A density functional theory study of Mn nanowires on the Si(001) surface

Abstract

The structure of experimentally observed Mn nanolines on the Si(001) surface is investigated using density functional theory (DFT) and the DFT + U method. A candidate line structure consisting of a two-atom sub-unit is proposed, based on total energy and appearance in simulated scanning tunnelling microscopy images. The electronic and magnetic properties of this structure are investigated. The atoms in the line are strongly antiferromagnetically coupled with individual Mn atoms having moments of 4 µ(B). The atoms in the sub-unit are seen to move further apart by 0.57 Å upon forcing ferromagnetic alignment.

Structure of Self-Assembled Mn Atom Chains on Si(001)

“…2(b)], excluding several other sites, in particular the C and H sites considered in Refs. [21,22]. As shown in Figs.…”

“…While they also run perpendicular to the Si dimer rows, their precise structure remains unsolved. Several models, from single-atom to trimer systems, have been proposed [21][22][23]. But they all fail to reproduce a striking asymmetry of the Mn chains observed in filled state STM images [ Fig.…”

“…E ads =Mn ¼ ðE surfaceþMn − E surface − nE isolated Mn Þ=n (where n is the number of Mn atoms) was used to calculate the adsorption energies of Mn atoms on the surface. DFT þ U calculations [31] were performed using Coulomb and exchange parameters of U ¼ 4.2 eV and J ¼ 1 eV (in accordance with prior calculations [22]). STM images were simulated using the Tersoff-Hamman method, as implemented in bSKAN33 [32].…”

“…Simple DFT calculations (Table I) find that the H' site is one of the most stable Mn adsorption sites, though slightly less stable than the H site. The addition of a local on-site shift, using DFT þ U [22], changes the ordering and makes the H' site most stable, while the simulated STM is hardly altered. However, our calculations show no energetic gain in forming a chain of atoms located at the H' site on one side of the dimer row, either with or without Hubbard U.…”

Atomic Structure of Mn Wires on Si(001) Resolved by Scanning Tunneling Microscopy

“…The periodic boundary conditions in the simulation further limit the appearance of wire asymmetries because the supercell has to be dipole free and charge transfer from one Mn wire edge to the other may be suppressed. Moreover, even if pure DFT simulations are accurate enough to describe the interaction and atomic structure of Mn on Si surfaces [22][23][24]28], the contraction of partially occupied d-shells is often underestimated, resulting in a modified orbital hybridization and therefore distorted simulated STM images. Nevertheless, we show that a closer look at the relaxed atom positions allows us to identify the origin of the observed asymmetry.…”