Magnon excitation with spin-polarized scanning tunneling microscopy

Using spin-polarized scanning tunneling microscopy, the local excitation of magnons in Fe and Co has been studied. A large cross section for magnon excitation was found for bulk Fe samples while for thin Co films on Cu(111) the cross section linearly scales with film thickness. Recording inelastic tunneling spectra with Fe coated W tips in a magnetic field, the magnonic nature of the excitation was proven. Magnon excitation could be detected without the use of a separating insulating layer opening up the possibility to directly study magnons in magnetic nanostructures via spin-polarized currents.     

Controlled low-temperature molecular manipulation of sexiphenyl molecules on Ag111 using scanning tunneling microscopy

A novel scanning tunneling microscope manipulation scheme for a controlled molecular transport of weakly adsorbed molecules is demonstrated. Single sexiphenyl molecules adsorbed on a Ag(111) surface at 6 K are shot towards single silver atoms by excitation with the tip. To achieve atomically straight shooting paths, an electron resonator consisting of linear standing-wave fronts is constructed. The sexiphenyl manipulation signals reveal a pi ring flipping as the molecule moves from the hcp to fcc site. Ab initio calculations show an incorporation of the Ag atom below the center of a pi ring.     

Spin-polarized scanning tunneling microscopy with antiferromagnetic probe tips

We have performed low temperature spin-polarized scanning tunneling microscopy (SP-STM) of two monolayers Fe on W(110) using tungsten tips coated with different magnetic materials. We observe stripe domains with a magnetic period of 50 +/- 5 nm. Employing Cr as a coating material we recorded SP-STM images with an antiferromagnetic probe tip. The advantage of its vanishing dipole field is most apparent in external magnetic fields. This new approach resolves the problem of the disturbing influence of a ferromagnetic tip in the investigation of soft magnetic materials and superparamagnetic particles.     

The image potential in scanning transmission electron microscopy and scanning tunneling microscopy

Abstract

We sketch developments in the theory of the self-energy of charged particles moving near condensed matter surfaces. Some applications to experimental results from spectroscopy with electrons localized in microprobe beams and to electrons tunneling across a gap between two metals are considered.     

First-principles theory of scanning tunneling microscopy

Scanning tunneling microscopy/spectroscopy (STM/STS), which has been so epoch-making in surface science experiments introduced many challenging problems also to the theory of condensed matter physics. Recent progress in theories of STM/STS contributed to revealing the relation between the atomic structure of the tip and the STM/STS data, and to clarify various strange phenomena observed. The present article reviews various important issues of the fundamentals of STM/STS from theoretical view points.

After surveying the so far presented theoretical approaches, the first-principles simulation method based on the microscopic electronic state of both the sample surface and the tip is introduced. Several examples of the simulation such as graphite and Si surfaces, are described. Some novel phenomena of the microscopic tunnel system of STM such as the negative differential resistance in STS and single electron tunneling through fine supported particles are also discussed, as well as the many-body effect or electron-phonon coupling effect on STM/STS.     

Charge manipulation and imaging of the Mn acceptor state in GaAs by cross-sectional scanning tunneling microscopy

An individual Mn acceptor in GaAs is mapped by cross-sectional scanning tunneling microscopy (X-STM) at room temperature and a strongly anisotropic shape of the acceptor state is observed. An acceptor state manifests itself as a cross-like feature which we attribute to a valence hole weakly bound to the Mn ion forming the (Mn²⁺3d⁵+hole) complex. We propose that the observed anisotropy of the Mn acceptor wavefunction is due to the d-wave present in the acceptor ground state.     

Atom manipulation with the scanning tunneling microscope: nanostructuring and femtochemistry

We briefly survey our recent studies on the ‘soft’lateral manipulation of atoms and small molecules with the scanning tunneling microscope (STM), whereby mainly the tip–surface forces are employed. Repulsive (pushing) as well as discontinuous (pulling) and continuous (sliding)attractive manipulation modes could be distinguished on Cu(211) for CO molecules and metal atoms, respectively. In the case of pulling of Cu atoms on Cu(111) even finer details could be discerned: the adparticle may show various movement patterns visiting different surface sites upon applying different tip forces. Lateral manipulation also allows modifications of the Cu(211) substrate itself in an atom-by-atom manner by releasing atoms from sixfold coordinated kink sites and even sevenfold coordinated regular step sites. Furthermore, investigations concerning controled vertical manipulation with emphasis on ‘picking-up’ single CO molecules are reported. The mechanism behind vertical transfer of CO molecules relates to ultrafast chemical processes. Vertical manipulation implies, besides extending the possibilities for the build-up of nanostructures, the important possibility of creating structurally and compositionally well-defined tips, which may eventually lead to chemical sensitivity with the STM.     

