Insulator-like behavior coexisting with metallic electronic structure in strained FeSe thin films grown by molecular beam epitaxy

Abstract

This paper reports that ~10-nm-thick iron selenide (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 atomic composition ratios ([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 E a between the Se-rich and Fe-rich regions; i.e., E a were small (a few meV) up to [Fe]/[Se]=1.1 2 but jumped up to ~25 meV at higher [Fe]/[Se]. The film with [Fe]/[Se]=1.1 had the smallest E a of 1.1 meV and exhibited an insulator-superconducting transition at 35 K with zero resistance under gate bias.The large E a of the Fe-rich films was attributed to the unusual lattice strain with tensile in-plane and relaxed out-of-plane strains. The large E a 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. The high potential barrier of Fe-rich films is tentatively attributed to the presence of large amounts of excess Fe, which could plausibly cause a broad superconducting transition without zero resistance under gating.

Stabilization and heteroepitaxial growth of metastable tetragonal FeS thin films by pulsed laser deposition

“…Figure 6 shows the dependence of ρ of the t-FeS films grown under the optimum conditions on five kinds of single-crystal substrates on T. Even though it was reported that t-FeS bulk exhibits superconductivity at 4 K [22], none of the thin films exhibited superconductivity down to 2 K; instead, the films displayed insulator-like behavior as previously suggested [20] and as observed for very thin strained FeSe films [30,33,34].…”

“…The a-axis lattice parameters of the films (we can estimate them only for the films on LSAT and CaF 2 ) were small (a = 3.668 Å on CaF 2 ) compared with that of a t-FeS single crystal (3.683 Å [24]). The above strain trend for out-of-plane and in-plane directions is completely opposite to that of ~10 nm-thick t-FeSe on STO [30]. We are unsure of the exact reason of such unusual lattice variation; however, the misoriented in-plane lattice, excluding t-FeS on CaF 2 , and an interface layer (see that will be displayed in Fig.…”

Fe-based superconducting thin films—preparation and tuning of superconducting properties

“…That might be explained by a strong dependence on the exact stoichiometry. Hanzawa et al [271] pointed out recently that the strain state in very thin MBE-grown FeSe films depends strongly on the Fe:Se ratio, which was related to excess interstitial Fe or disordered Fe vacancies. Kawaguchi et al [215] have shown the dependence of the Poisson ratio of BaFe 2 (As 1-x P x ) 2 films on MgO on the P content x, more or less linearly increasing from around 0.2 at x=0 to 0.35 at x=0.…”

Microstructural control of the transport properties of β-FeSe films grown by sputtering

“…The key question is which type of disorder originate the localization. Recently, the formation of potential barriers in the conduction band has been suggested as the origin of the insulator-like behavior 43 . In this scenario, despite a metallic electronic structure observed by ARPES, the carriers cannot move freely due to the influence of the potential barriers.…”