Characterization of YBa2Cu3O7/PrBa2Cu3O7 superlattices

We investigated the structural and superconducting properties ofc-axis oriented (YBa₂Cu₃O₇) _nY /(PrBa₂Cu₃O₇) _npr superlattices with thicknesses of the individual layers down to one unit cell (10≥nY≥1; 18>nPr≥ 1). By transmission electron microscopy and X-ray diffraction we find an excellent structural quality of the samples, though the quantitative analysis shows the existence of defects. In superlattices with decoupled YBa₂Cu₃O₇ layers of two unit cell thickness we find a highT _c value of 75 K. We probed the flux line structure in the superlattices by measurements of the critical current density in magnetic fields. The experiments show that the flux-line dynamics is dominated by the movement of pancake vortices.     

Raman scattering studies of ultrathin-layer YBa2Cu3O7/PrBa2Cu3O7 superlattices

We present Raman scattering studies ofc-oriented ultrathin-layer superconducting (YBa₂Cu₃O₇) _m /(PrBa₂Cu₃O₇) _n superlattices. For the superlattice with (m=2,n=1) sequence, Raman spectra reveal a new line in the spectral region around 320 cm^?1. It is interpreted as a mode representing a combination of IR optical phonons of the Y-sublayers with an admixture of aB _1g type Raman active vibration in the Pr sublayers. This new line, which is similar to those from the interior of the Brillouin zone of the original lattice, does not exhibit superconductivity-induced self-energy effects, although its counterpart in the pure substance does. No additional line is found in the (m=1,n=2) superlattice in the same region, supporting our interpretation for the (m=2,n=1) sample.     

Field modulated microwave absorption in YBa2Cu3O7/PrBa2Cu3O7 multilayers

We describe field modulated microwave absorption measurements on YBa₂Cu₃0₇ / PrBa₂Cu₃O₇ multilayers with PrBa₂Cu₃O₇ thickness in the range 2.5 to 10 nm and with a fixed YBa₂Cu₃O₇ thickness of 10 nm. The Fourier spectrum of the reflected microwave power reveals one fundamental frequency which appears at T_c and even harmonics of the modulation field frequency at lower temperatures. The determination of the irreversibility line near T_c for different values of the PrBa₂Cu₃O₇ thickness and inferred from the vanishing of the nonlinear response signal at 2 in the presence of superimposed dc and modulation fields is reportedit.     

Reaction kinetics in the superconducting YBa2Cu3O7-x and SmBa2Cu3O7x

The compounds under investigation were prepared with nominal composition MBa₂ Cu₃ O_7-x, M being Y or Sm, through solid state reaction starting from Y₂O₃, Sm₂O₃, BaCO₃ and CuO powders. Reactions were investigated by thermo-gravimetric and differential thermal analysis in the temperature range 900–980°C. Reaction kinetics of MBa₂Cu₃O_7-x solid state synthesis were evaluated according to the mathematical relationships theoretically derived for phenomenological models. In the Y-based system the phase boundary model provides a reaction kinetics suitable to describe the process, while a three dimensional diffusion model fits the data in the Sm-based system. The influence of the mixing method on the reaction kinetics was evaluated. Microstructural features of superconducting samples were analyzed.     

Specific heat of YBa2Cu3O7

Specific heat measurements, including measurements in magnetic fields and at both low temperatures and nearT _c , on a number of YBa₂Cu₃O₇ samples have revealed several correlations among strongly sample-dependent parameters. These correlations suggest that the sample dependence of the parameters reflects a sample dependence of the volume fraction of superconductivity, which is in turn correlated with a low concentration of Cu²⁺ moments. The correlations give a criterion for recognizing the values of the parameters characteristic of the fully superconducting material. Preliminary results on the effects of sample heat treatment are reported. New data on the “linear term” is presented and discussed.     

