• Volume 14, Issue 3

June 1991,   pages  531-887

• Foreword

• Evidence of charge localization from photoinduced infrared absorption in BaBiO3

Photoinduced infrared activities were studied in BaBiO3 and the results compared with those of La2CuO4 and YBa2Cu3O6+x. The spectral range of investigation covered the interval from 50 to 4000 cm−1. Clear evidence of lattice relaxation around the photoinjected charges was observed in the phonon spectral range with the bleaching of IR phonon bands. On the high energy side, we observed the rising edge of a broad electronic absorption. The results give an evidence of the suppression of the charge density wave due to the lattice relaxation around the photoinjected carriers. Moreover the low-energy charge excitation in the near infrared appears as a common characteristic of both layered and three-dimensional highTc superconductors, and may be relevant in the pairing mechanism.

• Raman and infrared studies of cupric oxide

Polarized Raman and fourier-transform infrared (FTIR) measurements have been made on a single crystal of CuO. Group theory predicts nine vibrations of which three (Ag, 2Bg) are Raman-active and six (3Au, 3Bu) are infrared-active. We have observed three Raman modes at 296 (Ag), 346 (Bg1) and 636 (Bg2) cm−1. We have also observed six infrared modes at 146 (Bu3), 164 (Au2), 355 (Au3), 480 (Bu1), 542 (?) and 603 (Bu2) cm−1. The normal frequencies and eigenvectors have been calculated using Wilson’s FG method; a good fit between theory and experiment has been obtained.

• Characteristics of laser-induced plasma from highTc superconductor

The spectroscopic analysis of the emission from the plasma produced by irradiating a highTc superconducting GdBa2Cu3O7 target with a high power Nd:YAG laser beam shows the existence of the bands from different oxides in addition to the lines from neutrals and ions of the constituent elements. The spectral emissions by oxide species in laser-induced plasma show considerable time delays as compared to those from neutral and ionic species. Recombination processes taking place during the cooling of the hot plasma, rather than the plasma expansion velocities, have been found to be responsible for the observed time delays in this case. The decays of emission intensities from various species are found to be non-exponential.

• Oxygen isotope shifts of the Raman modes of YBa2Cu3O7−x

Raman scattering experiments have been carried out on sintered pellets of YBa2Cu316O7−x and YBa2Cu318O7−x obtained both by gas exchange and by growth with substituted oxides. The frequencies of the modes at 340, 435 and 502 cm−1, which involve motion of the oxygen atoms and which shift significantly upon oxygen isotope substitution, have been measured for several sets of samples. The measured frequency shifts indicate that the isotope exchange on the O(2) and O(3) sites is more complete than the exchange on the O(4) site. The 502 cm−1 line of the18O samples is observed to be broadened and this is attributed to18O-16O disorder on the O(4) sites. The results are discussed with reference to previous measurements of site activation energies and models for the exchange mechanism.

• High-Tc superconductivity in thed-p electron system

The relaxation time with spin flipτs and the parametersξ, δ, χ of superconducting phase have been calculated on the basis of the kinematical mechanism of superconductivity in strongly correlated oxide models. An inter-relation between the superconducting gap Δ0 and the specific heat jump Δc allowing the experimental verification was obtained and the Ginsburg-Landau equation derived.

• The role of basal-plane oxygen atoms in determining the ferroelastic and microstructural properties of Y-Ba-Cu-O

The basal-plane oxygen atoms in YBa2Cu3O7−x behave like a lattice gas, with very high diffusivity, especially for oxygen-deficient specimens. Implicit in this behaviour is the property that even a small amount of stress applied along theb-axis (b &gt;a) can make these oxygen atoms hop from the (0,1/2,0) sites to the (1/2,0,0) sites. This is suggested as the primary mechanism responsible for the ferroelastic switching observed in this crystal. Since the material is an oxide of a mixed-valence element (Cu), the common occurrence of overall nonstoichiometry is only to be expected. Also, as discussed by Khachaturyan and others, except at very high temperature, the oxygen atoms and the vacancies will always have a tendency for ordering and/or precipitation into configurations which approach near-perfect stoichiometry locally. However, not all evidence for ordering is in conformity with the predictions of Khachaturyan’s concentration-wave model. The experimental data are examined critically. Further experiments to resolve the discrepancies are suggested.

• Superstructures in Ba0·6K0·4BiO3−y and BaPb1−xBixO3−y

The perovskite compounds Ba0·6K0·4BiO3 and BaPb1−xBixO3 (x=0·9, 0·5, 0·25) have been investigated by high resolution electron microscopy and selected area electron diffraction. Several superlattices produced byin situ electron beam reduction have been observed. Ordered atomic arrangements in the reduced materials are discussed.

