The Influence of Bi Doping on the In Situ Growth of TlBa2Ca2Cu3O9 High-Tc Films

The in situ process—laser ablation in combination with thermal evaporation of Tl₂O—has turned out to be a preparation method for single-phase and epitaxial TlBa₂Ca₂Cu₃O₉ (1223) thin films with T _c values up to 109 K. It was found by several groups that a partial substitution of Tl by Bi simplifies the phase development of the 1223 compound in the usual two-step process. We have investigated the influence of the Bi doping on the in situ growth. X-ray measurements show that the films consisted mainly of the 1223 compound. In 300-nm thin films there was no evidence of a Bi amount in the crystal structure, but thinner films (80 nm) show a small amount of Bi. We concluded that Bi doping supports the phase development of the 1223 compound only in an early stage of the film growth. The Bi-doped films have higher T _c values up to 114 K, higher j _c values up to 6 × 10⁵ A/cm² (77 K, 0 T), and lower surface resistances of 56 m (77 K, 87 GHz) than the undoped films.     

Temperature-dependent collective pinning exponent in Bi2Sr2Ca2Cu3O10+x tapes and bulk samples with preferential crystallite orientation

The low-voltage-level current-voltage characteristics of Bi₂Sr₂Ca₂Cu₃O_10+x tapes and bulk samples with preferential crystallite orientation were measured in applied magnetic fields both parallel and perpendicular to the samples' surfaces. The data were interpreted within the framework of the collective pinning model. The collective pinning exponent changes from 1 to 0.5 (close toT _c), while in the case of unoriented samples it is constant (and equal to the theoretical value 1/2) as shown in an earlier paper. We suggest that this temperature dependence of is due to the change of the dimensionality of the intragranular vortices.     

X-Ray Diffraction Studies on Yttrium-Doped La0.67Ca0.33MnO3

Polycrystalline samples of yttrium-doped La_0.57Ca_0.33MnO₃ (LCMO) have been prepared. Structure characterizations have been carried out at room temperature by X-ray diffraction with Rietveld refinements. The octahedral tilting distortion increases with the increase of yttrium content or the decrease of the average radius of La-site cations, La. The difference between the two crystallographically distinct Mn—O—Mn bond angle increases with increasing Y content. The average Mn—O bond distance is relatively constant across the entire series, but individual Mn-O bond distances and the Mn—O—Mn bond angles show the presence of the cooperative Jahn–Teller effect at room temperature in yttrium-doped LCMO.     

X-Ray Diffraction Studies on LaBa2Cu3Ox Cuprates with Iron Substitution

LaBa₂Cu_3–y Fe _y O _x ceramic samples with y = 0.00–1.50 are synthesized by the solid-state reaction technique. Rietveld analysis for X-ray diffraction is performed on these iron-doped samples. A BaCuO₂ impurity phase and a ceramic cuprate phase coexist in each sample. An orthorhombic-to-tetragonal (OT) phase transition occurs in the doping range of 0.03y0.06, and a tetragonal-to-orthorhombic (TO) one occurs in the doping level of 0.10y0.25. There is a jump in the structural parameters due to the iron doping. The occupancy of oxygen at the O(4) site, which is in the La plane at z = 1/2, increases with increase in iron content. These results may relate to the iron preferential occupancy for the Cu(1) site at the lower doping level, and for Cu(2) sites at the higher doping level.     

Anisotropy in ac losses near the irreversibility field of Bi2Sr2CaCu2O8

The ac susceptibility under a biased dc field near the irreversibility field (H _irr) of a Bi₂Sr₂CaCu₂O₈ single crystal has been measured. The frequency dependence, the ac-power dependence, and the nearly lossless character of the vs.H _dc curve forHa-axis have been roughly explained from a reversible (elastic) fluxoid motion, while those forH c-axis have been explained from a thermally assisted flux-flow (TAFF) model. The obtained parameters are discussed in relation to anisotropic flux-pinning mechanisms in the layered structure of this compound.     

