EAGER: CRYO: Development of a sub-Kelvin Refrigerator using Magnetic Field Activated Solid-State Thermal Switches based on Thermal Chiral Anomaly

这个探索性研究（EAGER）项目的早期概念资助是为了将开关的优化范围从已知的工作温度高于4K扩展到低于1k，然后构建一个小型绝热消磁制冷（ADR）原型并建立概念验证。基于超导体的量子计算机必须被冷却到远低于1开尔文的温度。只有两种技术可以做到这一点，稀释冷藏和ADR。稀释制冷使用氦的稀有同位素He3，而氦本身已经是一种稀缺气体和不可再生资源。因此，氦供不应求，其稀缺性很可能会限制量子计算机的普及。替代方案ADR是一种热泵，其功能类似家用冰箱，但使用磁力“做功”。家用冰箱冷却里面的东西（热负荷），并将热量排除到房间（散热器）。它的工作流体是一种制冷剂，比如氟利昂，它被压缩，加热，膨胀，冷却。类似地，ADR中的工作流体是一种磁性盐丸，它交替地与热负荷热接触，将热负荷的热量排出，然后与散热器接触，在那里它拒绝从负载中吸收的热量。这项工作是通过交替地在盐丸上施加磁场来完成的，磁场加热盐丸，然后移除磁场，冷却盐丸。盐丸在与散热器接触时暴露在现场。当盐丸与负载接触时，电场被移除。现有的传统ADR使用氦气交换气体作为热接触：循环它们需要将氦气泵入和泵出系统，这是缓慢的，并且限制了循环时间。这个项目是开发一对固态热开关，连接盐丸，一个到水槽，另一个到负载。该设计需要相同的磁场来完成药丸中的磁性工作，并自动操作两个开关。热开关是一种装置，当它们“关闭”时让热量流动，但当它们“打开”时阻挡热量。它们由导热系数在低和高之间切换的材料制成。在这里要开发的开关中，热量将由电子携带。在几乎所有的固体中，电子传导在磁场中会减弱，磁场会使电子运动偏转，从而产生磁阻。一个值得注意的例外是“热手性异常”或“重力异常”。在这个最近发现的效应中，在Weyl半金属（一类拓扑固体）中，沿特定晶体学方向平行施加温差，大大增加了导热性。该材料从低场的低导热系数切换到高场的高导热系数。但周期性ADR需要两个开关，一个用于负载，一个用于接收。结果表明，同样的材料，如果磁场方向相反，就不是Weyl半金属，而是“拓扑绝缘体”。它现在给出了相反的效果：当场高时开关关闭，当场低时开关打开。通过将两者结合并将其与盐丸结合，理论上可以制造出达到量子计算所需温度的ADR。该奖项反映了美国国家科学基金会的法定使命，并通过使用基金会的知识价值和更广泛的影响审查标准进行评估，被认为值得支持。

This EArly-concept Grant for Exploratory Research (EAGER) project is to extend the optimization of the switches from temperatures above 4K, where it is known they work, to below 1 K, then to build a small prototype adiabatic demagnetization refrigeration (ADR) and establish the proof of concept. Quantum computers based on superconductors must be cooled to temperatures far below 1 Kelvin. Only two technologies exist to do that, dilution refrigeration and ADR. Dilution refrigeration uses a rare isotope, He3, of helium, which itself is already a scarce gas and a non-renewable resource. He3 is thus in such short supply that its scarcity may well limit the spread of quantum computers. The alternative, ADR, is a heat pump that functions like a household refrigerator but uses magnetic “work”. A household refrigerator cools its contents (the heat load) and rejecting the heat to the room (the heat sink). Its working fluid is a refrigerant, like freon, that is alternatively compressed, which heats it up, and expanded, which cools it down. Similarly, the working fluid in an ADR is a pill of a magnetic salt that is alternatively put in thermal contact with the heat load, whose heat it drains, and then with a heat sink, where it rejects the heat it has taken from the load. The work is done by alternatingly applying a magnetic field to the salt pill, which heats it up, and then removing the field, which cools the pill down. The salt pill is exposed to the field while in contact with the heat sink. The field is removed while the salt pill is in contact with the load. Existing conventional ADR’s use helium exchange gas as a thermal contact: cycling them requires pumping helium in and out of the system, which is slow and limits the cycle time. This project is to develop a pair of solid-state heat switches that connect the salt pill, one to the sink and the other to the load. The design calls for the same field to do the magnetic the work in the pill and operate both switches automatically.Heat switches are devices that let heat flow when they are “closed” but block heat when they are “open”. They are made from materials that switch between a low and a high thermal conductivity. In the switches to be developed here, heat will be carried by electrons. In almost all solids, electron conduction is decreased in a magnetic field that deflects the electron motion sideways, creating magneto-resistance. One notable exception is the “Thermal Chiral Anomaly” or “Gravitational Anomaly”. In this recently discovered effect, a temperature difference applied parallel to a magnetic field along a specific crystallographic direction in Weyl semimetals, a class of topological solids, greatly increases the thermal conductivity. The material switches from a low thermal conductivity at low field to a high one at high field. But a cyclical ADR needs two switches, one to the load and one to the sink. It turns out that the same materials, with the field oriented along the opposite direction, is not a Weyl semimetal but a “topological insulator”. It now gives the opposite effect: the switch is closed when the field is high and open when the field is low. By combining both and integrating them with a salt pill, an ADR can be made that theoretically can reach the temperatures needed for quantum computing.This award reflects NSF's statutory mission and has been deemed worthy of support through evaluation using the Foundation's intellectual merit and broader impacts review criteria.