Structural Phase Transitions: a Limiting Factor for Quantum Criticality and Superconductivity in La_1.6−xNd_0.4Sr_xCuO₄

One branch of the La-214 family of cuprate superconductors, La_1.6−xNd_0.4Sr_xCuO₄, has been of great interest as it displays the full complexity of the canonical hole-doped high TC phase diagram. The strong dependence of the electronic properties on the crystal symmetry has motivated careful diffraction studies of the La_1.6−xNd_0.4Sr_xCuO₄ structural phase diagram.

We present our recent findings on the structural transitions in the La_1.6−xNd_0.4Sr_xCuO₄ system. Our group has synthesized single-phase polycrystalline samples with x spanning from 0.01 to 0.40, and grew large single crystals of La_1.6−xNd_0.4Sr_xCuO₄ for select x. To the best of our knowledge, such high Sr concentrations have not been achieved thus far. Systematic neutron and X-ray diffraction studies were performed at both low and high temperatures. These analyses allowed us to follow the sequence of structural phase transitions and to propose an updated La_1.6−xNd_0.4Sr_xCuO₄ phase diagram that now covers Sr doping concentrations up to x = 0.40. Furthermore, our findings suggest that the persistence of the high temperature tetragonal structure, HTT, down to low temperatures when x = 0.27 might be a limiting factor for both superconductivity and the putative quantum critical point proposed for La_1.6−xNd_0.4Sr_xCuO₄.