S. B. Samanta

High resolution STM/STS studies of Bi-O redox layers and superconductivity in pure and substituted Bi-2122 high-Tc cuprate

Abstract Effects arising from the intercalation of excess oxygen in Bi-O redox layers of the Bi-2122 compound have been studied with a STM/STS technique which has been successfully used for obtaining atomically resolved images. The excess oxygen is incorporated by substituting Dy 3+ at the Ca 2+ -site as well as by oxygenating the pure sample. It has been possible to detect and directly observe the excess oxygen atoms in the Bi-O layer which is in agreement with the information inferred from neutron diffraction and HRTEM studies of other workers. The STS studies show that there is a gradual increase in its room temperature energy gap and a decrease in the LDOS, indicating the Bi-O layer to become insulating with intercalation of excess oxygen. Depletion of excess oxygen through nitrogenation or Pb substitution at the Bi-site produces the reverse effect. The importance of improving the metallicity of the redox layer is elaborated and it is demonstrated that the T c of the bulk Bi-2122 phase can be significantly sharpened and raised to 96 K by depleting the excess oxygen from the layer. The redox reaction and the influence of the resistive state of the Bi-O layers in relation to the observed superconductivity results are discussed.

High-Resolution STM/STS Studies of Doping Effects on Bi-O Layers in Bi-2122 High-Tc Cuprate

Effects arising from the presence of excess oxygen in the Bi-O layer of Bi-2122 compound have been studied with the STM/STS technique which has been effectively used for obtaining atomically resolved images. The excess oxygen is incorporated by substituting Dy3+ at the Ca2+-site as well as by oxygenating the pure sample. It has been possible to detect and directly observe the excess oxygen atoms in the Bi-O layer which is in accord with the information inferred from neutron diffraction and HRTEM studies of other workers. The controversial issue of whether the Bi-O layer is conducting or insulating has been discussed in the light of the SIS findings on doped samples. The results show that there is a gradual increase in its room temperature gap and a decrease in the LDOS, indicating the Bi-O layer to become insulating with incorporation of excess oxygen.

STM studies of growth spirals in bulk single crystals of superconducting Y-123 and Bi-2122

Abstract The STM observations of growth spirals in bulk single crystals of Y-123 and Bi-2122 high Tc cuprates are reported for the first time. The density of screw dislocations present is of the same order as recently reported for sputtered epitaxial thin films of Y-123. Growth features are briefly discussed and it is argued that the observed dislocation density is still too low for realizing optimum flux pinning in these HTSC systems.

STM resolved step heights of growth spirals in YBa2Cu3O7−y crystals

Abstract The growth features of bulk single crystals of YBa2Cu3O7 (Y-123) heve been further investigated using the scanning tunnelling microscopy (STM) technique and the step heights of terraces associated with the growth spirals, observed in the a-b plane, have been measured. The results show that a growth step of height equal to the c-parameter of the unit cell comprises substeps and, in the ultimate resolution, six such substeps are observed. This provides the first piece of evidence of the crystals growing, layer by layer, in the framework of the spiral growth mechanism.

A new route to study fluorination of high-T c superconductors: Crystal growth with fluorine ion as additive

After the discovery of the high temperature superconductor YBa2Cu307_y (YBCO), much research effort was devoted to the effect of cationic [1] and anionic [2] substitutions on the superconductivity. One of the most important areas of study was substituting oxygen by fluorine by (i) solid state reaction [3], (ii) ionic implantation [4] and (iii) solid-gas reaction [5]. After extensive investigation, most researchers agreed that the fluorination did not make any important improvement to the superconductivity under such solid state conditions and relatively low temperature. In this letter, we report the influence of fluorine ion as additive on the crystal growth from high temperature solution. A small amount of BaF2 greatly improved the crystal growth and the superconducting transition temperature (Te) by reducing aluminium contamination of Cu-O chains from the alumina crucible. The conductance spectra for Cu-O chains by scanning tunnelling microscopy (STM) showed that such contamination made the chain layer more insulating. No Fwas found in the crystal. A growth model with a monomolecular layer spiral dislocation mechanism is proposed, which enables to understand the function of F-. High quality crystals with sharp superconducting transition at 94 K have been obtained in a yttria crucible by using BaF2 as additive. The starting materials for the crystal growth were YBCO, BaCO3, CuO and BaF2. The crystal growth was carried out by spontaneous nucleation from a self-flux system with a small amount of BaF2 as additive. In order to select an appropriate system for the crystal growth, phase diagrams [6] of the flux system and the growth system were explored by improved thermogravimetric analysis (ITGA) and differential thermal analysis (DTA), from which the eutectic composition of 0.28 BaO + 0.72 CuO was chosen as solvent and the solute concentration and growth temperature were introduced from the liquidus range of the growth system. Alumina or yttria crucibles were used as containers for the crystal growth. In fact, most compounds, including BaF2, are not suitable as flux for the crystal growth of YBCO [7] because of its low stability. The crystal could be grown mainly from the self-flux system. However, we could reduce the concentration of BaF2 in the self-flux to such a low level as an additive that YBCO remains the major crystalline phase. Sometimes small amounts of a particular additive can dramatically affect the nucleation and crystal growth, resulting in improvements of crystal size or quality a n d modification of the habit. We have used B203, BaF2, Bi203 and BaC12 as additives for the crystal growth. Only BaF2 achieved improvements of both crystal growth and superconductivity [8]. Table I shows the effects of various concentrations of BaF2 on the cyrstal growth under the same solute concentration. The optimum amount of BaF2 appears

