Quantum Phase Transitions and Quantum Criticality in Helium Films

从历史上看，量子流体，即接近绝对零度的氦液体，提供了简单的模型系统，在凝聚态物理中关键概念的发展中发挥了关键作用。对超流性和破缺规范对称性的理解；关联费米子标准模型的发展；第一个非传统超流/超导体；拓扑激发在二维物理中的中心作用：所有这些发现和见解都源于对氦的研究。量子流体的研究也推动了产生和测量低温、强磁场的技术的发展，以及一系列新的测量技术和仪器。我们建议研究各种低维氦模型系统，以解决理解强关联量子物质的基本问题。我们将研究氦-3(费米子)膜和氦-4(玻色子)膜。这些薄膜在原子平坦的石墨表面上以原子层的形式生长，而氦层所经历的晶格势可以产生三角形的超晶格结构。这些层的密度基本上可以连续变化，以在不同的量子力学基态之间进行调节。这些可能包括理论上提出但尚未明确实现的基态。我们将更详细地研究不同基态之间的量子相变。我们将研究二维氦-3费米液体和二维量子自旋液体之间的Mott跃迁，以及三角形晶格上空穴掺杂自旋液体的性质。我们将尝试稳定正方形格子上的Mott绝缘体，并进行类似的实验。在相应的氦-4薄膜中，我们将研究超流-绝缘体的转变，并研究可能的2D超固态行为。我们将开发一种高度有序的石墨衬底，以期优化条件，在此条件下寻找氦-3流体单层中非传统超流动性的圣杯。我们将研究我们最近发现的氦-3双层重费米子系统的量子临界性。我们将把纳米通道中的氦-3作为一维费米子系统来研究，以及可能实现的Luttinger液体。这些关于费米子和玻色子冷原子的实验是在均匀的低维系统上进行的，在热力学平衡下，在200微开尔文到4K的范围内精确测量温度。最低温度将由我们实验室的核绝热退磁低温恒温器产生。将使用一系列高精度的实验探头来研究这些系统。我们实验室开发的敏感核磁共振技术，基于超导量子干涉器件(SQUID)对进动磁信号的检测，将用于测量磁化率、磁化强度和自旋动力学。我们将把热容的测量扩展到最低温度，以便获得系统的熵并探索元素的激发。超流密度和响应的任何耗散分量将由高质量的扭转机械谐振器测量。我们将合作开发基于石墨烯的具有宽带SQUID放大器检测的纳米机械谐振器。该项目预计将导致对强关联物质物理学中一些最核心的问题的基本见解，并对理解更复杂的具有潜在技术意义的材料产生影响。该项目将在一个重要的科学前沿--低温前沿推动新仪器和测量技术的创新。就像在任何前沿科学中一样，我们可能会遇到意想不到的事情。

Historically quantum fluids, the helium liquids near absolute zero, have provided simple model systems which have played a crucial role in the development of key concepts in condensed matter physics. The understanding of superfluidity and broken gauge symmetry; the development of the standard model of correlated fermions; the first unconventional superfluid/superconductor; the central role of topological excitations in two dimensional physics: all these discoveries and insights arose from the study of helium. The study of quantum fluids has also fuelled developments in techniques for producing and measuring low temperatures, high magnetic fields, and a host of novel measurement techniques and instrumentation. We propose to study a variety of low dimensional helium model systems to address fundamental issues in the understanding of strongly correlated quantum matter. We will study helium-3 (fermion) films and helium-4 (boson) films. These films grow as atomic layers on the atomically flat surface of graphite, and the lattice potential experienced by a helium layer can give rise to a triangular superlattice structure. The density of these layers can be varied essentially continuously to tune between different quantum mechanical ground states. These may include ground states theoretically proposed but yet to be unambiguously realized. We will study the quantum phase transitions between different ground states in some detail. We will study the Mott transition between a 2D helium-3 Fermi liquid and a 2D quantum spin liquid and the properties of the hole-doped spin liquid on a triangular lattice. We will attempt to stabilise a Mott insulator on a square lattice and perform a comparable experiment. In the corresponding helium-4 film we will study the superfluid-insulator transition, and investigate possible 2D supersolid behaviour. We will develop a highly ordered graphite substrate with a view to optimising conditions under which to search for the holy grail of unconventional superfluidity in a helium-3 fluid monolayer. We will investigate quantum criticality in the helium-3 bilayer heavy fermion system recently discovered by us. And we will study helium-3 in nano-channels as a one dimensional fermion system, and a possible realization of a Luttinger liquid. These experiments on fermionic and bosonic cold atoms are performed on uniform low dimensional systems in thermodynamic equilibrium at precisely measured temperatures in the range 200 microKelvin to 4K. The lowest temperatures will be produced by nuclear adiabatic demagnetization cryostats in our laboratory. A range of high precision experimental probes will be employed to study these systems. Sensitive NMR techniques developed in our laboratory, based on the detection of the precessing magnetic signal by SQUIDs (Superconducting Quantum Interference Devices), will be used to measure magnetic susceptibility, magnetization and spin dynamics. We will extend measurements of the heat capacity to the lowest temperatures in order to access system entropy and probe the elementary excitations. The superfluid density, and any dissipative component of the response, will be measured by high quality torsional mechanical resonators. We will collaborate on developing graphene based nano-mechanical resonators with wide-bandwidth SQUID amplifier detection. The project is expected to lead to fundamental insights into some of the most central issues in the physics of strongly correlated matter, and impact on the understanding of more complex materials of potential technological relevance. The project will drive innovation of new instrumentation and measurement techniques at an important scientific frontier; the low temperature frontier. As in any frontier science we may encounter the unexpected.