Emergence of superconductivity on a SrTiO₃ surface by electric-field charge accumulation

We briefly review principles and main features of an electric double layer transistor (EDLT) as well as electric field induced superconductivity in SrTiO₃. EDLT is a field-effect transistor that employs an electrolyte as a gate dielectric. An electric double layer between a semiconductor and the electrolyte attains much higher breakdown field than the maximum of a solid gate dielectric, resulting in high density charge accumulation up to 10 ¹⁴cm^-2. That density is sufficient for inducing new physical phases, such as superconductivity and ferromagnetism, on various oxide systems. We employed a surface of a SrTiO₃ single crystal as a semiconductor channel. We have demonstrated insulator-tosuperconductor transition by electric field-effect without chemical doping. Charge carrier density was linearly increased from zero to 10¹⁴ cm^-2 with increasing gate bias to 3.5 V. Superconducting critical parameters, such as critical temperature T_c, critical magnetic field H_c, and critical current density J_c were examined as a function of carrier density by varying gate bias. T_c was almost constant as a function of the carrier density, contrasting to bell-shaped dependence of H_c and J_c. Temperature dependence of I-V curve shows the BKT-type transition, which indicates two-dimensional superconductivity in the electric field induced superconductivity.