In crystalline solids, the interactions of charge and spin can result in a variety of emergent quantum ground states, especially in partially filled, topological flat bands such as Landau levels or in “magic angle” graphene layers. Much less explored is rhombohedral graphite (RG), perhaps the simplest and structurally most perfect condensed matter system to host a flat band protected by symmetry. By scanning tunneling microscopy, we map the flat band charge density of 8, 10, 14, and 17 layers and identify a domain structure emerging from a competition between a sublattice antiferromagnetic insulator and a gapless correlated paramagnet. Our density matrix renormalization group calculations explain the observed features and demonstrate that the correlations are fundamentally different from graphene-based magnetism identified until now, forming the ground state of a quantum magnet. Our work establishes RG as a platform to study many-body interactions beyond the mean-field approach, where quantum fluctuations and entanglement dominate.

Friction stir welding of a low carbon thin steel plate was performed with preheating the base material to increase the joining performance. The rotational speed of the tool was set from 1500 to 2000 rpm with a constant travel speed of 15 mm/min and preheating temperature of 50 to 150 °C. The tensile strength of 340 MPa was achieved for the preheated specimen compared with 310 MPa for the non-preheating specimen at the welding speed of 15 mm/min. Electron microscopic images of the preheated joint revealed a remarkable plasticized material flow and dynamic recrystallization, which resulted in the grain refinement that had increased the joining strength. The weld thinning issue was almost eliminated in the preheated welded specimen. However, by increasing the preheat temperature further, the tensile strength decreases due to the formation of defects from excessive heat. The preheated sample fractured at the base metal, revealing a ductile fracture mode.