SIN feature observed in intrinsic Josephson junction characteristics for overdoped Bi_2Sr_2CaCu_2O_(8+δ)

摘要

We have fabricated mesa structures made of 5 to 10 intrinsic Josephson junctions (IJJs) from overdoped and slightly overdoped Bi_2Sr_2CaCu_2O_(8+δ) (BSCCO) crystals and observed in their tunneling spectra a characteristics structure or a broad peak within the superconducting gap. We propose that this structure results from the inclusion of nonsuperconducting regions in the superconducting layer of IJJs. The cause of the structure is presumed to be the tunneling current through a superconductor/insulatorormal-metal (SIN) junction and the voltage drop due to the current flow in the normal regions after the tunneling through the SIN junctions. The broad peak located at a higher than Δ/e position reflects the current path through the normal region. All these features imply that the superconducting state is inhomogeneous. The experimental results are compared with numerical calculations based on a simple model to obtain a good agreement. From the comparison, it is concluded that the nonsuperconducting and superconducting regions coexist in overdoped BSCCO on a ～ 50 nm scale.