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.     

Mapping the first electronic resonances of a Cu phthalocyanine STM tunnel junction

Using a low temperature, ultrahigh vacuum scanning tunneling microscope (STM), dI/dV differential conductance maps were recorded at the tunneling resonance energies for a single Cu phthalocyanine molecule adsorbed on an Au(111) surface. We demonstrated that, contrary to the common assumption, such maps are not representative of the molecular orbital spatial expansion, but rather result from their complex superposition captured by the STM tip apex with a superposition weight which generally does not correspond to the native weight used in the standard Slater determinant basis set. Changes in the molecule conformation on the Au(111) surface further obscure the identification between dI/dV conductance maps and the native molecular orbital electronic probability distribution in space.     

Surface scattering via bulk continuum states in the 3D topological insulator Bi2Se3

We have performed scanning tunneling microscopy and differential tunneling conductance (dI/dV) mapping for the surface of the three-dimensional topological insulator Bi(2)Se(3). The fast Fourier transformation applied to the dI/dV image shows an electron interference pattern near Dirac node despite the general belief that the backscattering is well suppressed in the bulk energy gap region. The comparison of the present experimental result with theoretical surface and bulk band structures shows that the electron interference occurs through the scattering between the surface states near the Dirac node and the bulk continuum states.     

Electronic density oscillations in gold atomic chains assembled atom by atom

Linear Au chains two to 20 atoms long were constructed on a NiAl(110) surface via the manipulation of single atoms with a scanning tunneling microscope. Differential conductance (dI/dV) images of these chains reveal one-dimensional electronic density oscillations at energies 1.0 to 2.5 eV above the Fermi energy. The origin of this delocalized electronic structure is traced to the existence of an electronic resonance measured on single, isolated Au atoms. Variations in the wavelength in dI/dV images of an eleven-atom chain taken at different energies revealed an effective electronic mass of 0.4+/-0.1 times the mass of a free-electron.     

Wave-function mapping of InAs quantum dots by scanning tunneling spectroscopy

Scanning tunneling spectroscopy is used to investigate the single-electron states and the corresponding squared wave functions of single and freestanding strain-induced InAs quantum dots grown on GaAs(001). Several peaks are found in dI/dV curves, which belong to different single-electron states. Spatially resolved dI/dV images reveal (000), (100), (010), (200), and (300) states, where the numbers describe the number of nodes in [11;0], [110], and [001] directions, respectively. The total number and energetic sequence of states is different for different dots. Interestingly, the (010) state is often missing, even when (200) and (300) states are present. We interpret this anisotropy in electronic structure as a consequence of the shape asymmetry of the dots.     

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.     

扫描隧道显微术中的微分谱学及其应用

文章介绍了扫描隧道显微术中微分谱学的原理及其在实验中的诸多应用。微分谱（dI／dV谱）和dI／dV成像可用来研究电子局域态密度在能量和空间的分布，即微分谱固定空间一点，反映电子态密度以能量为变量的分布；而dI／dV图像则反映某给定能量的电子局域态密度以空间为变量的分布，二次微分谱（d^2I／dV^2谱）和二次微分成像可以用来反映分子的非弹性隧穿过程，从而研究分子的振动态。     

Even-odd effect in Andreev transport through a carbon nanotube quantum dot

We have measured the current (I)-voltage (V) characteristics of a single-wall carbon nanotube quantum dot coupled to superconducting source and drain contacts in the intermediate coupling regime. Whereas the enhanced differential conductance dI/dV due to the Kondo resonance is observed in the normal state, this feature around zero-bias voltage is absent in the superconducting state. Nonetheless, a pronounced even-odd effect appears at finite bias in the dI/dV subgap structure caused by Andreev reflection. The first-order Andreev peak appearing around V=Delta/e is markedly enhanced in gate-voltage regions, in which the charge state of the quantum dot is odd. This enhancement is explained by a "hidden" Kondo resonance, pinned to one contact only. A comparison with a single-impurity Anderson model, which is solved numerically in a slave-boson mean-field approach, yields good agreement with the experiment.     

单层硅烯表面的CoPc分子吸附研究

由于低维材料表面上的单原子和分子具有丰富的物理化学性质,现已经成为量子器件及催化科学等领域的研究热点.单层硅烯在不同的衬底制备温度下,表现出丰富的超结构,这些超结构为实现有序的单原子或分子吸附提供了可靠的模板.利用原位硅烯薄膜制备,分子沉积,超高真空扫描隧道显微镜以及扫描隧道谱,本文研究了Ag(111)衬底上3种硅烯超结构((4×4),(■×■),(2■×2■))的电子态结构,表面功函数随超结构的变化,以及CoPc分子在这3种超结构硅烯上的吸附行为.研究结果表明,这3种超结构的硅烯具有类似的电子能带结构,且存在电子从Ag(111)衬底转移到硅烯上的可能性,从而导致硅烯的表面功函数增大,表面功函数在原子级尺度上的变化对分子的选择性吸附起着重要作用.此外,还观察到分子与硅烯的相互作用导致CoPc分子的电子结构发生对称性破缺.     

