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.     

自旋极化扫描隧道显微术

自旋极化扫描隧道显微术是一种新兴的表面自旋分辨技术，文章主要介绍了自旋极化的扫描隧道显微镜和扫描隧道谱实现表面自旋分辨的原理以及在各种磁性表面研究中的应用，采用自旋极化技术的扫描隧道显微镜可以测量表面磁结构，其空间分辨可以达到原子尺度，分辨率超过其他磁显微技术，而自旋极化扫描隧道谱不但可以分辨空间精细磁畴结构，而且能研究表面态的交换劈裂，文章作者还进一步提出了利用自旋极化扫描隧道显微镜实现自旋注入的设想。     

通过单分子化学调控单个离子磁性--单分子"手术"开启分子磁性"开关"

报道了如何通过改变单个磁性离子的化学环境来调控其白旋性质．利用扫描隧道显微镜，对吸附于Au（111）表面的单个钴酞菁分子进行化学修饰，通过在针尖上施加一定的电压脉冲，将分子配合体外层的8个氢原子“剪裁”掉，使其与金衬底形成稳定的化学键合．在这个新的人造分子结构中，其中心钴离子的电子态在费米面上出现强烈的共振峰；通过理论的模拟和分析，发现出现这一共振峰的原因是，脱氢和与衬底的化学结合使中心钴离子自旋性质发生改变，从而导致了输运特性中近藤效应（Kondo effect）的出现．     

Kondo effect of single Co adatoms on Cu surfaces

The Kondo resonance of Co adatoms on the Cu(100) and Cu(111) surfaces has been studied by scanning tunneling spectroscopy. We demonstrate the scaling of the Kondo temperature T(K) with the host electron density at the magnetic impurity. The quantitative analysis of the tunneling spectra reveals that the Kondo resonance is dominated by the Cu bulk electrons. While at the Cu(100) surface both tunneling into the hybridized localized state and into the substrate conduction band contribute to the Kondo resonance, the latter channel is found to be dominant for Cu(111).     

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.     

Structure and electronic properties of imperfect oxides and nanooxides

Results of studies of the structure and physicochemical properties of perfect (defectless) and imperfect (containing point defects) nanoparticles of metal oxides by the methods of scanning tunnel microscopy (STM) and spectroscopy (STS) are summarized and reviewed. Using nanooxides of various metals (Al, Pt, W, and Ti), as examples, it was demonstrated that the modern STM and STS methods make it possible to measure the electronic spectra of perfect nanoparticles, detect individual and associated point defects, determine spatial distributions of defects, derive spatial and energetic distributions of electrons trapped on point defects (anionic vacancies, intercalation atoms, and adatoms), detect single electron spins (paramagnetic surface complexes), measure phonon and vibration spectra of individual nanoparticles and surface complexes, determine (from measured electronic and phonon spectra) the chemical composition and atomic structure of individual nanoparticles. It was emphasized that modern versions of the STM-STS methods can constitute the foundation of new analytical methods needed in nanometrology and nanodefectoscopy.     

低维原子/分子晶体材料的可控生长、物性调控和原理性应用

本文介绍了高鸿钧课题组在物理所20年来的部分代表性工作.研究的主要方向为低维纳米功能材料的分子束外延可控制备、生长机制、物性调控及其在未来信息技术中的原理性应用.从材料的可控制备入手,结合第一性原理的理论计算,阐明材料生长机制和结构与物性的关系,进而实现物性调控和原理性应用.主要内容有:1)纳米尺度"海马"分形结构的形成及其生长机制;2)STM分辨率的提高及最高分辨Si(111)-7×7原子图像的获得;3)固体表面上功能分子的吸附、组装及其机制;4)稳定、重复、可逆的纳米尺度电导转变与超高密度信息存储;5)固体表面上单分子自旋态的量子调控及其原理性应用;6)原子尺度上朗德g因子的空间分辨及其空间分布不均匀性的发现;7)晶圆尺寸、高质量、单晶石墨烯的制备及原位硅插层绝缘化;8)几种新型二维原子晶体材料的可控构筑及其物性调控;9)"自然图案化"的新型二维原子晶体材料及其功能化.这些工作为低维量子结构的构造、物性调控及其原理性应用奠定了基础.     

Theory of the fano resonance in the STM tunneling density of states due to a single kondo impurity

The conduction electron density of states nearby single magnetic impurities, as measured recently by scanning tunneling microscopy (STM), is calculated, taking into account tunneling into conduction electron states only. The Kondo effect induces a narrow Fano resonance in the conduction electron density of states. The line shape varies with the distance between STM tip and impurity, in qualitative agreement with experiments, but is very sensitive to details of the band structure. For a Co impurity the experimentally observed width and shift of the Kondo resonance are in accordance with those obtained from a combination of band structure and strongly correlated calculations.     