Low-temperature specific heat of YBa2Cu3O7, YBa2Cu3O6, Y2BaCuO5, YBa3Cu2O7, BaCuO2, and CuO

We have measured the low-temperature specific heat (1.3T20 K) and the dc magnetic susceptibility (100T250 K) of eight samples of the high-T _c superconductor Y _x Ba_3–x Cu₃O_7– (x=0.9, 1.0, 1.1) and of two samples of nonsuperconducting YBa₂Cu₃O₆₊. We have also performed specific heat measurements on the possible impurity phases: YBa₃Cu₂O₇, Y₂BaCuO₅, CuO, and BaCuO_2+x . The superconducting samples all have a nonzero, sample-dependent linear term * and an upturn inC/T at very low temperature. We show that this anomalous behavior is at least partly due to the presence of a small amount (1%) of BaCuO_2+x impurity phase in the measured samples. This is evidenced by the correlation between * and the Curie component of the susceptibility, which is proportional to the amount of paramagnetic impurities.     

Superconducting and non-superconducting PrBa2Cu3O7

Superconducting properties of Pr123 and microscopic structural differences between the superconducting and non-superconducting Pr123 crystals are reviewed. It is shown that the variety of physical properties of Pr123 is closely related to Cu deficiency at the chain site and our experimental results are consistently explained with a model that the Pr 4f-O 2p hybridization forms a narrow band and carriers are localized by a kind of atomic disorder in non-superconducting crystals.     

Vortex motion in coupled DyBa2Cu3O7/(Y0.6Pr0.4)Ba2Cu3O7 heterostructures

Multilayer structures containing 24 Å thick DyBa₂Cu₃O₇ layers, separated by 96 Å of an (Y_0.6Pr_0.4)Ba₂Cu₃O₇ alloy, are studied to investigate the effect of coupling on vortex dynamics. With the magnetic field perpendicular to theab plane, and as a function of the number of superconducting layers in the structure, we find that the activation energy for flux motion increases, first linearly, and then saturates. This linear increase is taken as evidence that pancake vortices belonging to different DyBa₂Cu₃O₇ layers are stacked and have a coupled motion. Above a characteristic number of superconducting layers,N _c , shear of the vortex structures becomes important and the thermally activated process only displaces a stack ofN _c pancake vortices, meaning that the vortex lattice is turning three dimensional. In these structures we findN _c to be 2 to 3.     

Effect of columnar defects on theJ c anisotropy of YBa2Cu3O7−δ thin films and YBa2Cu3O7−δ /PrBa2Cu3O7−δ multilayers

Epitaxial YBa₂Cu₃O_7?δ thin films (YBCO) and YBa₂Cu₃O_7?δ /PrBa₂Cu₃O_7?δ multilayers (Y/Pr) were irradiated with high-energy heavy ions (770 Mev²⁰⁸Pb) under various directionsφ relative to thec-axis. The irradiation resulted in columnar defects tilted byφ from thec-axis. The angular dependence of their pinning activity was studied by measuring the anisotropy of the critical current density. TheJ _c (B, T,?) behavior of the irradiated YBCO thin films showed an additional peak, which exceeds the intrinsic pinning peak, exactly at the irradiation direction. The Y/Pr multilayers, however, showed an isotropicT _c -enhancement by a factor of 5, without any additional structure in theJ _c(B, T,?) curve.     

Glassy behavior of low-temperature energy relaxation in YBa2Cu3O7 and YBa2Cu3O6

Measuring the power release after rapid cooling a YBa₂Cu₃O₇ sample (m=42.85 g, T_c=91 K) from the equilibrium temperature T₁ (2.35 KT₁15.1 K) to T₀=1.5 K, we observed a time dependence typical of a glass: is proportional to t^–1. The results allow us to determine the linear term of the heat capacity (0.8 mJ/mole · K²) due to the two-level systems. While the low-temperature heat capacity anomaly noticeably decreases, the power release is essentially unchanged after oxygen reduction of the sample.     

