Conductivity and Hall coefficient inLa2CuO4+ynear the insulator-metal transition

Measurements are reported of the conductivity, its anisotropy, and the in-plane Hall coefficient for a single crystal of ${\mathrm{La}}_2\mathrm{Cu}{\mathrm{O}}_{4+y}$ with $y$ varied from 0%-1.5%. The conductivity is thermally activated with a single activation energy at high temperatures and with a continuously decreasing activation energy below ∼50 K. The anisotropy is as large as 500 at high temperatures, falls sharply to 10-20 at ∼50 K, and then is only weakly $T$ dependent at lower temperatures. The Hall coefficient is simply activated above ∼50 K (with an activation energy similar to that of the in-plane conductivity), and has a peak at lower temperatures. All these features are evidence for a change in the transport mechanism near 50 K. The low-temperature conductivity is consistent with thermally assisted tunneling between localized states at the Fermi energy. The high-temperature process arises from thermal activation of carriers into a band of highly anisotropic states. The low mobility associated with these states makes it impossible, however, to determine whether they arise from impurities or are states near a band or mobility edge.