Publications (40)49.41 Total impact
- [Show abstract] [Hide abstract] ABSTRACT: The antiferromagnetic structure of the high-temperature superconductor HoBa2Cu4O8 with Tc=77 K was determined by means of neutron diffraction at T=9 mK. Imperfect three-dimensional antiferromagnetic ordering of the Ho3+ ions corresponding to propagation vector K=(0, 1/2 , 0) is found below TN=100 mK with magnetic moments of magnitude 1.5 mu B at saturation and oriented parallel to the a-axis. The antiferromagnetic sublattice is characterized by stacking faults corresponding to a finite correlation length xi c=18 AA along the c-axis. The majority of the 17 crystal-field level within the ground-state J-multiplet of the Ho3+ ions is established by inelastic neutron scattering, which allowed an unambiguous determination of the nine assignable crystal-field parameters. The observed temperature dependence of the order parameter is found to be in excellent agreement with mean-field calculations including crystal-field, exchange and hyperfine interactions.
- [Show abstract] [Hide abstract] ABSTRACT: Slope changes of the lattice constants and dc magnetization measurements as a function of the oxygen content of powder 123 samples, indicate the existence of three phases with phase boundaries at x ≅ 6.75 and x ≅ 6.91. In addition, a splitting of the derivative of the magnetization curve appears at x ≥ 6.95. These results are discussed in terms of a three-phase model. It is shown that oxygen inhomogeneities of the grains cannot be the reason for the double peaks in specific heat and magnetization.
- [Show abstract] [Hide abstract] ABSTRACT: The temperature dependence of the crystal structure and in particular the lattice contraction of a ceramic powder sample of 44Ca substituted YBa2Cu4O8 with Tc = 87 K was investigated in the temperature range of 23 to 300 K, using the high-resolution reverse Fourier time-of-flight (RFTOF) diffractometer `Mini-Sfinks'. For the substitution the 44Ca isotope has been used for better neutron scattering contrast to Y and Ba. In the investigated temperature range the Y0.9Ca0.1Ba2Cu4O8 system has been confirmed to crystallize with orthorhombic structure according to the space group Ammm. Ca atoms are verified to occupy Y positions. Similar to previous observations of a phase transition of Ca doped YBa2Cu4O8 at 150 K, the lattice parameter b stays approximately constant at temperatures below 166 K, in contrast to YBa2Cu4O8. The orthorhombicity is found to decrease both with increasing temperature and due to the Ca substitution.
- [Show abstract] [Hide abstract] ABSTRACT: An overview is presented of the phase diagram, crystal growth and the structural and physical characterization studies of the double chain compounds YBa2Cu4O8 (124) and Y2Cu7O14+x (247). The stability regions of the YBa2Cu3O7-x (123), 247 and 124 phases (Po2≤ 3000 bar) depend on the metal ratio. Different compositional sections of the P-T-X phase diagram have been investigated. The thermodynamics of the oxygen exchange reaction of the orthorhombic 123 with gaseous oxygen has been studied in the pressure range 10-3–100 bar. Highly perfect ceramic 247 material with Tc of 93 has been obtained. X-ray single crystal structure analysis of 124 and 247 shows, that structural changes and incorporation of impurities occur only in the “123” unit, whereas the “124” unit remains stable. In the case of the 247 compound not only oxygen nonstoichiometry, but also misorientation of oxygen, copper nonstoichiometry together with the incorporation of impurities, like and Al, strongly affect Tc. Furthermore, the phase diagram for the crystal growth process at high Po2 is also presented. The 124 single crystals were grown in l(Tc=72 K) and in Y2O3 and ZrO2 crucibles (Tc=80 K). Doping with Ca gives rise to an increase of Tc (Tc=84 K). Melting of bulk 124 samples at Po2 = 1000–2700 bar results in textured ingots consisting of several mm large 124 grains with Jc=5x104A/cm2 at 5K.
