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  • 1
    Electronic Resource
    Electronic Resource
    Positron annihilation study of the electronic structure of CeCu6 and LaCu6 (1995)
    Springer
    Il nuovo cimento della Società Italiana di Fisica 17 (1995), S. 449-458 
    Details
    ISSN: 0392-6737
    Keywords: Positron annihilation ; Narrow-band systems ; heavy-fermion metals ; intermediate-valence solids ; Measurement of Fermi surface parameters (including dHvA magnetoacoustic, magnetoresistance oscillation and cyclotron resonance studies etc.)
    Source: Springer Online Journal Archives 1860-2000
    Topics: Physics
    Notes: Summary The results of a prelimianry study of the electronic structure of the heavy-fermion system CeCu6 via the two-dimensional angular correlation of electron-positron annihilation radiation (2D ACAR) technique were compared with those obtained for the isostructural reference non-f-electron material LaCu6. The spectra relating to the 2 compounds displayed extremely similar anisotropic structures, with features related to the symmetry of the Brillouin zone. At present, these structures cannot however be attributed to Fermi-surface or positron wave function effects or to anisotropies of some full valence band. In the case of LaCu6, data processing prior to the LCW procedure via new filtering techniques revealed a reasonably good agreement between the LCW data and LAPW Fermi-surface calculations while in the case of CeCu6 it appeared difficult to relate the results, still of controversial interpretation, to any theoretical model.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1007/BF02451733
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  • 2
    Electronic Resource
    Electronic Resource
    Spontaneous breaking of time-reversal symmetry in the pseudogap state of a high- T c superconductor (2002)
    [s.l.] : Nature Publishing Group
    Nature 416 (2002), S. 610-613 
    Details
    ISSN: 1476-4687
    Source: Nature Archives 1869 - 2009
    Topics: Biology , Chemistry and Pharmacology , Medicine , Natural Sciences in General , Physics
    Notes: [Auszug] A change in ‘symmetry’ is often observed when matter undergoes a phase transition—the symmetry is said to be spontaneously broken. The transition made by underdoped high-transition-temperature (high-Tc) superconductors is unusual, in that it is not a mean-field ...
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1038/416610a
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  • 3
    Electronic Resource
    Electronic Resource
    BSCCO Superconductors: Hole-Like Fermi Surface and Doping Dependence of the Gap Function (1999)
    Springer
    Journal of low temperature physics 117 (1999), S. 365-369 
    Details
    ISSN: 1573-7357
    Source: Springer Online Journal Archives 1860-2000
    Topics: Physics
    Notes: Abstract We use the gradient of the energy-integrated angle resolved photoemission (ARPES) intensity in order to define precisely the Fermi surface (FS) in BSCCO superconductors. We show that, independent of the photon energy, the FS is a hole barrel centered at (π, π). Then, the superconducting gap along the FS is precisely determined from ARPES measurements on overdoped and underdoped samples of Bi2212. As the doping decreases, the maximum gap increases, but the slope of the gap near the nodes decreases. Though consistent with d-wave symmetry, the gap with underdoping cannot be fit by the simple cos(k x) − cos(k y) form. A comparison of our ARPES results with available penetration depth data indicates that the renormalization of the linear T suppression of the superfluid density at low temperatures due to quasiparticle excitations around the d-wave nodes is large and doping dependent.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1023/A:1022569616840
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  • 4
    Electronic Resource
    Electronic Resource
    Positron lifetime studies of free volume hole size distribution in amorphous and in semi-crystalline polymers (1996)
    Springer
    Journal of radioanalytical and nuclear chemistry 211 (1996), S. 69-76 
    Details
    ISSN: 1588-2780
    Source: Springer Online Journal Archives 1860-2000
    Topics: Chemistry and Pharmacology , Energy, Environment Protection, Nuclear Power Engineering
    Notes: Abstract Positron lifetime spectroscopy has been applied to estimate the free-volume hole size distribution in glassy polycarbonate (PC) and polystyrene (PS) as well as in plastically deformed and undeformed, semi-crystalline polyethylene (HDPE). The hole radius density distribution is determined from the ortho-positronium lifetime distribution which is obtained via a Laplace-inversion of the positron lifetime spectrum. The hole volume density distribution and the number density distribution of holes is estimated from the hole radius density distribution. In PC and in PS all of the distributions may be well approximated by a single Gaussian. The hole radius and the hole number density distributions have centres 〈r〉 and 〈v n 〉 at 0.29 nm and 0.1 nm3 in PC, and at 0.28 nm and 0.09 nm3 in PS. The FWHM of the corresponding distributions are 0.042 nm and 0.040 nm3 (PC), and 0.039 nm and 0.34 nm3 (PS), respectively. Both, the shape and the width of the distributions correlate well with the free volume theory of BUECHE. In PE the lifetime spectra consist of four components. The o-Ps lifetime distribution is bimodal and may be attributed to o-Ps annihilation in the crystalline and in the amorphous phase of the polymer. The corresponding hole size distributions show definite changes of their position and width following plastic deformation which we attribute to homogeneous crystal lattice dilatation and/or a local disorder in the crystals and to an increase in the eccentricity of holes in the amorphous phase.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1007/BF02036257
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  • 5
    Electronic Resource
    Electronic Resource
    Positron and positronium annihilation studies of phase behaviour in confined geometry (1996)
    Springer
    Journal of radioanalytical and nuclear chemistry 210 (1996), S. 255-271 
    Details
    ISSN: 1588-2780
    Source: Springer Online Journal Archives 1860-2000
    Topics: Chemistry and Pharmacology , Energy, Environment Protection, Nuclear Power Engineering
