Harmonic voltage response to AC current in the nonlinear conductivity of iridium oxide Ca5Ir3O12

We have carried out harmonic voltage response experiments by application of AC current along the c-axis of Ca5Ir3O12, which has a nonlinear electrical conductivity in a non-ordered state. This AC current method can allow us to investigate a detail of the nonlinear conductivity by application of small current. We observed the harmonics up to 7th order below 200 K. The results reveal that the nonlinear conductivity exists even in application of current close to zero. In addition, the temperature dependence of the resistance R0 estimated at the zero current limit is expressed by ln R0 ∝T-2/3, which is explained by an adaptation of Efros-Shklovskii variable range hopping or Fogler-Teber-Shklovskii variable range hopping. As a field assisted motion of charge career occurs in hopping conduction, from this analysis result, the nonlinear conductivity comes form the field-assisted hopping conduction.