Resistivity of nonmetallic La2−ySryCu1−xLixO4−δ single crystals and ceramics

Abstract

The resistivity of nonmetallic ${\mathrm{La}}_{2-y}{\mathrm{Sr}}_y{\mathrm{Cu}}_{1-x}{\mathrm{Li}}_x{\mathrm{O}}_{4-\delta }$ single crystals and ceramics accurately follows the functional form $\exp \left({\left(\frac{T_0}{T}\right)}^{\frac{1}{4}}\right)$, characteristic of variable-range hopping. For each of the crystals grown from Li-containing flux and CuO flux, pure reduced ceramics, and ceramics containing 0.025 to 0.2 mole% Li, the values of $T_0$ are in the range 0.3-7 × ${10}^6$ K. The hopping conductivity shows that the crystals, which also manifest the two-dimensional quantum spin fluid state, antiferromagnetism, and the tetragonal-to-orthorhombic transition, are nonmetallic because the electronic states at the Fermi energy are localized. No evidence of a large gap is observed, and all samples, including the reduced ceramic, are $p$ type, leading to the suggestion that in the nonmetallic state as well as the superconductor, the Fermi energy lies near the top of the band of singly occupied states (lower Hubbard band). The suppression of superconductivity by Li impurities is discussed.