- [Show abstract] [Hide abstract] ABSTRACT: By means of a quantitative analysis of extensive elastic neutron scattering experiments performed on theT c =78 K superconductor DyBa2Cu4O8 it is shown that two-dimensional (2D) antiferromagnetic Dy ordering in the (a,b)-plane remains stable in the whole temperature range fromT N =(1.10±0.02) K down to 7 mK. The magnetic difference pattern with good angular resolution fits well to the Warren equation for scattering on 2D systems with powder averaging. Consequently, the fitted sublattice magnetisation is found to have a 2D Ising character. Moreover, the Dy3+ crystal-field levels were determined by inelastic neutron scattering. From the resulting crystal-field parameters we calculate the sublattice magnetisation of Dy3+ at saturation to beμ Dy,CEF=6.0μ B in good agreement withμ Dy,obs=(5.9±0.5)μ B as measured by neutron diffraction.
- [Show abstract] [Hide abstract] ABSTRACT: The anisotropic response of an YBa2Cu4O8 single crystal to superconducting order is studied using an ultrahigh resolution capacitance dilatometer. A shortening of the crystal below Tc is observed along the a-axis, whereas little or no response is seen along the b- and c-axis lattice directions. The dependence of Tc on uniaxial pressure, dTc/dpi(i = a,b,c), is determined using the thermal expansion data and one of the Ehrenfest relationships. We find that dTc/dpa = +(3.7-5.0) K/GPa, dTc/dpb = +(0.3- 0.4) K/GPa, and dTc/dpc = (0+/-0.7) K/GPa, which shows that the large hydrostatic pressure dependence of Tc results mainly from the uniaxial effect along the a-axis.
- [Show abstract] [Hide abstract] ABSTRACT: To show that single-crystal x-ray-diffraction data can reveal the presence of two apical oxygen sites in YBa2Cu3O7-based samples, the structure of a copper-deficient 1:2:3 sample, YBa2Cu2.78O7, has been determined. Small crystals were grown under high oxygen pressure. Gandolfi and precession techniques showed that the symmetry is tetragonal with parameters: a=3.8764(4) Å and c=11.625(1) Å. High-order reflection scans, carried out during the single-crystal intensity-data collection, revealed that some of these reflections were broadened indicating that the crystal was twinned and consequently of lower symmetry than tetragonal. Except for the Cu vacancies the structure of YBa2Cu2.78O7 is similar to that of the 1:2:3 compound. All the vacancies were found to be on chain Cu(1) sites. This means that every either fourth or fifth Cu(1) site is empty and there exist two apical O(1) sites and two Cu(2)-O(1) distances. No long-range superstructure was detected either by x-ray diffraction or electron diffraction. The two O(1)-site feature is revealed qualitatively by the large thermal motion of O(1) along the c axis and quantitatively by electron density maps. The relationship between this structure and the double-well model deduced by EXAFS data for the apical oxygen is discussed.
- [Show abstract] [Hide abstract] ABSTRACT: An overview is presented of our phase diagram, crystal growth, structural, surface and physical characterization studies of double chain compounds YBa2Cu4O8 (124) and Y2Ba4Cu7O15−x (247). The stability regions of 123, 247 and 124 phases (up to 3000 bar O2) are appreciably different for different compositions. The thermodynamics of the oxygen exchange reaction of the orthorhombic 123 with gaseous oxygen has been investigated in the pressure range 10−3–100 bar. Thermodynamics and mechanism of crystal growth have been studied by STM, AFM, SEM, X-ray and ED AX. New results of flux growth phase diagram studies are also presented. Melting of bulk 124 samples under Po2 = 1000 − 2700 bar results in ingots with Jc=4×104A/cm2 at 5K. X—ray single crystal structure analysis of 247 shows, that structural changes and incorporation of impurities occur only in the single chain “123” unit, whereas the double chain “124” unit remains stable.