• Crystal chemistry of Ruddlesden-Popper type structures in highTc ceramic superconductors

Similar structural patterns have been noticed in the systems La-Cu-O, La-Ni-O and Bi and Tl-containing superconducting oxides. The formation of Ruddlesden-Popper type layers (alternating slabs of rocksalt and perovskite structures) is seen in these oxides which is similar in many respects to what is seen in the system Sr-Ti-O. However, there are some significant differences, for example the rocksalt and perovskite blocks in new superconducting compounds are not necessarily electrically neutral, unlike in the Sr-Ti-O system. It, thus, becomes necessary to create oxygen vacancies in the basic perovskite (figure 1) structure of Bi-containing compounds, when the width of the perovskite slab changes on addition of extra Cu-O planes. Results of our atomistic simulations suggest that these missing oxygen ions allow the Cu-O planes to buckle in Bi-series of compounds. This is also supported by the absence of buckling in the Sr-Ti-O series of compounds and the first member of Bi-containing compounds in which there are no missing oxygen ions. We present additional results on the phase stability of polytypoid structures in La-Cu-O system and defect chemistry of compounds of La-Ni-O system.

• Electron microscopic observations of Bi and Tl bearing cuprate high temperature superconductors

The second generation of high temperature superconductors typified by Bi2Sr2Can−1CunO2n+4 and Tl2Ba2Can−1CunO2n+4 exhibits curious structural properties which have direct relevance to the superconducting behaviour particularly transition temperature (Tc). The present paper reports on investigations of structural properties at microlevel in Bi-bearing HTCS. We have found curious structural characteristics which manifests itself in the form of transformation fromap×ap×c to (2)1/2ap×(2)1/2ap×c through the loss of calcium atoms and the formation of five-fold modulated phase alongb through the loss of Ca and Sr atoms. We have also found the evidence of high periodicities (n=4) Bi2Sr2Ca3Cu4O12. The microstructural characteristics of HTCS showing the higherTc(R=0) ∼ 120 K exhibits unusual characteristics.

• About copper valence and superconductivity

We describe the copper valence in superconductors based on our arguments on La2O3 crystal structure. In order to explain the two oxygen sites in La2O3, it has been supposed that OII1− ions occupy the tetrahedral site while OII2− occupy the octahedral site. Oxygen ions in tetrahedral site form a covalent bond with lanthanum, which can be written [LaO]1+. Considering the chemical and crystallographic properties of Tl and Bi compounds, [TlO]1+ and [BiO]1+ groups appear as defined by strong covalent bonds between Bi or Tl and O. This leads to the supposition that the four oxygen ions which coordinate to copper in Tl and Bi copper oxide-based superconductors are OII1− ions. The bivalent character of copper is then obtained through covalent bonds. For La2−xSrxO4 compounds, copper is supposed to have valence three, but spectroscopic studies point out bivalent copper. We show that krypton shell of Sr is responsible for the lack of one unit of valence as expected from krypton compounds for example KrF2.

• Tc and crystal structure distortions of high-temperature superconductors

The influence of orthorhombic distortions of CuO2 layer onTc is considered within the framework of kinematic mechanism. The dependence of the interaction constant on orthorhombicity is calculated. It is shown that orthorhombic distortions may result in the increase ofTc by 10% in accordance with experiment.

• Inelastic neutron scattering from Tl2CaBa2Cu2O8

Coherent inelastic neutron scattering measurements have been carried out on the high temperature superconductors Tl2CaBa2Cu2O8 (Tl-2122,Tc=107 K) and YBa2Cu3O7 (Y-123,Tc=92 K), at the Dhruva reactor at Trombay. The density of phonon states in Tl-2122 is enhanced at 6–17 meV and reduced at 40–70meV compared to that in Y-123.

• Crystal field excitations in electron superconductors

We have measured the complete crystalline electric field (CEF) spectrum of Nd3+ in superconducting Nd1·85Ce0·15CuO4 and Nd2CuO3·7F0·3, and in the non-superconducting parent compound, Nd2CuO4, by neutron inelastic scattering. The best description of the ground-state excitations for both compounds is achieved by the addition of a molecular field parameter to the CEF hamiltonian that takes account of exchange interactions from Nd or Cu spin ordering.

• High energy crystal field excitations in PrxY1−xBa2Cu3O7−δ

We have measured the high energy crystalline electric field transitions in samples of PrxY1−xBa2Cu3O7−δ withx ≈ 0·5 andx ≈ 1. The fully Pr-doped samples exhibits three strong transitions with excitation energies of about 65, 85 and 105 meV, plus three more weaker ones at 113, 123 and 132meV. Magnetic intensity is observed in the same energy range in thex=0.5 sample but is largely structureless in character, with only one clear peak at about 109meV. The large number of transitions would seem to indicate a Pr valence of 3+, rather than 4+, but the intrinsically broad character of the excitations is evidence for a dynamically mixed valence state.

