Electron tunneling in oxide superconductors

Electron tunneling experiments in very well-characterized polycrystalline samples of Ba_1–xK_xBiO₃ and Bi₂Sr₂Ca₂Cu₃O_10– show very clear features of the energy gap. The energy gap 2 and the ratio 2/K _B T _c were determined. In the Ba_1–xK_xBiO₃ compound we also study the behavior of the tunneling conductance in the normal state.     

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 .     

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.     

Isotope coefficient in high Tc superconductors

An exact expression for the isotope coefficient (), within the conventional BCS theory, has been derived for any arbitrary electronic density of states and the mass dependence of carrier concentration. It is shown that the effect of the mass dependence of the carrier concentration within the van-Hove scenario (VHS) can explain some anomalous features of forLa ₂ CuO ₄ based superconductors, not explained by earlier theories based on VHS. These anomalous features are the asymmetry of about the optimum concentration (n _m) corresponding to the maximum critical temperature, minimum in aboveT _m , and the value of less than 0.5 belown _m .The financial support from Fundacao de Arapaio a pesquisa do estado de Sao Paulo (FAPESP) is gratefully acknowledged.     

Novel ceramic substrates for high Tc superconductors

A group of complex perovskite oxides REBa₂NbO₆ (RE=La and Dy) have been synthesized and developed for their use as substrates for both YBa₂Cu₃O_7-δ and Bi(2223) superconductors. These materials have a complex cubic perovskite (A₂BB′O₆) structure with lattice constants,a=8·48?8·60 Å. REBa₂NbO₆ did not show any phase transition in the temperature range 30–1300°C. The thermal expansion coefficient, thermal diffusivity and thermal conductivity values of REBa₂NbO₆ are favourable for their use as substrates for highT _c superconductors. The dielectric constant and loss factor of REBa₂NbO₆ are in a range suitable for their use as substrates for microwave applications. Both YBa₂Cu₃O_7-δ and Bi(2223) superconductors did not show any detectable chemical reaction with REBa₂NbO₆ even under extreme processing conditions. Dip coated YBa₂Cu₃O_7-δ thick films on polycrystalline REBa₂NbO₆ substrate gave aT _c(0) of 92 K and a current density of ～1·1×10⁴ A/cm² and Bi(2223) thick film on polycrystalline REBa₂NbO₆ substrate gave aT _c(0) of 110 K and a current density of ～ 4×10³ A/cm² at 77 K and zero magnetic field. A laser ablated YBa₂Cu₃O_7-δ thin film deposited on polycrystalline REBa₂NbO₆ substrate gave aT _c(0) of 90 K and a current density of ～5×10⁵ A/cm².     

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.     

Point contact spectroscopy in Fe-based superconductors: Recent advancements and future challenges

Iron-based superconductors (FeSC) present an unprecedented variety of features both in the superconducting and in the normal state. Different families differ in the value of the critical temperature, in the shape of the Fermi surface, in the existence or absence of quasi-nesting conditions, in the range of doping in which the antiferromagnetic (AFM) and the superconducting phase coexist and in the structure of the order parameter in the reciprocal space, and so on. In this paper the most important results of point-contact spectroscopy (PCS) in Fe-based superconductors are reviewed, and the most recent advances are described with the aim to discuss the future perspectives and challenges of this spectroscopic technique in the characterization of the superconducting properties of these complex compounds. One of the main challenges, faced so far only by a few researchers in the PCS field, is to fully explore the phase diagram of these materials, as a function of doping or pressure, to understand the interplay between superconductivity and magnetism, the effect of intrinsic or extrinsic inhomogeneities, the role of spin fluctuations (SFs) in the pairing, the symmetry and the structure of the order parameter(s).     

Specific-heat anomaly near superconducting transition in high temperature superconductors

A brief appraisal of the current state of understanding on the specific heat anomaly nearT _c in the oxide superconductors is presented. Spectacular specific heat double transitions are explained with plausible models.     

Superconductivity and stoichiometry in the BSCCO-family materials

We report on magnetization, c-axis and ab-plane resistivity, critical current, electroni band structure and superconducting gap properties. Bulk measurements and photoemission data were taken on similar samples.     

Orbital pairing effects in fullerenes

We propose a theory of superconductivity for a crystal having multiple band structure. The theory is valid for the parameter(k _F)/1 when the splitting between bands(k _F ) is small in comparison with the phonon frequency. The theory may be applicable to the doped fullerenes where it is widely supposed that pairing occurs through high-energy intramolecular phonons. As in semiconductors, the bunch of bands is treated by ascribing the highest spin to electrons. We derive the analytic expression for the critical temperature, which strongly depends on the value of the total spin of the Cooper pair, which may be equal toY=0,1,.... In all cases the order parameter is a vector with components proportional to spherical harmonics and at the same time the superconducting gap has no zeros. The data may be fitted to doped fullerenes, if the superconductivity arises fromd-pairing.     

Evidence for optimal isometric bond order in high-temperature superconductors

Critical results from chemical structure studies of high-temperature superconductors are elucidated which show a tendency toward isometric bond order which correlates positively with increasing critical temperature. Evidence from phonon, interstitial charge density, oxygen ordering and pressure effects, and lattice instability studies is summarized, and congruence with configuration interaction symmetry and bipolaron models is noted. Prospective high-temperature superconductors are identified.     

