Thickness dependence of superconductivity and superconductor–insulator transition in ultrathin FeSe films on SrTiO ₃ (001) substrate

Abstract

Interface-enhanced high-temperature superconductivity in one unit-cell (UC) FeSe film on SrTiO 3 (001) (STO) substrate has recently attracted much attention in condensed matter physics and material science. Here, by ex situ transport measurements, we report on the superconductivity in FeSe ultra-thin films with different thickness on STO substrate. We find that the onset superconducting transition temperature (T c ) decreases with increasing film thickness of FeSe, which is opposite to the behavior usually observed in traditional superconductor films. By systematic post-annealing of 5 UC FeSe films, we observe an insulator to superconductor transition, which is accompanied with a sign change of the dominated charge carriers from holes to electrons at low temperatures according to the corresponding Hall measurement.2

Superconductivity above 28 K in single unit cell FeSe films interfaced with GaO2− layer on NdGaO3(1 1 0)

“…The opposite behaviour seen here reflects the decaying influence from the substrate. Previous transport measurements have revealed that thicker FeSe films on SrTiO3 also exhibit lower transition temperatures as well as wider transition temperature regions, suggesting that only the first UC of FeSe at the interface is superconducting [35,39]. The same thickness dependence seen in FeSe/NdGaO3(110) system, therefore, indicates here also only the first UC FeSe becomes superconducting.…”

Enhanced superconductivity induced by several-unit-cells diffusion in an FeTe/FeSe bilayer heterostructure

“…Similarly, the predominance of hole-type carriers in PLD-grown multilayer FeSe thin films grown on CaF 2 substrates was also verified by angle-resolved photoemission spectroscopy (ARPES) measurements by Shen et al [73]. In other words, the enhanced T c in #FT-300 could be excluded from the heavily electron-doped HTS scenarios [25,30,44,47,65,66]. In addition, the temperature dependence of the normalized resistance [y axes in Figs.…”

“…For example, an alkali-metalcoating (Li, K) process [38][39][40][41][42][43] was found promising for introducing massive electron doping into multilayer FeSe films on STO. Wang et al [44] reported that a postannealing procedure is another effective way to introduce electron charge carriers into a multilayer FeSe system and realize an insulatorsuperconductor transition. A novel method was also proposed by Shiogai et al [45,46].…”

“…Quite a few works have explained the disappearance of superconductivity in ultrathin Fe-Ch films. For instance, it is well accepted that severe thermal and quantum fluctuations at low dimensionality usually suppress the T c of a superconducting film [44]. Tensile stress, which is detrimental to the superconductivity of Fe-Chalcogenide films [59], tends to be induced when the thickness of FeSe 0.5 Te 0.5 is lower than 30 nm [56].…”

Detection of Bosonic Mode as a Signature of Magnetic Excitation in One-Unit-Cell FeSe on SrTiO3

“…However, systematic investigations on the bosonic excitation mode remain scarce [13] in the recently discovered one-unit-cell (1-UC) FeSe on SrTiO 3 (001) (STO) [14]. The 1-UC FeSe/STO grown by molecular-beam epitaxy (MBE), which ranks the highest critical temperature (T c ) (typically 65 K) in iron-based superconductors, has triggered tremendous research interest in condensed-matter physics [15][16][17][18][19][20][21][22][23]. Photoemission spectroscopy suggests that the Fermi surface consists of only electron pockets at M points [15], different from that found in bulk FeSe and iron pnictides with additional hole pockets at Γ point.…”