纳米隧道结的量子电容现象

利用STM针尖和二维Au纳米团簇构造的双隧道结，通过对单电子隧穿谱的测量，研究了纳米隧道结的电容随隧道结宽度的变化，发现电容随结宽度的变化偏离了经典电容的行为，为纳米隧道结的量子电容效应提供了实验证据。     

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.     

Nonclassical behavior in the capacitance of a nanojunction

The capacitance of a nanojunction formed by a scanning tunneling microscope (STM) tip and a two-dimensional gold cluster was measured through the single electron tunneling spectroscopy of a double-barrier tunnel junction. By decreasing the STM tip-cluster separation, it was observed that the capacitance first increases and then decreases at short separation. This characteristic clearly deviates from the classical behavior and provides evidence for potential quantum effects on the capacitance.     

MANIPULATION OF ATOMS, MOLECULES AND CLUSTERS FOR CONSTRUCTION OF NANOSYSTEMS

Controlled manipulations with the scanning tunneling microscope (STM) down to the scale of small molecules and single atoms allow to built molecular and atomic nanosystems, leading to the fascinating possibility of creating manmade structures on atomic scale. Here we present a short review on atomic scale manipulation investigations. Upon soft lateral manipulation of adsorbed species, in which only tip/particle forces are used, three different manipulation modes (pushing, pulling, sliding) can be discerned. We show that even the manipulation of highly coordinated native substrate atoms is possible and we demonstrate how this can be applied as local analytic and synthetic chemistry tools, with important consequences on surface structure research. Vertical manipulation of Xe and CO is demonstrated, leading to improved imaging with functionalized tips. With CO deliberately transferred to the tip, we have also succeeded to perform vibrational spectroscopy on single molecules. Furthermore, we describe how we have reproduced a full chemical reaction with single molecules, whereby all basic steps, namely preparation of the reactants, diffusion and association, are induced with the STM tip. Finally, we have extended the manipulation techniques to large specially designed molecules by performing lateral manipulation in constant height and realizing the principle of a conformational molecular switch.     

Controllable dissipative tunneling in an external electric field

We have studied the problem of controllable dissipative tunneling in the system of tunnel-binding quantum dots (quantum molecules) and in the “AFM/STM cantilever tip-quantum dot” system, which was simulated by a double-well oscillator potential interacting with a heat-bath in an external electric field. We show that theoretical results qualitatively describe some experimental I–V curves for “the AFM/STM cantilever tip-zirconium quantum dot” system. These experimental curves were obtained in the Research Institute of Physics and Technologies at the State University of Nizhniy Novgorod.     

Manipulating Ge quantum dots on ultrathin SixGe1−x oxide films using scanning tunneling microscope tips

Germanium quantum dots (QDs) were extracted from ultrathin Si_xGe_1−x oxide films using scanning tunneling microscope (STM) tips. The extraction was most efficiently performed at a positive sample bias voltage of +5.0 V. The tunneling current dependence of the extraction efficiency was explained by the electric field evaporation transfer mechanism for positive Ge ions from QDs to STM tips. Ge QDs (∼7 nm) were formed and isolated spatially by extracting the surrounding Ge QDs with an ultrahigh density of >10¹² cm⁻². Scanning tunneling spectroscopy of the spatially-isolated QDs revealed that QDs with an ultrahigh density are electrically-isolated from the adjacent dots.     

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。     

基于原子操纵技术的人工量子结构研究

扫描隧道显微镜原子操纵技术是指利用扫描探针在特定材料表面以晶格为步长搬运单个原子或分子的技术.它是纳米尺度量子物理与器件研究领域一种独特而有力的研究手段.利用这种手段,人们能够以原子或分子为单元构筑某些常规生长或微加工方法难以制备的人工量子结构,通过对格点原子、晶格尺寸、对称性、周期性的高度控制,实现对局域电子态、自旋序、以及能带拓扑特性等量子效应的设计与调控.原子操纵技术与超快测量及自动控制技术的结合,使得人们能够进一步研究原子级精准的量子器件,因而该技术成为探索未来器件新机理、新工艺的重要工具.本文首先简介原子操纵方法的发展过程和技术要点,然后分别介绍人工电子晶格、半导体表面人工量子点、磁性人工量子结构、人工结构中的信息存储与逻辑运算、单原子精度原型器件等方面的最新研究进展,以及单原子刻蚀和自动原子操纵等方面的技术进展,最后总结并展望原子操纵技术的应用前景和发展趋势.     

