Cryogenic STM/STS observation on oxide superconductors

The topographic and electronic properties of the surfaces of (001) and (110) oriented YBa₂Cu₃O_y, epitaxial films have been probed by atomic resolution STM/STS at 4.2 K. The STM image on the (001) surface clearly revealed the atomic corrugation of the tetragonal lattice with an average spacing of 0.4 nm. while on the (110) surface the orthorhombic atomic lattice, corresponding to the Cu atoms of both CuO₂ and CuO chain planes, was observed. The STS result on the (001) surface indicated the semiconducting nature of the terminating layer. As the tunneling tip came closer to the surface, however, the shape of the tunneling spectrum became more metallic and showed a superconducting energy gap, which seems to arise from the underlying superconducting layer. On the other hand, the tunneling spectra on the (110) surface indicated superconducting gap structures, independent of the tip-sample distance.     

STM Observations of 2D Kr and Xe Adsorbed on Graphite

Crystalline structures of two dimensional rare-gas solids physisorbed on a graphite surface are studied with a low-temperature scanning tunneling microscope (STM) at T=4 K. We have obtained atomically resolved STM images of monolayer krypton (Kr) for the first time as well as those of xenon (Xe). It was observed that the 2D structure of Xe is destroyed with small tip-sample separation. Distinct changes in the local density of states were observed in tunneling spectra after the adsorption. For a multi-layer Xe film, a characteristic telegraph pattern of the tunneling current was also observed, which probably indicates single atom dynamics.     

Tunneling Conductance Dependence of the Tunneling Spectra in Bi2Sr2CaCu2O8

We have measured the tunneling spectra in Bi₂Sr₂CaCu₂O₈ with a scanning tunneling microscope(STM) at various tip-sample distances by changing the tunneling conductance in a controlled manner. When the tunneling conductance is increased from 1×10^–9 to 1×10^–5 S, the spectra do not show changes in particular. However, the gap value decreases steeply and the asymmetric back ground density of states turns inverted V-shaped one above 6×10^–4 S. The changes in the tunneling spectra at the high tunneling conductances are explained by the enhancement of the local carrier density induced by the pressure that the STM tip applied to the sample.     

Low Temperature Scanning Tunneling Spectroscopy Studies of High Jc NdBa2Cu3O7 ? δ Single Crystals

We report low temperature scanning tunneling spectroscopy (STS) studies on NdBa₂Cu₃O_{7 –} (Nd123) single crystals. Two characteristic spectra with a nearly ohmic background spectrum and a V-shaped background are observed on the cleaved surfaces. The former spectum shows an inhomogeneous distribution of a superconducting energy gap (2) for STS differential conductance map. The latter shows a homogeneous distribution. It is probable that those differences are attributed to a tunneling current from different surface layers. The temperature dependence of tunneling conductance spectrum with a V-shaped background reveals that the superconducting gap disappears around T _c, and no pseudogap behavior exists above T _c.     

Low Temperature Scanning Tunneling Spectroscopy Studies of High J c NdBa2Cu3O7 − δ Single Crystals

We report low temperature scanning tunneling spectroscopy (STS) studies on NdBa₂Cu₃O_{7 ? δ} (Nd123) single crystals. Two characteristic spectra with a nearly ohmic background spectrum and a V-shaped background are observed on the cleaved surfaces. The former spectum shows an inhomogeneous distribution of a superconducting energy gap (2Δ) for STS differential conductance map. The latter shows a homogeneous distribution. It is probable that those differences are attributed to a tunneling current from different surface layers. The temperature dependence of tunneling conductance spectrum with a V-shaped background reveals that the superconducting gap disappears around T _c, and no pseudogap behavior exists above T _c.     

Thermochemical hole burning performance of TCNQ-based charge transfer complexes with different electrical conductivities

Thermochemical hole burning (THB) memory is an ultrahigh density data storage technique based on the scanning tunneling microscope (STM). It utilizes the STM current to induce localized thermochemical decomposition of TCNQ-based charge transfer (CT) complexes and sequentially create nanometer-sized holes as information bits. The writing reliability and hole size depend on many factors, including the properties of the storage materials and the STM tip, and the tip-sample distance and interaction. We have found here that for the high electrical conductivity CT complexes, the hole size (represented by volume) monotonically decreases with the tip displacement increasing in the direction of leaving the sample; but for low electrical conductivity samples, the hole size first increases and then decreases with the tip displacement increasing in the same direction. Subsequent experiments and analyses indicate that the surface deformation induced by the tip-sample interaction and the heat conduction of the metal tip account for such a unique phenomenon.     