Phonon Frequency Spectrum HTSC YbBa2Cu3O7

Since the discovery of the high T _c superconductivity by Bednorz and Muller, several workers around the world have studied several systems to reach a really high T _c superconductor. As the strong electron-phonon coupling may be one of the possible origins of the high T _c , a knowledge about the phonon in these materials is essential. In order to investigate phonon spectra, one has to study Raman and infrared spectra of these systems. But there is a little information about the Raman spectra and infrared absorption spectra are available in the literature. Hence, a theoretical evaluation of phonon frequencies of high temperature superconductors assume importance. Due to strong covalent bonding nature in high temperature superconductors, a normal coordinate analysis using Wilson??s FG matrix is applied to evaluate Phonon frequencies. The normal coordinate analysis of optically active lattice vibrations will be useful for the theoretical interpretation of vibrational spectrum at the center of Brillouin zone in high T _c superconductors. Hence, an attempt is made to evaluate all the phonon frequencies of HTSC YbBa₂Cu₃O₇ through normal coordinate analysis. Calculations of lattice dynamics is also performed by use of the modified three body force shell model. The present approach leads to a better understanding of phonon frequency of high T _c superconductor YbBa₂Cu₃O₇ under investigations in the present work. This calculations yield not only the zone center phonon modes but also the stable dispersion curves through out the Brillouin zone. It also supports strong electron-phonon interactions in high T _c superconductors.     

Crystal-chemical anomalies in nonsuperconducting PrBa2Cu3O7

Studies of the crystal chemistry of nonsuperconducting PrBa₂Cu₃O₇ indicate that this compound is strictly isostructural with its superconducting RBa₂Cu₃O₇ (R = Y, rare earth) analogs. Crystallographically, Pr is present in the trivalent state according to the structural trends exhibited by the RBa₂Cu₃O₇ series as a function of R³⁺ ionic radius. The sole structural anomaly attributable to the presence of Pr³⁺ in the YBa₂Cu₃O₇ structure is a next-next-nearest neighbor effect and consists of an unusually short axial Cu-O distance, i.e., a short bond length between the in-plane copper and the chain oxygen. The correlation of this anomaly with the nonmetallic/nonsuperconducting properties of PrBa₂Cu₃O₇ supports a variety of literature reports, both theoretical and experimental, suggesting that the apical oxygen in the YBa₂Cu₃O₇ structure plays a critical role in mediating the appearance of superconductivity. The mechanism by which the f-electrons in Pr³⁺ (f ²) interact with the Cu-O manifold to produce the nonmetallic behavior of PrBa₂Cu₃O₇ remains unknown; however, superconductivity is “turned back on” for Nd³⁺ (f ³), immediately next to Pr and just slightly smaller. Careful comparative studies of superconducting NdBa₂Cu₃O₇ and nonmetallic PrBa₂Cu₃O₇ are needed to elucidate the critical difference in the behavior of the f-electrons and may shed light on the fundamental mechanism of high-temperature superconductivity in copper oxides.     

Critical current in YBa2Cu3O7 ceramics

Measurements of critical currents using a four-point direct current (dc) method and an alternating magnetic field method have been performed on several superconducting YBa₂Cu₃O₇ ceramics at 77 K. In the presence of a constant magnetic field, the critical currents obtained with the alternating field method are several orders of magnitude larger than the critical currents measured by the dc method. Also, we observed a minimum in the dc critical current as a function of applied transverse magnetic field. Several authors have suggested that these ceramics behave as individual superconducting grains coupled by Josephson junctions. In this paper, we explain the two observations above using that model.     

