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

Abstract

Bulk FeSe is superconducting with a critical temperature $T_c$ $\cong$ 8 K and SrTiO${}_3$ is insulating in nature, yet high-temperature superconductivity has been reported at the interface between a single-layer FeSe and SrTiO${}_3$. Angle resolved photoemission spectroscopy and scanning tunneling microscopy measurements observe a gap opening at the Fermi surface below $\approx$ 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 (LE-$\mu$SR) to detect and quantify the supercarrier density and determine the gap symmetry in FeSe grown on SrTiO${}_3$ (100). Measurements in applied field show a temperature dependent broadening of the field distribution below $\sim$ 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 $n_s^{2D}\simeq 6\times 10^{14}$ cm${}^{-2}$ at $T\to 0$ K, which is a factor of 4 larger than expected from ARPES measurements of the excess electron count per Fe of 1 monolayer (ML) FeSe. The temperature dependence 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 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.

9 pages, 8 figures

References in corpus (11)

• High-temperature superconductivity in iron-based materials
• Superconductivity in single-layer films of FeSe with a transition temperature above 100 K
• Magnetic Penetration Depth in Unconventional Superconductors
• Electric-field-induced superconductivity in electrochemically-etched ultrathin FeSe films on SrTiO3 and MgO
• Interface induced high temperature superconductivity in single unit-cell FeSe films on SrTiO3(110)
• Universal Substrate Effect on the Superconductivity of FeSe Monolayer Films
• High-temperature superconductivity in single-unit-cell FeSe films on anatase TiO2(001)
• Intrinsic Paramagnetic Meissner Effect due to s-wave Odd-Frequency Superconductivity
• High-temperature superconductivity in one-unit-cell FeSe films
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• The Meissner effect in a strongly underdoped cuprate above its critical temperature

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• Superconductivity in monolayer FeSe enhanced by quantum geometry
• Heterostructural one-unit-cell FeSe/SrTiO3: from high-temperature superconductivity to topological states
• Probing the Berezinskii-Kosterlitz-Thouless vortex unbinding transition in two-dimensional superconductors using local noise magnetometry
• Studies of superconductivity of Fe chalcogenides in films grown by PLD technique
• Study on fluctuations of interface-enhanced superconductivity in ultrathin FeSe/SrTiO3 by the Nernst effect
• Robustness of superconducting properties to transition metal substitution and impurity phases in Fe1-xVxSe