ortho-II Phase Research Articles

Formation of metastable phases in Zr-ion-irradiated Al2O3 upon thermal annealing.

Formation of metastable phases in Zr-ion-irradiated corundum alumina (Al2O3) upon thermal annealing was examined using transmission electron microscopy. A metastable cubic spinel phase was formed in the topmost layer of the as-irradiated microstructure. During thermal annealing at temperatures ranging from 1073 to 1273 K, this spinel layer grew in extent via an unusual corundum-to-spinel phase transformation. A normal spinel-to-corundum phase transformation was observed at post-irradiation annealing temperatures greater than 1473 K. In addition, ZrO2 nanocrystals embedded in α-Al2O3 were observed to form at these higher temperatures. High-resolution transmission electron microscopy observations and electron diffraction experiments revealed that the structure of the ZrO2 precipitates observed in this study are consistent with a high-pressure metastable orthorhombic phase of ZrO2 known as the Ortho-I phase.

Re-examination of high pressure orthorhombic-I phase of ZrO2 by Raman spectroscopy

Currently there is ambiguity about the space group of the first high pressure phase of ZrO2. Early Raman spectroscopic measurements on a single crystal in a diamond anvil cell pointed to a structural transformation at 3.5 GPa, and later in-situ X-ray diffraction studies concluded that this structure is orthorhombic with space group Pbcm. From a subsequent analysis of neutron diffraction patterns of ortho-I phase quenched from 6 GPa and 600 °C it was concluded that the space group of this phase is Pbca and not Pbcm. We have re-investigated this using Raman spectroscopy and factor group analysis, and show that the space group indeed is Pbcm and not Pbca.

Cation off-stoichiometric effect on superconducting GdBa2Cu3O7-δ thin films investigated by Raman spectroscopy

The superconducting properties of cation off-stoichiometric GdB2Cu3O7-δ (Gd123) thin film grown by pulsed laser deposition method show a strong dependence on target compositions and particularly get worse in case of a ratio Ba/Gd > 2. We investigated the effect of cation off-stoichiometric target sample on lattice properties of Gd123 thin films using Raman spectroscopy. In addition to five Ag modes related to ortho-I phase, some additional peaks are seen in the films of Ba/Gd > 2. The result implies that the superconducting properties of the films are strongly influenced by the cation off-stoichiometric effect and in connection with the existence of additional peaks.

Oxygen chain disorder as the weak scattering source inYBa2Cu3O6.50

The microwave conductivity of an ultra-pure single crystal of YBa$_2$Cu$_3$O$_{6.50}$ has been measured deep in the superconducting state as a continuous function of frequency from $0.5\to20$ GHz. Conductivity spectra were first measured at four temperatures below 10 K after having prepared the crystal in the so called ortho-II phase in which the CuO chain oxygen are ordered into alternating full and empty chains. These spectra exhibit features expected for quasiparticle scattering from dilute weak impurities (small scattering phaseshift) in an otherwise clean d-wave superconductor. The measurements were repeated on the same crystal after heating and then rapidly quenching the sample to reduce the degree of oxygen order in the CuO chains. With the increased disorder, the conductivity spectra retain the distinctive weak-limit scattering features, but have increased widths reflecting an increase in quasiparticle scattering. These measurements unambiguously establish that CuO chain oxygen disorder is the dominant source of in-plane quasiparticle scattering in high purity YBCO.

Raman study of twin-free ortho-IIYBa2Cu3O6.5single crystals

The polarized Raman scattering spectra from freshly cleaved ab, ac, and bc surfaces of high quality twin-free YBa2Cu3O6.5 ortho-II single crystals Tc=57.5 K and T=0.6 K were studied between 80 and 300 K. All 11 Ag Raman modes expected for the ortho-II structure as well as some modes of B2g and B3g symmetries were identified in close comparison to predictions of lattice dynamical calculations. The electronic scattering from the ab planes is strongly anisotropic and decreases between 200 and 100 K within the temperature range of a previously reported pseudogap opening. The coupling of phonons to Raman active electronic excitations manifested by asymmetric Fano profiles of several modes also decreases within the same range. Among the findings that distinguish the Raman scattering of the ortho-II phase from that of the ortho-I phase is the unusual relationship xx − yy between the elements of the Raman tensor of the apex oxygen Ag mode.

