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Charge order at magnetite Fe3O4(001): surface and Verwey phase transitions

At ambient conditions, the Fe$_3$O$_4$(001) surface shows a $(\sqrt{2}\times \sqrt{2})R45^o$ reconstruction that has been proposed as the surface analog of the bulk phase below the Verwey transition temperature, T$_V$. The reconstruction disappears at a high temperature, T$_S$, through a second order transition. We calculate the temperature evolution of the surface electronic structure based on a reduced bulk unit cell of P2/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 T$_S$, introducing a competition between surface and bulk charge orders below T$_V$.