Nanostructure morphology and atomically resolved images of chains and planes of YBCO single crystals grown with different additives

Abstract Bulk single crystals of superconducting YBCO (YBa 2 Cu 3 O 7 ), grown with incorporation of different additives like BaF 2 , B 2 O 3 and Bi 2 O 3 , have been investigated to gain an insight into the problem of combating T c degradation due to aluminium contamination from the crucible material. Electron probe microanalysis (EPMA) along with T c values of the above crystals, shows that, with BaF 2 introduced as additive, the contamination due to aluminium is significantly reduced, which improves crystal growth and superconductivity. The crystals have been subjected to high resolution imaging of planes and chains using STM, while the ensuing changes in the conductance spectra of chains due to Al contamination were studied by scanning tunnelling spectroscopy (STS). The contamination of CuO chains is corroborated by chains becoming quasi-insulating on account of substitutional disorder, and also by the increase in their surface roughness at the atomic level. The effect of additives is discussed and the observed T c variation is linked with the relative changes in the adulteration and disordering of CuO chains. The results are in general accord with the role of interlayer coupling between CuO 2 planes and other effects that are known to control the T c of layered cuprates.

Some novel results of scanning tunneling microscopy (STM) study of cadmium iodide crystals

A combined optical, SEM and STM study of growth spirals on the richly polytypic CdI2 crystals has yielded some novel results. The STM photographs directly reveal the atomic arrangement in the unit cell, in exact conformation with the known structural data. The STM has also enabled an accurate measurement of the interstep spacings, which are all found to be an integral multiple of the unit cell a-dimension. The interstep spacing is found to be variable for steps of the same height, which apparently arise from differences in supersaturation during crystal growth.

High-resolution studies of C60 films by scanning tunnelling microscopy

Scanning tunnelling microscopic studies of C60 films containing conducting islands, under ambient conditions, have revealed the characteristic FCC lattice of the C60 clusters with intermolecular distances in various planes along different crystallographic directions closely matching with the diffraction data. The diameter of the cluster is close to 0.71 nm, i.e. the value given by the positions of the carbon atoms on the C60 shell, and not influenced by their Ti-electron lobes. The studies have revealed the nanometric defect structures in the C60 lattice which are found consistent with the FCC structure. The cage morphology of the buckyballs in the form of hexagons and pentagons forming the well-known icosahedral structure of the C60 molecule has been resolved in the finest details with bond lengths in a fair accord with the reported data. The resolved structure forming the lattice shows an orientational disorder which has been frozen from a state in which each molecule at the ambient tem perature is undergoing a rotational motion which is uncorrelated with the motion of its neighbour, in conformity with diffraction and NMR results. Various possible causes of retardation and freezing of the rotational diffusion, relevant to the samples studied are discussed which also include the distortions in the carbon cages detected in the present study.

Direct observation of carbon cage C60 buckyballs by scanning tunnelling microscopy

Abstract Submolecular features of the C60 buckyballs have been resolved at the ambient temperature by scanning tunnelling microscopy (STM). The possible causes of freezing of the rotational motion of the C60 molecules are discussed.

Micro-Raman spectroscopy of Bi2Sr2CaCu2O8+δ intercalated with nickel phthalocyanine/benzene

Abstract Micro-Raman measurements on pure Bi2Sr2CaCu2O8+δ (host) intercalated with conjugated ring-shaped molecules (guest) like nickel phthalocyanine (NiPc) and benzene have been carried out. Drastic changes due to intercalation have been observed in the peak frequencies, intensities and band widths of several vibrational modes of both the guest and the host species. The anomalous variations in the peak frequencies and intensities of C = N aza group stretching and C-C pyrrole ring stretching vibrations of the NiPc molecule as well as the vibrational modes of oxygen in the Cu-O planes of the host material reveal the possibility of charge transfer processes in the intercalated compounds from the guest to the host species. The large increase in the bandwidths of many vibrational modes has been related to local distortions and to the inhomogeneous distribution of guest species in the intercalated samples. The contention of charge transfer due to intercalation is also substantiated from STM/STS studies. The suppression of the superconductivity due to intercalation has been explained in terms of the charge transfer mechanism.