The origin of the 60 K plateau inYBa2Cu3O6+x

A model for the charge transfer mechanism inYBa2Cu3O6+x high-Tc cuprate based on the critical chain length concept is proposed to account for the 60 and 90 K plateaus inthe Tc(x) dependence. It has been shown that, when the statistics of CuO chain formationwas described in terms of the two-dimensional asymmetric next-to-nearestneighbor Ising (ASYNNNI) model, at any constant temperature below the topof ortho-II phase there exists a uniquely defined value of critical chain lengthlcr(T) that yields aconstant doping p(x)≈const over the regime of ortho-II phase (related to the 60 K plateau ofTc(x)), while the 90 K plateau coincides with the monotonically increasingp(x) over the optimaldoping level p = 0.16 in the regime of the ortho-I phase. Short length chains(l<lcr(T)), together withthe first lcr(T)−2 holesin longer chains (l≥lcr(T)), are taken as not capable of attracting electrons fromCuO2 planes. It is shownthat only a part (≈41%) of the remaining l−lcr(T)+1 holes in the long chains can capture electrons. The results obtainedindicate that the ASYNNNI model and two-plateau-like behavior ofTc(x) inYBa2Cu3O6+x are closely connected.

Oxygen chain disorder as a weak-limit scatterer in YBa 2Cu 3O 6.50

The real part of the microwave conductivity of an ultra-pure sample of YBa 2Cu 3O 6.50 deep in the superconducting state has been measured as a continuous function of frequency. Conductivity spectra were first measured with the Cu–O chain oxygens atoms highly ordered in the ortho-II phase (alternating full and empty chains). The measurements were then repeated after heating and quenching the sample to reduce the degree of oxygen order in the Cu–O chains which lowered T c from 55 K to 49 K. Disordering the Cu–O chains produced conductivity spectra that were more than twice as broad as those of the ortho-II ordered phase. In both cases the width of the conductivity spectra increases approximately linearly with temperature. Moreover, within our experimental resolution we find that the low-temperature quasiparticle spectral weight does not depend on the degree of oxygen orderer in the Cu–O chains. Each of these features indicates that oxygen disorder in the Cu–O chains acts as a source of weak-limit scattering for quasiparticles propagating in the CuO 2 planes.

Optical conductivity of ortho-IIYBa2Cu3O6.5

The ortho-II phase of $\mathrm{Y}{\mathrm{Ba}}_{2}{\mathrm{Cu}}_{3}{\mathrm{O}}_{6.5}$ is characterized by a periodic alternation of empty Cu and filled $\mathrm{Cu}\ensuremath{-}\mathrm{O}$ $b$-axis chains doubling the unit cell in the $a$ direction. The extra oxygen in the full chains gives rise to an attractive potential for the holes in the planes. The planar bands split in two with a gap opening at the new Brillouin zone boundary ${k}_{x}=\ifmmode\pm\else\textpm\fi{}\ensuremath{\pi}∕2$, which we estimate from local-density approximation calculations. Using a planar model which treats the $d$-wave superconductivity in a mean field approximation, we show that interband transitions produce a strongly anisotropic feature in the optical conductivity controlled by a region in $\stackrel{P\vec}{k}$-space close to $(\ensuremath{\pi}∕2,\ensuremath{\pi}∕2)$. The edge position of this feature gives information on the temperature dependence of quasiparticle spectrum in this region. Bilayer splitting would show up as a double edge shape.

