Low temperature electron spin resonance of the Kondo ion in a heavy fermion metal: YbRh2Si2

We report an electron spin resonance (ESR) study on single crystals of the heavy fermion metal YbRh2Si2 which shows pronounced non-Fermi liquid behavior related to a close antiferromagnetic quantum critical point. It is shown that the observed ESR spectra can be ascribed to a bulk Yb3+ resonance. This is the first observation of ESR of the Kondo ion itself in a dense Kondo lattice system. The ESR signal occurs below the Kondo temperature (T(K)) which thus indicates the existence of large unscreened Yb3+ moments below T(K). We observe the spin dynamics as well as the static magnetic properties of the Yb3+ spins to be consistent with the results of nuclear magnetic resonance and magnetic susceptibility.     

A local view of the Kondo effect: Scanning tunneling spectroscopy

The fascinating many-body physics involved in the interaction of a single magnetic impurity with the conduction electrons of its nonmagnetic metallic host is reflected in unconventional phenomena in magnetism, transport properties and the specific heat. Characteristic low-energy excitations, termed the Kondo resonance, are generally believed to be responsible for this striking behaviour. However, in spite of an intense research for over more than 30 years, a direct spectroscopic observation of the Kondo resonance on individual magnetic adatoms withstood experimental efforts hitherto. The development of low-temperature scanning tunneling microscopes (STM) operating under ultrahigh vacuum (UHV) conditions has provided new opportunities for investigating locally the electronic structure at surfaces. At low temperatures, due to the reduced broadening of the Fermi level of the STM tip and the sample, rather high energy resolution (≤ 1 meV) in scanning tunneling spectroscopy (STS) is achievable. Moreover, the absence of diffusion together with the spatial resolution of the STM enables detailed studies of electronic states on and near single adsorbed atoms and other nanoscale structures. Recently, for the first time, two such STS/STM experiments spatially resolved the electronic properties of individual magnetic adatoms displaying the Kondo effect. In particular, the observed Fano lineshape of the Kondo resonance reveals unambiguously the details of the quantum mechanical interference between the localized orbital and the conduction electrons on an atomic length scale [1,2]. This achievement of the detection of individual magnetic atoms with atomic resolution opens new perspectives for probing magnetic nanostructures.     

Tunneling interferometry and measurement of the thickness of ultrathin metallic Pb(111) films

Spectra of the differential tunneling conductivity for ultrathin lead films grown on Si(111) 7 × 7 single crystals with a thickness of 9 to 50 ML have been studied by low-temperature scanning tunneling microscopy and spectroscopy. The presence of local maxima of the tunneling conductivity is characteristic of such systems. The energies of maxima of the differential conductivity are determined by the spectrum of quantum-confined states of electrons in a metallic layer and, consequently, the local thickness of the layer. It has been shown that features of the microstructure of substrates, such as steps of monatomic height, structural defects, and inclusions of other materials covered with a lead layer, can be visualized by bias-modulation scanning tunneling spectroscopy.     

The passivation of atomic scale defects present on III–V semiconductor laser facets: an STM/STS investigation

The work presented in this paper is based on the use of scanning tunnelling microscopy (STM) and scanning tunnelling spectroscopy (STS) to study the passivation of atomic scale defect-induced surface states on cleaved III–V (1 1 0) surfaces. This is based on the use of thin Si layers deposited in situ on to the atomically clean surface. The simultaneous STM and STS measurements allowed direct correlation of the structural and electronic properties at the nanoscopic level. The preferential adsorption of Si clusters onto surface defects was achieved using elevated temperature growth on the GaAs(1 1 0) substrate. The STS results clearly indicated local electronic passivation of both step defects and vacancy clusters when the interface is formed at 280 °C. This observation was also confirmed on a macroscopic level using X-ray photoelectron spectroscopy (XPS) under identical conditions. The results are interpreted in terms of the surface bonding of Si with the defect sites. Furthermore, this STM/STS study has been extended to real laser devices where comparable defect features are observed. The implications of defect passivation in nanotechnology are also discussed.     

