Electric field effect on the transport properties of ultrathin Y0.9Pr0.1Ba2Cu3Oy channels in the insulating phase of superconductor-insulator transition

The electric field effect on the transport properties of ultrathin Y0.9Pr0.1Ba2Cu3Oy channels was examined as a function of channel thickness and channel length. Samples having a planar YBa2Cu3Ox ultrathin Y0.9Pr0.1Ba2Cu3Oy- YBa2Cu3Ox structure with a gate electrode on the backside of their SrTiO3 substrate were used. The channel layer exhibited superconducting properties when its thickness was more than 9 nm, whereas channels less than 7.5 nm thick were in the insulating phase of the superconductor-insulator transition. The modulation ratio of the conductance of the insulating (I)-phase channels was a few orders of magnitude higher than the modulation ratio of the carrier density. The field-effect mobility increased with increasing channel thickness when the film was in the insulating phase. The sheet conductance of channels shorter than 10 μm was higher than that of a 100-μm-long channel of the same thickness due to a size effect on the transport of the I-phase Y0.9Pr0.1Ba2Cu3Oy channel. The field-effect mobility was also enhanced by the size effect. These results imply that the performance of possible field-effect transistors with high-temperature superconductors may be much improved by utilizing an I-phase channel whose length is short enough for the size effect to be evident.