Tunneling Spectra for Quasi–One-Dimensional Organic Superconductors

The tunneling conductance spectra of a normal metal/insulator/quasi–one-dimensional superconductor is calculated using the Blonder-Tinkham-Klapwijk formulation. The pairing symmetry of the superconductor is assumed to be p or d, and f-wave. It is found that there is a well-defined zero-bias peak in electron tunneling along the direction parallel to the chains or normal to these when the transmitted quasiparticles feel different sign of the pair potential. The actual line shape of the spectra is sensitive to the nodes of the pair potential on the Fermi surface.     

Andreev Bound-State Tunneling and ESR Spectroscopy of High-Temperature Superconductors and Observations of Broken Time-Reversal Symmetry

An emphasis on reliable materials growth and development of new fabrication techniques has allowed us to investigate the electronic structure of high-temperature superconductors by planar quasiparticle tunneling and electron paramagnetic resonance (EPR) spectroscopies. The quasiparticle (QP) density of states (DoS) is investigated by tunneling into oriented thin films of Y₁Ba₂Cu₃O₇ (YBCO) and single crystals of Ba₂Sr₂Ca₁Cu₂O₈ (BSCCO). Data are obtained as a function of crystallographic orientation, temperature, doping, damage, and applied magnetic field. These data demonstrate that the observed zero-bias conductance peak (ZBCP) is composed of Andreev bound states (ABS) which intrinsically form at a symmetry-breaking interface of an unconventional superconductor, for example, a (110)-surface of d-wave YBCO. Tunneling into doped or ion-damaged YBCO provides a measure of the QP scattering rate below T _c. An applied field causes Doppler shift of the ABS, arising from the scalar product between the QP velocity and the superfluid momentum, v _F·P _s, observed as a splitting in the ZBCP. Magnetic hysteresis of the splitting is consistent with the effects of strong vortex pinning near the interface. The directional field dependence shows that the ABS is highly anisotropic in its transport. These results, plus in-plane crystallographic orientational dependence on single-crystal BSCCO, demonstrate the d-wave symmetry of this superconductor. Below 8 K and in zero applied field, the ZBCP splits, indicating a transition into a superconducting state with spontaneously broken time-reversal symmetry (BTRS). EPR experiments are used to detect directly the spontaneous formation of the magnetic moments in the BTRS state.     

Quasi-Two-Dimensional Organic Superconductors

A review on selected normal-state and superconducting properties of the quasi-two-dimensional organic superconductors is given. Thereby, the focus is laid on the charge-transfer salts based on bisethylenedithio-tetrathiafulvalene, or ET for short, the building block of most of the to-date known organic superconductors. Some basic features of the crystallographic structure, the highly anisotropic electronic band structure for some materials, as well as unusual electronic-transport properties are highlighted. The principal phase diagram of the κ-phase salts and the fundamental difficulties of a purely electronic explanation of the observed features are discussed. A brief overview on the anisotropic superconducting properties as well as the still very controversial notion on the nature of the superconducting state is presented.      

STM tunneling spectroscopy on high Tc superconductors

STM tunneling spectroscopy has been performed on the bulk single crystals of BiSrCaCuO (BSCCO) and the epitaxial thin films of YBaCuO (YBCO) at cryogenic temperatures. The STM images and tunneling spectra observed on the (001) surfaces can be classified into three cases; 1) Atomic image is visible. However, the tunneling spectrum shows semiconducting or smeared superconducting gap structures, depending on the tip-sample distance. 2) Clear atomic image can not be obtained. But, the tunneling spectrum shows flat bottom region with quite low zero bias conductance. 3) Tunneling spectra demonstrate gapless behavior, independent of the tip-sample separation. These observations support the quasi-2D electronic picture in whichs-wave like 2D superconducting layers are coupled with each other through the Josephson effect.     

Deposition and STM characterization of luminescent organic molecules on metal substrates

The porphyrin-based molecules including H₂TBPP, ZnTBPP and P_tTBPP as well as the perinone derivatives (PD) molecules were deposited on Au(100), Au(111) and Cu(100) substrates in the form of single molecule, molecular line, submonolayer, monolayer and multilayer. The features of the performed molecular nanostructures were characterized by an ultrahigh vacuum scanning tunneling microscopy (STM) at room temperature. The observed molecular topographies matched very well with the molecule structures in spite of the fact that the molecular adsorption states were influenced by different metal surface. STM-induced photon emission was generated from the surface of H₂TBPP multilayer structures on Au(100) in tunneling junction when the applied bias exceeds the “turn-on” voltages ∼− 1.6 V or + 1.3 V at 0.5 nA.     