• NMR studies of Nd2−xCexCuO4

Copper NMR has been studied as a function of temperature in a number of superconducting Nd2−xCexCuO4 samples. The electric field gradient is very small and the Knight shift is 2380 ppm at room temperature, both of these implying that the copper is in a Cu+ state. The Knight shift decreases with temperature particularly belowTc. The spin contribution to the Knight shift is estimated to be ∼ 200 ppm (about a factor ten smaller than in YBa2Cu3O7) indicating thatNs(EF) at the copper sites is small in this material.

• Magnetic resonance in high-temperature superconductors

Since the discovery of high-temperature superconductors a new microwave absorption has been found. The microwaves are absorbed in the flux-quantized eigenstates so that the absorption is proportional to the Josephson current and hence it varies as the gap of the superconductor. This absorption is found in the electron-paramagnetic resonance configuration. The flux-quantized fields are found in small domains of the size of 10−6 cm. A giant moment is found to occur. The necessary theory as well as experiments in YBa2Cu3O7−δ type compounds are described. The Cu2+ electron-paramagnetic resonance gives an anisotropic exchange narrowed line with anisotropicg-values withg-shift proportional to susceptibility. The symmetry of theg-value also reflects the symmetry of the superconducting gap.

• CESR in superconductors: Relevance of highTc copper oxide systems

CESR is a highly effective tool to study the interaction among conduction electrons in normal metals. In superconductors belowTc it can reveal vital information concerning the pairing interactions. A comparative study of highTc and conventional superconductors is presented and it is shown that the disappearance of CESR in the superconducting state is a feature common to both conventional and highTc superconductors and establishes the importance of exchange interactions in pairing. This is supported by experimental observations.

• Electron paramagnetic resonance studies in the oxides Bi2Sr2Ca1−xGdxCu2Oy

We have recently reported our results on magnetic susceptibility and microwave absorption studies on Bi2Sr2Ca1−xGdxCu2Oy and Bi2Sr2Ca1−xYxCu2Oy. In the present work two important observations of these investigation, viz, the dependence ofTc on Gdconcentration and the absence of EPR-signals of Gd and Cu, are considered in greater detail. It is suggested that the microwave absorption by fluxon lattice might be playing one of the important and effective pathways for electron spin relaxation resulting in nonobservation of EPR signals.

• Electron spin resonance studies of some cuprate(II)systems

The electron spin resonance (ESR) of a number of ternary oxides of copper (II) has been studied at both 77 K and 300 K. Materials investigated include those which might be considered 1-dimensional analogues of La2CuO4, namely A2CuO3 (A=Ca,Sr, Ba) plus the related material SrCuO2. The two oxygen rich derivatives of Ba2CuO3 have also been studied in view of their different oxygen environments around copper. Also other alkaline earth cuprates such as the recently described Sr14Cu24O41-x and Bi2Cu2O5, both pure and doped with Sr2+ are included. Compounds containing extended Cu-O chains are found to be essentially ESR silent both at room temperature and 77 K, regardless of whether the chains are one-dimensional or form two-dimensional sheets as in Ca(Sr)CuO2. In Sr14Cu24O41−x, which contains both Cu-O chains and chains of edge-shared CuO4 square-planar units there is an asymmetric signal which sharpens on cooling.

• Thermal and magnetic measurements on YBa2Cu3O7−y

Specific heat measurements in zero and7T magnetic fields from 0·4 to 100 K, and magnetic susceptibility measurements aboveTc on a series of polycrystalline samples of YBa2Cu3O7−y (YBCO) reveal a number of regularities. The size of ΔC(Tc)/Tc for the samples [ΔC(Tc) is the jump in the specific heat atTc] appears to vary linearly with the low-temperature value of the Debye Θ, with the entropy change between ≈ 85 K andTc induced by the application of a 7Г magnetic field, and with the variation of the observed low temperatureγ with magnetic field, dγ/dH. On the other hand the temperature-independent part of the magnetic susceptibility aboveTc appears to be essentially independent of ΔC(Tc)/Tc. These results are consistent with the idea that samples of YBCO belowTc are a mixture of superconducting and normal phases.

• Low temperature heat capacity measurements on HoBa2Cu3O7−δ

Specific heats were measured on two samples of HoBa2Cu3O7−δ in the temperature range 1·7 K to 10 K. In addition to the known Schottky behaviour, a peak in the specific heat curve was observed near 7·9 K in both the samples. This peak is probably due to impurity contribution and the specific heat measurements were undertaken in holmium oxide (Ho2O3), which was the suspected impurity. However, no peak was observed in the specific heat curve of holmium oxide

• DTA studies of formation of LaBa2Cu3Ox

The 1:2:3 composition in the La-Ba-Cu-O system has been investigated to understand the formation pathways of the LaBa2Cu3Ox using the static air differential thermal analysis (DTA) as the principal probe and X-ray diffraction and EPR as supporting probes. The major endothermic events observed in DTA are attributed toγα type phase transition of BaCO3 and the formation of “123” phase. It was found necessary to dry the La2O3 at high temperatures to make it crystalline and reactive. BaCo3 was prepared afresh to achieve optimum reactivity. Even after four DTA runs the compound was not single phase as seen in XRD which shows the presence of BaCuO2 as the impurity. The Ba(NO3)2-based mixture appears to react slower than the BaCO3-based mixture.