Au(111)表面吸附单个八乙基钴卟啉分子的电子态和输运性质调控

通过低温超高真空扫描隧道显微镜及其谱学方法研究并展示了分子配体在调控表面吸附的单个八乙基钴卟啉(CoOEP)分子的电子态和输运性质中的重要作用. 通过单分子剪裁可以脱去该分子外围的甲基, 并在中心钴原子的微分电导谱中观察测到d轨道共振到近藤共振的演变. 实验结果结合第一性原理的理论计算研究表明, 在脱去甲基前后中心钴原子的化学环境和磁矩均未发生显著变化, 这一演变可以通过一个简化模型来阐释并被归结为脱去甲基后分子配体与衬底成键改变了体系隧穿参数所导致. 此外, 实验结果表明CoOEP分子配体的输运性质可受到分子间距离和范德华相互作用的显著调控. 在CoOEP低聚体中位于分子之间的乙基被抬高, 同时在其微分电导谱谱中0–0.8 V区域内新出现一个强的共振峰. 这一新的共振峰表现出等间距的多峰细节, 其峰间距与卟啉环和乙基之间的C–C键伸缩模式能量符合. 这一新共振峰的出现被归结为由于分子局部与衬底耦合减弱形成双结隧穿体系所导致的振子态隧穿峰.     

Electronic stabilization of the Si(111)5 × 2-Au surface: Pb and Si adatoms

Using scanning tunneling microscopy/spectroscopy (STM/STS), angle resolved photoemission spectroscopy (ARPES) and first-principles density functional theory (DFT), we study the structural and the electronic properties of the Si(111)5 × 2-Au surface decorated with Pb adatoms. The STM topography data reveal that Pb adatoms form a similar superstructure to that observed in the case of Si adatoms on a bare Si(111)5 × 2-Au surface. The DFT calculations show that preferential adsorption sites of Pb atoms are located near the double Au chain. Bias dependent STM topography and spectroscopy together with the DFT calculations allow us to distinguish Pb from Si adatoms. Both the Si and Pb adatoms modify the electronic properties in the same way, which confirms the electronic origin of the stabilization of the surface.     

Exchange interaction between single magnetic adatoms

The magnetic coupling between single Co atoms adsorbed on a copper surface is determined by probing the Kondo resonance using low-temperature scanning tunneling spectroscopy. The Kondo resonance, which is due to magnetic correlation effects between the spin of a magnetic adatom and the conduction electrons of the substrate, is modified in a characteristic way by the coupling of the neighboring adatom spins. Increasing the interatomic distance of a Cobalt dimer from 2.56 to 8.1 A we follow the oscillatory transition from ferromagnetic to antiferromagnetic coupling. Adding a third atom to the antiferromagnetically coupled dimer results in the formation of a collective correlated state.     

Point defects along metallic atomic wires on vicinal Si surfaces: Si(5 5 7)–Au and Si(5 5 3)–Au

Point defects on the metallic atomic wires induced by Au adsorbates on vicinal Si surfaces were investigated using scanning tunneling microscopy and spectroscopy (STM and STS). High-resolution STM images revealed that there exist several different types of defects on the Si(5 5 7)–Au surface, which are categorized by their apparent bias-dependent images and compared to the previous report on Si(5 5 3)–Au [Phys. Rev. B (2007) 205325]. The chemical characteristics of these defects were investigated by monitoring them upon the variation of the Au coverage and the adsorption of water molecules. The chemical origins and the tentative atomic structures of the defects are suggested as Si adatoms (and dimers) in different registries, the Au deficiency on terraces, and water molecules adsorbed dissociatively on step edges, respectively. STS measurements disclosed the electronic property of the majority kinds of defects on both Si(5 5 7)–Au and Si(5 5 3)–Au surfaces. In particular, the dominating water-induced defects on both surfaces induce a substantial band gap of about 0.5 eV in clear contrast to Si adatom-type defects. The conduction channels along the metallic step-edge chains thus must be very susceptible to the contamination through the electronic termination by the water adsorption.     

铜箔上生长的六角氮化硼薄膜的扫描隧道显微镜研究

利用扫描隧道显微镜研究了采用化学气相沉积法在铜箔表面生长出的高质量的六角氮化硼薄膜. 大范围的扫描隧道显微镜图像显示出该薄膜具有原子级平整的表面, 而扫描隧道谱则显示, 扫描隧道显微镜图像反映出的是该薄膜样品的隧穿势垒空间分布. 极低偏压的扫描隧道显微镜图像呈现了氮化硼薄膜表面的六角蜂窝周期性原子排列, 而高偏压的扫描隧道显微镜图像则呈现出无序和有序排列区域共存的电子调制图案. 该调制图案并非源于氮化硼薄膜和铜箔衬底的面内晶格失配, 而极有可能来源于两者界面处的氢、硼和/或氮原子在铜箔表面的吸附所导致的隧穿势垒的局域空间分布.     