Electronic properties of CuO2 planes

We study a 3-band model of CuO₂ with bare bones interactions: hopping from copper ions to nearest-neighbor oxygenst _pd only, a two-body interaction on the copper ionsU _dd only, and an overlap copper-oxygen parameter _pd. In the limitt _pd U _dd ^1/2, t^*=t _pd ²/2U _dd is the unit of energy with 8_pd ² as the only parameter of significance. If the two-body interaction is invariant under particle-hole interchange, the low-lying states (energy O(t^*)) can be described by conserved particles and can all be classified. They are quite distinct from the high-lying states (energies O(U _dd)). The dynamics of the conserved fermion-like elementary particles are well described by a modifiedt-J model with extended hopping and nearest-neighbor superexchange attraction. This is a scenario known to be favorable to high-temperature superconductivity, but it must be noted that both the hopping range and the exchange are functions of 8_pd ². Moreover, if the Hamiltonian isnot invariant under particle-hole interchange the dynamics becomes much more complex and possibly more inimical to high-temperature superconductivity. This may provide an explanation for the deleterious effect on superconductivity of very small concentrations of certain impurities.     

Electronic Raman scattering of superconducting YBa2Cu3Oy single crystals

TheA _1g andB _1g low-energy Raman continua of YBa₂Cu₃O _y (Y123) single crystals, withy=7.0, 6.99, and 6.93, have been investigated. It is found that the peak frequency of theA _1g continuum is equal to 310±10 cm^–1 and independent of oxygen concentration fory in the above range. The central frequency of the broad peak in theB _1g continuum, however, shifts from about 470 cm^–1 fory7.0 to 550 cm^–1 fory6.93. Thus, a relatively small change in oxygen concentration results in a significant redistribution of the states contributing to theB _1g continuum. Assuming the low-energy portions of the continua are electronic in origin, the Raman spectra have been calculated and the results compared to the experimental spectra. It is suggested that the Raman continua arise, at least in part, from scattering across a spin fluctuation-induced pseudogap.     

Electronic Inhomogeneity and Possible Pseudogap Behavior of Spin Susceptibility in the Electron-Doped Superconductor Sr0.93La0.07CuO2: 63Cu NMR Study

The magnetic susceptibility, NMR spectra, nuclear spin-lattice relaxation rate (T ₁ ^–1) and the echo-decay rate (T ₂ ^–1) of ⁶³Cu were measured for the electron-doped infinite-layer superconductor Sr_0.93La_0.07CuO₂/T _c ^onset = 42.4 K). The results obtained revealed a clear tendency toward frustrated phase separation in this nominally underdoped high-T _c material. Above T _c the ⁶³Cu Knight shift is found to decrease upon cooling giving an evidence for a pseudogap-like decrease of the spin susceptibility. It is shown that unusual anisotropy of the ⁶³Cu Knight shift in the electron-doped CuO₂ layer can be understood as a compensation effect between the isotropic hyperfine coupling, mediated by the 4s Fermi-contact and 3d core-polarization exchange interactions, and the anisotropic on-site spin-dipolar hyperfine interaction of the Cu nuclei with the itinerant carriers, whose states near the Fermi energy have a sizeable admixture of Cu(4p_z) and/or Cu(3d_z ²) orbitals.     

SrCuO2 and related high-pressure phases

Treatment at high pressures and temperatures of 6 GPa and 1120 K, typically, have proved to be useful to find new layered complex cupric oxides. Among these, SrCuO₂ crystallizing in the infinite-layer structure may be considered to be the key material not only because of its simple composition and structure but also because of the fact that interesting homologous series of high-pressure phases like Sr _n+1Cu _n O_2n+1+ and Sr _n–1Cu _n+1O _2n converge to SrCuO₂ asn increases.     

Structural stabilities, electronic and optical properties of CaF2 under high pressure: A first-principles study

The structural stabilities, electronic and optical properties, the pressure-induced metallization for CaF₂ have been studied by using the density functional theory calculations. The ground phase is predicted to transform into Pnma structure at 8.1 GPa, which is well consistent with the experimental findings. Above 278 GPa, Pnma-CaF₂ transform into P6₃/mmc phase. The calculated structural data for and pnma phases are in very good agreement with experimental values. The electronic band structures show that Pnma and P6₃/mmc phases of CaF₂ are insulators at the transition pressure. Upon further compression, the band gap of P6₃/mmc decreases with pressure, and CaF₂ is predicted to undergo metallization around 2250 GPa. The possible reason for the metallization was discussed. All CaF₂ polymorphs have ionic character between Ca–F bond with the analysis of the charge–density distribution and density of states.     

