Prediction of a Surface State and a Related Surface Insulator-Metal Transition for the (100) Surface of Stochiometric EuO

Abstract

We calculate the temperature and layer-dependent electronic structure of a 20-layer EuO(100)-film using a combination of first-principles and model calculation based on the ferromagnetic Kondolattice model. The results suggest the existence of a EuO(100) surface state which can lead to a surface insulator-metal transition.In the recent past many theoretical and experimental research works have been focussed on the intriguing properties of rare earth metals and their compounds. Among others, the extraordinary surface magnetic properties of the lanthanides [1], as e. g. an enhanced Curie temperature of the Gd(0001) surface compared to that of bulk Gd [2], have provoked numerous research activities. Concerning the interplay between electronic structure and exceptional magnetic properties at the Gadolinium surface a Gd(0001) surface state [3,4] is believed to play a crucial role and its temperature dependent behaviour has been discussed intensely, e. g, [5].Rare-earth materials are so-called local-moment systems, i.e. the magnetic moment stems from the partially filled 4f-shell of the rare-earth atom being strictly localized at the ion site. Thus the magnetic properties of these materials are determined by the localized magnetic moments. On the other hand, the electronic properties like electrical conductivity are borne by itinerant electrons in rather broad conduction bands, e.g. 6s, 5d for Gd. Many of the characteristics of local-moment systems can be attributed to a correlation between the localized moments and the itinerant conduction electrons. For this situation the ferromagnetic Kondo-lattice model (FKLM) which is also referred to as the s-f model has been proven to be an adequate description. In this model, the correlation between localized moments and conduction electrons is represented by an intraatomic exchange interaction.In this work we will introduce a multi-band FKLM and use it to calculate the temperature and layer-dependent bandstructure of EuO films. For all the calculations, the surfaces of the films are parallel to the fcc(100) crystal plane and the films will be referred to as EuO(100) films. These films consist of n equivalent parallel layers. The lattice sites within the film are indicated by a greek letter α, β, γ, . . . , denoting the layer, and by a latin letter i, j, k, . . . , numbering the sites within a given layer. The different subbands of the conduction bands of EuO will be denoted by the indices m, m ′ .The Hamiltonian for the multi-band FKLM,consists of three parts. The first,contains the kinetic energy of the conduction band electrons. c + iαmσ (c iαmσ ) is the creation (annihilation) operator of an electron with spin σ from the m-th subband at the lattice site R iα . The T mm ′ ijαβ describe the hopping between the m-th subband at the lattice site R iα and the m ′ -th subband at the lattice site R jβ . These hopping integrals have to be determined within a first-principles band-structure calculation.The second part of the Hamiltonian represents the system of the localized f-momen...

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