Demonstrating the model nature of the high-temperature superconductor HgBa2 CuO4+δ

The compound HgBa2 CuO4+δ (Hg1201) exhibits a simple tetragonal crystal structure and the highest superconducting transition temperature (Tc) among all single Cu-O layer cuprates, with Tc =97 K (onset) at optimal doping. Due to a lack of sizable single crystals, experimental work on this very attractive system has been significantly limited. Thanks to a recent breakthrough in crystal growth, such crystals have now become available. Here we demonstrate that it is possible to identify suitable heat treatment conditions to systematically and uniformly tune the hole concentration of Hg1201 crystals over a wide range, from very underdoped (Tc =47 K, hole concentration p≈0.08) to overdoped (Tc =64 K, p≈0.22). We then present quantitative magnetic susceptibility and dc charge transport results that reveal the very high-quality nature of the studied crystals. Using x-ray photoemission spectroscopy on cleaved samples, we furthermore demonstrate that it is possible to obtain large surfaces of good quality. These characterization measurements demonstrate that Hg1201 should be viewed as a model high-temperature superconductor.