Surface lattice vibration and electron–phonon interaction in Pb(111) ultrathin superconducting films from first principles: both with and without Si substrate
Surface lattice vibration and electron–phonon interaction in Pb(111) ultrathin superconducting films from first principles: both with and without Si substrate
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DOI：
10.1088/1367-2630/13/9/093023
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发表时间：
2011-09
影响因子：
3.3
通讯作者：
GuiQin Huang
中科院分区：
文献类型：
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作者：
GuiQin Huang
用第一性原理计算了三种Pb超薄膜的晶格振动和电子-声子耦合。对于独立薄膜，我们的计算表明，表面声子的软化和硬化共存。得到的极电位耦合λ和超导转变温度TC均小于它们的体值。对于（1×1）-Pb/Si（111）薄膜，六边形密堆积（hcp）堆垛得到的λ和TC值大于面心立方（fcc）堆垛得到的λ和TC值。在hcp堆叠中，当N=4 ML（单层）时获得的TC值最大，这与实验研究非常一致。因此，我们的计算表明，Si（111）表面的外延超薄Pb薄膜可能具有hcp结构而不是fcc结构。最后，研究了4/3 ML Pb在Si上的条纹状不相称相的表面重构和EP耦合，该相的估计TC与实验值非常接近。我们的计算清楚地表明，超薄膜的动力学性质和EP耦合在很大程度上取决于层间弛豫和层与衬底原子在界面处的成键环境。
The lattice vibration and electron–phonon (EP) coupling of three types of Pb ultrathin films were calculated from first principles. For free-standing films, our calculations showed that softening and stiffening of the surface phonons coexist. The obtained EP coupling λ and the superconducting transition temperature TC are smaller than their bulk values. For (1×1)-Pb/Si(111) films, the obtained λ and TC values in hexagonally close-packed (hcp) stacking are larger than those in face-centered-cubic (fcc) stacking. The largest TC value is obtained for N=4 ML (monolayers) in hcp stacking, which is very much consistent with the experimental study. Hence, our calculations imply that epitaxial ultrathin Pb films on the surface of Si(111) may possess an hcp rather than an fcc structure. Finally, the surface reconstruction and EP coupling of the striped incommensurate phase with 4/3 ML Pb on Si were studied, and the estimated TC in this phase is very close to its experimental value. Our calculations show clearly that the dynamical properties and EP coupling of ultrathin films depend strongly on the interlayer relaxation and the bonding environment between overlayer and substrate atoms at the interface.