MgB2 Superconducting Tips for Scanning Tunneling Microscopy Study

We report a simple method for the fabrication of reproducible, clean, and stable MgB₂ superconducting tips. The quality of these tips has been verified by imaging the surface of a thin Au(111) film sample, using a low temperature scanning tunneling microscopy (STM). Using the MgB₂ superconducting tip, high-quality semiatomically resolved STM surface images of the thin Au(111) film sample have been observed, which unambiguously indicates that the fabrication of relatively superconducting MgB₂, suitable for use as STM tips, is feasible.     

MgB2 Superconducting Tips for Scanning Tunneling Microscopy Study

We report a simple method for the fabrication of reproducible, clean, and stable MgB₂ superconducting tips. The quality of these tips has been verified by imaging the surface of a thin Au(111) film sample, using a low temperature scanning tunneling microscopy (STM). Using the MgB₂ superconducting tip, high-quality semiatomically resolved STM surface images of the thin Au(111) film sample have been observed, which unambiguously indicates that the fabrication of relatively superconducting MgB₂, suitable for use as STM tips, is feasible.     

STM Spectroscopy on Anisotropic Superconductors

The superconducting phase of organic superconductors -(BEDT-TTF)₂Cu(NCS)₂ and (MDT-TTF)₂AuI₂ was investigated by the electron tunneling spectroscopy using low temperature STM. The tunneling differential conductance at the lateral surface of -(BEDT-TTF)₂Cu(NCS)₂ varies its shape depending on the tip direction. The in-plane anisotropy of the conductance is well explained by the d-wave symmetry with line nodes along the direction 45° from the c-axis. For (MDT-TTF)₂AuI₂, the tunneling conductance at T = 1.4 K shows the superconducting energy gap structure clearly. The finite conductance inside the gap edge suggests the gap anisotropy. The tunneling spectrum is explained by the d-wave pairing. The obtained gap ₀ = 2 meV (2₀/kT _c = 12) is larger than that of the weak coupling limit. The pseudogap structure is observed near T _c .     

New approach towards imaging <Emphasis Type="Italic">λ</Emphasis>-DNA using scanning tunneling microscopy/spectroscopy (STM/STS)

A new methodology to anchor λ-DNA to silanized n-Si(111) surface using Langmuir Blodget trough was developed. The n-Si (111) was silanized by treating it with low molecular weight octyltrichlorosilane in toluene. Scanning tunneling microscopy (STM) image of λ-DNA on octyltrichlorosilane deposited Si substrate shows areas exhibiting arrayed structures of 700 nm length and 40 nm spacing. Scanning tunneling spectroscopy (STS) at different stages depict a broad distribution of defect states in the bandgap region of n-Si(111) which presumably facilitates tunneling through otherwise insulating DNA layer.     

Studies of 2D Cryocrystals by STM Techniques

Scanning tunneling microscopy (STM) studies of two-dimensional (2D) cryocrystals ⁴ He, Kr, Xe) physisorbed on graphite surfaces are presented. Individual helium atoms, usually thought to be invisible with STM, were recently observed on graphite surfaces at a density corresponding to the commensurate solid. Here we show that a local elastic deformation seems to be the principal mechanism responsible to render the atoms visible. Recent tight-binding calculations of the local density of states (LDOS) of graphite which predict the appearance of an energy gap support this picture. I-z curve measurements for the case of ⁴ He show a sharp drop (increase) of the tunneling current I at a certain tip-surface distance z during retraction (approach) of the tip. This drop (increase) may be associated with the tunneling of a single He adatom, opening new possibilities to study the quantum tunneling of atoms via STM.     