Tunnelling in Organic Superconductors

We present a simple scheme for computing the full current-voltage characteristics for tunnelling experiments within the framework of the non-equilibrium Keldysh Green function formalism. This formalism is flexible enough to address different pairing symmetries combined with magnetic fields at arbitrary bias voltages. We show how to apply these results to probe for the symmetry of the superconducting order parameter in the Bechgaard salts using tunnelling experiments.     

STM manipulation of molecular moulds on metal surfaces

Molecular Landers are a class of compounds containing an aromatic board as well as bulky side groups which upon adsorption of the molecule on a surface may lift the molecular board away from the substrate. Different molecular Landers have extensively been studied as model systems for nanomachines and the formation of molecular wires, as well as for their function as “molecular moulds”, i.e., acting as templates by accommodating metal atoms underneath their aromatic board. Here, we investigate the adsorption of a novel Lander molecule 1,4-bis(4-(2,4-diaminotriazine)phenyl)-2,3,5,6-tetrakis(4-tert-butylphenyl)benzene (DAT, C₆₄H₆₈N₁₀) on Cu(110) and Au(111) surfaces under ultrahigh vacuum (UHV) conditions. By means of scanning tunneling microscopy (STM) imaging and manipulation, we characterize the morphology and binding geometries of DAT molecules at terraces and step edges. On the Cu(110) surface, various contact configurations of individual DAT Landers were formed at the step edges in a controlled manner, steered by STM manipulation, including lateral translation, rotation, and pushing molecules to an upper terrace. The diffusion barrier of single DAT molecules on Au(111) is considerably smaller than on Cu(110). The DAT Lander is specially designed with diamino-triazine side groups making it suitable for future studies of molecular self-assembly by hydrogen-bonding interactions. The results presented here are an important guide to the choice of substrate for future studies using this compound. This article is published with open access at Springerlink.com These authors contributed equally to this work     

Organic Superconductors as Stages for Exploration of Superconductivity in High Magnetic Fields

The low dimensionality of organic superconductors leads to pronounced anisotropy in the upper critical field. In the vicinity of the field direction parallel to the superconducting plane, the critical field shoots out due to the suppression of the orbital pair-breaking effect. Organic superconductors are suitable for the study of the high-field state related to the spin effect under an aligned field, since they are of a high crystalline quality. The reported experimental results covering the behavior at low temperatures are reviewed first, and the breakthrough of the BCS Pauli paramagnetic limit is discussed. The potential of the low-dimensional organic superconductor for study of the effect of electronic spectrum quantization is argued.     

The Mechanism of Surface Decomposition of Tl- and Hg-Containing Superconductors

The surface chemistry of various Tl- and Hg-containing superconducting oxides has been studied by high resolution transmission electron microscopy. The mechanism of surface decomposition of these materials is discussed. High chemical reactivity of the Tl-O and Hg-O layers seem to be the main course of the decomposition. It is proposed that a surface substitution of Tl and Hg may stabilise the structures without losing superconducting properties.     

Self-assembly of truxene derivatives investigated by STM

Truxene and its derivatives were synthesized, and their assembly structures were studied by scanning tunneling microscopy (STM) technique. Parallelogram-shaped dimers with different contrast can be resolved for two-dimensional assembly of truxene molecules. Moreover, the side-chain-effect is introduced into the system to tune and control the assembling structures of the truxene for one of the two truxene derivatives, hexatetradecyltruxene (truxene-C₁₄) with six C₁₄ side chains, showing stripe assembling structures with different contrast. While for the other one, 2,7,12-tertriazine-5,5′,10,10′,15,15′-hexatetradecyltruxene (tertriazine-truxene-C₁₄) form the intermolecular multi-hydrogen bonds. Hydrogen bond interaction influences the molecular orientation and conformation of these two-dimensional supramolecular architectures. These results demonstrate the role of hydrogen bonds in molecular assembly formations and what is more, would be beneficial for understanding and fine tuning of the assembling of truxene-related opto-electronic materials.     

Pairing insights in iron-based superconductors from scanning tunneling microscopy

Scanning tunneling microscopy (STM) has made tremendous progress in the study and understanding of both classical and unconventional superconductors. This has motivated a rapidly growing effort to apply the same techniques to the iron-based high-T_c superconductors since their discovery in 2008. Five years have brought exciting advances in imaging and spectroscopic investigation of this new class of materials. In this review, we focus on several recent STM contributions to the identification of the gap symmetry and pairing glue. We highlight the unique capabilities and challenges still ahead for STM studies of iron-based superconductors.     

