E. Jilek

ETH Zurich, Zürich, ZH, Switzerland

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Publications (35)23.01 Total impact

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    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.
    Journal of Physics Condensed Matter 12/1998; 6(22):4147. · 2.22 Impact Factor
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    ABSTRACT: A broadening of the magnetisation transition and lattice constant anomalies in the nearly oxygen stoichiometric YBa2Cu3O7-x are observed. These anomalies together with others (resistivity, cp, muon depolarization rate) could be explained by the existence of two different phases in the overdoped range (x>6.90).
    Physica C Superconductivity 12/1994; 235:425-426. · 0.72 Impact Factor
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    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.
    Physica C Superconductivity 01/1994; 227:285-290. · 0.72 Impact Factor
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    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.
    Physical review. B, Condensed matter 03/1993; 47(6). · 3.66 Impact Factor
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    ABSTRACT: By means of a quantitative analysis of extensive elastic neutron scattering experiments performed on theTc=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 fromTN=(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 beµDy,CEF=6.0µB in good agreement withµDy,obs=(5.9±0.5)µB as measured by neutron diffraction.
    Zeitschrift für Physik B Condensed Matter 01/1993; 91(2):149-153.
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    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.
    Physica C Superconductivity 01/1993; 209(4):591-596. · 0.72 Impact Factor
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    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.
    Applied Superconductivity 01/1993; 1:333-349.
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    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.
    Physica C Superconductivity 01/1993; 205:259-265. · 0.72 Impact Factor
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    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.
    Physica C Superconductivity 06/1992; 196:164-170. · 0.72 Impact Factor
  • High Pressure Research 05/1992; 10(1):486-491. · 0.90 Impact Factor
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    ABSTRACT: High resolution neutron diffraction studies were performed on carefully prepared powder samples of the high-Tc 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 TN~1.1 K. It appears to be stable down to T = 7 mK.
    Physica B Condensed Matter 01/1992; 180:414-416. · 1.28 Impact Factor
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    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.
    Physica C Superconductivity 08/1991; 178(s 4–6):383–389. · 0.72 Impact Factor
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    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.
    Physica C Superconductivity 07/1991; 178:75-80. · 0.72 Impact Factor
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    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.
    MRS Online Proceeding Library 01/1991; 251.
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    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.
    Physica C Superconductivity 01/1991; 185:190-197. · 0.72 Impact Factor
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    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.
    Physica C Superconductivity 01/1991; 182:62-66. · 0.72 Impact Factor
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    ABSTRACT: Resistive measurements in a cryogenic diamond anvil cell show that the Tc of YBa2Cu4O8 can be increased by almost 30 K by applying a pressure of 12 GPa. The pressure derivative is, however, not constant. At p ⪷ 5 GPa, dTc/dp≊ 5 K/GPa. At higher pressures dTc/dp decreases gradually and Tc saturates at ≈ 108 K. This behaviour is reproduced by a phenomenological model where Tc depends only on the number of holes in the CuO2 planes.
    High Pressure Research 07/1990; · 0.90 Impact Factor
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    ABSTRACT: We have investigated the effect of temperature and 10 kbar pressure on the structure of the new high-temperature superconductor Y2Ba4Cu7O15 (Tc=68 K) in an attempt to relate the pressure dependence of Tc in such materials to changes in certain Cu-O distances. The orthorhombic Ammm X-ray model of Bordet et al., with lattice constant c≈50.3 Å, has been confirmed by our high-resolution neutron powder diffraction measurements. The structure can be described as alternate layers of single CuO-chain (YBa2Cu3O7) and double CuO-chain (YBa2Cu4O8) blocks. The thermal expansion is anisotropic, with the b-axis, parallel to the single and double Cu-O chains, remaining approximately constant for T<Tc. As in YBa2Cu4O8, Cu-O distances bridging chains and planes, as well as Ba-O distancesare significantly changed by pressure. However, in Y2Ba4Cu7O15 these changes are different for the single and double CuO-chain layers. A correlation with the pressure dependence of Tc is proposed.
    Physica C Superconductivity 05/1990; · 0.72 Impact Factor
  • Journal of the Less Common Metals 01/1990;
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    ABSTRACT: A brief overview is presented of some thermodynamic, structural and physical properties of the HTc compounds YBa2Cu3O7 (single chains; nonstoichiometric; Tc = 92 − 55 K), YBa2Cu4O8 (double chains; stoichiometric; Tc = 80 K) and Y2Ba4Cu7O15 (alternating single and double chains; nonstoichiometric; Tc = 70 − 14 K). This family of compounds is an excellent model system for the study of the influence of the chains in superconductivity.A great advantage of YBa2Cu4O8 is the high oxygen stability (up to 850 °C) and the narrow nonstoichiometric range, both due to the stability of the double chains. The oxygen stability keeps orthorhombicity and hinders a structural phase transition like that in YBa2Cu3O7. Therefore, single crystals of YBa2Cu4O8 (and Y2Ba4Cu7O15) do not have a high concentration of twins. Crystals without twins are easily grown and allow for the first time the investigation of the a-b anisotropy which is shown to be appreciable.Although we have put great efforts in the investigation of the thermodynamic phase diagram of the Y-Ba-Cu-O system, the slow kinetics do not allow yet an exact determination of the phase boundaries. The up to now existing data are discussed and the dependence of the ratio on the oxygen pressure is presented.
    Journal of the Less Common Metals. 01/1990;