“…Many different opposite models have been proposed based on structural and nonstructural experimental techniques such as low-energy electron diffraction ͑LEED͒, reflection highenergy electron diffraction ͑RHEED͒, medium-energy ion scattering ͑MEIS͒, real-space atom-resolved scanning tunneling microscopy ͑STM͒, Auger and photoemission spectroscopies, 16 -24 and also a few ab initio total-energy theoretical calculations. [25][26][27] These include ͑i͒ the double dimer row model ͑DDRM͒ with a surface terminated by a 2 3 monolayer ͑ML͒ Si, 16,12,20,[22][23][24] ͑ii͒ the single dimer row model ͑SDRM͒ terminated by a 1 3 -ML Si, 19 ͑iii͒ the alternate dimer row model ͑ADRM͒ predicted theoretically and having 2 ϫ3 reconstruction with a 1 3 Si ML coverage and asymmetric dimers, 25,26 ͑iv͒ another ADRM having a 3ϫ2 surface array and asymmetric dimers as established by atom-resolved scanning tunneling microscopy ͑STM͒, 21 and finally, ͑v͒ a two adlayer asymmetric dimer model ͑TAADM͒ predicted by ab initio pseudopotential total-energy and grand canonical potential calculations. 27 The TAADM is basically the ADRM 3ϫ2 model built on top of the DDRM.…”