Subgap density of states in superconductor-normal metal bilayers in the cooper limit

We present transport and tunneling measurements of Pb-Ag bilayers with thicknesses, d(Pb) and d(Ag), that are much less than the superconducting coherence length. The transition temperature, T(c), and energy gap, Delta, in the tunneling density of states (DOS) decrease exponentially with d(Ag) at fixed d(Pb). Simultaneously, a DOS that increases linearly from the Fermi energy grows and introduces states within the gap. The integrated subgap DOS approaches 40% of the normal state value in the lowest T(c) film investigated (T(c) approximately 0.1 T(Pb)(c,bulk)). This behavior suggests that a growing fraction of quasiparticles decouple from the superconductor as T(c)-->0. The linear dependence is consistent with the quasiparticles becoming trapped on integrable trajectories in the metal layer.     

二维铁电材料α-In2Se3 表面分子束外延生长Pb 纳米岛的STM 研究

铁电材料拥有自发电极化, 不同的极化方向会对异质结的电子结构产生可逆的和非易失性的影响. 本工作采用分子束外延技术在二维铁电材料α-In2Se3 衬底上成功制备了 Pb 纳米岛构建 Pb/α-In2Se3 超导铁电异质结,并通过扫描隧道显微镜表征了其表面原子结构与电子结构. 进一步的扫描隧道谱测量显示不同层厚 Pb 岛的量子阱态消失, 并且我们在4.5 K 的温度下没有观察到超导能隙, 表明铁电衬底会影响 Pb 岛的电子结构, 甚至其超导特性. 这些发现为理解铁电衬底对超导性的影响提供了参考, 并为调控低维量子材料中的电子结构及超导性提供了新的思路.     

Andreev reflection and tunneling spectrum in a superlattice of metal-superconductor junctions

The tunneling spectrum of an electron and a hole in a superlattice of NS junctions is computed using the BTK approach and the transfer matrix method. It shows sharp resonances at some energies above the superconducting gap. The sharper the resonance is the more layers the superlattice has. We find for the first time a mechanism to balance the incident and outgoing currents on the superlattice by averaging over the phase between the incident electron and the incident hole. This mechanism is more natural and physical than those in literatures.     

Ag nanostructures on Au(7 8 8): A self-assembled superlattice of metallic quantum resonators

We have studied by scanning tunneling microscopy (STM) the effect of the reconstruction of a stepped Au(1 1 1) surface on the growth of silver sub-monolayer deposition. For narrow terraces, the reconstruction is disturbed and its pattern changes, Ag growth is therefore influenced. Thus growth of Ag on Au(7 8 8) vicinal surface can be controlled and leads to the formation of a highly ordered superlattice of nanostructures. Moreover, we show by tunneling conductance images that Ag islands exhibit electronic confinement effects of the Shockley surface state. Due to the homogeneity of their shapes and sizes, all the nanostructures of the self-assembled superlattice should exhibit similar electronic properties.     

Tunneling characteristics of trilayer tunnel junctions

We report here the tunneling characteristics (d²V/dI²) of trilayer junctions of the form AlOxAgCuPb and AlOxCuAgPb. The Cu and Ag thicknesses both being 300 Å and the Pb thickness being 3000 Å. The two contributions to the tunneling density of states seen are: (i) the interference effect in the two normal layers showing structures associated with the Pb phonons [1]. This structure is very similar to the one of AlOxAgPb (or AlOxCuPb where the Ag (or Cu) thickness is 600 Å thus showing that the interference effect takes place in both normal metals, (ii) the induced pair potential in the normal layers produces a structure associated with the normal metal phonons. However, only the normal phonons of the metal next to the oxide are seen thus showing that the electron tunneling is a very local probe.     

Creation of an atomic superlattice by immersing metallic adatoms in a two-dimensional electron sea

Cerium adatoms, deposited on a Ag(111) surface, are found by low-temperature scanning tunneling microscopy to self-assemble into large ordered hexagonal arrays covering macroscopically the entire surface. We show that the 32 A periodicity of the superlattice is caused by the interaction of surface-state electrons with Ce adatoms and that the large-scale formation of the superlattice is governed by a subtle balance between the sample temperature, the surface diffusion barrier, and the concentration-dependent adatom interaction potential.     

