Phase separation in a high-pressure-oxygenated ${\mathrm{La}}_2$${\mathrm{CuO}}_{4+\mathrm{\delta }}$ crystal: Evidence from anisotropic electronic transport and magnetic susceptibility

We report magnetic susceptibility, resistivity, and thermoelectric power measurements performed on a high-pressure-oxygenated ${\mathrm{La}}_2$ ${\mathrm{CuO}}_{4+\mathrm{\delta }}$ single crystal. Distinct anomalies are present in all three quantities between 200 and 280 K that are attributed to a phase separation involving oxygen diffusion. Hysteresis present in the data near the phase-separation temperature indicates that the process is weakly first order. We suggest that this transition may be driven, in part, by interactions between excess oxygen ions and copper spin magnetism.