Hybrid circuits as a thermodynamic measurement platform for 2D materials

二维材料——原子级薄的晶体层——以及经常由它们构成的层层堆叠，已经成为设计奇异电子功能的最富有成效的平台之一。这种二维材料的堆叠仅靠范德华力（vdW）维系在一起，这种力也赋予壁虎攀爬光滑垂直表面的能力。由于可能的堆叠安排的多样性，所谓的vdW堆叠可以承载巨大范围的电子行为。即使是完全由二维碳材料石墨烯制成的堆栈，在两侧都有保护层，也可以表现出超导性或铁磁性，以及其他现象，所有这些都可以通过调整栅极电压在单个设备中实现。几乎所有对vdW异质结构的实验研究都是通过电阻测量来完成的，这反映了该技术在探测样品甚至到单分子尺度上的既定能力。尽管电导测量很方便，但在基本水平上，它们只记录了样品电子状态的一个方面——载流子在样品中移动时如何散射——而忽略了对理解其潜在性质至关重要的其他特性。热力学探针是电导测量的有力补充，可以（例如）测量熵、导热性或磁化。然而，在纳米级系统中实现这种探针极其困难，因此，在研究vdW堆栈时很少尝试。目前的提案提供了一种测量二维材料热力学性质的新方法，将我的研究小组目前独立的两个领域结合在一起：测量二维材料（如石墨烯）中的强相关或拓扑状态，以及使用热力学探针研究由传统半导体制成的量子器件。我们提出了一种混合方法，利用已经开发成温度计和化学势传感器的GaAs电路元件，并将它们与vdW堆栈集成。其结果将是在探测二维材料中的电子状态的热测量能力上的一个数量级的提高，为研究它们的行为打开了一个新的窗口。这项工作将我们与凝聚态物理领域一些最具活力的实验和理论社区联系在一起，为我们的HQP提供了一个极好的培训计划，并与世界各地精英研究机构的顶尖科学家直接联系。从二维材料器件中出现的发现的影响可能与它们所承载的电子状态类型一样广泛：从探索量子多体物理的多功能测试平台（这可能是理解高温超导等现象的关键），到建立基于拓扑状态的量子技术的新途径。

Two-dimensional materials - atomically thin crystal layers - and the layer-by-layer stacks that are frequently made from them, have emerged as one of the most fruitful platforms from which to engineer exotic electronic functionality. Such stacks of 2D materials are held together just by van der Waals (vdW) forces, the same forces that give geckos the ability to climb smooth vertical surfaces. Due to the multiplicity of possible stacking arrangements, so-called vdW stacks can host an enormous range of electronic behaviours. Even stacks made exclusively from the 2D carbon material graphene, with protective layers on either side, can exhibit superconductivity or ferromagnetism, among other phenomena, all accessible in a single device just by tuning a gate voltage. Almost all experimental investigations of vdW heterostructures are made using electrical resistance measurements, reflecting the established power of that technique to probe samples even down to the single-molecule scale. As convenient as conductance measurements are, at a fundamental level they record only one aspect of a sample's electronic state - how carriers scatter as they move through the sample - but are blind to other characteristics that are often crucial to understanding its underlying nature. Thermodynamic probes are a powerful complement to conductance measurements, enabling (for example) measurements of entropy, thermal conductivity or magnetization. Such probes are, however, extremely difficult to implement in nanoscale systems and, for that reason, are much less often attempted when investigating vdW stacks. The present proposal offers a fresh approach to measuring thermodynamic properties in 2D materials, bringing together two areas that are, for now, independent strengths of my research group: the measurement of strongly correlated or topological states in 2D materials such as graphene, and the use of thermodynamic probes for studying quantum devices built from conventional semiconductors. We propose a hybrid approach, exploiting GaAs circuit elements already developed as thermometers and chemical-potential sensors, and integrating them with a vdW stack. The result will be an order-of-magnitude improvement in the power of thermal measurements to probe electronic states in 2D materials, pushing open a new window into their behaviours. This effort joins us with some of the most vibrant experimental and theoretical communities in condensed matter physics, offering a superb training program for our HQP with direct connections to leading scientists at elite research institutions across the world. The impact of discoveries emerging from 2D material devices may be as broad as the types of electronic states they host: from a versatile testbed for exploring quantum many-body physics (which is likely to be the key to understanding phenomena such as high-temperature superconductivity), to establishing new avenues to quantum technologies based on topological states.