Intercalated europium metal in epitaxial graphene on SiC

X-ray magnetic circular dichroism (XMCD) reveal the magnetic properties of intercalated europium metal under graphene on SiC(0001). Intercalation of Eu nano-clusters (average size 2.5 nm) between graphene and SiC substate are formed by deposition of Eu on epitaxially grown graphene that is subsequently annealed at various temperatures while keeping the integrity of the graphene layer. Using sum-rules analysis of the XMCD of Eu M$_{4,5}$ edges at $T = 15$ K, our samples show paramagnetic-like behavior with distinct anomaly at T $\approx$ 90 K which may be related to the N{\`e}el transition, T$_N$ = 91 K, of bulk metal Eu. We find no evidence of ferromagnetism due to EuO or antiferromagnetism due to Eu$_2$O$_3$ indicating that the graphene layer protects the intercalated metallic Eu against oxidation over months of exposure to atmospheric environment.Comment: 6 pages, 5 figure

Energetics of Cu adsorption and intercalation at graphite step edges

To assess the energetics of Cu intercalation on defective graphite, the chemical potentials and binding energies for Cu at graphite step edges are calculated for three main configurations: an isolated atom, a chain, and an atom attached to a chain. As expected, for Cu interacting directly with a graphite step edge, the strength of interaction depends on the stability of the step, with Cu binding more strongly at a less-stable step. However, the relationship is reversed when considering binding of a Cu atom attached to a chain. Taken together, these trends mean that if the graphite step is less stable, as for the zigzag step, then decorating the step with a Cu chain facilitates intercalation by additional Cu atoms (which are less strongly bound to the decorated step). For more stable steps, intercalation is optimal without decoration. We also calculate the diffusion barrier for atomic Cu on top of the graphite terrace and, in the uppermost gallery, find values of 0.008 and 0.021 eV, respectively. These values are very small, indicating that the minimum barrier for a Cu atom to detach from a step and move to a terrace or gallery is dominated by the difference in binding energies. For intercalation, this minimum barrier is 1.4 to 3.1 eV and depends strongly on step configuration

Stochastic coarsening model for Pb islands on a Si(111) surface

The coarsening behavior of individual Pb islands on Si(111) surface has been studied by scanning tunneling microscopy. Traditionally island decay follows a smooth power-law dependence on the time until disappearance. In Pb/Si(111), some unstable islands are inactive for a long time but once their decay is triggered they suffer a “sudden death.” Four-layer islands are found to decay rapidly, increasing the area covered by seven-layer islands. All islands, decaying or otherwise, are accompanied by island size fluctuation which involve a large number of perimeter atoms moving collectively as a “quantized” unit. A stochastic model is developed to elucidate the mechanism behind this coarsening behavior of Pb islands. The distinct evolution of the islands with different heights is correctly predicted, and the size fluctuations of islands and the sudden death behavior observed in island coarsening are also recovered. The key ingredients are incorporation of accurate non-Gaussian statistics of the size fluctuations and also accounting for size changes in large quantized bursts