O. Pietzsch

Atom-specific spin mapping and buried topological states in a homologous series of topological insulators

A topological insulator is a state of quantum matter that, while being an insulator in the bulk, hosts topologically protected electronic states at the surface. These states open the opportunity to realize a number of new applications in spintronics and quantum computing. To take advantage of their peculiar properties, topological insulators should be tuned in such a way that ideal and isolated Dirac cones are located within the topological transport regime without any scattering channels. Here we report ab-initio calculations, spin-resolved photoemission and scanning tunnelling microscopy experiments that demonstrate that the conducting states can effectively tuned within the concept of a homologous series that is formed by the binary chalcogenides (Bi(2)Te(3), Bi(2)Se(3) and Sb(2)Te(3)), with the addition of a third element of the group IV.

A low-temperature ultrahigh vacuum scanning tunneling microscope with a split-coil magnet and a rotary motion stepper motor for high spatial resolution studies of surface magnetism

We present the design of a new ultrahigh vacuum scanning tunneling microscope (STM) which operates at T 270° about an axis perpendicular to the tip axis. This feature allows metal or molecular beam evaporation normal to the sample surface. Even more important, by means of this device tip and sample can be brought into a parallel or antiparallel magnetic configuration thus opening a novel approach to the study of magnetic phenomena on an atomic length scale. In addition, measurements of the magneto-optical Kerr effect can be carried out without removing the sample from the STM. Also a new tip exchange mechanism is described. The microscopic and spectroscopic performance of the new ins...

Modification of Electrical Properties of Graphene by Substrate-Induced Nanomodulation

A periodically modulated graphene (PMG) generated by nanopatterned surfaces is reported to profoundly modify the intrinsic electronic properties of graphene. The temperature dependence of the sheet resistivity and gate response measurements clearly show a semiconductor-like behavior. Raman spectroscopy reveals significant shifts of the G and the 2D modes induced by the interaction with the underlying grid-like nanostructure. The influence of the periodic, alternating contact with the substrate surface was studied in terms of strain caused by bending of graphene and doping through chemical interactions with underlying substrate atoms. Electronic structure calculations performed on a model of PMG reveals that it is possible to tune a band gap within 0.14-0.19 eV by considering both the periodic mechanical bending and the surface coordination chemistry. Therefore, the PMG can be regarded as a further step toward band gap engineering of graphene devices.

Controllable Magnetic Doping of the Surface State of a Topological Insulator

A combined experimental and theoretical study of doping individual Fe atoms into Bi(2)Se(3) is presented. It is shown through a scanning tunneling microscopy study that single Fe atoms initially located at hollow sites on top of the surface (adatoms) can be incorporated into subsurface layers by thermally activated diffusion. Angle-resolved photoemission spectroscopy in combination with ab initio calculations suggest that the doping behavior changes from electron donation for the Fe adatom to neutral or electron acceptance for Fe incorporated into substitutional Bi sites. According to first principles calculations within density functional theory, these Fe substitutional impurities retain a large magnetic moment, thus presenting an alternative scheme for magnetically doping the topological surface state. For both types of Fe doping, we see no indication of a gap at the Dirac point.

Interface-induced chiral domain walls, spin spirals and skyrmions revealed by spin-polarized scanning tunneling microscopy

The spin textures of ultra-thin magnetic layers exhibit surprising variety. The loss of inversion symmetry at the interface of the magnetic layer and substrate gives rise to the so-called Dzyaloshinskii-Moriya interaction which favors non-collinear spin arrangements with unique rotational sense. Here we review the application of spin-polarized scanning tunneling microscopy to such systems, which has led to the discovery of interface-induced chiral domain walls and spin spirals. Recently, different interface-driven skyrmion lattices have been found, and the writing as well as the deleting of individual skyrmions based on local spin-polarized current injection has been demonstrated. These interface-induced non-collinear magnetic states offer new exciting possibilities to study fundamental magnetic interactions and to tailor material properties for spintronic applications.

A low-temperature spin-polarized scanning tunneling microscope operating in a fully rotatable magnetic field.