Electronic structure of YBa2Cu3O7 and YBa2Cu3O6—charge-self-consistent band structure calculations

Charge self-consistent LCAO band structure (CSCBS) calculations are reported for orthorhombic YBa₂Cu₃O₇ and tetragonal YBa₂Cu₃O₆ assuming ordered vacancy models. The effective atomic charges are used to study the charge transfer. In YBa₂Cu(1)₁Cu(2)₂O₇, the two types of copper atoms have their energy bands almost overlapping with effective valency of each copper as 7/3 (or effective valency of each oxygen as approximately — 13/7), so that electron hopping can take place without any loss or gain of energy while in YBa₂Cu(1)₁Cu(2)₂O₆, Cu(1)₁ is monovalent and Cu(2)₂ are divalent with significant difference in their bands. Therefore, YBa₂Cu₃O₇ should conduct much better compared to YBa₂Cu₃O₆. This corroborates the experimental observations that YBa₂Cu₃O₇ is a (super)conductor while YBa₂Cu₃O₆ is not. The calculated effective charges and DOS support the above view.     

Theory of electron distributions and63Cu and17O nuclear quadrupole interactions in YBa2Cu3O7 and YBa2Cu3O6

Ab initio unrestricted Hartree-Fock Cluster investigations have been carried out on the electronic structures of the YBa₂Cu₃O₇ and YBa₂Cu₃O₆ systems. The results of these investigations provide satisfactory explanations of available⁶³Cu and¹⁷O nuclear quadrupole interaction data. The electron distributions obtained rule out the presence of Cu³⁺ ions and are supportive of the presence of Cu²⁺, Cu¹⁺, O¹⁻ ions in the O₇ system and Cu²⁺, Cu¹⁺ and O²⁻ in the O₆ system with actual charges departing significantly from the formal charges, especially in the O₇ system, indicating the importance of covalency effects. Suggestions will be made regarding possible sources that can bridge the remaining gap between theoretical and experimental results for the nuclear quadrupole interaction parameters.     

Improved Flux Pinning in Melt-Textured YBa2Cu3O7 − δ and ErBa2Cu3O7 − δ Through Chemical Additions

The effects of Ce-based additions (CeO₂ and BaCeO₃) in combination with MgO additions on the magnetic properties of melt-textured YBa₂Cu₃O_{7 ? δ} and ErBa₂Cu₃O_{7 ? δ} have been investigated. The additions lead to improvements in the magnetic properties of both RE-123 compounds compared to samples with either addition alone or with no additions. The Ce–Mg addition combination produces a “peak effect” in the magnetic hysteresis loop without significant T _c degradation. This is postulated to be due to the formation of a new type of pinning center. Both Ce and Mg ions are thought to substitute in the RE-Ba₂Cu₃O_{7 ? δ} lattice, creating defects that produce a “peak effect” in the magnetic hysteresis loop. Mg additions alone lead to a reduced T _c, while Ce additions restore the T _c and enhance the magnitude of the peak.     

Superconducting properties of YBa2Cu3O7-Au composites

By X-ray diffraction and photoelectron spectroscopy, we have demonstrated that Au is incorporated in the YBCO lattice up to a concentration 0.1. An XPS study gave evidence for an electronic valence state Au¹⁺. Its replacement in the Cu(1) site appears questionable due to the large difference in ionic radii (Cu²⁺0.073 nm; Au¹⁺0.137 nm). The so called irreversibility lines determined by AC susceptibility measurements are strongly affected by the doping. Up to the limit of solubility in the grains, an improvement in intra and intergrain coupling and flux pinning energy is observed, showing the beneficial interest of YBCO-Au composites.     

Effects of Ag/Cu substitution in YBa2Cu3O7 superconductors

We have substituted up to ca. 2 at.% of the Cu in orthorhombic YBa₂Cu₃O₇ by Ag. This substitution leads to a wider temperature range for the resistive transition to the superconducting state and a decrease in the fraction of the sample that is superconductive, as compared to the Ag-free material. Introduction of up to ca. 1.5 at.% Ag causes a slight lattice expansion. Higher nominal levels of Ag bring about the formation of additional phases, which, however, still show zero resistance, albeit at lower temperatures. We use solid state chemical arguments to infer that Ag substitutes for Cu in the CuO₄ chains of the structure, in which case our results suggest that optimal superconducting behaviour is obtained when these chains are intact.