Atomic resolution imaging of the (001) surface of UHV cleaved MgO by dynamic scanning force microscopy

The (001) surface of UHV cleaved single MgO crystals was imaged with dynamic mode scanning force microscopy. Large-scale images show various defects, like steps of mostly one atomic height, rectangular holes of nanometer size, and some complex adstructures. First time images with atomic resolution show one square ionic sublattice in its bulklike dimension with a corrugation of up to 40 pm along the <001> direction. Most images exhibit atomic point defects which appear as depressions including a few ionic lattice sites proving that point defects are stable on flat terraces.     

Direct visualization of structural defects in 2D semiconductors

Direct visualization of the structural defects in two-dimensional (2D) semiconductors at a large scale plays a significant role in understanding their electrical/optical/magnetic properties, but is challenging. Although traditional atomic resolution imaging techniques, such as transmission electron microscopy and scanning tunneling microscopy, can directly image the structural defects, they provide only local-scale information and require complex setups. Here, we develop a simple, non-invasive wet etching method to directly visualize the structural defects in 2D semiconductors at a large scale, including both point defects and grain boundaries. Utilizing this method, we extract successfully the defects density in several different types of monolayer molybdenum disulfide samples, providing key insights into the device functions. Furthermore, the etching method we developed is anisotropic and tunable, opening up opportunities to obtain exotic edge states on demand.     

钴原子及其团簇在Rh(111)和Pd(111)表面的扫描隧道显微学研究

利用扫描隧道显微镜和扫描隧道谱(STM/STS)及单原子操纵,系统研究了单个钴原子(Co) 及其团簇在Rh (111)和Pd (111)两种表面的吸附和自旋电子输运性质. 发现单个Co原子在Rh (111)上有两种不同的稳定吸附位,分别对应于hcp和fcc空位, 他们的高度明显不同,在针尖的操纵下单个Co原子可以在两种吸附位之间相互转化. 在这两种吸附位的单个Co原子的STS谱的费米面附近都存在很显著的峰形结构, 经分析认为Rh (111)表面单个Co原子处于混价区,因此这一峰结构是d轨道共振 和近藤共振共同作用的结果.对于Rh (111)表面上的Co原子二聚体和三聚体, 其费米面附近没有观测到显著的峰,这可能是由于原子间磁交换相互作用 和原子间轨道杂化引起的体系态密度改变所共同导致.与Rh (111)表面不同, 在Pd (111)表面吸附的单个Co原子则表现出均一的高度.并且对于Pd (111)表面所有 单个Co原子及其二聚体和三聚体,在其STS谱的费米面附近均未探测到显著的电子结构, 表明Co原子吸附于Pd (111)表面具有与Rh (111)表面上不同的原子-衬底相互作用与自旋电子输运性质.     

Kondo effect of molecular complexes at surfaces: ligand control of the local spin coupling

The spin state of single magnetic atoms and molecules at surfaces is of fundamental interest and may play an important role in future atomic-scale technologies. We demonstrate the ability to tune the coupling between the spin of individual cobalt adatoms with their surroundings by controlled attachment of molecular ligands. The strength of the coupling is determined via the Kondo resonance by low-temperature scanning tunneling spectroscopy. Spatial Kondo resonance mapping is introduced as a novel imaging tool to localize spin centers in magnetic molecules with atomic precision.     

Insight into the f-derived Fermi surface of the heavy-fermion compound YbRh2Si2

Angle-resolved photoelectron spectroscopy (ARPES) was used to study the Fermi surface of the heavy-fermion system YbRh(2)Si(2) at a temperature of about 10 K, i.e., a factor of 2 below the Kondo energy scale. We observed sharp structures with a well-defined topology, which were analyzed by comparing with results of band-structure calculations based on the local-density approximation (LDA). The observed bulk Fermi surface presents strong similarities with that expected for a trivalent Yb state, but is slightly larger, has a strong Yb-4f character, and deviates from the LDA results by a larger region without states around the Γ point. These properties are qualitatively explained in the framework of a simple f-d hybridization model. Our analysis highlights the importance of taking into account surface states and doing an appropriate projection along k(z) when comparing ARPES data with results from theoretical calculations.     