A new scanning tunneling microscope for spin-polarized experiments has been developed. The microscope is operated at 4.7 K in a superconducting triple axis vector magnet providing the possibility for measurements depending on the direction of the magnetic field. In single axis mode the maximum field is 5 T perpendicular to the sample plane and 1.3 T in the sample plane, respectively. In cooperative mode fields are limited to 3.5 T perpendicular and 1 T in plane. The microscope is operated in an ultrahigh vacuum system providing optimized conditions for the self-assembled growth of magnetic structures at the atomic scale. The available temperature during growth ranges from 10 up to 1100 K. The performance of the new instrument is illustrated by spin-polarized measurements on 1.6 atomic layers Fe/W(110). It is demonstrated that the magnetization direction of ferromagnetic Fe and Gd tips can be adjusted using the external magnetic field. Atomic resolution is demonstrated by imaging an Fe monolayer on Ru(0001).

Imaging magnetic nanostructures by spin-polarized scanning tunneling spectroscopy

Abstract Spin-polarized scanning tunneling spectroscopy (SP-STS) using Gd- and Fe-coated probe tips is investigated. The antiferromagnetic domain structure of perpendicularly magnetized Fe nanowires is imaged by using Gd-coated tips. The influence of an external magnetic field on the nanowire domain structure is demonstrated. Magnetic saturation of the sample is observed at about 450 mT. While Gd-coated tips are sensitive to the out-of-plane magnetization direction, the Fe thin film tips were found to be magnetized in-plane, i.e. perpendicular to the tip axis. Our results indicate that the shape anisotropy of our thin film tips plays a minor role compared to the interface anisotropies at the tip apex.

Complex magnetic order on the atomic scale revealed by spin-polarized scanning tunnelling microscopy

A fundamental understanding of magnetic phenomena requires the determination of spin structures down to the atomic scale. The direct visualization of atomic-scale spin structures has been accomplished by combining the atomic resolution capability of Scanning Tunnelling Microscopy (STM) with spin sensitivity, based on vacuum tunnelling of spin-polarized electrons. The resulting technique, Spin-Polarized Scanning Tunnelling Microscopy (SP-STM), nowadays provides unprecedented insight into collinear and non-collinear spin structures at surfaces of magnetic nanostructures and has already led to the discovery of new types of magnetic order at the nanometre scale. Several examples of complex magnetic order as revealed by atomic-resolution SP-STM will be reviewed.

Spin–orbit induced local band structure variations revealed by scanning tunnelling spectroscopy

Scanning tunnelling spectroscopy (STS) of thin Fe films on W(110) shows that the electronic structures of magnetic domains and domain walls are different. This experimental result is explained on the basis of first-principles calculations. A detailed analysis reveals that the spin–orbit induced mixing between minority dxy+xz and minority dz2 spin states depends on the magnetization direction and changes the local density of states in the vacuum detectable by STS. The effect scales in second or fourth order with the magnetization angle relative to the easy axis. Our finding implies that nanometre-scale magnetic structure information can be obtained even by using non-magnetic probe tips. Magnetization dependent measurements show that the canting of adjacent spins has no major influence on the electronic structure of the sample.

Non-collinear magnetic order in nanostructures investigated by spin-polarized scanning tunneling microscopy*

The successful conjunction of the ultimate spatial resolution capability of the scanning tunneling microscope (STM) with the sensitivity to the spin of the tunneling electrons has opened the door to investigations of magnetism at the nanoscale where the fundamental interactions responsible for magnetic order can be studied. Spin-polarized (SP) STM allows insight into a fascinating world with surprisingly rich magnetic phenomena. Ferromagnetic structures with magnetic domains are found at nanometer length scales, or 2D antiferromagnetically ordered monolayers (MLs) where the magnetization is reversed from one atom to the next. Such collinearly ordered states may be modified by the Dzyaloshinsky–Moriya (DM) interaction which can induce a small canting angle between neighboring atomic moments, thus giving rise to novel non-collinear spin spiral ground states. DM interaction is a result of electron scattering in a crystal environment with broken inversion symmetry. Spin spirals were observed in a variety of systems, like ultrathin Fe films, or MLs of Mn atoms on the (110) and (001) faces of a W crystal. Using a magnetically sensitive probe tip, individual Co atoms were assembled to form chains on top of a spin spiral. The magnetization orientation of each individual atom can be manipulated by repositioning it along the spin spiral.