Phase Of YBa Research Articles

Ultrafast pump-probe spectroscopic signatures of superconducting and pseudogap phases in YBa2Cu3O7−δ films

Femtosecond pump-probe spectroscopy is applied to identify transient optical signatures of phase transitions in optimally doped YBa2Cu3O7-{\delta} films. To elucidate the dynamics of superconducting and pseudogap phases, the slow thermal component is removed from the time-domain traces of photo-induced reflectivity in a high-flux regime with low frequency pulse rate. The rescaled data exhibit distinct signatures of the phase separation with abrupt changes at the onsets of TSC and TPG in excellent agreement with transport data. Compared to the superconducting phase, the response of the pseudogap phase is characterized by the strongly reduced reflectivity change accompanied by a faster recovery time.

MECHANISM OF QUENCHING OF SUPERCONDUCTIVITY IN THE YBa2Cu3O7-δ SYSTEM ON SUBSTITUTION OF Zn FOR Cu

Substituting Zn for Cu is known to rapidly quench superconductivity in the doped perovskites but the mechanism behind it is still not clearly understood. Single phase YBa 2( Cu 1-x Zn x)3 O 7-δ (x = 0.0 to 0.06) samples were synthesized and characterized using XRD, wet titration, resistivity, and ac susceptibility. Angle-integrated Valence Band Photoemission and Zn K-edge XAFS results clearly show that their oxygen stoichiometry (δ) changes on Zn substitution, thereby adversely affecting the density of free charge carriers and hence the normal state resistivity and the Tc. However, the observed changes in the two happen to be too large to be accounted for solely on the basis of changes in the oxygen stoichiometry δ. We find that the Zn cation acts as a strong "impurity" scattering centre in the YBCO lattice and causes local lattice distortion (LLD). It consequently induces local magnetic moment, seen in our dc susceptibility measurements. It is thus a composite of (Δδ), LLD and possibly also magnetic pair-breaking that is responsible for the rapid quenching of the superconductivity observed with Zn doping in this system.

Substitution of Co 3+ in YBa 2Fe 3O 8

The accommodation of Co in the oxygen-saturated solid-solution phase YBa 2(Fe 1− z Co z ) 3O 8+ w has been investigated by powder X-ray and neutron diffraction techniques, as well as by 57 Fe Mössbauer spectroscopy. Of the nominal composition range 0.00⩽ z⩽1.00 tested, the solid-solution limit under syntheses at 950°C in 1 bar O 2 is z=0.47(5). No symmetry change in the nuclear and magnetic structures is seen as a consequence of the Co substitution, and the Co atoms are distributed evenly over the two sites that are square-pyramidally and octahedrally coordinated for w=0. The oxygen-saturated samples maintain their oxygen content roughly constant throughout the homogeneity range, showing that Co 3+ replaces Fe 3+. Despite the nearly constant value of w, Mössbauer spectroscopy shows that the amount of tetravalent Fe slightly increases with increasing z, and this allows Co to adopt valence close to 3.00 to a good approximation. The magnitude of the antiferromagnetic moment (located in the a, b plane) decreases with z in accordance with the high-spin states of the majority Fe 3+ and Co 3+ ions. Bond-valence analyses are performed to illustrate how the structural network becomes increasingly frustrated as a result of the substitution of Fe 3+ by the smaller Co 3+ ion. A contrast is pointed out with the substitution of cobalt in YBa 2Cu 3O 7 where it is a larger Co 2+ ion that replaces smaller Cu 2+.

Charge and spin structure in of YBa(2)Cu(3)O(6.35).

Neutron scattering has been used to measure the charge and spin structure in the highly underdoped superconductor YBa(2)Cu(3)O(6.35). Incommensurate static charge ordering is found that remains at high temperatures. The magnetic pattern is complex with a resonance and incommensurate structure observed at low temperatures. The results clarify the role of striped phases in YBa(2)Cu(3)O(6+x) superconductors.

Neutron powder diffraction study of the crystal structure of the oxycarbonate phase YBa 2Cu 2.85(CO 3) 0.15O 6.73

The oxycarbonate phase YBa 2Cu 2.85(CO 3) 0.15O 6.73 has been analyzed by neutron powder diffraction. The phase crystallizes with the symmetry of space group P4/ mmm and with unit cell parameters a=3.8717(3), c=11.607(1) Å. Rietveld refinements show that the basic structure of the oxycarbonate is the same as that of the 123 superconductor YBa 2Cu 3O 6+ w . The CO 3 2− ions are located on the basal plane of the unit cell, with the carbon atoms replacing an equal number of “chain” copper atoms. The presence of the CO 3 2− defects explains why the oxygen stoichiometry of the oxycarbonate can be larger than seven atoms per formula unit.

