Growth and characterization of HgBa ₂ CaCu ₂ O6+δ and HgBa ₂ Ca ₂ Cu ₃ O8+δ crystals

We report the successful growth of sizable crystals of the cuprate superconductors HgBa ₂ CaCu ₂ O ₆ +δ and HgBa ₂ Ca ₂ Cu ₃ O ₈ +δ. These compounds are well known for their high optimal superconducting critical temperatures of Tc=128 K and 134 K at ambient pressure, respectively, and for their challenging synthesis. Using a conventional quartz-tube encapsulation method and a two-layer encapsulation method that utilizes custom-built high-pressure furnaces, we are able to grow single crystals with linear dimensions up to several millimeters parallel to the CuO2 planes. Extended postgrowth annealing is shown to lead to sharp superconducting transitions, indicative of high macroscopic homogeneity. X-ray diffraction and polarized Raman spectroscopy are identified as viable nondestructive methods to help separate the two compounds from synthesis products, as the latter often contain intergrowths of the former. Our work helps to remove obstacles toward the study of these model cuprate systems with experimental probes that require sizable high-quality crystals.