The effect of oxygen plasma generated by different microwave modes on NdBa 2Cu 3O 6+ δ superconductor: structure and IR studies

The effect of longitudinal and transverse modes of oxygen plasma generated by microwave source with frequency 2.45 GHz has been used to characterize polycrystalline samples of NdBa 2Cu 3O 6+ δ (Nd-123) high T c-superconductor. For this purpose we have developed a simple oxidation technique, used by us earlier, to investigate the effect of different modes on the appearance of superconductivity in Nd-123. Our main results show that the mode of microwave plasma and time of exposure can control the local arrangement of oxygen vacancies in CuO δ basal plane. Samples exposed to the transverse mode reveals a well ordered orthorhombic phase transition while the longitudinal modes caused the samples to have a non-superconducting tetragonal structure for all of the exposure time. The T c vs. δ curve does not show the typical 60 K plateau corresponding to the ortho-II phase. The IR spectrum show bands at shorter wavelengths which corresponding to the metal oxides.

Preparation and characterization of homogeneous YBCO single crystals with doping level near the SC-AFM boundary

High-purity and homogeneous YBa 2Cu 3O y single crystals with carrier doping level near the AFM-SC boundary have been obtained in the oxygen content range between y=6.340 and 6.370. The crystals are ortho-II phase at room temperature and undergo the orthorhombic to tetragonal transition at about 140 °C. They show sharp superconducting transitions, with T c between 4 and 20 K. T c changes by 0.8 K when the oxygen content y is changed by 0.001, and is also sensitive to annealing conditions near room temperature, due to the dependence of doping on oxygen ordering correlation lengths. Crystals with oxygen content y lower than 6.345 are non-superconducting.

Correlation between microstructure, DC resistivity and magnetoresistance of SrRuO3 films.

ABSTRACTWe have investigated the correlation between microstructure, DC resistivity and magnetoresistance of SrRuO3 thin films. The films were epitaxially grown by pulsed laser deposition on (001) SrTiO3 substrates in a temperature range of 690–810°C. According to x-ray measurements, the structure of all films is a mixture of highly oriented domains of strained orthorhombic phases (ortho-I and ortho-II) with different lattice parameters. Films deposited at 780°C show a minimum resistivity (270 μΩcm at 300 K) and a maximum magnetoresistance (8% at 5 K). These films consist mainly of ortho-I phase (a=0.393 nm). Films deposited at 690°C (predominantly ortho-II) have the highest resistivity (up to 1700 μΩcm at 300 K) and lowest magnetoresistance (3% at 5K).

Study of the chain oxygen order in underdoped YBa 2Cu 3O 6.44 by means of NQR investigation of Cu(1)

The Cu(1) NQR spectrum of the YBCO x superconductor is studied in a carefully annealed sample with x=6.44 (T c≈ 40K) belonging to the Ortho-II phase. The assignment of the peaks observed in the low frequency part of this spectrum (18–24.5 MHz) is attempted.

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.

Oxygen ordering and the 60 K plateau in the ortho-II phase of YBa 2Cu 3O 6+ x (0.48≤ x≤0.62)

A simple method for sample preparation based on copper oxide Cu 2O acting as an oxygen getter is presented, which results in very well-ordered ortho-II phases in both polycrystalline and single-crystal samples of the high T c superconducting compound YBa 2Cu 3O 6+ x (0.48≤ x≤0.62). It comes out from our measurements of the superstructure correlation lengths and the critical temperature in samples with various degrees of oxygen ordering that the 60 K plateau behaviour of the T c vs. x dependence is clearly related to the degree and quality of short-range oxygen ordering into the ortho-II phase. The best sample exhibits correlation lengths up to 70(2) unit cells along the a-axis and 5(1) unit cells along the c-axis, to be compared to ≈10 and ≈2 unit cells, respectively, for the best ones published to date. The superstructure peaks are resolution-limited in the k direction.