Construction of poly-naphthalocyanine linked by [4]-radialene-like structures on silver surfaces

Cyclic-conjugated linkages between planar-macrocydic molecules contribute to the robustness of the two-dimensional (2D) polymerization and extension of π-interactions. The fabrication of such linkages in 2D polymers remains challenging. Combining scanning tunneling microscope (STM) measurements and density functional theory (DFT) calculations, we demonstrate a linear polymerization of metal-free naphthalocyanine (NPc) molecules with [4]-radialene-like linkages on silver surfaces. Experimentally, by depositing NPc molecules on the Ag(110) surface and subsequent annealing up to 750 K, one-dimensional polymers are constructed along the [\(1\overline 1 0\)] direction. High-resolution STM images show a stem-leaf-like feature. STM simulations based on a linear polymer of NPc molecules linked by four-membered carbon rings, [4]-radialene-like structure, agree well with the experimental observations. DFT calculations reveal that the polymerization process includes detaching two-terminal H atoms of NPc molecules along [\(1\overline 1 0\)] direction, then bonding with a neighboring dehydrogenated NPc molecule by forming a four-membered ring. The dehydrogenation process can be promoted by on-surface impurities such as additional H atoms. Similar polymerizations have been achieved on Ag(111) surfaces in an amorphous way. Moreover, the energy gap of the NPc molecule decreases after linear polymerization, suggesting a red-shift for its optical absorption/scattering spectrum. Our study offers a new route to polymerize conjugated molecules with extended planar π-interactions.

     

锁定放大技术用于扫描隧道显微镜

介绍了锁定放大技术应用于扫描隧道显微镜(STM)中调制隧道间隙而获得物质表面原子的局域势垒分布的原理,并给出了高定向石墨的观测结果,与恒电流模式下STM图像相比较,具有分辨率高、针尖的影响小、抗干扰性能强的特点。另外,根据局域势垒分布还可区分不同的原子,给解释STM原子图像提供了一种手段。     

Direct STM mapping of the superconducting energy gap in single crystal BI2SR2CACU2O8+x

Scanning tunneling spectroscopy applied to BiO cleavage planes of 90 KT _c BSCCO 2212 at 4.2 K simultaneously provides topography and localdI/dV spectra (superconducting DOS). The spectra, which are similar to recent photoemission spectra, confirm a large gap parameter (x, y) associated with an apparently gapless DOS on the uppermost layer. Transverse spatial variations of on a 100 Å scale are attributed to variations in BiO metallicity, presumably originating in oxygen stoichiometry variations in an unannealed crystal. We identify two characteristicdI/dV spectral shapes with regions of metallic and nonmetallic BiO layers, and can relate these by the McMillan model of the superconducting proximity effect. A sharply peaked spectral shape, similar to that observed in photoemission, is predicted for the metallic BiO layer induced superconducting by its proximity to the underlying CuO₂ planes. The short mean free path and short coherence length imply that both tunneling and photoemission spectra are heavily weighted toward contributions from the BiO layer in fully metallic 2212. The present results and analysis thus suggest that the superconducting proximity effect influences the lineshapes seen by both techniques.     

Probing the Superconducting Gap from Tunneling Conductance on NdFeAsO<Subscript>0.7</Subscript> with <Emphasis Type="Italic">T</Emphasis><Subscript>C</Subscript>=51 K

We report the tunneling spectroscopy of an iron-based oxypnictide NdFeAsO_0.7 with T _C=51 K, measured by a mechanical point contact technique. Mainly two kinds of tunneling spectra have been observed reproducibly. One is tunneling conductance displaying sharp superconducting gap peaks at 6.0±1.0 mV, in which hump (or kink) structures are also observed at 20–30 mV. Another is that showing dominantly the larger superconducting gap Δ _L with sharp conductance peaks at 14±1.0 meV, in which the trace of a smaller gap (Δ _S=5–7 meV) is simultaneously observed. Our results give direct evidence for the existence of multiple gaps in the quasiparticle excitation spectrum of this multiband system, although the origin of the hump at 20–30 mV is still unclear.     