Electronic properties and interfacial coupling in Pb islands on single-crystalline graphene

Introducing metal thin films on two-dimensional (2D) material may present a system to possess exotic properties due to reduced dimensionality and interfacial effects. We deposit Pb islands on single-crystalline graphene on a Ge(110) substrate and studied the nano- and atomic-scale structures and low-energy electronic excitations with scanning tunneling microscopy/spectroscopy (STM/STS). Robust quantum well states (QWSs) are observed in Pb(111) islands and their oscillation with film thickness reveals the isolation of free electrons in Pb from the graphene substrate. The spectroscopic characteristics of QWSs are consistent with the band structure of a free-standing Pb(111) film. The weak interface coupling is further evidenced by the absence of superconductivity in graphene in close proximity to the superconducting Pb islands. Accordingly, the Pb(111) islands on graphene/Ge(110) are free-standing in nature, showing very weak electronic coupling to the substrate.     

High-resolution scanning tunneling spectroscopy of magnetic impurity induced bound states in the superconducting gap of Pb thin films

Tunneling spectra for individual atoms and dimers of Mn and Cr adsorbed on superconducting Pb thin films were measured by a low temperature scanning tunneling microscope. Multiple-resonance structures within the superconducting gap on the adsorbates were resolved and interpreted as the magnetic impurity-induced bound states associated with different scattering channels. The experiment demonstrates a spectroscopic approach to characterizing the spin states of magnetic structures and exploring the competition between superconductivity and magnetism at the nanometer scale.     

低维超导的实验进展

超导自发现以来, 已成为凝聚态物理领域最重要的方向之一. 近年来, 低维材料制备技术的进步使得一维或二维的超导特性实验研究成为可能. 本文在简要介绍超导现象的基础上, 重点回顾了近些年二维超导薄膜和一维超导纳米线的制备和电输运研究, 以及在低维超导体中发现的相移、近邻效应、铁磁超导相互作用和高温超导等新奇的现象, 并对该领域的进一步发展做出了展望.     

Resonant tunneling via localized impurity states in metal-insulator-metal junctions

Strongly temperature dependent peaks in the tunnel conductance of Al-Al oxide-Pd diodes appearing at low temperatures are interpreted in terms of resonant tunneling via impurity states. Such structure was not observed when non-transition metals (Ag, Au, Pb, Sn) were used as counter electrodes.     

Two-dimensional Sb cluster superlattice on Si substrate fabricated by a two-step method

Nanoclusters consisting of a few atoms have attracted a lot of research interests due to their exotic size-dependent properties. Here, well-ordered two-dimensional Sb cluster superlattice was fabricated on Si substrate by a two-step method and characterized by scanning tunneling microscopy. High resolution scanning tunneling microscope measurements revealed the fine structures of the Sb clusters, which consist of several Sb atoms ranging from 2 to 7. Furthermore, the electronic structure of the nanocluster displays the quantized energy-level which is due to the single-electron tunneling effects. We believe that the fabrication of Sb cluster superlattice broadens the species of the cluster superlattice and provides a promising candidate to further explore the novel physical and chemical properties of the semimetal nanocluster.     

超导交叉膜隧道结的电流-电压滞迴

本文指出对于超导交叉膜隧道结,跨越结的隧道电流除引起结区温升外,同时在结区产生不均匀的自场。这将使结区超导膜进入中间态。用这个物理模型不仅可以解释超导Pb膜的不可逆非平衡电压曲线的滞迴以及实验上出现的阈值电流I_(t3)的存在,而且还可以得出通常的隧道I-V曲线中出现滞迴的奇异现象。 关键词：     

Anomalous tunneling into magnetically ordered dilute alloys which have been induced into the superconducting state

Unusual tunneling characteristics for junctions consisting of Al, oxide and magnetic alloys, which are backed by Pb, are presented. The Pb induces the alloys to be superconducting. The alloys studied are 1500 ppm Fe in Au and 1200 ppm Co in Pd.     

Fluctuation dominated Josephson tunneling with a scanning tunneling microscope

We demonstrate Josephson tunneling in vacuum tunnel junctions formed between a superconducting scanning tunneling microscope tip and a Pb film, for junction resistances in the range 50-300 k Omega. We show that the superconducting phase dynamics is dominated by thermal fluctuations, and that the Josephson current appears as a peak centered at small finite voltage. In the presence of microwave fields ( f = 15.0 GHz) the peak decreases in magnitude and shifts to higher voltages with increasing rf power, in agreement with theory.     