Rare-earth ionic radius dependence of superconductivity in Pr-substituted and oxygen deficient R 1− xPr xBa 2Cu 3O 7− y

The suppression effect of superconductivity by Pr substitution correlates strongly with that by oxygen deficiency in the R 1− x Pr x Ba 2Cu 3O 7− y system, where R is Yb, Y, Ho, Dy, Gd and Nd. The larger the value of x C, the Pr content at which superconductivity disappears, the larger the value of y C, the oxygen deficiency at which superconductivity disappears. The decreasing rate in T C by Pr substitution (−d T C/d x) decreases with decreasing ionic radius of a rare-earth ion for the Ortho-I phase ( T C>90 K for x=0) and also with decreasing oxygen concentration (7− y). The −d T C/d x for the smaller rare-earth ions (R=Yb, Y, Ho and Dy) are nearly independent of ionic radius for the Ortho-II phase ( T C=60 K for x=0). The Pr content dependence of T C in (Y 0.625La 0.375) 1− x Pr x Ba 2Cu 3O 7− y , average rare-earth ionic radius of which is equal to that of Gd, coincides well with that in the Gd 1− x Pr x Ba 2Cu 3O 7− y . The weakened T C-suppression by Pr in the case of the smaller rare-earth ions is explained by considering the hole transfer effect from the Cu(1)O chain toward the Cu(2)O 2 plane according to the difference in ionic radius between the rare-earth ions and Pr ion.

Raman line shapes from sputtered thin films of Y(Pr)Ba2Cu3O6+δ: Fine structures and oxygen ordering

A temperature dependence study of Raman line shapes taken from superconducting and insulating Y(Pr)${\mathrm{Ba}}_{2}$${\mathrm{Cu}}_{3}$${\mathrm{O}}_{6+\mathrm{\ensuremath{\delta}}}$ thin films with well-characterized electrophysical properties and morphologies (a-axis or c-axis orientation) has been carried out using the excitation energies at 1.83 eV and 2.41 eV. Ba and Cu bands show doublets, with 113\char21{}119 ${\mathrm{cm}}^{\mathrm{\ensuremath{-}}1}$ and 146\char21{}152 ${\mathrm{cm}}^{\mathrm{\ensuremath{-}}1}$ components and have been calculated using a Green's function model. The shapes of the 113 and 152 ${\mathrm{cm}}^{\mathrm{\ensuremath{-}}1}$ lines are well described by Lorentzians whereas those of the other two components can be satisfactorily reproduced in the assumption that two phonons of bare frequencies 119 and 146 ${\mathrm{cm}}^{\mathrm{\ensuremath{-}}1}$ interact with a common continuum of excitations. The temperature dependence of the phonon-continuum coupling parameters allows us to ascribe the scattering continuum to low-energy electronic interband transitions. The model is able to explain qualitatively the overestimated mixing between pure Ba and plane Cu ${\mathrm{A}}_{\mathrm{g}}$ vibrational modes found in previous first-principles calculations. We also find that for the 1.83 eV exciting line, the ${\mathrm{B}}_{1\mathrm{g}}$ oxygen phonon at 335 ${\mathrm{cm}}^{\mathrm{\ensuremath{-}}1}$ presents a strong Fano effect and anomalous temperature dependence. We link Raman features occuring at \ensuremath{\approx}220\char21{}240, 270\char21{}290, and 560\char21{}596 ${\mathrm{cm}}^{\mathrm{\ensuremath{-}}1}$ to the existence of local variations in oxygen composition resulting from the conditions of deposition. These lines could originate from chain defects and be activated by a coupling between ${\mathrm{A}}_{\mathrm{g}}$ and ${\mathrm{B}}_{2\mathrm{g}}$-${\mathrm{B}}_{3\mathrm{g}}$ modes. The presence of microphases of oxygen-depleted domains (ortho-II phase) reproducing macroscopic oxygen disorder is proposed.