Preparation and X-ray characterization of highly ordered ortho-II phase YBa 2Cu 3O 6.50 single crystals

High purity, highly ordered and twin free ortho-II phase YBa 2Cu 3O 6.50 crystals have been successfully prepared and their structural properties characterized by X-ray diffraction. Removal of twin boundaries was found to be essential in order to obtain highly ordered ortho-II crystals. The ortho-II phase shows a sharp superconducting transition at T c=62 K, 5 K higher than the ortho-I phase of the same oxygen content. The ordering was found to be three-dimensional with long correlation lengths ξ a=148 Å, ξ b=430 Å and ξ c=58 Å. No doubling of the unit lattice along the c-direction was observed, indicating an attractive interaction between oxygen ions in adjacent basal planes.

Fluorohalogenation of superconducting oxide YBa 2Cu 3O 7− δ with ClF 3

The low- Tc phase YBa 2Cu 3O x ( x∼6.6, Tc∼60 K) which was sintered into a dense tablet was subjected to gaseous ClF 3, in order to investigate the mechanism of fluorohalogenation of a sintered YBa 2Cu 3O x . It was found that the fluorochlorinated high- Tc phase YBa 2Cu 3O x F y Cl z ( x∼6.6, y∼0.5, z∼0.1) was formed at the outer part of sample tablet, and that the high- Tc phase with high oxygen and low halogen contents ( x∼6.8, y∼0.1, z∼0.01) was also formed at the inner part of it, despite of no oxygen in the reaction atmosphere. Rietveld analysis of neutron powder diffraction data of the fluorochlorinated high- Tc phase YBa 2Cu 3O x F y Cl z (YBa 2Cu 3O 6.10F 0.75Cl 0.49, T onset=94.5 K) revealed that fluorine preferentially occupied the oxygen defect site and/or the vacant site along the Cu–O chain. On the other hand, chlorine occupied as Cl the site at (0, 0, 1/2) which was the center of the yttrium site surrounded with oxygen because Cl − was too large to occupy the oxygen defect site and/or the vacant site along the Cu–O chain. The ClF 3-treated sample was considered to consist of at least two phases of the orthorhombic YBa 2Cu 3O 6.35F 0.65Cl 0.49 ( Pmmm) and the tetragonal YBa 2Cu 3O 6.29F 1.17Cl 0.49 ( P4/ mmm). The chemical formula estimated by the Rietveld refinement was YBa 2Cu 3O 6.33F 0.80Cl 0.49 which was very close to the analyzed composition, YBa 2Cu 3O 6.35F 0.75Cl 0.49 in which chlorine was assumed to be zero-valence.

Y 2Ba 5(Sn 3− y− zCu yPt z)O x in textured YBCO superconductors

Ultrafine SnO 2 powder was made with grain size down to 20 nm. Such powder was introduced into the textured growth of Y 1+ x Ba 2Cu 3O 7+ δ (YBCO) superconductors. Crystalline inclusions, which were cubic in shape and from micrometer to submicrometer in size, were found trapped inside such grown YBCO superconductors. Determined by energy dispersive spectroscopy (EDS), and further confirmed by X-ray diffraction, the composition of the inclusions was Y 2Ba 5(Sn 3− y− z Cu y Pt z )O x . This Sn-based phase belongs to double perovskite structures. They were not trapped uniformly inside the YBa 2Cu 3O 7− δ (123), possibly caused by the agglomeration of the SnO 2 powder at the very beginning. Nevertheless, it is possible for fine Sn-based inclusions down to a few hundred nanometers to be trapped inside the 123. The addition of the ultrafine SnO 2 to YBCO textured superconductors without Pt did not seem to affect the size of Y 2BaCuO 5 (211) particles. For comparison, Y 2Sn 2O 7 was also introduced to YBCO textured superconductor. In this case, the so-called φ phase YBa 3Sn 2− x Cu x O 9− δ was found.

Paramagnetic Cu 2+ centers in oxycarbonate phase YBa 2Cu 2.95(CO 3) 0.35O 6.6 probed by EPR and magnetic susceptibility

Room temperature EPR and magnetic susceptibility studies are reported for the YBa 2Cu 2.95(CO 3) 0.35O 6.6 phase, an intermediate phase in the synthesis of YBa 2Cu 3O 7−δ by a sol-gel route. EPR measurements have revealed spectra of Cu 2+ ions in an orthorhombic local-crystal symmetry which are attributed to copper ions in a modified Cu(1) site. The presence of CO 2− 3 groups in the structure, surrounding the Cu(1) atoms, is at the origin of the EPR signal. The values of the g-tensor for this signal are as follows: g x = 2.045(5), g y = 2.117(5) and g z = 2.228(5). The d.c. magnetic susceptibility has shown a paramagnetic term superimposed on the diamagnetic contribution of the superconducting regions. The temperature dependence of the paramagnetic response is described by a perfect Curie law plus a constant term. The molar Curie constant is 1.1 × 10 −2 cm 3 K/mol Cu. The free spin concentration calculated from this Curie constant agrees well with the intensity of the EPR signal, suggesting it emanates from isolated Cu 2+ paramagnetic centers.

