Correlated electronic states for cryogenic refrigeration - fundamentals and applications

低温冷却技术使凝聚态物理学中一些最引人注目的科学发现成为可能，比如超导性、超流动性和量子霍尔效应。这些发现，像大多数低温下的研究一样，是通过利用原子携带的高熵，即氦同位素而获得的。是否有可能制定固态类似于这些技术，它们是否会开辟新的机会？我们的项目结合了基础研究、技术评估和仪器开发，以建立基于操纵电子而不是原子的冷却方法，从而使其易于小型化和大规模生产。这很重要，因为进入亚开尔文范围不再仅仅是基础研究的兴趣：量子工程，将量子效应用于新技术，依赖于安静的环境，这在固态设备中意味着低温。更多样化的冷却平台有助于量子技术的传播。固态冰箱可以与机械制冷机相结合，生产出小型、低成本、节能和无低温的平台，非常适合携带传感器和其他量子设备。我们表明，通过使用现有的相关电子金属，在相同的低施加磁场下，熵密度变化几乎比传统盐高一个数量级，并且不需要封装来延缓脱水，也不需要金属基础设施来促进热传导，这些措施不仅可以限制紧凑性，而且还可以限制长期可靠性。需要进行基础研究，以提供新的见解和开发固态制冷的新材料。我们考虑的冷却方法要么利用熵对磁场的依赖（磁热效应），要么利用热量随电荷电流传输（珀尔帖效应）。我们将通过在大范围的场、温度和压力下的多探针测量来研究在低温下放大这些效应的相关现象：(i)在金属稀土化合物中，稀土元素的f轨道可以承载磁矩，这可以形成金属自旋液体。这些系统中的磁矩比传统制冷剂中的磁矩密集得多，在传统制冷剂中，磁矩被高度稀释以避免磁有序。由于这种状态与低温下非常高且强场相关的熵有关，因此可以利用它进行冷却。我们将使用广泛的实验技术，包括热输运、热容和量子振荡测量，来研究金属自旋液态及其激发。（ii）在近藤绝缘体中，电子相互作用导致低温下的半导体行为。由于其小的能量间隙和窄的电子带，近藤绝缘体有利于珀尔帖冷却。此外，它们还能显示出更多有趣的现象，比如SmB6中受拓扑保护的表面态和体态的量子振荡。我们将研究近藤绝缘子中的热传递，并通过多探头测量探索近藤绝缘子的本质，当间隙在施加压力下变化时。结构不稳定性在具有复杂晶格结构的材料中普遍存在，它们可以通过改变成分或施加压力来控制。这为操纵声子谱和诱导影响声子平均自由程的介观织构开辟了进一步的选择。我们将研究晶格热导率和材料作为珀尔帖制冷剂的有效性的后果。在这个项目中获得的见解也将有助于改善高温下的固态制冷。

Low temperature cooling techniques have enabled some of the most dramatic scientific discoveries in condensed matter physics, such as superconductivity, superfluidity, and the quantum Hall effects. These discoveries, like most research at low temperatures, were made by exploiting the high entropy carried by atoms, namely the helium isotopes. Is it possible to formulate solid-state analogues to these techniques, and could they open up new opportunities? Our project combines fundamental research, technology evaluation and instrument development, in order to establish cooling methods which are based on manipulating electrons rather than atoms and which thereby lend themselves to miniaturisation and mass production.This is important, because access to the sub-Kelvin range is no longer of interest to fundamental research alone: quantum engineering, the use of quantum effects for new technologies, relies on quiet environments, which in solid state devices implies low temperatures. A more diverse arsenal of cooling platforms facilitates the spread of quantum technologies. Solid-state refrigerators can be combined with a mechanical cryocooler to produce small, low-cost, energy-efficient and cryogen-free platforms ideally suited to carrying sensors and other quantum devices. We show that significant miniaturisation is already possible by use of existing correlated electron metals with entropy density changes almost an order of magnitude higher than those of conventional salts for the same low applied magnetic fields, and not requiring encapsulation to retard dehydration nor metallic infrastructures to promote thermal conduction, measures that can limit not only compactness but also long-term reliability.Fundamental research is needed to provide new insights and to develop new materials for solid-state refrigeration. The cooling methods we consider either exploit the magnetic field dependence of the entropy (magnetocaloric effect), or heat is transported along with a charge current (Peltier effect). We will investigate correlated phenomena which amplify these effects at low temperature by multiprobe measurements over wide ranges of field, temperature and pressure:(i) In metallic rare earth compounds, the f-orbitals of the rare earth elements can host magnetic moments, which can form a metallic spin liquid. Magnetic moments in these systems are much more densely packed than in conventional refrigerants, in which moments are highly diluted to avoid magnetic order. Because this state is associated with a very high and strongly field-dependent entropy at low temperature, it can be exploited for cooling. We will use a wide range of experimental techniques, including thermal transport, heat capacity and quantum oscillation measurements, to investigate the metallic spin liquid state and its excitations. (ii) In Kondo insulators, electronic interactions cause semiconducting behaviour at low temperature. Because of their small energy gaps and narrow electronic bands, Kondo insulators are favourable for Peltier cooling. They can, moreover, display further intriguing phenomena, such as topologically protected surface states and quantum oscillations from bulk states in SmB6. We will examine thermal transport in Kondo insulators and explore the nature of the Kondo insulating state by multiprobe measurements, when the gap is varied under applied pressure.(iii) Structural instabilities are widespread in materials with complex lattice structures, and they can be controlled by varying the composition or the applied pressure. This opens up further options for manipulating the phonon spectrum and for inducing mesoscopic textures which affect the phonon mean free path. We will investigate the consequences for the lattice thermal conductivity and for the material's effectiveness as a Peltier refrigerant.The insights gained in this project will also help improve solid state refrigeration at elevated temperature.