Growth and characterization of Bi2Sr2Ca1Cu2O8+x single crystals

High-quality single crystals are well suited to the investigation of some intrinsic material properties. A modified Bridgman method using a sharp temperature gradient (300°C/cm) was used to grow Bi₂Sr₂Ca₁Cu₂O_8+x single crystals. Although the samples were contained in alumina ampoules, no aluminium contamination of the samples was detected. Blade-shaped crystals up to 7–8 mm length and 3–4 mm width could be grown by this method, although extraction from the matrix was difficult. Electron diffraction patterns of the [001] zone axis revealed a high degree of crystallinity. The narrowness of the superconducting transition temperature, as determined by ac susceptibility, also suggests the existence of well-formed crystalline domains. In order to determine the relative orientation of the crystalline domains, electron channeling patterns were recorded from several consecutive growth steps from a fracture surface. The poor contrast of these and Kikuchi patterns suggests the presence of a stacking structure. The results showed a [100] growth direction and (001) cleavage plane. Reversible oxygen loss at the peritectic decomposition temperature of 863°C was observed. Knoop indentation measurements showed that the crystals were quite soft, having a microhardness of 0.44 GPa.     

Normal-State Optical Response Functions of MgB2 Superconductor

The reflectivity of superconducting MgB₂ (T _c = 39 K) has been measured on a randomly oriented thin film at room temperature over a wide-range of frequencies, 20 < 100000 cm^–1. The conductivity shows highly metallic behavior but cannot be explained with a simple Drude model alone. The electronic contribution is analyzed by a generalized Drude model. The scattering rate 1/() and the mass renormalization ratio m*()/m = 1 + () exhibit clear frequency dependence. The electron–phonon coupling strength is estimated to be 1.5 ± 0.5 while the plasma frequency _p is 2.4 eV.     

Analysis of electronic structure and charge density of the high-temperature superconductor YBa2Cu3O7

Self-consistent linearized augmented plane wave (LAPW) method calculations of the band structure, density of states, Fermi surface, Coulomb potential, charge density, core-level shifts, and electron-phonon interaction are presented for Y₁Ba₂Cu₃O₇. The calculated Sommerfield parameter is 4.35 mJ(mole Cu)^–1 K^–2, roughly about a factor of 2 smaller than experimentally deduced values of the enhanced value=(1 + )₀, suggesting that the Fermi surface mass enhancement is of the order of unity. The crystal charge density is best represented by overlapping spherical ionic densities when the Cu and O ions are assigned charges of +1.62 and –1.69, respectively, corresponding to about 0.3 holes per oxygen atom. Core-level energies for the inequivalent atoms differ by as much as 0.45 eV for Cu and 0.7 eV for O, amounts which may be detectable by core-level spectroscopies. These results provide important information on the character and magnitude of ionic contributions to bonding in these materials. Within the rigid muffin-tin approximation, calculated McMillan-Hopfield parameters yield estimates for the electron-phonon strength that appear to be too small to account for the observedT _c. We point out an unusual band of oxygen-derived chain states below, but within 0.1 eV of, the Fermi level.     

Crystal structure analysis of Y2Cu2O5

Single crystals of Y₂Cu₂O₅ were obtained in the flux growth process by controlled heating of a mixture of Y₂O₃, BaO, and CuO in a molar ratio of 1820. These crystals were analyzed by a single-crystal X-ray diffraction analysis. The crystal contains polymeric chains of Cu₂O₅ interspersed by yttrium ions surrounded by octahedral arrangements of oxygen atoms. Crystal data: space group=Pna2₁,a=10.799(2) Å,b=3.4990(5) Å,c=12.459(2) Å,Z=4, 380 reflections,R=0.026,R _w=0.030.     

d-Wave superconductivity in thet-J model using a systematically improved Variational Monte Carlo method

We can construct unbiased systematic improvements of standard Variational Monte Carlo calculations by the use of wavefunctions modified by short range operators. Using the energy and spectral width of the original and improved wavefunctions we can judge the quality of the trial wavefunctions. For the t-J model we find the regions of phase separation and d-wave superconductivity with varying J/t and hole density S and extend the T=0 phase diagram into the physically interesting region of 0.1 – 0.2and J/t 0.4.Above the critical ratio of J/t¦ _c we find clear evidence of d_x2-y2-pairing and above a higher value J/t¦_s phase separation. At =0.16we obtain values J/t¦ _c=0.4and J/t¦ _s=0.7. Cluster sizes up to 50 sites are used.     