    Notes: Abstract In this paper we present a brief review of the current state of positron annihilation research into the phase behaviour of fluids confined within restricted boundaries. We summarise, in the form of selected examples, the work done so far on: (1) fluids confined in the nanometer-size pores of VYCOR glass, with particular emphasis on the confined phase diagram and the mechanisms behind phase transitions compared to bulk. (2) The adsorption/physisorption of gases on internal surfaces of grafoil and the potential of positron technique for revealing physical properties, such as the intricate molecular arrangements during phase transitions of the “layered” fluid.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1007/BF02056372
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  • 6
    Electronic Resource
    Electronic Resource
    Phase behaviour near the triple point of carbon dioxide confined in mesoporous VYCOR glass (1996)
    Springer
    Journal of radioanalytical and nuclear chemistry 211 (1996), S. 173-179 
    Details
    ISSN: 1588-2780
    Source: Springer Online Journal Archives 1860-2000
    Topics: Chemistry and Pharmacology , Energy, Environment Protection, Nuclear Power Engineering
    Notes: Abstract The phase behaviour of carbon dioxide confined in VYCOR glass at pressures below that of the bulk triple point (∼0.51 MPa) has been investigated. The temperature at which freezing occurs appears to be pressure dependent below 0.3 MPa. As experiments are performed at successively lower pressures, the confined phase transitions gradually disappear, due to either partial pore filling, or the proximity of the confined triple point.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1007/BF02036270
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  • 7
    Electronic Resource
    Electronic Resource
    Anomalous freezing and melting behaviour of capillary confined CO2 (1996)
    Springer
    Journal of radioanalytical and nuclear chemistry 210 (1996), S. 575-582 
    Details
    ISSN: 1588-2780
    Source: Springer Online Journal Archives 1860-2000
    Topics: Chemistry and Pharmacology , Energy, Environment Protection, Nuclear Power Engineering
    Notes: Abstract In this paper we present our recent positron annihilation study of the liquid»solid phase boundary for CO2 confined in nanometer pores of VYCOR glass. We find that CO2 remains liquid in the pores far below the bulk freezing temperature and there is pronounced hysteresis between freezing and melting compared to that seen at the gas-liquid boundary in the pores. On freezing we see evidence of open space created in the pores. This leads to complex melting behaviour possibly involving the formation of gas-liquid interfaces. We see that frezing in the pores is totally irreversible, so that any solid which forms (no matter how small) remains stable up to the higher melting temperature. In contrast melting is more reversible (possibly indicating nucleation centres which permit immediate re-freezing). Finally, the pre-frozen state in the pores is different to the post-melted state.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1007/BF02056398
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  • 8
    Electronic Resource
    Electronic Resource
    Adsorption and desorption of binary mixtures in confined geometry: A pilot positron annihilation study (1996)
    Springer
    Journal of radioanalytical and nuclear chemistry 211 (1996), S. 165-171 
    Details
    ISSN: 1588-2780
    Source: Springer Online Journal Archives 1860-2000
    Topics: Chemistry and Pharmacology , Energy, Environment Protection, Nuclear Power Engineering
    Notes: Abstract In confined (nanometer-scale) geometry, the effects of substrate forces and finite size produce a shift of the gas ⇔ liquid phase boundary from that found in corresponding bulk. The pore transitions also show marked hysteresis. The phase behaviour of a binary gas mixture in confined geometry is likely to depend on the miscibility of the system and the interaction between the substrate and the individual fluid molecules/atoms. Here, we present the results of a pilot positronium annihilation study of the condensation and evaporation of argon-nitrogen mixtures confined in ∼4 nm diameter cylindrical pores in VYCOR glass.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1007/BF02036269
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  • 9
    Electronic Resource
    Electronic Resource
    The temperature dependence of the local free volume in polyethylene and polytetrafluoroethylene: A positron lifetime study (1998)
    Bognor Regis [u.a.] : Wiley-Blackwell
    Journal of Polymer Science Part B: Polymer Physics 36 (1998), S. 1513-1528 
    Details
    ISSN: 0887-6266
    Keywords: free volume ; thermal expansion ; positron lifetime measurements ; semicrystalline polymers ; polyethylene ; polytetrafluoroethylene ; Physics ; Polymer and Materials Science
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology , Physics
    Notes: Positron lifetime measurements, performed in the temperature range 80-300 K, are reported for polyethylene (PE) and polytetrafluoroethylene (PTFE). The lifetime spectra have been analyzed using the data processing routines LIFSPECFIT and MELT. Two long-lived components appear, which are attributed to pick-off annihilation of ortho-positronium in crystalline regions and at holes in the amorphous phase. The ortho-positronium lifetimes, τ3 and τ4, are used to estimate the crystalline packing density and the size of local free volumes in the crystalline and amorphous phases. The interstitial free volume in the crystals exhibits a weak linear increase with the temperature which is attributed to thermal expansion of the crystal unit cell. In the amorphous phase, the hole volume varies between 0.053 and 0.188 nm3 (PE) and between 0.152 and 0.372 nm3 (PTFE). Its temperature variation may be fitted by two straight lines, the intersection of which is used to estimate a glass transition temperature of Tg = 195 K for both PE and PTFE. The slopes of the free volume in the glassy and crystalline phases with the temperature correlate well with each other. The coefficients of thermal expansion of the hole volume are compared with the macroscopic volume change below and above the glass transition. From this comparison a fractional hole volume at Tg of 4.5 (PE) and 5.7% (PTFE) and a number of 0.73 (PE) and 0.36 (PTFE) × 1027 holes/m3 is estimated. Finally, it is found that the intensity of o-Ps annihilation in crystals shows a different temperature dependence to that in the amorphous phase. © 1998 John Wiley & Sons, Inc. J Polym Sci B: Polym Phys 36: 1513-1528, 1998
    Additional Material: 11 Ill.
    Type of Medium: Electronic Resource
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