Direct evidence of superconductivity and determination of the superfluid density in buried ultrathin FeSe grown on SrTiO3

Bulk FeSe is superconducting with a critical temperature ${T}_c\phantom{\rule{4pt}{0ex}}\ensuremath{\cong}8$ K and ${\mathrm{SrTiO}}₃$ is insulating in nature, yet high-temperature superconductivity has been reported at the interface between a single-layer FeSe and ${\mathrm{SrTiO}}₃$. Angle-resolved photoemission spectroscopy and scanning tunneling microscopy measurements observe a gap opening at the Fermi surface below $\ensuremath{\approx}60$ K. Elucidating the microscopic properties and understanding the pairing mechanism of single-layer FeSe is of utmost importance as it is a basic building block of iron-based superconductors. Here, we use the low-energy muon spin rotation/relaxation technique to detect and quantify the supercarrier density and determine the gap symmetry in FeSe grown on ${\mathrm{SrTiO}}₃$ (100). Measurements in applied field show a temperature-dependent broadening of the field distribution below $\ensuremath{\sim}60$ K, reflecting the superconducting transition and formation of a vortex state. Zero-field measurements rule out the presence of magnetism of static or fluctuating origin. From the inhomogeneous field distribution, we determine an effective sheet supercarrier density ${n}_s^{2\text{D}}\ensuremath{\simeq}6\ifmmode\times\else\texttimes\fi{}{10}¹⁴\phantom{\rule{4pt}{0ex}}{\mathrm{cm}}^{\ensuremath{-}2}$ at $T\ensuremath{\rightarrow}0$ K, which is a factor of 4 larger than expected from ARPES measurements of the excess electron count per Fe of 1 monolayer FeSe. The temperature dependence of the superfluid density ${n}_s(T)$ can be well described down to $\ensuremath{\sim}10$ K by simple $s$-wave BCS, indicating a rather clean superconducting phase with a gap of 10.2(1.1) meV. The result is a clear indication of the gradual formation of a two-dimensional vortex lattice existing over the entire large FeSe/STO interface and provides unambiguous evidence for robust superconductivity below 60 K in ultrathin FeSe.