• Thermogravimetric method for determination of oxygen stoichiometry in superconducting 1:2:3 and metal-substituted 1:2:3 compounds

The paper presents a modified thermogravimetric method for determination of oxygen stoichiometry of the oxide superconductors YBa2Cu3O7−δ and metal-substituted compounds of the type YBa2−xLaxCu3O7+δ and YBa2(Cu1−xMx)3O7−δ (M=Mg/Zn) between 25° and 930°C.

• Impedance and heat capacity anomalies in Y-Ba-Cu-O ceramics atT&gt;Tc

Peculiarities of rf and thermodynamic properties of Y-Ba-Cu-O ceramics in the normal state are detected. These include frequency oscillations of resistance in the range ∼ 107 to 2×109 Hz, dielectrization of ceramic properties below 106 Hz, temperature dependence of impedance for different frequencies, intercorrelating anomalies of temperature dependences of heat capacity and ohmic resistance.

• Effect of heat treatment procedures on site preferences of Fe in YBa2Cu3O7−D

Mössbauer spectra of YBa2Cu2·85Fe0:15O7−d prepared using different heat treatment procedures have been obtained. The transition temperature varies from 23 to 53 K, depending on the procedure adopted for preparation. Associated with this are changes in the lattice structure. Mössbauer spectra reveal population of the four components and their relation to the decrease in the transition temperatures.

• Doppler-broadened positron annihilation studies in Y-Ba-Cu-O, Tl-Ca-Ba-Cu-O and Bi-Ca-Sr-Cu-O superconductors

Doppler-broadened annihilation radiation spectra have been measured as a function of temperature from 77 K to 300 K, for several high temperature oxide superconductors viz single-phase YBCO, single- and mixed-phase Tl-Ca-Ba-Cu-O and Bi-Ca-Sr-Cu-O compounds. The temperature-dependent parameters extracted respond to a change at the onset of superconducting transition. The observations point to involvement of oxygen valence electrons at the onset of superconducting transition. Also a possible structural change and/or increase in electron density at the oxygen vacancy/defect sites seem to accompany the transition. In addition, the parameters derived are seen to be sensitive to the presence of more than one superconducting phases in mixed phase samples.

• Interpretation of Mössbauer spectra of YBa2Cu3−xFexO7−d

We have studied the Mössbauer spectra of YBa2Cu3−xFexO7−d (11 compositions) and present here the results obtained from quantitative analyses of the paramagnetic and magnetic spectra.

• 151Eu Mössbauer studies on Zn-doped EuBa2Cu3O7−y

151Eu Mössbauer studies have been performed on the compounds EuBa2(Cu1−xZnx)3O7−y withx=0·0, 0·025, 0·05, 0·075 and 0·1. The parent compound, EuBa2Cu3O7−y is superconducting with a transition temperature (Tc) of 88 K.Tc is depressed as Zn is substituted for Cu in this system and the compounds withx&gt;0·05 do not show superconductivity down to 12 K.151Eu Mössbauer studies at 295 K show a single Mössbauer line in all the compounds (whether superconducting or not) with isomer shift value typical of Eu3 + ion. Further, the isomer shift values are nearly independent ofx and the temperatures down to 10 K. These observations imply that the Cu-O network responsible for superconductivity is very weakly coupled to the Eu sublattice.

• Structural studies on Fe-substituted Y-Ba-Cu-O superconductors by Mössbauer spectroscopy

In this paper we report preparation and structural aspects of YBa2Cu2·97Fe0·03O7−x with different oxygen stoichiometry. The samples were prepared by standard solid-state reaction. The samples were characterized by X-ray powder diffraction and oxygen contents were estimated from iodometric titration method. The superconducting transition temperatureTc is measured using the standard four-probe technique. Mössbauer studies of YBa2Cu2·97Fe0·03O7−x with different oxygen stoichiometry were carried out and the results show that Fe substitutes both Cu(2) and Cu(1) sites and clear asymmetry in the doublets except for the tetragonal case. Also our results show the progressive oxygenation inb-axis as oxygen stoichiometry is increased.

• Mössbauer studies of Gd-doped bismuth-based ceramic superconductors

The results of155Gd Mössbauer studies (above 1·5K) of the oxides, Bi3·5Gd0·5Ca3Sr3Cu4Oy and Bi2Sr2Ca1−xGdxCu2Oy (x=0·25 and 0·5) are reported. The observed electric quadrupolar split resonance patterns suggest that Gd ions prefer to occupy Ca site in both series. The spectral features give evidence for paramagnetic relaxation of isolated Gd ions at low temperatures.