Temperature dependence of a single Kondo impurity

Recent advances in scanning tunneling microscopy have allowed the observation of the Kondo effect for individual magnetic atoms. One hallmark of the Kondo effect is a strong temperature-induced broadening of the Kondo resonance. In order to test this prediction for individual impurities, we have investigated the temperature dependent electronic structure of isolated Ti atoms on Ag(100). We find that the Kondo resonance is strongly broadened in the temperature range T = 6.8 K to T = 49.0 K. These results are in good agreement with theoretical predictions for Kondo impurities in the Fermi liquid regime, and confirm the role of electron-electron scattering as the main thermal broadening mechanism.     

Interpretation of orientational contrast in STM images of GaAs(1 1 0) cleavage surface

The electronic structure of GaAs(1 1 0) surface is analyzed using Density Functional Theory (DFT-GGA) in atomic orbital basis (LCAO). The surface orbitals and the corresponding local density of electronic states (LDOS) are calculated for purposes of interpreting STM images. We show how local atomic orbitals of surface atoms are related to tunneling channels for electrons in STM imaging. A destructive interference between orbitals of two neighbouring atoms increases the contrast between the two atoms, and this is reflected in directionality of STM patterns of GaAs(1 1 0) surfaces. We also discuss how the basic formalism of Tersoff-Hamann approach to STM simulation can be reformulated to reveal the role of phase difference between tunneling channels.     

Formation and local electronic structure of Ge clusters on Si（111）-7×7 surfaces

We report the formation and local electronic structure of Ge clusters on the Si(111)-7$\times $7 surface studied by using variable temperature scanning tunnelling microscopy (VT-STM) and low-temperature scanning tunnelling spectroscopy (STS). Atom-resolved STM images reveal that the Ge atoms are prone to forming clusters with 1.0~nm in diameter for coverage up to 0.12~ML. Such Ge clusters preferentially nucleate at the centre of the faulted-half unit cells, leading to the `dark sites' of Si centre adatoms from the surrounding three unfaulted-half unit cells in filled-state images. Bias-dependent STM images show the charge transfer from the neighbouring Si adatoms to Ge clusters. Low-temperature STS of the Ge clusters reveals that there is a band gap on the Ge cluster and the large voltage threshold is about 0.9~V.     

Three-dimensional Wentzel–Kramers–Brillouin approach for the simulation of scanning tunneling microscopy and spectroscopy

We review the recently developed three-dimensional (3D) atom-superposition approach for simulating scanning tunneling microscopy (STM) and spectroscopy (STS) based on ab initio electronic structure data. In the method, contributions from individual electron tunneling transitions between the tip apex atom and each of the sample surface atoms are summed up assuming the one-dimensional (1D) Wentzel–Kramers–Brillouin (WKB) approximation in all these transitions. This 3D WKB tunneling model is extremely suitable to simulate spin-polarized STM and STS on surfaces exhibiting a complex noncollinear magnetic structure, i.e., without a global spin quantization axis, at very low computational cost. The tip electronic structure from first principles can also be incorporated into the model, that is often assumed to be constant in energy in the vast majority of the related literature, which could lead to a misinterpretation of experimental findings. Using this approach, we highlight some of the electron tunneling features on a prototype frustrated hexagonal antiferromagnetic Cr monolayer on Ag(111) surface. We obtain useful theoretical insights into the simulated quantities that is expected to help the correct evaluation of experimental results. By extending the method to incorporate a simple orbital dependent electron tunneling transmission, we reinvestigate the bias voltage- and tip-dependent contrast inversion effect on theW(110) surface. STM images calculated using this orbital dependent model agree reasonably well with Tersoff-Hamann and Bardeen results. The computational efficiency of the model is remarkable as the k-point samplings of the surface and tip Brillouin zones do not affect the computational time, in contrast to the Bardeen method. In a certain case we obtain a relative computational time gain of 8500 compared to the Bardeen calculation, without the loss of quality. We discuss the advantages and limitations of the 3D WKB method, and show further ways to improve and extend it.     

The exchange interaction effect in the scanning tunneling spectroscopy of a two-orbital Anderson impurity on metallic surface

We investigate the scanning tunneling spectroscopy (STS) of a two-orbital Anderson impurity adsorbed on a metallic surface by using the numerical renormalization group (NRG) method. The density of state of magnetic impurity and the local conduction electron are calculated. We obtain the Fano resonance line shape in the STM conductance at zero temperature. For the impurity atom with antiferromagnetic inter-orbital exchange interaction and a spin singlet ground state, we show that a dip in the STM spectra around zero bias voltage regime and side peaks of spin excitation can be observed. The spin excitation energy is proportional to the exchange interaction strength. As the exchange interaction is ferromagnetic, the underscreened Kondo effect dominates the low energy properties of this system, and it gives rise to drastically different STM spectra as compared with the spin singlet case.     

二维磁结构的扫描隧道显微术研究

文章介绍了近年来利用扫描隧道显微术（STM）对表面和薄膜磁结构的研究进展。二维或表面磁结构可以通过在非磁性单晶上外延磁性单原子层薄膜形成，也可以在清洁的磁性单晶表面形成。利用磁性的STM针尖可以观测到原子分辨的表面磁结构。这将增进人们从纳米尺度对磁性的理解，并推动磁电子学的发展。