The spin-polarized scanning tunneling microscope (SP-STM) operated in the constant current mode is proposed as a powerful tool to investigate complex atomic-scale magnetic structures of otherwise chemically equivalent atoms. The potential of this approach is demonstrated by successfully resolving the magnetic structure of Cr͞Ag(111), which is predicted on the basis of ab initio vector spin-density calculations to be a coplanar noncollinear periodic 120 ± Néel structure. Different operating modes of the SP-STM are discussed on the basis of the model of Tersoff and Hamann. DOI: 10.1103/PhysRevLett.86.4132 PACS numbers: 75.30.Fv, 68.37.Ef, 75.70.Ak, 75.70.Rf Exploiting the spin, rather than the charge degrees of freedom, is the core vision behind the current excitement driving the rapid developments in magneto-and spin electronics. Some of the key issues relate to the understanding of the magnetic properties of nanoscale magnets with competing exchange interactions between neighboring atoms. Examples are (ultrathin) ferromagnetic films in contact with antiferromagnetic ones, as is common for exchangebias systems [1] used in the magnetic recording industry. In many cases, the geometrical arrangement of the atoms does not allow one to satisfy the competing exchange interactions between neighboring atoms, which leads to frustrated spin structures. Frustration gives rise to a wide variety of complex spin structures on the atomic scale, such as antiferromagnetism, spiral spin-density waves (SSDW), or general noncollinear states [2]. These spin structures are still poorly understood because of the inability of traditional techniques to spatially determine the magnetic structure. Even the currently most advanced techniques, the x-ray spectromicroscopy [3] and the spin polarized scanning tunneling microscope (SP-STM) in the spectroscopy mode [4,5], have no atomic-scale resolution.In this paper we introduce a new powerful method -the SP-STM operated in the constant current mode -to image complex magnetic structures at surfaces on the atomic scale. By applying the Tersoff-Hamann model [6] to the case of a SP-STM and considering the effect of the vacuum barrier on the lateral resolution of the STM, we show that in general the SP-STM image of any periodic magnetic superstructure of otherwise chemically equivalent atoms displays a pronounced pattern corresponding to the magnetic configuration and not to the geometric arrangement of the atoms. This is in contradiction to the conventional wisdom that spin polarization is a small effect and that therefore the nonspin-polarized STM image reflecting the atomic structure will be only slightly modulated in the SP-STM experiment.This approach opens up a new route for using the STM, namely, besides the exploration of the topological, chemical, and ferromagnetic structure of surfaces, and also the inherently much more difficult investigation of surfaces with noncollinear spin structures with ultimate, i.e., atomic, resolution. This new concept was very recently applied for the first ...

We report on the epitaxial fabrication and electronic properties of a topological phase in strained α-Sn on InSb. The topological surface state forms in the presence of an unusual band order not based on direct spin-orbit coupling, as shown in density functional and GW slab-layer calculations. Angle-resolved photoemission including spin detection probes experimentally how the topological spin-polarized state emerges from the second bulk valence band. Moreover, we demonstrate the precise control of the Fermi level by dopants.

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A method for the first-principles calculation of the transfer matrix is presented. The method is based on a Green-function formulation and allows one to relate the wave functions and their derivatives on boundaries at opposite sides of a film or junction of finite thickness. Both the underlying theory and an actual implementation in the full-potential linearized augmented plane wave method are described. Currently the embedding method is used to evaluate the Green-function matrix elements and in turn we show that the transfer matrix can be used to construct the embedding potential. Some possible applications of the transfer-matrix method such as the calculations of the complex band structure or the calculation of the transmission and reflection coefficients for ballistic transport are discussed. As a first example, complex band structures of Cu, Fe, and Si are presented.

We present an efficient method for calculating the conductance of ballistic electrons through an interface from first principles using the embedding approach of Inglesfield. In our method the Landauer-Büttiker formula for ballistic transport is expressed in terms of two quantities that are available in the embedded Green-function formalism without additional calculations. One is the embedding potential of bulk crystals on both sides of the interface and the other is the Green function in the interface region. As a proof of principle we calculate on the basis of the density-functional theory the spin-resolved electron transmission through a model system of ferromagnetic Co monolayers sandwiched between bulk Cu crystals. The relationship between our formulation and the Green-function formulation of Baranger and Stone is discussed.