也可能发生这种转变。在我们最近的工作中，BKT 跃迁的特征首先在液氦薄膜中观察到，其微观起源在原子气体中得到了最直接的证实。然而，许多问题仍然悬而未决，特别是因为这种转变预计从根本上取决于粒子之间相互作用的强度，并且在以前的二维实验中无法可控地调整这种强度。其次，根据我们最近的理论建议，我们将开发直接测量原子气体超流体密度的实验方法。基础物理学往往是通用的，但不同子领域的实验方法往往有很大不同。因此，在液氦中，超流体密度是常规测量的，但凝聚密度很难提取。另一方面，超冷原子气体因直接观察玻色-爱因斯坦凝聚而闻名，但对其超流体密度的直接测量仍然难以实现。我们的工作将为在同一实验系统中的各种物理情况下测量和直接比较这两个量提供可能性。

项目成果

Adiabatic preparation of vortex lattices

• DOI: 10.1103/physreva.88.033603
• 发表时间: 2013-06
• 期刊: Physical Review A
• 影响因子: 2.9
• 作者: S. Baur;N. Cooper

Dynamic optical superlattices with topological bands

• DOI: 10.1103/physreva.89.051605
• 发表时间: 2014-05-30
• 期刊: PHYSICAL REVIEW A
• 影响因子: 2.9
• 作者: Baur, Stefan K.;Schleier-Smith, Monika H.;Cooper, Nigel R.
• 通讯作者: Cooper, Nigel R.

Collective modes of a two-dimensional spin-1/2 Fermi gas in a harmonic trap

谐波陷阱中二维自旋 1/2 费米气体的集体模式

• DOI: 10.48550/arxiv.1301.0358
• 发表时间: 2013
• 作者: Baur S

Radio frequency spectra of Feshbach molecules in quasi-two dimensional geometries

准二维几何中 Feshbach 分子的射频谱

• DOI: 10.48550/arxiv.1204.2704
• 发表时间: 2012
• 作者: Baur S

Radio-frequency spectra of Feshbach molecules in quasi-two-dimensional geometries

准二维几何中 Feshbach 分子的射频光谱

• DOI: 10.1103/physreva.85.061604
• 发表时间: 2012
• 期刊: Physical Review A
• 影响因子: 2.9
• 作者: Baur S