• Mössbauer study of 3d-ion-substituted YBa2(Cu, Fe, M)3O7−δ; M=V, Mn

57Fe Mössbauer investigations at 300K have been carried out on V,Mn substituted YBa2Cu3O7−δ system. Preliminary studies show that for small concentration of V (4%), Fe occupies preferentially the Cu(1) site whereas for the same concentration of Mn, Fe prefers the Cu(2) site. It is suggested that for low concentrations V has a preference for Cu(2) site whereas Mn prefers Cu(1) site in the 1-2-3 lattice.

• Effect of laser irradiation on the superconducting properties of high-Tc SmBa2Cu3Ox

Studies of the effect of high power laser (Q-switched Ruby laser, 694 nm, 30 ns) irradiation on the critical current density (Jc) and magnetic hysteresis at 77K and temperature variation of microwave induced d.c. voltage on SmBa2Cu3Ox ceramic samples have been performed. Irradiation did not substantially changeTc but caused a strong increase inJc and magnetic hysteresis at 77K. The microwave-induced d.c. voltage at 77K showed appreciable decrease after irradiation. SEM studies showed grain growth due to sintering which improves the interconnectivity among the superconducting grains. These are attributed to physical densification and consequent reduction in the number of weak links. The increase of magnetic hysteresis after laser irradiation is presumably connected with the creation of defects which act as pinning centres. Thermal modelling suggests that on irradiation the surface melts up to a depth of 1µ and laser-induced evaporation occurs at energy density of 2·5 J/cm2.

• Cross-sectional observation of Bi-O and Cu-O layers by STM/STS in Bi2Sr2CaCu2Oy

• Determination of the oxidizing ability of HTSC materials containing two elements with a variable degree of oxidation

The presence of mixed valence states of elements is supposed to be a common characteristic feature of the high-Tc superconductors, based on copper oxides. Methods have been suggested for the determination of Cu(III) in the Y-Ba-Cu-O ceramics and separate determination of two oxidants: Bi(V) and Cu(III), Tl(III) and Cu(III) in the corresponding materials. Compounds of Bi(V) oxidize in the ions of Mn(II) to MnO4 in the acidic medium. Active oxygen, bound to Cu(III) is isolated, not oxidizing Mn(II). MnO4 is titrated then with a solution of Fe(II) sulphate. For the determination of Tl(III) the sample is dissolved in the acid, Cu(III) is reduced to Cu(II) and Tl(III) is titrated with the hydroquinone solution. The compounds of Bi(V), Tl(III) and Cu(III), having high oxidation-reduction potentials can oxidize the complex Co(II)-EDTA to Co(III)-EDTA. The colour intensity of the latter is proportional to the content of Cu(III) or to the sum of Bi(V) and Cu(III) or Tl(III) and Cu(III). The content of Cu(III) in Bi- and Tl-containing materials is determined by the difference.

• Carrier density control in the La2−xSr1+xCu2O6+y system

The effect of strontium and oxygen content variation is systematically studied in the La2−xSr1+xCu2O6+y system. The reason for the absence of superconductivity in this system is discussed.

• A sequence of critical temperatures in YBCO between 20 and 70 K

Evidence has been found for a number of discrete and sharp transitions in the magnetic susceptibility transition of many single crystals of YBCO. These appear to favour certain temperatures and at least eight transitions are observed between 20 K and 70 K. One of these transitions dominates the behaviour of one of the individual crystals and X-ray studies indicate that the oxygenation value corresponds to O6·7. We suggest that these transitions are associated with different ordered states in the oxygen deficient lattice.

• Superconducting properties of the Bi-Ca-Sr-Cu-O system

The electrical properties of unquenched and rapidly quenched bulk samples of granular Bi2-Ca1-Sr2-Cu2-Ox system have been studied. Electrical resistivity measurements show superconductivity transition temperature (R=0) at 72 K, 80 K and 90·5 K. X-ray crystallographic studies show that most samples were of single-phase.

• Magnetic scattering and superconductivity in Nd1·86Ce0·14CuO4−y

Nd1·86Ce0·14CuO4−y is superconducting below about 20 K and electrons are considered to be responsible for superconductivity in these materials as in the case of Ln2−xCexCuO4−y and Ln2−xThxCuO4−y. Structurally these materials are not very different from thep-type superconductors La2−xSrxCuO4−y. In both these types of superconductors, the parent compounds are antiferromagnetically-ordered insulators. The induction of holes or electrons by substitution destroys magnetic interactions and brings about superconductivity. Peng and coworkers have studied the resistivity variation of both superconducting and nonsuperconducting Nd1·85Ce0·15CuO4−y and have found a decrease in resistivity with temperature, obeying a lnT dependence in the superconducting samples. Such a variation was not seen by them in the nonsuperconducting samples. They ascribe the lnT variation seen in their superconducting samples to arise from magnetic scattering of electrons. To study whether such an effect exists, Nd1·86Ce0·14CuO4−y was subjected to various annealing conditions and the resistivity behaviour is presented here. Our results differ from those of Penget al.

