Gate-induced superconductivity using field effect transistor devices has a strong advantage over the conventional bulk superconductivity owing to its controllability. To realize the gate-induced superconductivity, researchers have nowadays established two methods; one is to use gate dielectrics which enables carrier accumulation with high enough carrier density for inducing superconductivity. The other is to use channel materials with large lattice parameters, which enable the effective high band-filling with a rather low carrier density. The former was realized by ionic gating first on SrTiO₃ followed by many layered materials [1]. The latter was achieved in 2018 [2] on magic angle twisted bilayer graphene. Of particular importance of gate-induced superconductivity is that it is a unique platform of two-dimensional (2D) moderately clean superconductivity.

In this talk, we would like to discuss the uniqueness of 2D superconductivity from the view point of vortex physics and low carrier density superconductors. As for the vortex physics, we emphasize that the vortex liquid state can be a ground state of 2D superconductor with weak pinning [1, 3]. This ground sate are called quantum metal, failed superconductor, or Bose metal. We established a dynamical vortex phase diagram in the T-B-I space for relatively clean 2D superconductors [4].

Another advantage of the ionic gating is the tunability of carrier density, which allows us to approach not only the high but also the low-carrier density limit. On LixZrNCl superconductors, we found that superconductivity takes place at much lower temperature than the pair-forming temperature. In other words, superconductivity seemingly takes place as a BEC of preformed pairs in the pseudo-gap phase. T_BKT is well scaled as T_BKT/T_F = 0.125, where T_BKT and T_F is the Berezinsky-Kosterlitz-Thouless transition temperature and Fermi temperature. This scaling is in fair agreement with the theoretical upper bound of T_c in the 2D BCS-BEC crossover [5].

[1] Y. Saito et al., Nat. Rev. Mater. 2, 16094 (2016).
[2] Y. Cao et al., Nature 556, 43 (2018).
[3] Y. Saito et al., Science 350, 409 (2015).
[4] Y. Saito et al., Phys. Rev. Mat. 4, 074003 (2020).
[5] Y. Nakagawa et al., submitted.

Keywords: 2D superconductivity, Bose metal, BCS-BEC crossover