- [Show abstract] [Hide abstract] ABSTRACT: Low-temperature neutron powder diffraction investigations performed on YBa2Cu4O8 ceramic samples, with Tc increased to ~ 90 K by 44Ca and Ca substitution, show that the Ca is distributed on the Y sites rather than the Ba sites, according to the chemical formula Y1-xCaxBa2Cu4O8. Use of the isotope 44Ca (scattering length b=1.42 F) increased the contrast with Y (b=7.75 F) and Ba (b=5.07 F). Associated changes of the structure are discussed. In particular interatomic distances Cu(2)-O(1) and Cu(2)-O(2,3) appear to decrease with increasing Ca doping.
- [Show abstract] [Hide abstract] ABSTRACT: High resolution neutron diffraction studies were performed on carefully prepared powder samples of the high-T(c) superconductors REBa2Cu4O8 (RE = Dy, Er, Ho) ('R124') and on Nd2Ba4Cu7O15 ('Nd247') at temperatures down to 7 mK. The structure parameters of these compounds, which are isostructural to the R = Y systems (Ammm), were refined. In agreement with Zhang et al. two-dimensional (2D) antiferromagnetism exists in 'Dy124' below T(N) almost-equal-to 1.1 K. It appears to be stable down to T = 7 m K.
- [Show abstract] [Hide abstract] ABSTRACT: The thermodynamics of the oxygen exchange reaction of the orthorhombic YBa2Cu3O7-x phase with gaseous oxygen has been investigated in the pressure range 10-3-100 bar. Assuming that the oxygen exchange takes place at the O(4) and O(5) sites of the structure, thermodynamic functions of this reaction have been calculated as a function of the oxygen pressure. The thermodynamic stability range of the orthorhombic ``123'' is also discussed.
- [Show abstract] [Hide abstract] ABSTRACT: The application of high oxygen pressure modifies the thermodynamic equilibrium in the YBaCuO system. In general the influence of the gas pressure on the thermodynamic equilibrium is twofold. For the investigations under high pressure we have developed a two chamber autoclave, the internal chamber being a ceramic crucible. Using this autoclave for the annealing and melting of YBa2Cu3O7 samples, we have observed the decomposition and formation of other phases like Y2Ba4Cu7O15 and YBa2Cu4O8. They differ from 123 in having double instead of single CuO chains. This paper discusses further investigations on crystal growth and phase diagrams at 1<PO2<3000 bar.
- [Show abstract] [Hide abstract] ABSTRACT: A review of the phase diagram studies of the Y -Ba-Cu-O system under high oxygen pressure, is given. The structure and properties of the three superconducting phases are discussed together with some new defect structures.
- [Show abstract] [Hide abstract] ABSTRACT: A review is given of the up to now experimentally determined structural and physical properties of the double chain compounds YBa2Cu4O8, in pure (Tc=806K) and Ca-substituted form (Tc=990 K), as well as Y2Ba4Cu7O15+/-x (Tc=19-14 K). These compounds belong together with 123 to the Y2Ba4Cu6+nO14+n family of a HBC superconductors. The structure of the first compound (124) is very similar to that of 123 but contains only double chains. These are thermodynamically stable up to 850°C and stoichiometric, therefore, both O÷T phase transition and a-b twinning are supressed. The structure of the second compound (123.5 or 247) is also derived from that of 123, but it contains alternating single and double chains which make the properties more complex than those of 124. All three up to now known HTc-compounds of the Y-Ba-Cu-O system are classical examples of structure sensitive superconductors, so that in depth structural characterization is necessary for the understanding of the physical properties. Most sensitiv structural feature is the apex of the pyramid (Cu2-O1). Structural investigation show that its contraction can be correlated with the increase of Tc. Some properties of these compounds are not yet completely understood. Two examples are discussed here : a) The increase of Tc of 123.5 up to 94K, and b) the site of incorporation of Ca, in Ca-124, whose increase of Tc does not seem to be due to the substitution of Y by Ca, but of Ba by Ca. Evidence from various methods is presented. Last but not least, evidence is presented in this review, indicating that also other Y-Ba-Cu-O compounds, some with much higher Tc, may exist.