• Fluctuation-induced excess conductivity in the compounds CaREBaCu3O7−y (RE=La and Sm)

The compounds CaREBaCu3O7−y (RE=La and Sm) are tetragonal at room temperature withTc between 60 and 70 K. The single-phase compounds were prepared by solid-state reaction. The resistivity was measured by a four-probe technique in a continuous flow cryostat with the temperature being controlled to an accuracy of 10 mK. The resistivity vs temperature showed a break in slope around 180 K in CaLaBaCu3O7−y and around 220 K in CaSmBaCu3O7−y. The results were analysed for fluctuation conductivity from 180 K downwards. A plot of dρ/dT vsT showed a sharp peak atTm=69·69 K for La compound and 66·00 K for the Sm compound. Detailed analysis of the resistivity in the regionTon to 180 K was carried out using the procedure due to Veira and Vidal. The results are discussed in this paper.

• Effect of oxygen content on the electrical properties of YBa2Cu3O7−x single crystals

The effect of oxygen content in the single crystals of high-temperature superconductor YBa2Cu3O7−x on the electrical resistivity, the Hall effect in the plane perpendicular to thec axis and the energy gap Δ, measured with tunnelling electron microscope, has been studied. The distribution of the gap along the surface of the crystal was also studied. The results of the study on the relationship between the magnitude of the energy gap Δ and the superconducting transition temperatureTc of single crystals with various oxygen contents are approximated by the linear dependence 2Δav=4·4kTc.

• Superconducting gap in Nb seen by point contact spectroscopy

The nonlinear current voltage characteristics of a point contact convey information about various excitations in the metal. We have made a point contact study on a superconductor to see the band gap and on a normal metal to see the transport characteristics.

• High resolutionV-I characteristics measurements on highTc superconductors

V-I characteristics of sintered superconducting pellets of YBa2Cu3O7−δ was measured with a resolution one order of magnitude better than the usual dc techniques employing nanovoltmeters. For this purpose software-based lock-in-amplifier technique was developed and used. A square-wave excitation current was used. The voltage signal was digitized into a time series and Fourier-analysed on a computer in this technique. Our results show a very small frequency-dependent resistance in some samples which are nominally superconducting. These results help in checking the material quality and defining the critical current densities better.

• Thermoelectric power of YBa2Cu3O7−δ under pressure up to 9 GPa

Thermoelectric power (TEP) of two YBa2Cu3O7−δ compounds (with δ=0·17 and 0·21) was measured as a function of quasi-hydrostatic pressure up to 9GPa at 300K on samples with low porosity. In both cases TEP decreases with increasing pressure, at a rate ∼ 0·8 μVK−1/GPa. The data obtained under hydrostatic pressure up to 3 GPa are in good agreement with those under quasi-hydrostatic pressure. The TEP of both compositions is found to decrease linearly at a rate 0·8 μVK−1/GPa above 1·5 GPa.

• Resistive current states in narrow superconducting films

The I–V characteristics of superconducting channels are studied experimentally in the region of low (j ∼ jc) and high (j≫jc) transport currents. The analytical expressions obtained for the I–V characteristics by Ivlev and Kopnin provide a good description of the experiment. The theoretical results suggest the existence of two temperature regions where the relaxation of the charge disbalance proceeds in different ways.

• Microwave absorption studies in field-cooled hafnium doped YBaCuO

The differential microwave absorption is studied in the field-cooled samples. Flux-trapping during field-cooling is clearly demonstrated. Glassy behaviour starts appearing for fields greater than the lower intergranular critical field revealed by the difference between field-cooled and zero-field-cooled cases. Results can be understood in terms of super-conducting cluster model.

• Microwave absorption in YBa2Cu3Ox granular thin films at the dc magnetic fields

Localization of microwave absorption in YBa2Cu3Ox superconducting thin films in the presence of very low dc magnetic field was studied. The granular YBa2Cu3Ox thin films of submicrometer thickness had random orientation of grains, were inserted into a microwave cavity for maximum rf magnetic field. The microwave absorption in low dc magnetic field at temperatures just below the critical temperature shows properties which may be ascribed to superconducting grains. At lower temperatures the hysteretic effects and flux-trapping occur in the intergrain regions of the film.

• Effect of modulation amplitude on low-field microwave absorption in hafnium-doped Y-Ba-Cu-O

Low-field differential microwave absorption has been studied for hafnium-doped Y-Ba-Cu-O at the first harmonic of modulation field. The effect of modulation amplitude on peak height, peak position and the area under hysteresis curve has been discussed.