Coupled quantum dots: manifestation of an artificial molecule

Transport spectroscopy reveals the microscopic features of few-electron quantum dots which justify the nameartificial atoms. New physics evolve when two quantum dots are coupled by a tunneling barrier. We study, both theoretically and experimentally, the tunneling spectroscopy on a double quantum dot. A detailed lineshape analysis of the conductance resonances proves that off-resonant coherent interdot tunneling governs transport through this system, while tunneling into the double quantum dot occurs resonantly. This coherent interdot tunneling witnesses the evolution of a delocalized electronic state which can be compared to a valence electron of thisartificial molecule.     

Characterization of heat-assisted magnetic probe recording on CoNi/Pt multilayers

A method of heat-assisted magnetic recording (HAMR) potentially suitable for probe-based storage systems is characterized. In this work, field emission current from a scanning tunneling microscope (STM) tip is used as the heating source. Pulse voltages of 2–7 V were applied to a CoNi/Pt multilayered film fabricated on either bare silicon or oxidized silicon substrates. Different types of Ir/Pt and W STM tips were used in the experiment. The results show that thermally recorded magnetic marks are formed with a nearly uniform mark size of 170 nm on the film fabricated on bare silicon substrate when the pulse voltage is above a threshold voltage. The mark size becomes 260 nm when they are written on the identical film fabricated on an oxidized silicon substrate. The threshold voltage depends on the material work function of the tip, with W having a threshold voltage about 1 V lower than Pt. A synthesized model, which contains the calculation of the emission current, the simulation of heat transfer during heating, and the study of magnetic domain formation, was introduced to explain experimental results. The simulation agrees well with the experiments.     

STM studies on quantum wire structures in air and liquid helium

In this work, low temperature scanning tunneling microscopy (STM) studies on quantum wires are reported, which were fabricated by laser holography and wet chemical etching. Inverted heterostructures with thin and highly doped cap layers were used as substrates in order to keep the total tunneling barrier as small as possible. Current—voltage curves were measured on the wires and in the depleted areas between them. Between the wires, significant current is only observed for electrons which tunnel from the GaAs valence band into the STM tip, whereas symmetric curren voltage curves are observed on the wires. This behavior is ascribed to the influence of surface depletion and thus, a comparison of current imaging spectroscopy data taken at 300 K and in liquid helium directly yields the edge depletion width of the quantum wires.     

Multi-color tunneling quantum dot infrared photodetectors operating at room temperature

Quantum dot structures designed for multi-color infrared detection and high temperature (or room temperature) operation are demonstrated. A novel approach, tunneling quantum dot (T-QD), was successfully demonstrated with a detector that can be operated at room temperature due to the reduction of the dark current by blocking barriers incorporated into the structure. Photoexcited carriers are selectively collected from InGaAs quantum dots by resonant tunneling, while the dark current is blocked by AlGaAs/InGaAs tunneling barriers placed in the structure. A two-color tunneling-quantum dot infrared photodetector (T-QDIP) with photoresponse peaks at 6 μm and 17 μm operating at room temperature will be discussed. Furthermore, the idea can be used to develop terahertz T-QD detectors operating at high temperatures. Successful results obtained for a T-QDIP designed for THz operations are presented. Another approach, bi-layer quantum dot, uses two layers of InAs quantum dots (QDs) with different sizes separated by a thin GaAs layer. The detector response was observed at three distinct wavelengths in short-, mid-, and far-infrared regions (5.6, 8.0, and 23.0 μm). Based on theoretical calculations, photoluminescence and infrared spectral measurements, the 5.6 and 23.0 μm peaks are connected to the states in smaller QDs in the structure. The narrow peaks emphasize the uniform size distribution of QDs grown by molecular beam epitaxy. These detectors can be employed in numerous applications such as environmental monitoring, spectroscopy, medical diagnosis, battlefield-imaging, space astronomy applications, mine detection, and remote-sensing.     