Mapping itinerant electrons around Kondo impurities

We investigate single Fe and Co atoms buried below a Cu(100) surface using low temperature scanning tunneling spectroscopy. By mapping the local density of states of the itinerant electrons at the surface, the Kondo resonance near the Fermi energy is analyzed. Probing bulk impurities in this well-defined scattering geometry allows separating the physics of the Kondo system and the measuring process. The line shape of the Kondo signature shows an oscillatory behavior as a function of depth of the impurity as well as a function of lateral distance. The oscillation period along the different directions reveals that the spectral function of the itinerant electrons is anisotropic.     

Structure of the Rh2O3(0001) surface

We have studied the (0001) surface termination of Rh₂O₃ on a Rh(111) single crystal using a combination of high resolution core level spectroscopy, low energy electron diffraction, scanning tunneling microscopy and density functional theory. By exposing the Rh(111) to atomic oxygen we are able to grow Rh₂O₃ layers exposing the (0001) surface. The experiments support the theoretical predictions stating that the surface is terminated with an O–Rh–O trilayer yielding a RhO₂ termination instead of a bulk Rh₂O₃ termination. The structural details as found by the DFT calculations are presented and reasons for the previously observed strong differences in catalytic activity between the structurally similar RhO₂ surface oxide, and the Rh₂O₃(0001) surface are discussed.     

Relevance of ferromagnetic correlations for the electron spin resonance in Kondo lattice systems

Electron spin resonance (ESR) measurements of the ferromagnetic (FM) Kondo lattice system CeRuPO show a well defined ESR signal which is related to the Ce3+ magnetism. In contrast, no ESR could be observed in the antiferromagnetic (AFM) homologue CeOsPO. Additionally, we detect an ESR signal in ferromagnetic YbRh while it was absent in a number of Ce or Yb intermetallic compounds with dominant AFM exchange. Thus, the observation of an ESR signal in a Kondo lattice is neither specific to Yb nor to the proximity to a quantum critical point, but seems to be connected to the presence of FM fluctuations. These conclusions not only provide a basic concept to understand the ESR in Kondo lattice systems even well below the Kondo temperature (as observed in YbRh2Si2) but point out ESR as a prime method to investigate directly the spin dynamics of the Kondo ion.     

Atomic-registry-dependent electronic structures of sulfur vacancies in ReS2 studied by scanning tunneling microscopy/spectroscopy

Rhenium disulfide (ReS₂) is regarded as a promising candidate for optoelectronic applications (e.g., infrared photodetector), as it maintains a direct bandgap regardless of the number of layers unlike other typical transition metal dichalcogenides. Therefore, it is very important to understand and control the defects of ReS₂ for enhancing the performance of photodevices. In this work, we studied the electronic structures of ReS₂ affected by sulfur vacancies of different atomic registries at the atomic scale. The atomic and electronic structures of the mechanically exfoliated ReS₂ flakes were investigated using scanning tunneling microscopy (STM) and scanning tunneling spectroscopy (STS), and were confirmed using density functional theory (DFT) calculations. The atomic structural models indicate four distinguishable atomic registries of sulfur vacancies on one face of ReS₂. Energetically, these atomic vacancies prefer to locate on the bottom side of the top monolayer of ReS₂ flakes. Only two among four possible kinds of vacancies could be observed using STM and STS, and they were identified using additional DFT calculations. We believe that our results regarding the identification of the defects and understanding the corresponding effects for electronic structures will provide important insights to enhance the performances of ReS₂-based optoelectronic devices in the future.     

Fabrication of GaAs/SiO_2/Si and GaAs/Si heterointerfaces by surface-activated chemical bonding at room temperature

The room-temperature(RT) bonding mechanisms of Ga As/Si O_2/Si and Ga As/Si heterointerfaces fabricated by surface-activated bonding(SAB) are investigated using a focused ion beam(FIB) system, cross-sectional scanning transmission electron microscopy(TEM), energy dispersive x-ray spectroscopy(EDX) and scanning acoustic microscopy(SAM).According to the element distribution detected by TEM and EDX, it is found that an intermixing process occurs among different atoms at the heterointerface during the RT bonding process following the surface-activation treatment. The diffusion of atoms at the interface is enhanced by the point defects introduced by the process of surface activation. We can confirm that through the point defects, a strong heterointerface can be created at RT. The measured bonding energies of Ga As/Si O_2/Si and Ga As/Si wafers are 0.7 J/m~2 and 0.6 J/m~2. The surface-activation process can not only remove surface oxides and generate dangling bonds, but also enhance the atomic diffusivity at the interface.     

