摘要
Quantum size effect (QSE) on the electronic structures of nanoscale antiferromagnets with a monolayer (ML) thickness is of fundamental importance in antiferromagnetic (AFM) spintronics. Here, we have carried out systematic studies on the size-dependent unoccupied electronic states of ML Mn nanoislands on Ag(111) by employing scanning tunneling microscopy/spectroscopy (STM/STS) together with density-functional theory (DFT). According to bias-dependent height profiles, a lower apparent height has been found on the larger Mn island within a bias voltage range of +/- 1.0 eV, suggesting a smaller Mn-Ag interlayer distance. Additionally, a single broad dI/dU peak from small Mn islands gradually evolves into two distinct peaks at approximately 1.3 eV (peak 1) and 1.5 eV (peak 2) as the island size increases. On top of that, peaks 1 and 2 move about 0.37 +/- 0.05 eV and 0.30 +/- 0.03 eV toward lower energy positions when the area size of the ML Mn island increases. Given the projected density of states (PDOS) deduced from orbital-dependent electronic band structures of ML Mn/Ag(111), two unoccupied dI/dU peaks originate from the contributions of out-of-plane Mn-3d orbitals. Further PDOS comparison analyses reveal that the Mn-Ag interlayer coupling develops a stronger energy shift in unoccupied states than the Mn-Mn atomic bonding, yielding the two-peak feature in dI/dU spectra resolved experimentally.