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arXiv 1107.3774cond-mat.supr-con
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锰氧化物/铜氧化物薄膜外延层c轴邻近效应的扫描隧道谱研究

Scanning tunneling spectroscopy study of c-axis proximity effect in epitaxial bilayer manganite/cuprate thin films

I. Fridman, L. Gunawan, G. A. Botton, J. Y. T. Wei

摘要:

最近的实验研究表明,在由铁磁锰氧化物和超导铜氧化物组成的薄膜异质结构中存在着新的超导邻近效应。为了在显微镜下观察这种效应,我们对La $_{2/3}$ Ca $_{1/3}$ MnO $_3$ /YBa $_2$ Cu $_3$ O $_{7-δ}$(LCMO/YBCO)双层薄膜进行了扫描隧道光谱。采用脉冲激光烧蚀沉积法在SrTiO $_{3}$衬底上生长了不同厚度的$\emph{c}$轴取向薄膜。通过横截面透射电子显微镜证实了薄膜的异质外延性。在4.2 K下测量Tunnel光谱,并分析LCMO层上的配对间隙的签名。对于LCMO厚度小于5 nm的双层样品,观察到单层LCMO膜的不对称电导谱特征,没有明显的间隙结构。这些观察结果与涉及自旋单重态对的非常短距离的邻近效应一致,并且难以与涉及自旋三重态对的较长距离的邻近效应相协调。

英文摘要:

Recent experimental studies have indicated novel superconducting proximity effects in thin-film heterostructures comprising ferromagnetic manganites and superconducting cuprates. To look for such effects microscopically, we performed scanning tunneling spectroscopy on La$_{2/3}$Ca$_{1/3}$MnO$_3$/YBa$_2$Cu$_3$O$_{7-δ}$ (LCMO/YBCO) bilayer thin films. $\emph{c}$-axis oriented films of varying thickness were grown on SrTiO$_{3}$ substrates using pulsed laser-ablated deposition. Heteroepitaxiality of the films was confirmed by cross-sectional transmission electron microscopy. Tunneling spectra were measured at 4.2 K, and analyzed for signatures of a pairing gap on the LCMO layer. For bilayer samples with LCMO thickness down to 5nm, asymmetric conductance spectra characteristic of single-layer LCMO films were observed, showing no clear gap structures. These observations are consistent with a very short-range proximity effect involving spin-singlet pairs, and difficult to reconcile with longer-range proximity scenarios involving spin-triplet pairs.

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