Cracking self-assembled InAs quantum dots

We present a cross-sectional scanning tunneling microscopy (X-STM) investigation of InAs quantum dots in a GaAs matrix. The structures were grown by molecular beam epitaxy (MBE) at a low growth rate of 0.01 ML/s and consist of five layers of uncoupled quantum dot structures. Detailed STM images with atomic resolution show that the dots consist of an InGaAs alloy and that the indium content in the dot increases towards the top. The analysis of the height versus base-length relation obtained from cross-sectional images of the dots shows that the shape of the dots resembles that of a truncated pyramid and that the square base is oriented along the [010] and [100] directions. Using scanning tunneling spectroscopy (STS) we determined the onset for electron tunneling into the conduction and out of the valence band, both in the quantum dots and in the surrounding GaAs matrix. We found equal voltages for tunneling out of the valence band in GaAs or InGaAs whereas tunneling into GaAs occurred at higher voltages than in InGaAs.     

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。     

STM/STS实验中单电子隧道现象的观测

报道在低温(4.2K)条件下利用STM/STS实验方法对过渡族金属二硫族化合物1T-TaSe₂的观测结果,明确得到了1T-TaSe₂中电荷密度波超晶格以及原子晶格的直观图像,并且给出了有关电荷密度波能隙的隧道谱结果,通过选择高纯度Fc以及不锈钢金属材料作为探针,还得到了单个隧道结构中单电子隧道(SET)效应的实验证据,即“库仑阻塞”(Coulomb blocade)效应,并以此说明STM隧道结中探针表面氧化层对实验结果的影响。     

Transport properties of a lateral semiconductor quantum dot defined by a single connected metallic front-gate

We present studies on the electric transport in a lateral GaAs/AlGaAs quantum dot defined by a patterned single connected metallic front-gate. This gate design allows to easily couple a large number of quantum dots and therefore holds high potential in the design of new materials with tailor-made band structures based on quantum dot superlattices of controlled shape. Clear Coulomb diamond structures and well pronounced tunneling peaks observed in experiment indicate that single-electron control has been achieved. However, the dependence on electron density in the heterostructure embedding the dot, which is controlled by an additional back-gate, reveals that transport characteristics are strongly influenced supposedly by potential fluctuations in the dot and lead regions.     

Scanning tunneling microscope with long range lateral motion

We present our work on a recently built scanning tunneling microscope (STM), with coarse motion in two-dimensions. The tip of this STM can be translated a few millimeters in directions both parallel and perpendicular to the tip. This feature allows sampling of a larger area for experiments such as the study of how the electrical properties of charge density waves evolve between contacts, the proximity effect near a normal metal–superconducting interface, charge transport near the contact of a semiconductor interface, and for finding microscopically small samples like graphene. This STM is based on one of our previous one-dimensional designs. It utilizes orchestrated motion of six piezoelectric tubes in a slip–stick configuration in order to produce long range motion for the walker. This device is a single unit with a compact design making it very stable. It is stable enough to obtain atomic resolution on HOPG. It can operate in either a horizontal or vertical configuration and at cryogenic temperatures. It was designed entirely from non-magnetic materials for potential work in a magnetic field.     

Scanning tunneling microscopy with spin-polarized electrons

We present a short outline of the first STM experiments with spin-polarized electrons performed in ultrahigh vacuum by using ferromagnetic CrO₂ tips and a Cr(001) single crystal surface. A clear distinction can be made between topographic STM line scans obtained with a non-magnetic tungsten tip and those obtained with a ferromagnetic CrO₂ tip, which are modified due to an additional contribution from spin-dependent vacuum tunneling. STM therefore has the potential to measure the local electron spin polarization of the free surface as well as the spatial distribution of spins on the atomic scale.     

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.     

Spatially resolved STM light emission spectra of cleaved (1 1 0) AlGaAs/GaAs heterostructures

We have measured the spectra of light emitted from individual single GaAs quantum wells of cleaved (1 1 0) AlGaAs/GaAs heterostructures using the STM (scanning tunneling microscope) tip as a local electron injection source. The cross-sectional STM images of the quantum wells were obtained, and the light emission spectra were measured by locating the STM tip over individual quantum wells of interest. Single emission peaks were observed to shift to the high-energy side with decreasing well width. The peak positions agree with the calculated transition energies for the corresponding well width. The thermalization length of the injected electron was estimated by observing the change in the emission intensity as the tip is moved at different distances from a given well.