Crossover in charge transport from one-dimensional copper-oxygen chains to two-dimensional ladders in (La,Y)_y(Sr,Ca)_14-yCu ₂₄O₄₁

The charge transport in the copper-oxygen chain/ladder layers of (La,Y) y (Sr,Ca) 14-y Cu24 O41 is investigated along two crystallographic directions in the temperature range from 50 to 700 K and for doping levels from y≈6 (number of holes nh <1) to y=0 (number of holes nh =6). A crossover from a one-dimensional hopping transport along the chains for y≥3 to a quasi-two-dimensional charge conduction in the ladder planes for y≲2 is observed. This is attributed to a partial hole transfer from chains to ladders when the hole doping exceeds nh ≈ 4 and approaches fully doped value nh =6. For y≲2 a weak dielectric relaxation at radio frequencies and a microwave mode are detected, which might be recognized as signatures of a charge-density wave phase developed at short length scales in the ladder planes.