Phase stability of superconductive Y 1Ba 2Cu 4O 8

The stability of the 124 superconductive phase YBa 2Cu 4O 8 upon exposure to air and saturated humidity at ambient temperature has been studied by thermogravimetry, X-ray diffraction, and ac susceptometry. Extent of phase conversion was monitored by TG and confirmed by XRD and ac susceptometry. 124 samples upon prolonged exposure to air were found to be no longer phase-pure, with partial conversion to 123 and CuO. On oxygen annealing, reconversion of 123 + GuO to 124 was observed. However, upon prolonged exposure to saturated humid conditions, phase-pure 124 dissociated irreversibly into 211, GuO, and a highly disordered 124-like structure with planar defects along many hkl indices and was found to be nonsuperconducting even up to 60 K.

Seebeck effect in the mixed state of the 60-K phase of single-crystalYBa2Cu3Oy

The resistivity and the Seebeck effect in the mixed state were investigated for the 60-K phase YBa{sub 2}Cu{sub 3}O{sub y} single crystal. The measured Seebeck coefficient clearly shows an additional contribution at low temperatures, which may be evidence that there are two components (quasiparticles and vortices) carrying entropy in the mixed state. The ratio of the Seebeck coefficient to resistivity shows a remarkable field dependence, from which the lifetime enhancement of the quasiparticle below T{sub c} was derived. The enhancement is found to be less rapid in the 60-K phase than in the 90-K phase, which might come from the opening of the pseudogap above T{sub c}. {copyright} {ital 1997} {ital The American Physical Society}

Microwave-modulated nonresonant microwave absorption in YBa 2Cu 3O x high- Tc superconductors

Microwave-modulated non-resonant microwave absorption (MM-NRMA) were measured for a variety of YBa 2Cu 3O x superconducting samples such as a standard sintered YBa 2Cu 3O x sample, a sintered YBa 2Cu 3O x sample decomposed from YBa 2Cu 4O x phase, a quenched and melt growth YBa 2Cu 3O x sample with Y 2Ba 1Cu 1O 5 inclusions and an YBa 2Cu 3O x film sample prepared by pulsed laser ablation. The absorption spectra derived from the MM-NRMA were compared with the derivative of the absorption spectra derived from the standard field modulated NRMA (FM-NRMA). The experiment revealed that the MM-RNMA technique makes the interpretation for understanding the NRMA mechanism simpler, avoiding the ambiguity of the FM-NRMA, especially for the samples with strong pinning centers. The quenched and melt growth YBa 2Cu 3O x sample with strong pinning centers shows a large hysteresis in the MM-NRMA signal. The MM-NRMA signal for the sample was found to show an intense irregular signal caused by the magnetic field application.

A cubic phase in ceramic YBaCuO doped with Nb

The almost-single-phase ceramic c-YBa 2Cu 3− x (Nb x )O y ( x = 0.05−0.20) with cubic symmetry has been synthesized by thermolysis of an ammonium nitrate melt containing Y, Ba, Cu and Nb components. XRD patterns, thermogravimetric curves, far-infrared reflection spectra, and the temperature dependence of resistivity have been studied in the samples c-YBa 2Cu 2.86(Nb 0.14)O y and YBa 4Cu 2.86(Nb 0.14)O y prepared by this method. On this basis, the conclusion is made that c-YBa 2Cu 3− x (Nb x )O y is the well known cubic phase YBa 4Cu 3O y containing about 50% Ba vacancies and with the same lattice parameter a = 8.11 A ̊ .

Effects of solution parameters on the deposition of YBa 2Cu 3O x phase prepared by aerosol feed method in a cold plasma reactor

Films of the superconducting phase YBa 2Cu 3O x (YBCO) were deposited by CVD using an aerosol feed technique in a cold plasma reactor. Aerosol droplets containing the three precursors were generated by an ultrasonic nebulizer operating at 1.63 MHz, while a 50 kW RF generator, operating at 2.87 MHz, was used to simultaneously create the plasma and heat a stainless steel susceptor. The YBCO films were deposited on single crystal MgO substrates at an oxygen partial pressure of 1.3 kPa, at a substrate temperature of 850 °C, using a deposition period of 30 min. Four different solution concentrations were used to study the effect of concentration on the microstructure of the films. The dried particle size was determined to have a significant effect on the microstructure, composition and deposition rate of the films.