- [Show abstract] [Hide abstract] ABSTRACT: We report on electron energy-loss studies of Ca-doped YBa2Cu4O8. O 1s and Ca 2p absorption edges indicate that there is almost no change of the hole concentration upon Ca doping and that a major fraction of the Ca enters the Ba site. This suggests that the increase of Tc upon Ca doping is not caused by a formation of additional holes as expected for the replacement of trivalent Y by divalent Ca.
- [Show abstract] [Hide abstract] ABSTRACT: Stoichiometry-induced deviations of the perfect stacking order in Y-Ba-Cu-O 247 samples are investigated. Several new phases are observed which can be described as polytypoids or as shear structures of the 123 structure, and which differ from the known compounds by the number of 123 cells per unit cell, or by the number of CuO layers in an interleaving CuO multilayer.
- [Show abstract] [Hide abstract] ABSTRACT: dTc/dP of YBa2Cu4O8 single crystals has been measured up to 9 GPa under hydrostatic pressure. The large and positive initial value (dTc/dP)0=5.5 K/Gpa is confirmed. Tc(P) saturates above 5 GPa. Dramatic changes, in the Tc(P) behavior are observed when the pressure is non-hydrostatic. They are caused by the twinning of the crystals. Opposite to the case of YBa2Cu3O7, the results for hydrostatic pressure are well reproducible. This is probably due to the well defined oxygen stoichiometry of YBa2Cu4O8 single crystals.
- [Show abstract] [Hide abstract] ABSTRACT: A review of the phase diagram studies of the YBaCuO and GaN systems under high active gas (O2 or N2) pressure is given. The P, T equilibrium conditions for three superconducting phases existing in YBaCuO system vary as a function of Cu content. Application of high O2 pressure allows crystal growth of the 124 and 247 compounds and high N2 pressure allows crystal growth of GaN. We discuss also indications, that other YBaCuO compounds may exist.
- [Show abstract] [Hide abstract] ABSTRACT: Resistive measurements in a cryogenic diamond anvil cell show that the critical temperature Tc of YCa0.2Ba1.8Cu4O8 can be increased from 90 K to 99 K by applying a pressure of 8.8 GPa. At higher pressures Tc decreases. An inverted parabola is well fitted through the data. This behavior can be explained by assuming that the number of holes in the CuO2-planes increases linearly upon applying pressure and that Tc as a function of the number of holes nh in the CuO2-planes follows the (parabolic-like) Tc(nh) phase diagram line.
- [Show abstract] [Hide abstract] ABSTRACT: For the understanding of the Tc enhancement in Ca doped YBa2Cu4O8+/-y it is crucial to know which crystallographic site the Ca+2 ion is occupying. We investigated this problem by measuring the Cu nuclear quadrupole resonance (NQR) spectrum at 100 K in YBa2Cu4O8+/-y doped with 10% Ca (Tc = 91 K). The NQR lines of the plane and chain Cu sites appear at the same frequencies vq as in the pure compound, however, both lines are broadend by static defects. Within the linewidths no temperature dependence of vq was observed and no indication for magnetic ordering at the Cu sites was found. At 20 600 Mhz, near to the Cu(1) line, a resonance line has been detected which is absent in the pure YBa2-xCaxCu4O8+/-y spectrum. Based on NQR arguments we assign this new line to Cu(1) nuclei whose electric field gradients are disturbed by Ca+2 ions occupying Ba+2 sites. We concluded that a major fraction of Ca ions occupy Ba sites, therefore the increase of Tc by Ca doping cannot be explained by an increase of the hole concentration.
Zürich, Zurich, Switzerland
- Laboratory for Solid State Physics
University of Antwerp
Antwerpen, Flemish, Belgium
- Department of Physics