Superconductivity of vapour quenched beryllium and beryllium-based alloys

The properties of Be films, quench-condensed upon a³He cooled substrate, have been investigated by resistance and tunneling measurements. The superconducting transition temperature,T _c , of Be films increased with thickness and a thick film limit of 9.95 K could be estimated. Alloying with Al or Pb decreasedT _c. The ratios between energy gaps andT _c 's indicated that Be is a weak coupling superconductor, and no phonon induced structure could be traced in tunneling curves neither in pure Be nor in the Be based alloys. Resistance change during annealing as well as superconducting data indicated that the vapour quenched Be films were amorphous as deposited.     

Electrochemical formation of porous superlattices on n-type (1 0 0) InP

The present work deals with the electrochemical formation of superlattice structures on n-type (1 0 0) InP in HCl solutions. The superlattices consist of a stack of two layers with alternating high and low porosity on n-type material obtained by changing the anodizing current or the potential periodically in HCl solutions. The superlattice structures were characterized by scanning electron microscopy. The pore morphology and structure depend strongly on the electrochemical conditions. For anodization with low currents (e.g., 1 or 10 mA) or at low potentials (e.g., 1.5 V_Ag/AgCl), a porous layer with a facet-like structure was formed. For higher currents or potentials, such as 50 or 100 mA or 3 V_Ag/AgCl, respectively, a tree-like structure with random and/or tangled branches was observed. Finally, samples anodized at 5 V_Ag/AgCl, show a porous layer with a regular array of straight pores. The morphology and structure of the stacks of the porous layers can be controlled in the nanometer range, depending on the electrochemical conditions.     

Mode selective control of flow turbulence

We study spin transport in normal/ferromagnetic/normal/ferromagnetic.../normal graphene superlattices, which can be realized by putting a series of magnetic insulator bars on top of a graphene sheet. Owing to magnetic proximity effect, local exchange splittings will be induced in the graphene sheet, effectively forming a magnetic graphene superlattice. The spin polarization of tunneling conductance and the magneto resistance (MR) exhibit oscillatory behavior with the gate voltage. The superlattice structure leads to an enhanced spin polarization and MR ratio, making the magnetic graphene superlattice become very promising in spintronics applications.     

ARPES and STS investigation of noble metal Shockley states: Confinement in vicinal Au(1 1 1) surfaces and self-organized nanostructures

Spectroscopic effects associated with the superperiodic surface structure have been observed in Au(1 1 1) vicinal surfaces and nanostructured systems. In the vicinal Au(23 23 21) surface, high resolution angle resolved photoemission spectroscopy shows the opening of several gaps in the surface band structure, whereas scanning tunneling spectroscopy reveals the energy dependence of the electronic density. These combined spectroscopic data allow to determine the reconstruction potential by deducing their first Fourier components. We also demonstrate that due to the peculiar growth on this Au vicinal surface, we can obtain a self-assembled superlattice of triangular Ag islands. The high ordering of the nanostructures leads to homogenous electronic properties.     

Selective formation of ultrathin PbSe on Ag(111)

Two-dimensional (2D) semiconductors, such as lead selenide (PbSe), locate at the key position of next-generation devices. However, the ultrathin PbSe is still rarely reported experimentally, particularly on metal substrates. Here, we report the ultrathin PbSe synthesized via sequential molecular beam epitaxy on Ag(111). The scanning tunneling microscopy is used to resolve the atomic structure and confirms the selective formation of ultrathin PbSe through the reaction between Ag₅Se₂ and Pb, as further evidenced by the theoretical calculation. It is also found that the increased accumulation of Pb leads to the improved quality of PbSe with larger and more uniform films. The detailed analysis demonstrates the bilayer structure of synthesized PbSe, which could be deemed to achieve the 2D limit. The differential conductance spectrum reveals a metallic feature of the PbSe film, indicating a certain interaction between PbSe and Ag(111). Moreover, the moiré pattern originated from the lattice mismatch between PbSe and Ag(111) is observed, and this moiré system provides the opportunity for studying physics under periodical modulation and for device applications. Our work illustrates a pathway to selectively synthesize ultrathin PbSe on metal surfaces and suggests a 2D experimental platform to explore PbSe-based opto-electronic and thermoelectric phenomena.     

Electron tunneling through superconducting A1/monolayer/Pb junctions

The current-voltage characteristic of Al/adsorbed monolayer/Pb junctions was measured at 77, 4.2 and 1.8K at applied voltages from 1 to 3 mV. At 77K the current changes linearly with voltage whereas at 4.2 and 1.8 K the relationship becomes nonlinear. From the results at 1.8 K we obtain an approximate band gap for Pb equal to 2.6 meV. The observation of a nonlinear current-voltage characteristic at temperatures where Pb becomes superconducting is strong evidence that the observed current through the insulator is a tunneling current.