Insulator-like behavior coexisting with metallic electronic structure in strained FeSe thin films grown by molecular beam epitaxy
Insulator-like behavior coexisting with metallic electronic structure in strained FeSe thin films grown by molecular beam epitaxy
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DOI：
10.1103/physrevb.99.035148
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发表时间：
2019-01
影响因子：
3.7
通讯作者：
Kota Hanzawa;Y. Yamaguchi;Y. Obata;S. Matsuishi;H. Hiramatsu;T. Kamiya;H. Hosono
中科院分区：
文献类型：
--
作者：
Kota Hanzawa;Y. Yamaguchi;Y. Obata;S. Matsuishi;H. Hiramatsu;T. Kamiya;H. Hosono
本文报道了~10 nm厚的FeSe薄膜在其电阻率的温度依赖性方面表现出类似绝缘体的行为，尽管体相FeSe具有金属电子结构，这已被光电子能谱和第一性原理计算所证实。这种明显的矛盾可以用导带中形成的势垒来解释。用分子束外延方法制备了不同[Fe]/[Se]的超薄FeSe外延膜，并根据晶格应变和电学性质将其分为两类。晶胞参数a随[Fe]/[Se]增大而增大，晶胞参数c随[Fe]/[Se]增大而减小，当[Fe]/[Se]增大到1.1后，晶胞参数a趋于稳定，晶胞参数c开始减小。因此，当[Fe]/[Se]为1.1时，FeSe薄膜具有最大的应变晶格，但在结晶度和表面平整度方面具有最好的质量。[Fe]/[Se]为0.8~1.9的所有FeSe薄膜均表现出类似绝缘体的行为，但其电阻率的温度依赖关系在富Se区和富Fe区表现出不同的激活能Ea，即Ea在[Fe]/[Se]=1.1时很小(几meV)，但在较高的[Fe]/[Se]时跃升到~25 meV。当[Fe]/[Se]=1.1时，薄膜的电导率最小，为1.1 meV，在35K时表现出绝缘体-超导转变，栅偏压下的电阻为零。富铁薄膜的表观活化能大是由于异常的晶格应变引起的，既有面内拉伸应变，也有松弛的面外应变。与[Fe]/[Se]=1.1薄膜(~17 meV)相比，[Fe]/[Se]>；1.1薄膜具有较大的电子迁移率，与[Fe]/[Se]=1.1薄膜(~17 meV)相比，具有较低的迁移率，在渗流载流子传导的导带内具有约50 meV的高势垒。因此，尽管富铁薄膜具有金属电子结构，但仍表现出与半导体相似的绝缘体行为。
This paper reports that ~10-nm-thick FeSe thin films exhibit insulator-like behavior in terms of the temperature dependence of their electrical resistivity even though bulk FeSe has a metallic electronic structure that has been confirmed by photoemission spectroscopy and first-principles calculations. This apparent contradiction is explained by potential barriers formed in the conduction band. Very thin FeSe epitaxial films with various [Fe]/[Se] were fabricated by molecular beam epitaxy and classified into two groups with respect to lattice strain and electrical properties. Lattice parameter a increased and lattice parameter c decreased with increasing [Fe]/[Se] up to 1.1 and then a levelled off and c began to decrease at higher [Fe]/[Se]. Consequently, the FeSe films had the most strained lattice when [Fe]/[Se] was 1.1, but these films had the best quality with respect to crystallinity and surface flatness. All the FeSe films with [Fe]/[Se] of 0.8-1.9 exhibited insulator-like behavior, but the temperature dependences of their electrical resistivities exhibited different activation energies Ea between the Se-rich and Fe-rich regions; i.e., Ea were small (a few meV) up to [Fe]/[Se]=1.1 but jumped up to ~25 meV at higher [Fe]/[Se]. The film with [Fe]/[Se]=1.1 had the smallest Ea of 1.1 meV and exhibited an insulator-superconducting transition at 35 K with zero resistance under gate bias. The large Ea of the Fe-rich films was attributed to the unusual lattice strain with tensile in-plane and relaxed out-of-plane strains. The large Ea of films with [Fe]/[Se]>1.1 resulted in low mobility with a high potential barrier of ~50 meV in the conduction band for percolation carrier conduction compared with that of the [Fe]/[Se]=1.1 film (~17 meV). Therefore, the Fe-rich films exhibited remarkable insulator-like behavior similar to a semiconductor despite their metallic electronic structure.