Phase Of La2CuO4 Research Articles

On the role of electronic and chemical phase separation in La2CuO4 + δ

The mechanisms contributing to the metallization and the formation of the conducting and, at low temperature, superconducting phase in La2CuO4 + δ are investigated by conductivity and susceptibility experiments on single crystals with particular attention to the thermal history of the samples. The results give strong evidence that the phase formation follows the concept of percolative electronic phase separation.

Antiferromagnetism in the high-T c related compounds (invited)

Magnetic susceptibility measurements on La2CuO4−y have revealed maxima in temperature indicative of antiferromagnetic ordering. Powder diffraction measurements have demonstrated the existence of superlattice peaks at temperatures below those corresponding to the susceptibility maxima, and polarized neutron measurements have confirmed that the peaks are indeed of magnetic origin. The deduced antiferromagnetic structure in the orthorhombic (Cmca) phase of La2CuO4−y consists of ferromagnetic sheets of Cu atoms in the b-c plane which are alternating in sign along the a axis [100]. The moment is parallel to the c axis. The Neel temperature is extremely sensitive to the oxygen deficiency y and decreases rapidly to zero as y goes from 0.03 to 0. The saturated moment is ≂0.4 μB/Cu atom for samples with the highest TN (290 K). Antiferromagnetic spin fluctuations have also been observed by polarized neutron scattering in the paramagnetic phase above TN. Recent results obtained on magnetic ordering in the YBa2Cu3O6−y family of compounds are also discussed, together with the possible implications for novel superconductivity mechanisms.

Studies of New Superconductors Revive Old Questions

Lanthanum copper oxide, which is now regarded as the prototype for the new high‐temperature superconductors, undergoes a phase transition to an antiferromagnetic state. The critical temperature for this transition, at which the magnetic moments on copper ions begin to order antiferromagnetically, depends sensitively on the oxygen concentration. Confirmation of the existence of this antiferromagnetic phase in La2CuO4−y and the determination, using neutron scattering, of the arrangement of magnetic moments in it provided definitive evidence last summer for the importance of magnetic phenomena in the new superconducting oxides. Philip Anderson (Princeton University) had proposed in January 1987 that superconductivity in La2CuO4−y doped with barium or strontium, which had been confirmed only two months earlier, arose from novel, short‐range antiferromagnetic correlations between copper spins.

Thermal conductivity of La2CuO4, La2NiO4, and Nd2CuO4 in the semiconducting and metallic phases

Thermal conductivity of polycrystalline La2CuO4, La2NiO4, and Nd2CuO4 was measured in the temperature range 300–1000 K. No anomaly in thermal conductivity has been observed during the semiconductor-to-metal transition of La2NiO4 or in the metallic phases of La2CuO4 and La2NiO4. A change of slope has been found, however, in the thermal conductivity of La2CuO4 at the crystallographic transition. The thermal conductivity of the oxides is mainly phononic in this temperature range.