Influence of Cu site impurities on the thermal conductivity of YBa 2Cu 3O 7 − δ

The effect of Zn substitution at in-plane Cu sites in 90 K and 60 K phase YBa 2(Cu 1 − x Zn x ) 3O 7 − δ sintered materials (0 < x < 0.04) was investigated from the viewpoint of thermal conduction. The characteristic enhancement of the thermal conductivity κ just below T c in pure YBCO was rapidly suppressed in both phases by the Zn substitution. Based on the phonon heat conduction model, the disappearance of the κ enhancement is attributed to the depressed phonon-electron scattering caused by the Zn substitution.

A diffuse phase transition in superconducting YBa 2(Cu 1 − xMn x) 3O 7 − δ (MZn, Ni, Cr) compounds

Dielectric and magnetic behaviour of the YBa 2(Cu 1 − x M x ) 3O 7 − δ (MZn, Ni, Cr and x ⩽ 0.10) system was investigated. The results suggest a diffuse phase transition to a relaxor-type ferroelectric state. Taking into account the double-well model of high-temperature superconductivity and using double-well parameters from the Abrahams-Kurtz-Jamieson (AKJ) relation for ferroelectric phase transition in oxide perovskite, the unrenormalizable barrier heights are calculated. The values could be related to an excitonic mechanism of superconductivity. Small anomalies observed in the static dielectric constants coincide with the Néel points of the corresponding nonsuperconductive phases in YBa 2(Cu 1 − x M x ) 3O 7 − δ compounds.

The reaction between chlorine trifluoride and oxygen-deficient YBa 2Cu 3O 7−δ

On annealing in chlorine trifluoride gas (ClF 3), the Iow- T c phase YBa 2Cu 3O x ( x ~ 6.6, T c ~ 60 K) is readily converted into a high- T c phase exhibiting 90 K superconductivity. YBa 2Cu 3O 6.60 ( T onset = 63 K) began to react with ClF 3 at 423 K. The initial reaction rate was proportional to the ClF 3 pressure and the activation energy was calculated to be 17.1 kJ mol −1. The reaction process was represented by the rate equation, [1 − (1 − x)1/2] =Kt, derived from Jander's model for a gas/solid reaction, the reaction proceeding preferentially in a direction parallel to the CuO 2 network in the YBa 2Cu 3O x structure. Optimum annealing conditions required to obtain the high- T c single-phase structure were ClF 3 pressure = 2.7 kPa, temperature = 513 K, reaction time = 60 min. Under these conditions the crystal structure changed from pseudo-orthorhombic YBa 2Cu 3O 6.60 to the orthorhombic single phase of chemical composition YBa 2Cu 3O 6.17F 0.78Cl 0.52. The T onset value of this product was 94.5 K.

EFFECT OF THE SECOND PHASE ON CRITICAL CURRENT DENSITY IN Nb-ADDED YBCO FILMS

Thin films of YBCO with or without Nb added were prepared on (100) ZrO 2 single crystalline substrate by dc magnetron sputtering method. A phase of YBa 2 NbO 6 was found to form not only in the target materials of YBCO but also in the sputtered thin films. The second phase of YBa 2 NbO 6 in the superconducting films destroyed a match relation between the lattice of YBCO and the lattice of ZrO 2 substrate. The measurements of critical current density J c under magnetic field (H // c axis, up to 5T) showed that the J c in Nb-added YBCO films reduced more apparently than that in YBCO films. It is concluded that the existence of second phase of YBa 2 NbO 6 in superconducting phase does not play a rule of flux pinning.

Phonon thermal diffusivity and conductivity of oxygen deficient YBa 2Cu 3O 7−X

The thermal diffusivity α(T) and conductivity κ(T) were measured between 10 and 200K quasi-simultaneously on 60K phase and non-superconductive YBa 2Cu 3O 7−x(YBCO) sintered samples. The temperature dependence of the phonon mean free path l ph and the phonon scattering time τ ph was determined. Analyses based on the Tewordt-Wölkhausen theory gave the electron-phonon coupling parameter λ=0.65 for the 60K phase in contrast to the corresponding value λ=0.98 for the 90K phase.

Transport properties of Sb 2O 3 doped laser ablated YBa 2Cu 3O 7−δ

The effect of Sb 2O 3 addition on YBa 2Cu 3O 7−δ films grown on 〈100〉 LaAlO 3 by pulsed laser ablation has been studied. Transport measurement showed that though the critical temperature T c is slightly suppressed upon Sb 2O 3 doping, the critical current density J c increases up to 2.5 × 10 6 A cm −2 at 77 K for the critical concentration of 0.5 wt.% Sb 2O 3 and then decreases monotonically for further increase in Sb 2O 3 content. The increase in J c with 0.5 wt.% Sb 2O 3 addition is presumed to be due to the surfactant effect of Sb 2O 3 and the decrease in J c with the further increase of Sb 2O 3 content has been attributed to the formation of secondary insulating phase YBa 2SbO 6 as found by X-ray diffraction.