Temperature dependence of the Kondo resonance and its satellites in CeCu2Si2

We present high-resolution photoemission spectroscopy studies on the Kondo resonance of the strongly correlated Ce system CeCu2Si2. By exploiting the thermal broadening of the Fermi edge we analyze position, spectral weight, and temperature dependence of the low-energy 4f spectral features, whose major weight lies above the Fermi level E(F). We also present theoretical predictions based on the single-impurity Anderson model using an extended noncrossing approximation, including all spin-orbit and crystal field splittings of the 4f states. The excellent agreement between theory and experiment provides strong evidence that the spectral properties of CeCu2Si2 can be described by single-impurity Kondo physics down to T approximately 5 K.     

Transformations of C-type defects on Si(100)-2 × 1 surface at room temperature – STM/STS study

We present a scanning tunneling microscopy study of the C-type defects on the Si(100)-2 × 1 surface and their transformations into other defect forms at room temperature. A model of the C defect as a dissociated water molecule was adopted for interpretation of the observed transformations. We explained the transformations by hopping the H or OH between bonding sites on Si dimers. Newly, the most stable defect form, corresponding to the H and hydroxyl group adsorbed on the same dimer, is reported. Real time observations provided an explanation for the defect C2-C2 described earlier. A reversible transition of this defect into another form, not revealed yet, is presented. Electronic structure of the observed defects is studied by means of scanning tunneling spectroscopy. Measured spectra show semiconducting character of the C defect. Spectra of the other defect forms are discussed.     

Time-dependent tunneling spectroscopy for studying surface diffusion confined in nanostructures

By confining a diffusion atom in a nanometer region using surface potential heterogeneity, we have successfully employed a time-dependent tunneling spectroscopy to quantitatively study its random motion. A hopping rate in the range of 1-10(4) Hz, approximately 3 orders of magnitude faster than those accessible by the existing diffusion methods based on scanning tunneling microscopy, was demonstrated for single Cu atoms diffusing in the faulted half unit cell of Si(111)-(7 x 7). Our technique is potentially useful to detect fast diffusion processes such as H quantum diffusion at atomic scale.     

Photoemission insight into heavy-fermion behavior in YbRh2Si2

As shown by angle-resolved photoemission (PE), hybridization of bulk Yb 4f(2+) states with a shallow-lying valence band of the same symmetry leads in YbRh2Si2 to dispersion of a 4f PE signal in the region of the Kondo resonance with a Fermi-energy crossing close to Gamma[over ]. Additionally, renormalization of the valence state results in the formation of a heavy band that disperses parallel to the 4f originating signal. The symmetry and character of the states are probed by circular dichroism and the photon-energy dependence of the PE cross sections.     

STM/STS characterization of platinum silicide nanostructures grown on a Pt(1 1 1) surface

Formation of the platinum silicides nanostructures and their electronic properties have been studied using scanning tunneling microscopy and scanning tunneling spectroscopy. The investigated structures have been grown by solid state epitaxy upon deposition of the Si atoms (coverage about 0.2 ML) and sequential annealing at temperature range 600-1170 K. The formation of the Pt₂Si and PtSi islands was investigated until the Si atoms embedded into the Pt substrate at the 1170 K. The images of the silicides structures and Pt substrates with atomic resolution have been recorded. The evolution of the spectroscopic curves both for substrates and nanostructures, corresponding to the structural and sizes changes, have been shown.     

Atomic structure of a regular Si(2 2 3) triple step staircase

An atomically accurate regular triple step array with a period of 4.8 nm has been fabricated on the vicinal Si(5 5 7) surface. Its atomic structure was studied on different length scales by scanning tunneling microscopy, low energy electron diffraction and photoelectron spectroscopy. These complementary methods allowed to identify the average orientation of the regular triple step staircase as Si(2 2 3) and to give a deeper insight into the atomic arrangement of this structure.