超导体热力学状态变化中的能量节约与可逆性：焦耳加热与磁卡伦冷却
Energy conservation and reversibility during thermodynamic changes of state in superconductors: Joule heat vs. magnetocaloric cooling
AI总结：
研究超导体热力学状态变化中的能量守恒与可逆性，揭示焦耳加热与磁卡伦冷却的相互作用及温度不均匀性。
摘要：
在超导体的迈斯纳相中，外部恒定磁场由由库珀对形成的循环持续零电阻超电流屏蔽。然而，该相内的热力学状态变化，如冷却或加热，不可避免地会诱导由热激发的非配对电荷载体产生的正常电流，这些电流由局部磁场的时间依赖性变化引起。它们不仅导致磁场分布偏离教科书迈斯纳轮廓，还引起耗散的焦耳加热。这与真正热力学超导态的预期可逆性形成鲜明对比，这一事实长期以来在文献中被忽视。我们证明这些正常电流也产生磁卡伦冷却，总和即时且精确地补偿了耗散的热量，从而确保整体能量守恒和可逆性。然而，焦耳加热和磁卡伦冷却过程在空间上是分开的，因此应导致温度不均匀性。我们假设现实材料参数量化这些效应，并得出结论，它们目前难以用现有实验技术测量。显著的温度梯度仅在向超导态的第一性相变处预期出现，其中不连续的磁通量排斥应诱导比在迈斯纳相深处大得多的正常电流。我们还论证超导体中的基本物理与热磁发电机本质上是相同的，其中电磁机械工作可以从受热循环作用的磁化物质中提取，其中磁卡伦冷却由来自外部热库的热量平衡。
英文摘要：
In the Meissner phase of a superconductor, an external constant magnetic field is shielded by circulating persistent zero-resistance supercurrents that are formed by Cooper pairs. However, a thermodynamic change of state within this phase, such as cooling or heating, inevitably generates normal currents of thermally excited unpaired charge carriers, induced by the time-dependent variations in the local magnetic field. They not only lead to deviations of the magnetic-field distribution from textbook Meissner profiles but also cause dissipative Joule heating. This sharply contradicts the expected reversibility of a truly thermodynamic superconducting state, a fact that has largely been overlooked in the literature. We show that these normal currents also produce a magnetocaloric cooling, which in total instantaneously and precisely compensates for the dissipated heat, thus ensuring overall energy conservation and reversibility. However, the Joule heating and magnetocaloric cooling processes are spatially distinct and should therefore lead to temperature inhomogeneities. We quantify these effects assuming realistic material parameters and conclude that they are challenging to measure with current experimental techniques. Significant temperature gradients are expected only directly at the first-order transition to the superconducting state, where the discontinuous flux expulsion should induce normal currents that are much larger than those deep in the Meissner phase. We also argue that the underlying physics in superconductors is fundamentally identical to that of thermomagnetic generators, where electromechanical work can be extracted from magnetized matter subjected to thermal cycles, and where the magnetocaloric cooling is balanced by the heat supplied from an external thermal reservoir.
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