Meissner and mesoscopic superconducting states in 1–4 unit-cell FeSe films

We report a detailed investigation on the superconductivity in eight 1–4 unit-cell (UC) FeSe films on $\mathrm{SrTi}{\mathrm{O}}_3$ substrates by measuring their magnetization and sheet resistance in a field between $5\times {10}^{-2}$ and $7\times {10}^4\mathrm{Oe}$ over the last one and a half years as a function of temperature and frequency, from 2 to 300 K and from 0 to 1.5 kHz, respectively. The results show that samples of 1–4-UC FeSe films all display a complex superconducting structure, i.e., a Meissner state but populated with weak links below 20 K, and an unusual superconducting mesostructure up to 45 K, strongly suggesting that superconductivity may be of an interfacial nature. A model is proposed to account for the magnetizations in such a superconducting mesoscopic structure, similar to the Andreev reflection between the normal and superconducting carriers. Above 45 K, collective glasslike excitations are evident, although their nature is yet to be determined. The complex superconducting structure observed is consistent with the challenges in synthesizing ultrathin FeSe films with a superconducting temperature much higher than that of a bulk FeSe.

Figure 1

Ac magnetic susceptibility ${\chi }_{\mathrm{ac}}^{\prime }$ of the background of the sample (Si/10-UC FeTe/STO), at $H_{\mathrm{ac}}=1.5\mathrm{Oe}$ and $\omega =1488\mathrm{Hz}$. The error bars here and in Figs. 2, 3, 4, 7–12, and 14 represent the data spreads over 50 independent measurements. The inset represents the schematic structure for the Si/10-UC FeTe/STO.

Figure 2

ZFC ${\chi }_{\mathrm{dc}}$ and FC ${\chi }_{\mathrm{dc}}$ at $H_{\mathrm{dc}}=5\mathrm{Oe}$, and ${\chi }_{\mathrm{ac}}^{\prime }$ at $H_{\mathrm{ac}}=1.5\mathrm{Oe}$ and $H_{\mathrm{dc}}=0$ of a 3-UC FeSe film.

Figure 3

$H_{\mathrm{dc}}$ effect on ${\chi }_{\mathrm{ac}}^{\prime }$ at $H_{\mathrm{ac}}=1.5\mathrm{Oe}$ and $\omega =1488\mathrm{Hz}$ for a 3-UC film. The inset represents $m^{\prime }$ vs $H$ for $H_{\mathrm{ac}}=0–1.5\mathrm{Oe}$ and $H_{\mathrm{dc}}=0$ at 2 K.

Figure 4

The ${\chi }_{\mathrm{ac}}^{\prime }$ at $\omega =1488\mathrm{Hz}$ for a 3-UC fresh film and an aged film, and a 4-UC fresh film.

Figure 5

The sheet resistance $R_s$ of an aged 3-UC film and an aged 4-UC film at $H=0$. The red dashed lines are used for $T_c$ extrapolation.

Figure 6

The aging of the resistive $T_c$ onset (squares) and the magnetic $T_c$ onset (circles) for a 3-UC film (open symbols) and a 4-UC film (solid symbols).

Figure 7

ω dependence of $\Delta {\chi }_{\mathrm{ac}}^{\prime }$ for a 3-UC film at different temperatures: (a) 2 K log scale; (b) 20 K linear scale; (c) 80 K log scale; and (d) 40 K linear scale. Black dots: raw data; black lines: $\Delta {\chi }_{\log }^{\prime }+\Delta {\chi }_{\mathrm{lin}}^{\prime }$; red lines: $\Delta {\chi }_{\log }^{\prime }$; blue lines: $\Delta {\chi }_{\mathrm{lin}}^{\prime }$.

Figure 8

ω dependence of the χ″ for a 3-UC film at different temperatures. (a) 2 K; (b) 20 K; (c) 40 K; and (d) 80 K. Black dots: raw data; red lines: linear fitting.

Figure 9

(a) $\Delta {\chi }_{\log }^{\prime }$ vs $T$. The data at 2 K are reduced by a factor of 5 to fit the scale. (b) $\Delta {\chi }_{\mathrm{lin}}^{\prime }$ vs $T$ at $\omega =1488\mathrm{Hz}$ for a 4-UC film.

Figure 10

$m^{\prime }$ vs $H_{\mathrm{ac}}$ at 1488 Hz and $H_{\mathrm{dc}}=0$ for a 3-UC film at different temperatures. The data at 10 K are reduced by a factor of 10 to fit the scale.

Figure 11

Evolution of $\Delta {\chi }_{\mathrm{lin}}^{\prime }$ with time for a 4-UC film at different temperatures.

Figure 12

The $\Delta {\chi }_{l\mathrm{og}}^{\prime }$ vs ω at 2 K for a 4-UC film of different ages.

Figure 13

The proposed model for the quasilinear component in the observed ac response in FeSe films.

Figure 14

The ac phase angles of the moment of a Ag disk, a Pb sheet, an YBCO film, a 3-UC fresh film, and an aged film.