In 1933, Meissner and Ochsenfeld reported the expulsion of magnetic flux—the diamagnetic Meissner effect—from the interior of superconducting lead. This discovery was crucial in formulating the Bardeen-Cooper-Schrieffer (BCS) theory of superconductivity. In exotic superconducting systems BCS theory does not strictly apply. A classical example is a superconductor-magnet hybrid system where magnetic ordering breaks time-reversal symmetry of the superconducting condensate and results in the stabilization of an odd-frequency superconducting state. It has been predicted that under appropriate conditions, odd-frequency superconductivity should manifest in the Meissner state as fluctuations in the sign of the magnetic susceptibility, meaning that the superconductivity can either repel (diamagnetic) or attract (paramagnetic) external magnetic flux. Here, we report local probe measurements of faint magnetic fields in a Au / Ho / Nb trilayer system using low-energy muons, where antiferromagnetic Ho (4.5?nm) breaks time-reversal symmetry of the proximity-induced pair correlations in Au. From depth-resolved measurements below the superconducting transition of Nb, we observe a local enhancement of the magnetic field in Au that exceeds the externally applied field, thus proving the existence of an intrinsic paramagnetic Meissner effect arising from an odd-frequency superconducting state.
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机译：1933年，Meissner和Ochsenfeld报告了驱逐磁通量 - 从超导铅的内部驱逐磁通型效果。这一发现对于制定Bardeen-Cooper-Schrieffer（BCS）的超导性理论至关重要。在异国情调的超导系统中，BCS理论不会严格适用。经典实例是超导体 - 磁体混合系统，其中磁性排序破坏超导冷凝物的时间反转对称性，并导致奇数频率超导状态的稳定。已经预测，在适当的条件下，奇数频率超导性应该在喵喵圈状态下表现为磁性敏感性符号中的波动，这意味着超导性可以击退（抗磁的）或吸引（顺磁）外部磁通量。在这里，我们使用低能量μON向AU / HO / NB三层系统中的微弱磁场报告局部探测测量，反铁磁共孔（4.5≤nm）破坏Au中接近诱导对称的近距离对称的时间反转对称。从低于NB的超导转变的深度分辨测量，我们观察到超过外部施加的场的AU中磁场的局部增强，从而证明了由奇数频率超导状态产生的内在顺磁性发光效果的存在。
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