Structural distortion and suppression of superconductivity in stoichiometric B1-MoN epitaxial thin films
Structural distortion and suppression of superconductivity in stoichiometric B1-MoN epitaxial thin films
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DOI：
10.1103/physrevb.73.052504
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发表时间：
2006-02-01
影响因子：
3.7
通讯作者：
Yamanaka, S
中科院分区：
文献类型：
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作者：
Inumaru, K;Baba, K;Yamanaka, S
采用脉冲激光沉积(PLD)技术，在973K下，在α-Al_2O_3(001)和MgO(001)衬底上外延生长了具有NaC l结构(B1-MON)的氮化钼薄膜。高结晶度的外延薄膜使我们能够确定三维晶胞参数，这是因为理论计算表明，预测的超导体B1-MON在弹性方面不稳定，不受四方和三角扭曲的影响。在α-Al_2O_3(001)晶面上生长了B1-MON相(成分为Mo1N0.98)，其(111)面平行于衬底表面。用多轴衍射仪进行的X射线衍射分析只发现了一个很小的三角晶格变形[a=0.4219(3)nm，α=89.28(5)度]，沿垂直于衬底表面的[111]方向扩展。在氧化镁衬底上生长的薄膜具有MoN_(1.03)组分，由于与氧化镁衬底晶格(a=0.4213 nm)相吻合，薄膜呈现轻微的四方形变(a=0.4213，c=0.424 nm)。这两种化学计量比薄膜在2K以上没有超导电性，在α-Al_2O_3(001)上以较高的沉积速率获得了较低的氮含量(MoN_(0.86))薄膜。相应的薄膜具有小得多的晶格常数[a=0.4184(3)nm]和相似的变形[α=89.41(5)度]。B_1-MoN_(0.86)薄膜具有超导电性，转变温度T-c=4.2K，前化学计量相超导电性的抑制可以用晶格膨胀来解释。
Molybdenum nitride films with the NaCl structure (B1-MoN) were epitaxially grown on alpha-Al2O3(001) and MgO(001) substrates at 973 K by pulsed laser deposition (PLD) under nitrogen radical irradiation. The highly crystalline epitaxial films enabled us to determine the three-dimensional cell parameters, which was motivated by theoretical calculations that B1-MoN, a predicted superconductor, is elastically unstable against tetragonal and trigonal distortions. On alpha-Al2O3(001), the B1-MoN phase (composition, Mo1N0.98) was grown with its (111) planes parallel to the substrate surface. X-ray diffraction analysis with a multiaxes diffractometer detected only a small trigonal lattice distortion [a=0.4219(3) nm, alpha=89.28(5)degrees] with an expansion along the [111] direction perpendicular to the substrate surface. The film grown on MgO(001) had the MoN1.03 composition and showed a slight tetragonal distortion (a=0.4213 and c=0.424 nm) due to fitting to the MgO substrate lattice (a=0.4213 nm). These two stoichiometric films showed no superconductivity above 2 K. A lower nitrogen content (MoN0.86) film was obtained on alpha-Al2O3(001) using a higher deposition rate. The corresponding film had a much smaller lattice constant [a=0.4184(3) nm], and a similar distortion [alpha=89.41(5)degrees]. The B1-MoN0.86 film showed superconductivity with a transition temperature T-c=4.2 K. The suppression of the superconductivity of the former stoichiometric phase can be interpreted in terms of the lattice expansion.