Abstract
We track the systematic variation in magnetic susceptibility (χ), thermal conductivity (κ), anisotropic resistivity (ϱ), and thermoelectric power ( S) of crystalline La 2CuO 4+ δ as a function of light hole doping. The level of hole doping (δ) was carefully varied by anneals under controlled oxygen-partial-pressure performed in such a way that the antiferromagnetic ordering temperature T N ranged from 250 to 310 K. χ data indicate that increased oxygen hole doping acts to reduce T N below the δ = 0 value of 320 K. Anomalies are present in κ at T n that result from Cu spin fluctuations about the magnetically ordered ground state. Anomalies are also present in ϱ and S that occur at T N for air-annealed samples wherein T N = 250 K; when more lightly hole-doped so that T N >250 K, the anomalies in ϱ and S do not occur at T N, but instead occur at T<250 K. In analogy with heavily oxygen-loaded La 2CuO 4+ δ , these transport anomalies may result from phase separation involving the diffusion of non-stoichiometric, excess oxygen.
Highlights
Extensive research carried out during the past three years indicates that La2CuOq+d is remarkable material [ I]
The resistivity is extremely anisotropic at room temperature and becomes less so as the material is cooled into the non-Arrhenius low-temperature regime
Resistivity, thermoelectric power and thermal conductivity measurements indicate that charge transport in lightly-doped LazCu04+6 involves carrier hopping between localized states
Summary
Extensive research carried out during the past three years indicates that La2CuOq+d is remarkable material [ I]. The Cu moments undergo two-dimensional short-range magnetic order at temperatures far above room temperature [ 2,3 1, while complete three-dimensional antiferromagnetic order sets in at TNz 328 K [2,4]. When the material is hole-doped by replacing 10% of the lanthanum by strontium, it becomes metallic and undergoes a “high-temperature” superconducting transition at T,E 40 K [ 5 1. Strontium doping acts to destroy long range antiferromagnetic Cu order. La,CuO,+, can be hole-doped as well by adding excess oxygen to the system (i.e. 6> 0). The presence of the excess oxygen produces frustration in the antiferromagnetic copper-spin sublattice, resulting in the reduction of TN below 328 K [ 2,7 1.
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