Hc2 of MgCNi3 Determined from the Specific Heat and Resistivity Measurements

H _c2 of MgCNi₃ has been determined from the specific heat C and resistivity measurements in the same sample. The results from are nearly identical with those determined from the anomaly in C. Furthermore, utilizing the relation (H) H and the value of d/dH, the obtained value of H _c2 is the same as that by the WHH model, if the spin paramagnetic effect and the spin–orbit interaction are taken into account. The results of this comparison have strong implications on the order parameter of MgCNi₃.     

Upper critical and irreversibility fields in selectively doped YBa2Cu3O7

The electrical resistivity of (Y_1–xPr_x)Ba₂Cu₃O_7–gd and YBa₂(Cu_1–xZn_x)₃O_7– thin films and (Y_1–x. Tb_x)Ba₂Cu₃O_7– thin films and single crystals has been measured as a function of 0xx_crit, 2KT300K and OTH20T. The samples were oriented withc-axis parallel to applied fields. Upper critical field H_c2 and irreversibility field H_irr values have been determined from these measurements. Increased Tb doping appears to shift H_irr to higher temperatures. This coupled with observed twin peaks in magnetization measurements reflects an enhancement of flux pinning. Unlike Tb which does not appear to alterT _c , Pr and Zn doping of this system tends to depress bothT _c and the slope of the mean field normal phase-mixed phase boundary line (dH _c2/dT).     

Anisotropy in the resistive superconducting transition under magnetic fields in single crystal Pb2Sr2Ho0.5Ca0.5Cu3O8

Anisotropie properties of the single crystal Pb₂Sr₂Ho_0.5Ca_0.5Cu₃O₈ have been investigated by measuring the electrical resistivity in theab-plane _ab (H, ,T), which depends on the angle between theab-plane and the magnetic-field direction, in various constant fieldsH perpendicular to the current direction. All the angle-dependent values of _ab (H, ,T) at a constant temperature are scaled to be on one curve as a function of reduced field. The anisotropic parameter (m _c ^* /m _ab ^* )^1/2 is estimated as 12–13, which is larger than that of YBa₂Cu₃O₇ and much smaller than that of Bi₂Sr₂CaCu₂O₈. It has been concluded that the anisotropy does not always depend on the thickness of the blocking layer but seems to depend on the overlap of the electronic wave functions along thec-axis. Anisotropy in the pinning potential has also been discussed from the resistive tail in the temperature dependence of _ab (H,,T).     

Fabrication, characterisation and application of (8 mol% Y2O3) ZrO2 thin film on Na3Zr2Si2PO12 substrate for sensing CO2 gas

A planar type CO₂ gas sensor employing (8 mol% Y₂O₃) ZrO₂ (YSZ) thin film on Na₃Zr₂Si₂PO₁₂ (Nasicon) substrate with Na₂CO₃ as an auxiliary electrode has been fabricated and tested in laboratory environment between 700–900 K. The YSZ thin film was fabricated on Nasicon and alumina substrate using radio frequency (RF) magnetron sputtering. The film was examined using SEM and X-ray diffraction (XRD) after treating the Nasicon-YSZ bi-layer structure at 1300 K for 2 h. The results indicate that a crack free YSZ film was produced on Nasicon surface that was well bonded to the substrate. The conductivity of sputtered YSZ thin film measured by ac-impedance spectroscopy has been found to be higher than that of YSZ pellet by approximately half an order of magniture. The bi-electrolyte planar sensor displays rapid response (t₉₅ 200 s) to CO₂ compared to the tube type sensor (t₉₅ 700 s) and the measured open circuit voltage of the electrochemical CO₂ sensor has been found to be Nernstian at all temperatures.     

Band structure of Bi2Sr2CuO6: Strong effects due to structural modulation

The electronic structure of idealized tetragonal Bi₂Sr₂CuO₆ is compared with that of an orthorhombic 2×2 model that includes atomic displacements due to the structural modulation. The electronic structure, particularly that related to Bi and O(3) atoms, is strongly modified near the Fermi energy. In particular the shape and topology of the Bi-O(3) related Fermi surfaces near the ¯M point are changed. With the modulation, bands occur near the Fermi energy over a significant part of the zone near this point. A method is proposed to distinguish between lattice distortions and antiferromagnetic correlations as the cause of recently observed shadow Fermi surfaces.