Two dimensionality in electric field induced superconductivity

Applying electric field is recognized as a useful tool for search of novel superconductors by using the ionic gating. The method allows us to accumulate carrier density exceeding 1 $\times$ 10$^{14}$ cm$^{-2}$, which is sufficiently large enough for inducing superconductivity. Although such superconductivity has been demonstrated in several systems, physical properties have not been well investigated. In this presentation we will report on the phase diagram and two-dimensional (2D) nature of electric field induced superconductivity. We fabricated electric double layer transistor on superconductivity on ZrNCl [1] and MoS$_2$ [2] using mechanical exfoliation followed by electron beam lithography. First we have established a relation between ${\it T}_c$ and the sheet carrier density on both compounds, and compared with the bulk phase diagram. Second, we measured angle dependence of upper critical field and confirmed the 2D nature of superconductivity in both compounds. We have obtained the thickness of superconductivity as 1-2 nm in both compounds. \\[4pt] [1] J. T. Ye et al., Nat. Mater. 9, 125 (2010).\\[0pt] [2] J. T. Ye et al., Science 338, 1193 (2012).