Layer buckling and absence of superconductivity in LaNiO2

The recent observation of unconventional superconductivity in thin films of LaNiO2 (critical temperature, Tc~10 K) and in bulk single crystals of La3Ni2O7 (327) under pressure (Tc~80 K), has brought to light a long sought-after class of superconducting nickelates. Through structural measurements in the 327-system, it was shown that the absence of superconductivity is related to bending of the O-Ni-O bonds. Similarly, the bond bending may be linked to the absence of superconductivity in bulk LaNiO2. Neutron diffraction was used on bulk non-superconducting La(1-x)Sr(x)NiO(2) samples to show that the layers are naturally buckled, creating a Ni-O-Ni bond angle of 177 at 2 K and ambient pressure. The buckling angle increases to 170 on warming to room temperature. Furthermore, a broad paramagnetic continuum is observed that decreases in intensity on cooling from room temperature signaling a possible transition to a coherent state. However, no antiferromagnetic (AFM) peaks are detected, although enhancement of ferromagnetic (FM) correlations cannot be excluded.