Imaging of Friedel oscillation patterns of two-dimensionally accumulated electrons at epitaxially grown InAs(111) A surfaces

The local density of states (LDOS) at the epitaxially grown InAs surface on a GaAs(111) A substrate were characterized using low-temperature scanning tunneling microscopy. Using dI/dV signal mapping, LDOS standing waves were clearly imaged at point defects and within nanostructures. Measurement of the wavelength as a function of bias voltage showed a nonparabolic dispersion relation for the conduction band. The observed wave features originate from the Friedel oscillations of the two-dimensional electron gas in the semiconductor surface accumulation layer.     

Confined Shockley surface states on the (111) facets of gold clusters

Combining low-temperature scanning tunneling microscopy and spectroscopy with high-resolution ultraviolet photoemission, we have revealed a confined Shockley surface state on the (111) facets of gold clusters with about N=10(4) atoms grown in nanopits on highly oriented graphite. With tunneling spectroscopy, we observed energy dependent nodal patterns in the dI/dV maps, which are in quantitative agreement with the two-dimensional confinement of the surface state within the hexagonal facet area. The results indicate that the lattice of the ionic cores influences the electronic properties of the clusters significantly.     

Determining the spin polarization of surfaces by spin-polarized scanning tunneling spectroscopy

During the last few years, spin-polarized scanning tunneling microscopy and spectroscopy has been developed as a reliable tool to image surface magnetic domain structures of bulk materials as well as thin films and nanostructured systems. In principle, this technique also allows for the determination of the energy-resolved spin polarization of the sample P_S(E) with nanometer resolution, information which might play a crucial role in understanding systems like, for example, non-magnetic adatoms on magnetic surfaces. A main problem in quantifying P_S(E), however, arises from the fact that, in contrast to planar junctions, the tip–sample distance generally varies with the magnitude and direction of the surface magnetization, since the distance is controlled indirectly by the tunneling current that is itself spin-polarized. We employ a simple model of the tunneling process to investigate this issue and show that a normalization of the dI/dU spectra with the total conductance I/U is insufficient to correct for their distance dependence. Received: 2 September 2002 / Accepted: 2 September 2002 / Published online: 5 March 2003 RID="*" ID="*"Corresponding author. Fax: +49-40/42838-5311, E-mail: kubetzka@physnet.uni-hamburg.de     

Local electronic structure and Fano interference in tunneling into a Kondo hole system

Motivated by the recent success of local electron tunneling into heavy-fermion materials, we study the local electronic structure around a single Kondo hole in an Anderson lattice model and the Fano interference pattern relevant to STM experiments. Within the Gutzwiller method, we find that an intragap bound state exists in the heavy Fermi liquid regime. The energy position of the intragap bound state is dependent on the on-site potential scattering strength in the conduction and f-orbital channels. Within the same method, we derive a new dI/dV formulation, which includes explicitly the renormalization effect due to the f-electron correlation. It is found that the Fano interference gives asymmetric coherent peaks separated by the hybridization gap. The intragap peak structure has a lorenzian shape, and the corresponding dI/dV intensity depends on the energy location of the bound state.     

Image charge effect on the light emission of rutile TiO_2(110) induced by a scanning tunneling microscope

The plasmon-enhanced light emission of rutile TiO_2(110) surface has been investigated by a low-temperature scanning tunneling microscope(STM). We found that the photon emission arises from the inelastic electron tunneling between the STM tip and the conduction band or defect states of TiO_2(110). In contrast to the Au(111) surface, the maximum photon energy as a function of the bias voltage clearly deviates from the linear scaling behavior, suggesting the non-negligible effect of the STM tip on the band structure of TiO_2. By performing differential conductance( dI/dV) measurements, it was revealed that such a deviation is not related to the tip-induced band bending, but is attributed to the image charge effect of the metal tip, which significantly shifts the band edges of the TiO_2(110) towards the Femi level(E_F) during the tunneling process. This work not only sheds new lights onto the understanding of plasmon-enhanced light emission of semiconductor surfaces, but also opens up a new avenue for engineering the plasmon-mediated interfacial charge transfer in molecular and semiconducting materials.     

