Molecule-based magnetocalorics

在开尔文和亚开尔文温度下，磁挫折以及量子临界点的设计可能是巨大磁热效应的来源。本提案旨在系统地研究受挫的分子偶联方案及其磁热特性，并提出在T-B平面某些区域磁冷却应用的少数自旋系统的有希望的结构。重点将放在基态或场致基态下的大规模简并的产生上。应研究所得等熵，并讨论和建议单次和循环冷却过程。另一个有趣的方面是，T-B平面上的一些点可以同时通过绝热过程和等温过程连接起来。关键的优点是等温熵变和绝热温度变化，冷却速率是对场的导数。同样重要的是在较低温度的等温过程步骤中可以吸收的热量。本提案的第二部分致力于研究各向异性（单离子和交换）的影响，特别是各向异性偶极相互作用对磁热学性质的影响。各向异性倾向于分裂能级简并，从而减小等温熵随场的变化，并倾向于降低冷却速率。我们想要研究比其他结构更坚固的结构来抵抗这种影响。另一方面，各向异性允许构建通过重定向工作的磁热过程。这构成了分子磁制冷剂领域的一个全新方向。这类过程应进行调查和讨论。作为最后一部分，我们将开始研究使用Lindbladformalism冷却过程的实际动力学，以描述在磁热循环过程中与热浴的耦合。特别是观察到，有时两个不同的过程连接到同一状态，可以进一步研究。找出现实的过程在这些路径上是否运行不同将是一件有趣的事情。为了不将我们的提议与其他努力混淆，我们想声明，我们不是设计或研究真正的化学材料，而是未来可能实现为磁性分子的交换相互作用方案。我们还想声明，我们不针对室温应用或在十开尔文范围内的气体液化应用，因为分子由于其小（开尔文-）能量尺度而不适合用于此目的。

Magnetic frustration as well as the design of quantum criticalpoints can be the source of huge magnetocaloric effects atKelvin and sub-Kelvin temperatures. This proposal aims at asystematic investigation of frustrated molecular couplingschemes with respect to their magnetocaloric properties and the suggestion of promising structures of few spin systemsfor magnetic coolingapplications in certain regions of the T-B plane. Thefocus will be put on the generation of massive degeneracieseither in the ground state or in a field-induced groundstate.The resulting isentropes shall be investigated and single-shot as well as cyclic cooling processes shall bediscussed and suggested. One additional interesting aspect is given by thefact that some points in the T-B plane can be connected byadiabatic as well as isothermal processes at the same time.Key figures of merit are the isothermal entropy change and theadiabatic temperature change of which the cooling rate is thederivative with respect to the field. Also important is theamount of heat that can be absorbed during the lower temperatureisothermal process step. The second part of this proposal is devoted to the investigationof the influence of anisotropy (single-ion as well as exchange) and in particular of theanisotropic dipolar interaction on magnetocaloricproperties. Anisotropy tends to split degeneracies of levelswhich decreases the isothermal entropy change with field andtends to decrease the cooling rate. We want to investigatestructures that are more robust than others against sucheffects. On the other hand anisotropy allows to constructmagnetocaloric processes that work by reorientation. Thisconstitutes a completely new direction in the field of molecularmagneto-refrigerants. Processes of this kind shall beinvestigated and discussed.As a last part we would like to start investigations ona realistic dynamics of cooling processes using the Lindbladformalism in order to describe the coupling to heat baths in thecourse of a magnetocaloric cycle. Especially the observation,that sometimes two different processes connect the same state,could be studied further. It would be interesting to find out whether realisticprocesses run differently on those paths.In order to not confuse our proposal with other efforts we wouldlike to state that we do not design or investigate real chemicalmaterials, but schemes of exchange interactions that can bepossibly realized as magnetic molecules in the future. We wouldalso like to state that we do not aim at room temperatureapplications orapplications in the few-ten Kelvin range for gas liquification,since molecules are not appropriate for this purpose due to their small (Kelvin-) energy scale.