The crystal field in rare earth based high-temperature superconductors

Neutron spectroscopy is a powerful tool to determine unambiguously the crystal-field (CF) potential in rare-earth (R) based high-T _c superconducting materials. This technique provides detailed information on the electronic ground state of the R ions which is important to understand the thermodynamic magnetic properties as well as the observed coexistence between superconductivity and long-range magnetic ordering of the R ion sublattice at low temperatures. Moreover, the decay of the antiferromagnetic state of the parent compound as well as the evolution of the superconducting state upon doping can be directly and quantitatively monitored. It is found that the observed CF spectra separate into different local components whose spectral weights distinctly depend on the doping level, i.e., there is clear experimental evidence for cluster formation. The onset of superconductivity can be shown to result from percolation which means that the superconductivity is an inhomogeneous materials property. Since the linewidths of CF transitions directly probe the static electronic susceptibility, we discuss temperature-dependent experiments of the relaxation rate of CF excitations in both optimally doped and underdoped regimes. It is shown that there is clear evidence for the opening of an electronic gap in the normal state of underdoped superconductors. Furthermore, the relaxation behavior appears to be extremely dependent upon the energy at which the static susceptibility is being probed. The main observed features can be reproduced by considering a strongly anisotropic gap function.     

Effect of oxygen deficiency (δ) on transition temperature of yttrium cuprate superconductors

The nature of pairing mechanism as well as transition temperature of yttrium cuprates is discussed using the strong coupling theory. An interaction potential has been developed for the layered structure with two conducting CuO₂(a–b) layers in a unit cell. The interaction potential properly takes care of electron-electron, electron-phonon and electron-plasmon interactions. Furthermore, the electron-phonon coupling parameter (λ), the modified Coulomb repulsive parameter (μ*) and the 2D acoustic phonon (plasmon) energy as a function of oxygen deficiency is worked out. Finally, the superconducting transition temperature (T _c) is then evaluated by using these coupling parameters and obtainedT _c = 95(92)K for Y(Yb)Ba₂Cu₃O_7−δ superconductors withδ = 0·0. The model parameters estimated from the layered structure approach are consistent with the strong coupling theory. The result deduced on the variation ofT _c withδ are in fair agreement with the earlier reported data on yttrium cuprates. The analysis of the above results are discussed.     

Bi2212 c-Axis Twist Bicrystal and Artificial and Natural Cross-Whisker Josephson Junctions: Strong Evidence for s-Wave Superconductivity and Incoherent c-Axis Tunneling

We discuss the latest data from c-axis twist Josephson junction experiments on Bi₂Sr₂CaCu₂O (Bi2212) bicrystals and on artificially and naturally formed cross whiskers. All three experiments provide strong evidence for a dominant s-wave order parameter for . The bicrystal and natural cross-whisker data also provide strong evidence for incoherent c-axis tunneling across the intrinsic Bi2212 junctions, consistent with a strongly inhomogeneous material.     

反铁电晶体Cs3H（SeO4）2对称性和序参量的研究

在反铁电晶体Cs3H（SeO4）2顺电．反铁电相变中，对称性所属点群也随之发生相应的改变。通过对X射线衍射实验结果的分析，我们认为Cs3H（SeO4）2晶体反铁电相对称性所属点群为P2（C2）。选用轴矢量尺作为序参量来描述Cs3H（SeO4）2晶体反铁电相变中对称性的变化，应用居里原理，恰能得到Cs3H（SeO4）2晶体反铁电相的正确对称性所属点群P2（C2）。     

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     

WOx薄膜结晶特性的HREM和STM研究

对反应溅射沉积的ＷＯｘ薄膜在电致变色反应中结晶性能的变化进行了研究。结果表明：电致变色反应增加了薄膜的晶体特性,使着退色ＷＯｘ薄膜的选区电子衍射呈现明显的多晶环特性,而原始沉积态ＷＯｘ薄膜为非晶光晕衍射特性。ＳＴＭ分析得知,电致变色反应使薄膜的表面形貌也发生了剧烈的变化。     

Neutron crystal-field spectroscopy in underdoped and overdoped copper oxide superconductors

In many cuprate superconductors rare-earth (R) ions can be placed close to the superconducting copper oxide planes; thus, the crystal-field interaction at the R site constitutes an ideal probe of the local symmetry as well as the local charge distribution and thereby monitors directly changes of the carrier concentration induced by doping. For several compounds the crystal-field spectra observed by inelastic neutron scattering separate into different local components whose spectral weights distinctly depend on the doping level, i.e., there is clear experimental evidence for the formation of clusters which make the systems inhomogeneous. Moreover, it is found that the intrinsic linewidths of crystal-field transitions vary with temperature, which is essentially a reflection of the density of states associated with the charge carriers at the Fermi energy. Linewidth studies can therefore reveal information about the energy gap. For underdoped systems there is evidence for the formation of a pseudogap atT* >T _c .     

Creation of nanostructures on nickel thin films by STM

A scanning tunneling microscope (STM) has been used to create nanostructures on nickel thin films. A systematic procedure for the creation of such structures has been developed. Various possible mechanisms involved in such creations have been discussed. Based on our observations, a field-induced suction of the plastically deformed surface has been proposed.     

Mesoscopic modelling of high TC superconductors

Advances in experimental techniques, such as magneto-optical imaging, have influenced the way modelling studies are to be approached. In particular, these studies should be coupled with the fundamental understanding of flux lattices, vortex motion, melting and pinning, obtained from phenomenological theories and via various measurement techniques, such as Hall probe arrays, scanning superconducting quantum interference devices and by Lorentz microscopy. There are a range of models and questions to be addressed such as those concerning critical current density in relation to thin films. The ultimate goal is to obtain a cohesive picture of the properties of high T_C superconductors that will be useful from the standpoint of superconducting technology.