Bis(phthalocyaninato)yttrium grown on Au(111): Electronic structure of a single molecule and the stability of two-dimensional films investigated by scanning tunneling microscopy/spectroscopy at 4.8 K

Scanning tunneling microscopy/spectroscopy (STM/STS) at 4.8 K has been used to examine the growth of a double-decker bis(phthalocyaninato)yttrium (YPc₂) molecule on a reconstructed Au(111) substrate. Local differential conductance spectra (dI/dV) of a single YPc₂ molecule allow the characteristics of the highest occupied molecular orbital (HOMO) and the lowest unoccupied molecular orbital (LUMO) to be identified. Furthermore, lateral distributions of the local density of states (LDOS) have also been obtained by dI/dV mapping and confirmed by first principles simulations. These electronic feature mappings and theoretical calculations provide a basis for understanding the unique STM morphology of YPc₂, which is usually imaged as an eight-lobed structure. In addition, we demonstrate that bias-dependent STM morphologies and simultaneous dI/dV maps can provide a way of understanding the stability of two-dimensional YPc₂ films.      

NaCl multi-layer islands grown on Au(111)-([Formula: see text]) probed by scanning tunneling microscopy

The growth of multi-layer NaCl islands on Au(111)-([Formula: see text]) surfaces was investigated using scanning tunneling microscopy (STM). We observed that the aspect of the NaCl islands drastically differs depending on the tunneling conditions. It is therefore possible to observe the layers forming an NaCl island or to image the gold reconstruction below the first NaCl layer. Atomically resolved STM images obtained on the first NaCl layer demonstrate that NaCl grows as an epitaxial crystalline film on Au(111)-([Formula: see text]). STM images also suggest that some NaCl layers can be non-crystalline.     

Nanostructured H(3+x)PW(12-x)NbxO40 (x = 0-3) Keggin heteropolyacid catalysts

Nanostructured H(3+x)PW(12-x)NbxO40 (x = 0, 1, 2, 3) Keggin heteropolyacid (HPA) catalysts were investigated by scanning tunneling microscopy (STM) and tunneling spectroscopy to probe their redox property and oxidation catalysis. STM image showed that the HPAs formed two-dimensional well-ordered monolayer arrays on graphite surface. In tunneling spectra of the HPAs deposited on graphite, they exhibited a distinctive current-voltage behavior referred to as negative differential resistance (NDR). NDR peak voltage measured atop HPA molecule was then correlated with reduction potential and absorption edge energy determined by electrochemical method and UV-visible spectroscopy, respectively. It was revealed that NDR peak voltage of the HPAs appeared at less negative voltage with increasing reduction potential and with decreasing absorption edge energy. In order to correlate NDR peak voltage of H(3+x)PW(12-x)NbxO40 Keggin HPAs with oxidation catalysis, oxidative dehydrogenation of isobutyraldehyde to methacrolein was carried out as a model reaction. NDR peak voltage of the HPAs appeared at less negative voltage with increasing yield for methacrolein.     

Phonon structure in the tunneling characteristics of thin proximity effect sandwiches

The tunneling spectra of Al-oxide-N-S proximity effect junctions show phonon structure not only from the superconducting layerS, but also from the normal metal layerN. We report experimental results for junctios with Ag, Cu, and Al normal metal layers (100–400 Å thick) and Pb or Sn superconducting layers (3000 Å thick). We compare these experimental results with the McMillan geometrical resonance model of the proximity effect and with an extended McMillan tunneling model. The extension includes the effect of the frequency-dependent electron-phonon interaction in the normal metal. Finally, we estimate the electron-phonon coupling constant for the normal metals from the size of their phonon structure in the tunneling spectra.Research supported by National Science Foundation under Grant #NSF DMR 73-7518 A01.Research supported by National Science Foundation under Grant #NSF DMR 75-19544.Research supported by National Science Foundation under Grant #GH 37239.     

Real Space Imaging of the Electronic States in Underdoped Ca2−x Na x CuO2Cl2 Single Crystals

Scanning tunneling microscopy (STM) has been performed on underdoped Ca_2–x Na _x CuO₂Cl₂ (Na-CCOC) single crystals to investigate the electronic states of doped Mott insulators near the metal-insulator transition. STM images taken at 7 K show patch-like or river-like irregular features superposed on the atomic corrugations. The irregular structure has a characteristic length scale of 20 Å, which is unchanged in different samples and at different doping levels studied (0.08<x<0.12). Bias voltage dependence of the STM image suggests that Na-CCOC consists of two distinct phases with different electronic states.