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Bulk FeSe is superconducting with a critical temperature $T_c$ $\cong$ 8 Kand SrTiO$_3$ is insulating in nature, yet high-temperature superconductivityhas been reported at the interface between a single-layer FeSe and SrTiO$_3$.Angle resolved photoemission spectroscopy and scanning tunneling microscopymeasurements observe a gap opening at the Fermi surface below $\approx$ 60 K.Elucidating the microscopic properties and understanding the pairing mechanismof single-layer FeSe is of utmost importance as it is a basic building block ofiron-based superconductors. Here, we use the low-energy muon spinrotation/relaxation technique (LE-$\mu$SR) to detect and quantify thesupercarrier density and determine the gap symmetry in FeSe grown on SrTiO$_3$(100). Measurements in applied field show a temperature dependent broadening ofthe field distribution below $\sim$ 60 K, reflecting the superconductingtransition and formation of a vortex state. Zero field measurements rule outthe presence of magnetism of static or fluctuating origin. From theinhomogeneous field distribution, we determine an effective sheet supercarrierdensity $n_s^{2D} \simeq 6 \times 10^{14}$ cm$^{-2}$ at $T \rightarrow 0$ K,which is a factor of 4 larger than expected from ARPES measurements of theexcess electron count per Fe of 1 monolayer (ML) FeSe. The temperaturedependence of the superfluid density $n_s(T)$ can be well described down to$\sim$ 10 K by simple s-wave BCS, indicating a rather clean superconductingphase with a gap of 10.2(1.1) meV. The result is a clear indication of thegradual formation of a two dimensional vortex lattice existing over the entirelarge FeSe/STO interface and provides unambiguous evidence for robustsuperconductivity below 60 K in ultrathin FeSe.

Direct evidence of superconductivity and determination of the superfluid density in buried ultrathin FeSe grown on SrTiO$_3$