Effective thickness of two-dimensional superconductivity in a tunable triangular quantum well of SrTiO₃

We report two-dimensionality and effective thickness of the superconductivity of a SrTiO₃ single-crystal surface induced by electric double-layer gating. The carrier density was tuned from 3 × 10¹³ to 1.1 × 10¹⁴ cm^-2 by gating, where superconductivity appears with Tc of around 0.4 K. Typical two-dimensional behavior perfectly described by the Ginzburg-Landau equation was observed in the angular and temperature dependence of the upper critical magnetic field. The effective thickness of the superconducting layer remains nearly invariant, ranging from 11 to 13 nm, with increasing charge carrier density. This invariance contradicts the expected reduction in the thickness of the accumulation layer in a triangular quantum well model. This unexpected invariance of the superconducting layer thickness is probably a unique nature for a two-dimensional electron system in the incipient ferroelectric SrTiO ₃.