• On the measurement of microwave absorption of bulk YBaCuO superconductors in X-band (8–12 GHz)

Microwave absorption at the surface of highTc YBaCUO superconducting sample has been determined in X-band by measuring VSWR. Power reflectivity &gt;98% has been observed in the frequency range of 8·2–10·5 GHz indicating very low absorption at the surface. At some of the frequencies, however, negligible microwave loss has been observed.

• Nonlinear behaviour of high-Tc superconducting bridges at microwaves

We have experimentally investigated the phenomenon of coherent hypervortices motion in highTc superconducting Y-Ba-Cu-O thin film bridge-like structures. Narrow band internal radiation at frequencyf=21 GHz with intensityP=3×10−11 W was detected. The radiation processes, occurring around the singularity of negative differential resistance at current-voltage characteristics of bridge-like structures allows one to consider their further application for sensitive microwave reception.

• Investigation of high temperature superconductivity through microwave absorption method

The field dependence and near zero magnetic field microwave absorption as a function of rf power in YBa2Cu3O7−δ and Bi-Ca-Sr-Cu-O has been studied using a varian ESR spectrometer. A model of microwave absorption built on diamagnetic tensor susceptibility has been proposed which explains the observed results satisfactorily.

• Decimeter-wave impedance measurement of Y- and Tl-based superconductors with and without an external magnetic field

The real part of low-temperature surface impedance of samples of high-Tc superconductors in the YBa2 Cu3O6+x ceramic system was studied by the method of registration of the ownQ-factor of coaxial shortringed cylindrical resonator constructed entirely from examined material. For ceramic samples of high-Tc superconductors in the Tl2Ba2Ca2Cu3O system the same measurements were made with the help of coaxial Al resonator. The wavelength of radio frequency field equals 70cm. The dependence of the impedance of Y-based superconductors on weak magnetic field at helium temperature is close to the square law. The magnitude of impedance at temperature of liquid helium for Y-based superconductors composes 0,04 ohm, for Tl-based superconductors −0,01 ohm respectively which correlates with the statement of the presence of giant resonant absorption of radio frequency field power.

• ESR studies of superconducting cuprates: Low magnetic field effects at X-band and 300 MHz

Low field microwave absorption in cuprate superconductors belowTc is reported in a range of materials using ESR spectrometers operating at X-band and 300 MHz. The radiofrequency apparatus facilitated a detailed study of the 0–100G region where the absorption is most intense and we include here a brief discussion of the effects of incident microwave power and sample temperature on the intensity and position of the signal.

• Thermopower measurements on BiSrCaCuO system

We report thermoelectric power measurements on BiSrCaCuO (2212) superconducting oxide and compare the observed behaviour with that of the YBaCuO (123) system. The relative importance of phonon drag and diffusion thermopower contributions is evaluated for the two systems by analysing the data using a generalized expression of the typeS=αT+β/T. The thermopower enhancement effect just aboveTc gives parameter values comparable to those of YBa2Cu3O7−x system.

• Thermoelectric power of single-phase samples of Tl2CaBa2Cu2Oy and Ba2CaSr2Cu2Oy

Single-phase 2122 samples of thallium and bismuth superconductors were made by the precursor matrix method. The thermopower of these samples was measured in the temperature range 250 K-Tc. The thermopower was positive and decreased linearly with increasing temperature aboveTc (onset). The exponential enhancement of thermopower seen in the undoped and doped YBCO was not observed in these samples. The linear variation of thermopower can be explained on the basis of either a two-band model or a narrow band model.

• Thermopower of non-superconducting and superconducing Nd1·85Ce0·15CuO4−y samples

The thermopower of single-phase samples of Nd1·85Ce0·15CuO4−y was measured from 250 K down to 10 K. The as-prepared sample was not superconducting. It had a negative thermopower at 250 K, whose magnitude increased as temperature was decreased to 95 K. A further reduction in temperature caused a decrease in magnitude of thermopower. The sign of the thermopower changes to positive at 12 K. The superconducting sample also showed the same behaviour but the change of sign now occurred at 40 K. Below 40 K, the thermopower showed a positive peak and reduced to zero at the superconducting transition. These results were compared with previous studies.

• Low percolation concentration for zero thermoelectric power in Y1Ba2Cu3O7−x

Effect of oxygen deficiency on the thermoelectric power and the resistive transition is studied by changing the cooling rate of the sintered specimens of R1Ba2Cu3O7−x where R=Y, Gd and Sm. Slow-cooled (60°C/h) specimens are superconducting and show positive TEP with a broad peak aboveTc. It is a decreasing function of temperature at higher temperatures. Pair fluctuation aboveTc is thought to be responsible for this peak. Highly O2-deficient sample of Y123 has a finite resistivity atTc and yet shows a zero TEP. It is suggested that the percolation concentration of the superconducting phase for zero TEP should be much smaller than that required for the electrical conduction.