Interference and Interaction in multi-wall carbon nanotubes

We report equilibrium electric resistance R and tunneling spectroscopy (dI/dV)measurements obtained on single multi-wall nanotubes contacted by four metallic Au fingers from above. At low temperature quantum interference phenomena dominate the magnetoresistance. The phase-coherence (l_φ)and elastic-scattering lengths (l_e)are deduced. Because l_e is of order of the circumference of the nanotubes, transport is quasi-ballistic. This result is supported by a dI/dV spectrum which is in good agreement with the density of states (DOS) due to the one-dimensional subbands expected for a perfect single-wall tube. As a function of temperature T the resistance increases on decreasing T and saturates at ≈1–10 Kfor all measured nanotubes. R(T) cannot be related to the energy-dependent DOS of graphene but is mainly caused by interaction and interference effects. On a relatively small voltage scale of the order ≈10 meV, a pseudogap is observed in dI/dV which agrees with Luttinger-liquid theories for nanotubes. Because we have used quantum diffusion based on Fermi-liquid as well as Luttinger-liquid theory in trying to understand our results, a large fraction of this paper is devoted to a careful discussion of all our results. Received: 17 May 1999 / Accepted: 18 May 1999 / Published online: 4 August 1999     

Spatial fluctuations of the density of states in magnetic fields observed with scanning tunneling spectroscopy

Scanning tunneling spectroscopy images on n-InAs(110) exhibit a strong magnetic field dependent contrast on the 50 nm length scale, indicating fluctuations in the density of states of the sample. The contrast is correlated to previously observed Landau oscillations in dI/dV curves. Its origin is a spatial fluctuation of the Landau level energy of 3-4 meV caused by the inhomogeneous distribution of dopant atoms. Besides inducing large-scale fluctuations in the density of states, dopants preserve their ability to scatter electron waves. The resulting wave pattern is found to depend on the magnetic field. It is suggested that the dependence is guided by the condensation of the electronic states on Landau tubes.     

Topology-induced spin frustrations at the Cr(001) surface studied by spin-polarized scanning tunneling spectroscopy

The magnetic structure of the Cr(001) surface was investigated by spin-polarized scanning tunneling spectroscopy by making use of the spin-polarized surface state located close to the Fermi level. Periodic alternations of the intensity of the surface state peak in local tunneling spectra measured above different ferromagnetic terraces separated by monatomic steps confirm the topological antiferromagnetic order of the Cr(001) surface. Screw dislocations cause topology-induced spin frustration, leading to the formation of domain walls with a width of about 120 nm.     

Enhanced tunnel magnetoresistance due to spin dependent quantum well resonance in specific symmetry states of an ultrathin ferromagnetic electrode

Spin dependent quantum well resonance has been investigated in fully epitaxial magnetic tunnel junctions with Fe(001)/MgO(001)/ultrathin Fe(001)/Cr(001) structure. The dI/dV spectra clearly show the resonant peaks which shift systematically depending on the thickness of an ultrathin electrode as predicted in ab initio calculation [Zhong-Yi Lu et al, Phys. Rev. Lett. 94, 207210 (2005)]. The magnetotransport is strongly modulated at the same bias voltage as the resonant peaks. This control of the magnetotransport in magnetic tunnel junctions at a specific bias voltage will contribute to the development of active spintronic devices.     

Density of states of amorphous GdxSi1-x at the metal-insulator transition

We have determined the electronic density of states of amorphous Gd xSi (1-x), N(GdSi)(E), in the vicinity of the metal-insulator transition by measuring the tunneling conductance dI/dV across a Gd xSi (1-x)/oxide/Pb tunnel junction at low T (T approximately 100 mK). By applying a magnetic field we can tune through the metal-insulator transition and simultaneously measure the transport and N(E) on a single sample. We find a smooth transition from a metal with strong Coulomb interactions to a developing Coulomb gap in the insulating regime. In the metallic region N(GdSi)(0) scales approximately with sigma(2).     

Negative-differential conductivity measured on a germanium layer on Si(001)

The morphology and the electronic structure of heteroepitaxial germanium layers grown pseudomorphically by solution epitaxy on Si(001) has been investigated by scanning tunneling microscopy (STM) and scanning tunneling spectroscopy (STS). A significant decrease of tunneling current at a sample voltage of 1.5 V is observed in areas of 0.5 nm diameter between dimer rows. This decrease is due to a negative-differential conductivity at a tunnel diode configuration consisting of a surface defect structure of the germanium layer and the STM tungsten tip.