Advances and challenges in the single-crystal synthesis of infinite-layer and Ruddlesden-Popper phase nickelate single crystals

High Tc superconductivity remains one of the main focuses in condensed matter research, in which, rare-earth nickel oxides have emerged as a new class with potential for unconventional superconducting behavior. Here, two types of structures have drawn particular interest: The first are nickelates with the infinite-layer crystal structure, such as Nd_0.8Sr_0.2NiO₂, with superconducting transition temperatures up to 20 K. The second type was found in Ruddlesden-Popper phase nickelates, such as La₃Ni₂O₇, which under hydrostatic pressure realize a remarkably high Tc of 80 K. Many details of these systems are not yet fully understood especially as for the first type only thin films show superconductivity up to now. In my talk I will show the challenges of optimal doping perovskite single crystals, and show the advancements we have done in obtaining optimal reduced crystals as well as the effects of intercalation on these single crystals. Furthermore, I present our findings on La₃Ni₂O₇, where sizable single crystals can be readily grown using the optical floating zone method at 15 bar oxygen pressure. Here, we recently observed that these crystals exhibit multiple crystallographic phases and a pronounced sensitivity to oxygen stoichiometry, affecting their physical properties and fascinating pressure dependence as will be highlighted in the talk.