Superconducing HgBa 2 CuO 4+x single crystals grown in an Al 2 O 3 crucible using a high-pressure furnace (argon pressure = 10.1 kbar) show a clean and smooth surface. Investigations by scanning tunnelling (STM), scanning force (SFM) and friction force (FFM) microscopies reveal an atomically flat surface with steps ∼4 nm high in the [001 ] direction. At the surface, particles are observed that may have been caused by ageing under ambient conditions. These particles exhibit a contrast reversal in FFM images, implying that they consist of a different material than the underlying matrix. Occasionally, free step-flow during crystal growth is hindered by atomic point defects, resulting in local step bending. Such processes may explain the micrometre-sized indents found along the steps. On an atomic scale, STM measurements reveal an approximately square atomic lattice (a = 4.0-4.4 A) and an unexpected domain structure with edge dislocations and point defects.
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