Hole doped state in extremely lightly doped lanthanum copper oxide

An effective approach to understanding cuprate physics is to study the physical properties of cuprates at the initial doping level. Experimental studies in the extremely lightly doped region will provide intrinsic initial doping behaviors of cuprates. This information is important for understanding the fundamental mechanism of high temperature superconductivity. In this study, by preparing the samples through controlling the post heat-treatment process and adjusting the hole concentration using the electrochemical method, we investigated the magnetic properties of La2CuO4+delta in the extremely lightly doped region. We find: (1) A ferromagnetic-like anomaly coupled with the three-dimensional anti ferromagnetic transition can be observed after annealing the sample in flowing oxygen at a temperature range from 1040°C to 800°C, which is comparable to the coupling energy of a two-dimension anti ferromagnetic lattice. This result suggests that the ferromagnetic-like anomaly corresponds to an electronic quantum state formed at a temperature where the two-dimensional antiferromagnetic correlation begins to develop. (2) The Neel temperature can be expressed as 1-TN(ph)/TN(0) ≈ 6.2 ph+ (ph/0.025) 2 in the sample prepared with hole concentration steps of 0.001 using the electrochemical method. The Neel temperature in both cation and anion doped La2CuO4 follows the same expression. These results suggest that the suppression of the Neel temperature by doped holes is attributed to a combinatorial effect of dilution and finite size and that the combinatorial effect is an intrinsic property of the doped-hole state. (3) The Neel temperature remained constant at around 250 K when the hole concentration reached ∼0.01 in our La 2CuO4+delta samples. This suggests that there is an intrinsic, critical hole concentration in the La2CuO4+delta system. When the critical hole concentration is reached, the electronic phase separation takes place and the La2CuO4+delta system enters the miscibility gap.