Charge order at magnetite Fe₃O₄(0 0 1): surface and Verwey phase transitions

Abstract

At ambient conditions, the Fe3O4(001) surface shows a ( √ 2 × √ 2)R45 o reconstruction that has been proposed as the surface analog of the bulk phase below the Verwey transition temperature, TV . The reconstruction disappears at a high temperature, TS, through a second order transition. We calculate the temperature evolution of the surface electronic structure based on a reduced bulk unit cell of P 2/m symmetry that contains the main features of the bulk charge distribution. We demonstrate that the insulating surface gap arises from the large demand of charge of the surface O, at difference with that of the bulk. Furthermore, it is coupled to a significant restructuration that inhibits the formation of trimerons at the surface. An alternative bipolaronic charge distribution emerges below TS, introducing a competition between surface and bulk charge orders below TV .

Direct observation of subsurface charge ordering in Fe3O4(001) by scanning tunneling microscopy/spectroscopy

“…These dI/dV signals may include contributions from the surface Fe B atoms, however all surface Fe B atoms exclusively exhibited an Fe 3+ charge state. 4,9,16,20,24) If the spectra included mainly the surface Fe B LDOS, such a visible difference in the LDOS would not be observed. Therefore, the dI/dV spectra in Fig.…”

“…• surface reconstruction and the surface semiconducting nature have been proposed to be derived from the charge-ordered electronic states of the subsurface Fe B atoms. 8,9,20) Figure 1(b) shows the proposed model of the subsurface charge-ordered structure. The Fe 2+ -Fe 2+ and Fe 3+ -Fe 3+ dimers aline in an alternating manner along the [1][2][3][4][5][6][7][8][9][10] direction in the subsurface layer.…”

“…• surface reconstruction pattern. Although a few theoretical studies have investigated subsurface electronic structures, 8,9) the literatures contain no related experimental studies. This experimental exploration can ad- * E-mail address: hiura@nano.ist.hokudai.ac.jp • reconstructed unit cell is indicated by the black square.…”

“…4(c). The line profile was taken along the [1][2][3][4][5][6][7][8][9][10] direction, i.e., the direction perpendicular to the Fe B rows. This result indicates that the periodicities of high(low)-peak to high(low)-peak and high(low)-peak to low(high)-peak are ∼1.2 and ∼0.6 nm, respectively.…”

Electron-transfer-induced metallic electronic states in a H/Fe3O4(001) film subsurface

“…4(b). Given that DFT calculations predict the DOS of O(S) atoms to be almost zero near the Fermi level, 29) this highest DOS should reflect the greater reduction in subsurface Fe atoms than in Fe B (S-1) atoms. This metallization of subsurface Fe atoms induced by H adsorption is similar to the DFT result for the C/Fe 3 O 4 (001) surface, 10) which showed a metallic state in subsurface Fe atoms and a semiconducting state in surface Fe atoms.…”