• Hall effect and thermoelectric studies on Pb and Ag-doped Bi2Sr2Ca2Cu3Ox superconductors

Hall effect and thermoelectric power measurements at room temperature in the magnetic field of 18 kOe on Ag- and Pb-doped Bi2Sr2Ca2Cu3Ox samples indicate that the density of charge carriers decreases with increase in Ag concentration whereas the reverse is observed with Pb. From the reduced scattering factor, it is assumed that the scattering of the charge carriers occurs due to acoustic phonons in the normal state at room temperature.

• XANES study of oxygen doping in 1-2-3 system

Studies on X-ray absorption near-edge structure (XANES) using full multiple scattering approach on YBa2Cu3O7−x withx=0,0·5 and 1, CuO and Cu2O are reported. The MS-XANES calculations confirm that removal/addition of oxygen betweenx=0·0 and 1·0 appears to be confined to Cu chain sites only.

• An X-ray spectral study of the influence on the electronic structure of highTc superconductors of the parameters determining the superconducting transition temperature

CuLα and OKα X-ray fluorescence spectra, corresponding to the Cd3d-Cu2p and O2p-O1s transitions, and XANES and EXAFS spectra (CuK, YK, BiL, PbL) have been obtained for the following series of samples: (i) YBa2Cu3O7−δ (0&lt;δ&lt;0·8), (ii) La2−xSrxCuO4 (0&lt;x&lt;0·3) and (iii) Bi2(Sr, Ca)n+1 Cun O2n+4 (n=1,2,3), Bi1·8Pb0·3Sr1·9Ca2Cu3Oy. A correlation has been found between the decrease in the superconducting transition temperature (Tc) with variation in the stoichiometric parametersδ andx in series (i) and (ii), and the shift of the maximum in their CuLα spectra and the relative increase in the number of occupied states Cu3d in the upper part of the valence band. The changes in the distribution of the Cu3d and O2p densities and in the spectral parameters of the samples are discussed.

• Polarization dependence of EXAFS and XANES spectra of superconducting films on the basis of Y-Ba-Cu-O

Superconducting films of Y-Ba-Cu-O have been studied by EXAFS and XANES spectroscopy. Films of thickness 1000 Å obtained by laser sputtering onto sapphire substrates hadTc of about 90 K, ΔTc of about 2 K and the$$\bar c$$ axis orientation normal to the substrate surface. The EXAFS and XANES spectra were measured in the region of Y and CuK edges at different orientations of the sample with respect to the polarization vectorĒ. The spectra were measured in the surface-sensitive total electron yield detection mode. The observed differences between the spectra are discussed and compared with theoretical calculations.

• On the effects of Ti, Zr and Ce substitution in YBa2Cu3O7−δ: transport, XRD, XPS and XANES studies

Results of the XRD, XPS, XANES, iodometric titration and transport studies on freshly prepared Ce, Zr and Ti substituted 1-2-3 samples are reported here. It is argued that at least 2% of Ce, Zr and Ti ions go into the 1-2-3 lattice. It is found that Zr, Ti ions exist in 4+ valence state, Ce is in predominantly 4+ (mixed valence) state and valence of Cu remains unaffected on substitution.

• Vibrating reed studies on high-Tc superconductors

We present vibrating reed (VR) measurements on single crystal and ceramic “1-2-3” and melt-processed polycrystalline Bi-based compounds in a wide range of temperature (4·2–100 K) and magnetic field (B=0–4 T). The “depinning line” (DL) determined by the VR technique is equivalent to the “irreversibility line” determined by magnetization and susceptibility measurements. A comparison of the results on single crystal and polycrystalline 1-2-3 compounds indicates that the VR technique is sensitive to the intragranular properties of the polycrystalline reed. It is found that the DL for 1-2-3 compounds is much steeper than that for Bi-based compounds, reflecting an intrinsically different pinning in both the materials, in agreement with the measured elastic coupling (Labusch constant α(B, T)).

• Elasticity studies of aluminium-substituted highTc superconducting materials

Longitudinal and shear wave velocities of Al-substituted Bi-Sr-Ca-Cu-O superconducting materials prepared by the ceramic technique were measured by the pulse transmission technique. Using the data, several elasticity parameters like Young’s modulus, rigidity modulus, Poisson’s ratio, Debye temperature etc were evaluated. Variation of elasticity parameters with composition has been explained in terms of binding forces between various atoms of the superconducting materials. Temperature variation of longitudinal wave velocity has also been measured and attempts have been made to explain the anomalies at 200 K.

• # Bulletin of Materials Science

Current Issue
Volume 42 | Issue 6
December 2019

• # Editorial Note on Continuous Article Publication

Posted on July 25, 2019