  Intercalation of metal atoms at the SiC(0001)-graphene (Gr) interface can
provide confined 2D metal layers with interesting properties. The intercalated
Pb monolayer (ML) has shown the coexistence of the Gr(10x10)-moire and a stripe
phase, which still lacks understanding. Using density functional theory
calculation and thermal annealing with ab initio molecular dynamics, we have
studied the formation energy of SiC(0001)/Pb/Gr for different coverages of
intercalated Pb. Near the coverage of a Pb(111)-like ML mimicking the
(10x10)-moire, we find a slightly more stable stripe structure, where one half
of the structure has compressive strain with Pb occupying the Si-top sites and
the other half has tensile strain with Pb off the Si-top sites. This stripe
structure along the Gr zigzag direction has a periodicity of 2.3 nm across the
[1-210] direction agreeing with the previous observations using scanning
tunneling microscopy. Analysis with electron density difference and density of
states show the tensile region has a more metallic character than the
compressive region, while both are dominated by the charge transfer from the Pb
ML to SiC(0001). The small energy difference between the stripe and
Pb(111)-like structures means the two phases are almost degenerate and can
coexist, which explains the experimental observations.