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The suppression effect of superconductivity by Pr substitution correlates strongly with that by oxygen deficiency in the R1−xPrxBa2Cu3O7−y system, where R is Yb, Y, Ho, Dy, Gd and Nd. The larger the value of xC, the Pr content at which superconductivity disappears, the larger the value of yC, the oxygen deficiency at which superconductivity disappears. The decreasing rate in TC by Pr substitution (−dTC/dx) decreases with decreasing ionic radius of a rare-earth ion for the Ortho-I phase (TC>90 K for x=0) and also with decreasing oxygen concentration (7−y). The −dTC/dx for the smaller rare-earth ions (R=Yb, Y, Ho and Dy) are nearly independent of ionic radius for the Ortho-II phase (TC=60 K for x=0). The Pr content dependence of TC in (Y0.625La0.375)1−xPrxBa2Cu3O7−y, average rare-earth ionic radius of which is equal to that of Gd, coincides well with that in the Gd1−xPrxBa2Cu3O7−y. The weakened TC-suppression by Pr in the case of the smaller rare-earth ions is explained by considering the hole transfer effect from the Cu(1)O chain toward the Cu(2)O2 plane according to the difference in ionic radius between the rare-earth ions and Pr ion.

Raman study of the oxygen anharmonicity in YBa2Cu3Ox (6.4

A systematic Raman study in the visible and the near IR has been carried out on $\mathrm{Y}{\mathrm{Ba}}_{2}{\mathrm{Cu}}_{3}{\mathrm{O}}_{x}$ samples, with isotopic substitution of $^{18}\mathrm{O}$ for $^{16}\mathrm{O}$, and for oxygen concentrations in the range $x=6.44\ensuremath{-}6.98$. A small but clear deviation from harmonic behavior has been detected for the frequency shift of the in-phase mode and, to a less extent, of the apex mode, relative to the out-of-phase phonon. The deviation increases towards low concentrations of oxygen. A disagreement of the present work for the apical mode, with a previous investigation using IR excitation, is attributed to the contribution of several phases to the apex mode in the underdoped region and the possible IR resonance of the ortho-II phase.

Mean-field and Monte Carlo calculations of the three-dimensional structure factor forYBa2Cu3O6+x

We develop a general mean-field matrix theory for calculating phase boundaries and structure factors for a class of lattice gas Hamiltonians. We investigate the oxygen ordering in YBa{sub 2}Cu{sub 3}O{sub 6+{ital x}} by applying the theory and by Monte Carlo simulation using an extension to three dimensions of the two-dimensional anisotropic next nearest neighbor lattice gas model. The calculation of the structure factor in three spatial dimensions on a 256{times}256{times}16 system has been implemented on a massively parallel computer, the Connection Machine CM2. This allows for an extremely accurate determination of the line shape of the structure factor in the entire reciprocal space in excellent agreement with the mean-field prediction. We report on the results for an oxygen stoichiometry of {ital x}=0.4, {ital x}=0.5, and {ital x}=0.6 for temperatures between {ital T}=450 and 800 K. Using a sum-rule argument we have calculated the fluctuation-renormalized transition temperature into the ortho-II phase, {ital T}{sub {ital c}}, in good agreement with Monte Carlo simulation results. This allows an analytic description of the dependence of {ital T}{sub {ital c}} on the interaction {ital V}{sub 4} in the third dimension and on the oxygen stoichiometry {ital x}. An explanation for the experimental observation of themore » lack of a Bragg peak in the ordered ortho-II phase of YBa{sub 2}Cu{sub 3}O{sub 6+{ital x}} is offered in terms of scattering from platelike antiphase oxygen-ordered domains. {copyright} {ital 1996 The American Physical Society.}« less

Doping dependence of the oxygen isotope effect in YBa 2Cu 3O x

Measurements of the total oxygen isotope shift α 0 and the condensate density of n of YBa 2Cu 3O x (6.53 < x < 6.99) are presented. The T c( n) dependence exhibits a two-plateau behavior which is associated with the ortho-I and ortho-II phase, respectively. Around optimum doping ( x ≅6.9) and at the onset of the 60 K plateau where T c only weakly depends on n the small α 0 ≅ 0.025 is associated to changes in the phonon spectrum. A further decrease of T c leads to a rapid increase of α 0 ∝ ∂T c/∂ n suggesting that α 0 is related to a change of the condensate density n upon isotope exchange.