Conductivity of YBa2Cu3O6+x single crystals close to the superconductor-nonsuperconductor transition

We report conductivity measurements of YBa2Cu3O6+x single crystals with x approximate to 0.4 close to the transition from the superconducting to the nonsuperconducting state. The transition takes place upon decreasing the concentration of carriers, which is realized without changing the oxygen content in the sample but due to the effect of ordering in the Cu-O chains. It is shown that the transition to a nonsuperconducting metal takes place first, and only with further decrease of the concentration of carriers YBa2Cu3O6+x becomes dielectric.