基于小波变换的STM图像处理

根据扫描隧道显微镜(STM)的特点,阐述了将小波变换应用于扫描隧道显微镜的降噪、增强及融合的方法,对于STM扫描获得的石墨原子图像,选用不同的小波基函数和分解层数进行分解和重构,结果表明,利用小波变换对扫描隧道显微镜图像进行处理是有效的、可行的,图像质量得到了明显提高。     

Admixture of an s-Wave Component to the d-Wave Gap Symmetry in High-Temperature Superconductors

Neutron crystal-field spectroscopy experiments in the Y- and La-type high-temperature superconductors HoBa₂Cu₃O_6.56, HoBa₂Cu₄O₈, and La_1.81Sr_0.15Ho_0.04- CuO₄ are reviewed. By this bulk-sensitive technique, information on the gap function is obtained from the relaxation behavior of crystal-field transitions associated with the Ho³⁺ ions which sit as local probes close to the superconducting copper-oxide planes. The relaxation data exhibit a peculiar change from a convex to a concave shape between the superconducting transition temperature T _c and the pseudogap temperature T ^* which can only be modeled satisfactorily if the gap function of predominantly d-wave symmetry includes an s-wave component of the order of 20–25%, independent of the doping level. Moreover, our results are compatible with an unusual temperature dependence of the gap function in the pseudogap region (T _c≤T≤T ^*), i.e., a break up of the Fermi surface into disconnected arcs.     

Si(337)表面的STM研究

利用超高真空扫描隧道显微镜 (UHVSTM) ,研究了Si(337)高指数面的形貌和原子结构 ,发现Si(337)表面平台的生长与退火速度有关 ,相对较慢的退火速度有利于生长尺寸更大、结构更完整的平台。高分辨STM结果显示Si(337)表面平台原子结构为 (5 5 1 2 )的周期性结构 ,表明Si(337)表面退火过程中稳定性较差 ,易重构成临近的更稳定的高指数面     

单层聚酰亚胺LB膜的STM成象

扫描隧道显微镜(STM)被用来表征聚酰亚胺LB膜的形貌及分子排列结构。本文介绍了单层聚酰亚胺LB膜样品制备过程,所用的STM系统及STM实验。给出了该LB膜亚胺化前后的STM图象。结果表明,所制备的LB膜的聚合链排列有序,测得链间距即横向周期～7(?),纵向上所谓的“之”字型结构的周期为11(?)。这些数值与根据分子面积在理论上所预估的结果相符。     

Temperature Dependence of the STM Morphology and Electronic Level Structure of Silver‐Containing DNA

Understanding the effect of external conditions, temperature in particular, on novel nanomaterials is of great significance. The powerful ability of scanning tunneling microscopy (STM) to characterize topography and electronic levels on a single molecule scale is utilized herein to characterize individual silver‐containing poly(dG)–poly(dC) DNA molecules, at different temperatures. These measurements indicate that the molecule is a truly hybrid metal–organic nanomaterial with electronic states originating from both the DNA and the embedded silver. The temperature dependence of this density of states (DOS) leads to the temperature dependent STM apparent height of the molecule—a phenomenon that has not been observed before for other complex nanostructures.     

Mapping High-Temperature Superconductors—A Scientometric Approach

This study has been carried out to analyze the research field of high-temperature superconductivity and to demonstrate the potential of modern databases and search systems for generating meta-information. The alkaline earth (A2) rare earth (RE) cuprate high-temperature superconductors as a typical inorganic compound family and the corresponding literature were analyzed by scientometric methods. The time dependent overall number of articles and patents and of the publications related to specific compound subsets and subject categories are given. The data reveal a significant decrease of basic research activity in this research field. The A2 RE cuprate species covered by the CAS compound file were analyzed with respect to the occurrence of specific elements in order to visualize known and unknown substances and to identify characteristic patterns. The quaternary and quinternary cuprates were selected and the number of compound species as a function of specific combinations of A2 and RE elements is given. The Cu/O and RE/A2 ratios of the quaternary cuprate species as a function of A2 and RE atoms are shown. In addition, the research landscape of the MgB₂ related publications was established using STN AnaVist, an analysis tool recently developed by STN International.     

Layered Superconductors in an External Magnetic Field

Presence of localized magnetic moments and inhomogeneites leads to an unconventional behavior of the upper critical field for layered superconductors. In addition, one can observe a large diamagnetic moment at H H _c2 and in the normal resistive state.     

Real and Marginal Isotope Effects in Cuprate Superconductors

We critically review recent and earlier results on isotope effects in cuprate superconductors and emphasize that the sample preparation and the isotope exchange and back exchange are crucial in understanding and interpreting the data. Only extremely careful preparation techniques yield reliable results and permit differentiation between real isotope effects and marginal ones. The former are substantial and highlight the lattice vibrational importance in cuprate superconductors.