用扫描隧道显微镜(STM)观察纳米粉体的形貌

将不导电的纳米粉体压成薄片,在薄片的表面溅射上金膜,再经过适当的处理,将薄片固定在样品台上,这样制得样品可用于纳米粉体形貌,由于用于ＳＴＭ观察的样品必须导电这一限制,使得样品表面溅射的金膜厚度对成像有很大影响,此类工作可安排在学生物理实验的小型科研实验课程中。     

Images of crystalline alkanes obtained with scanning tunneling microscopy

Uncoated layered alkane (paraffin) objects with a thickness of up to a hundred nanometers have been imaged by scanning tunneling microscopy. This is surprising in view of the generally assumed excellent insulating nature of paraffins. Tilted layers were observed for n-alkanes of various chain lengths. The chemical nature of the imaged objects was demonstrated by the changes of structure observed near the respective melting temperature.     

Contrast mechanisms in photothermal scanning tunneling microscopy

By irradiation of the tunneling junction of a scanning tunneling microscope with intensity-modulated laser light a gap-width modulation due to thermal expansion of tip and sample was produced. Photothermal images were obtained by spatial mapping of the resulting modulation of the tunneling current or its logarithm. The various mechanisms responsible for the observed contrast are discussed quantitatively. In case of a highly corrugated gold film on mica the contrast arises mainly from either the current variations caused by the non-zero reaction time of the current control loop or from a geometry factor. In both cases the images reflect certain properties of the sample topography. On the other hand, for a liquid-crystal film adsorbed on graphite a contrast on a molecular scale was found which is attributed to variations of the effective barrier height.     

Theoretical aspects of scanning tunneling microscopy andspectroscopy

In spite of the huge potential of scanning tunneling microscopy (STM), a firm interpretation of experimental data is often difficult. Theoretical simulation of STM images andSTS spectra plays a very important role to derive detailed information from experiments. In the present article, a method of the first-principles simulation based on the local density functional approach is introduced, andapplied to some interesting surface systems. It is clarified how the atomistic structure of the tip influences the STM image. An example is presented in which a naive interpretation of the STM image fails. The exotic phenomenon of transparency of the adsorbed molecule is discussed.     

Controlled lateral manipulation of single molecules with the scanning tunneling microscope

We report on the first successful lateral manipulation of molecules and controlled formation of nanostructures with the Scanning Tunneling Microscope (STM) at temperatures above 4 K as used by Eigler and collaborators. Among the first structures, we built the letters F and U forming the logo of our university at 30 K with CO molecules on a Cu(211) substrate. Our method to manipulate the molecules is analogous to that employed successfully up to now only by Eigler and co-workers. First experiences concerning the manipulation of the CO molecules on the highly anisotropic substrate are presented and the crucial role of the tip composition in imaging the CO molecules is demonstrated.     

用扫描隧道显微镜操纵Cu亚表面自间隙原子

在超高真空环境下使用扫描隧道显微镜研究了吸附有双甘氨肽分子的Cu(001)表面.在一定的 偏压条件下，针尖在该表面扫描后会形成纳米尺度的Cu团簇，这些团簇可以根据意愿排列成 字母或图形.团簇的高度同偏压、隧道电流以及时间等条件有密切关系.在室温下可以稳定存 在的团簇为制造纳米器件提供了技术上的可能性.实验结果表明，形成团簇的Cu原子不是来 自Cu衬底表面或是针尖.化学吸附在Cu表面的双甘氨肽分子，受到隧道电场的作用会在Cu表 面形成张应变场，Cu亚表面自间隙原子在张应变场作用下迁移到表面是形成团簇的原因. 关键词： 扫描隧道显微镜 纳米尺度Cu团簇 自间隙原子     

Recent advances in spin-polarized scanning tunneling microscopy

Considerable progress in the field of spin-polarized scanning tunneling microscopy (SP-STM) has been achieved recently by gaining a high degree of control with regard to properties of the tunneling tip. It is found that by choosing the appropriate material for the magnetic thin film coating of the tip sensitivity to either the samples in-plane or perpendicular magnetization component can be achieved. Using SP-STM in external magnetic fields, domains and domain walls of two atomic layers thick Fe on a W(110) substrate are studied. A residual domain of enhanced stability against remagnetization is observed. Furthermore, a new imaging mechanism is identified which allows the use of even non-magnetic tungsten tips to observe contrast between magnetic domains and domain walls. The effect exploited is a modification of the electronic band structure which is induced by spin–orbit coupling. PACS 75.50.Ak; 75.75.+a; 68.37.Ef