Equal-spin Andreev reflection and long-range coherent transport in high-temperature superconductor/half-metallic ferromagnet junctions
Equal-spin Andreev reflection and long-range coherent transport in high-temperature superconductor/half-metallic ferromagnet junctions
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DOI：
10.1038/nphys2318
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发表时间：
2012-07-01
期刊：
影响因子：
19.6
通讯作者：
Villegas, Javier E.
中科院分区：
文献类型：
--
作者：
Visani, C.;Sefrioui, Z.;Villegas, Javier E.
传统的超导性与铁磁性是不相容的，因为磁场倾向于使电子自旋极化，并使反向自旋的单重态库珀对分裂(1)。然而，超导相关性在强铁磁体中远距离渗透的可能性已经被实验证明，实验发现，被铁磁间隔器隔开的超导电极之间存在约瑟夫森耦合（2-7）。这被认为是超导体/铁磁（S/F）界面上出现等自旋三重态配对的证明，它不受交换场的影响，因此可以长距离传播到F（参考文献8）。这种效应对自旋电子学的应用具有很大的兴趣和潜力(9)。然而，潜在的微观机制的光谱特征仍然难以捉摸。这里我们确实展示了这种类型的证据，特别是在S/F系统中，可能出现的等自旋三重态配对是有争议的（10-12）：异质结构结合了半金属F （La0.7Ca0.3MnO3）和d波S （YBa2Cu3O7）。我们在S/F界面上发现了准粒子和电子干扰效应，直接证明了超导相关在La0.7Ca0.3MnO3上的远距离传播，并暗示了非常规等自旋Andreev反射的发生。这可以理解在这种异质结构中观察到的不寻常的接近行为（12,13）。
Conventional superconductivity is incompatible with ferromagnetism, because the magnetic exchange field tends to spin-polarize electrons and breaks apart the opposite-spin singlet Cooper pairs(1). Yet, the possibility of a long-range penetration of superconducting correlations into strong ferromagnets has been evinced by experiments that found Josephson coupling between superconducting electrodes separated afar by a ferromagnetic spacer(2-7). This is considered a proof of the emergence at the superconductor/ferromagnetic (S/F) interfaces of equal-spin triplet pairing, which is immune to the exchange field and can therefore propagate over long distances into the F (ref. 8). This effect bears much fundamental interest and potential for spintronic applications(9). However, a spectroscopic signature of the underlying microscopic mechanisms has remained elusive. Here we do show this type of evidence, notably in a S/F system for which the possible appearance of equal-spin triplet pairing is controversial(10-12): heterostructures that combine a half-metallic F (La0.7Ca0.3MnO3) with a d-wave S (YBa2Cu3O7). We found quasiparticle and electron interference effects in the conductance across the S/F interfaces that directly demonstrate the long-range propagation across La0.7Ca0.3MnO3 of superconducting correlations, and imply the occurrence of unconventional equal-spin Andreev reflection. This allows for an understanding of the unusual proximity behaviour observed in this type of heterostructures(12,13).