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See detailCauchy-like relation between elastic constants in amorphous materials.
Krüger, Jan-Kristian UL; Baller, Jörg UL; Britz, Th et al

in Physical Review. B, Condensed Matter and Materials Physics (2002), 66(1), 012206-012210

It is well known that in addition to the longitudinal modulus, viscoelastic liquids show a shear stiffness at sufficiently high probe frequencies due to structural relaxations. For probe frequencies that ... [more ▼]

It is well known that in addition to the longitudinal modulus, viscoelastic liquids show a shear stiffness at sufficiently high probe frequencies due to structural relaxations. For probe frequencies that are large compared to the structural relaxation frequency, the measured elastic longitudinal and shear moduli become so-called clamped properties (c11∞ and c44∞, respectively). During freezing or polymerization of amorphous liquids, these clamped moduli behave in a strongly nonlinear fashion as a function of temperature or polymerization time. Based on Brillouin spectroscopy data we will show that there exists a linear relation between c11∞ and c44∞ over a large temperature or polymerization time range. Surprisingly, the parameters of this linear relation between the elastic moduli vary only little for different materials. Implications for the nonlinear elastic behavior at the glass transition will be discussed on the basis of mode Grüneisen parameters. [less ▲]

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See detailAnalysis of the small-angle neutron scattering of nanocrystalline ferromagnets using a micromagnetics model
Weissmüller, J.; Michels, Andreas UL; Barker, J. G. et al

in Physical Review. B, Condensed Matter and Materials Physics (2001), 63

In ferromagnets with a nonuniform magnetocrystalline and/or magnetoelastic anisotropy, such as nanocrystalline (nc-) or cold-worked (cw-) polycrystalline materials, the static magnetic microstructure ... [more ▼]

In ferromagnets with a nonuniform magnetocrystalline and/or magnetoelastic anisotropy, such as nanocrystalline (nc-) or cold-worked (cw-) polycrystalline materials, the static magnetic microstructure gives rise to strong elastic magnetic small-angle neutron scattering (SANS). The paper explores a method for analyzing field-dependent SANS data from such materials in terms of a model based on the theory of micromagnetics. Samples of cw Ni and of electrodeposited nc Ni and nc Co were characterized by x-ray scattering and magnetometry, and were investigated by SANS both with and without polarization of the neutron beam. The variation of the differential scattering cross section with the scattering vector and with the applied magnetic field is well described by the model. Also, experimental results for the exchange stiffness constant A and for the spin-wave stiffness constant D obtained from the analysis are found to agree with literature data obtained by inelastic neutron scattering on single-crystal specimens. The model supplies an “anisotropy field scattering function” that contains information on the magnitude of the magnetic anisotropy in the material, and on the characteristic length scales on which the anisotropy changes direction. The results suggest that the anisotropy may be strongly nonuniform in each crystallite, possibly due to twinning, and that some magnetic moments in the Ni samples are strongly pinned at defects. [less ▲]

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See detailExistence of An Intrinsic Glass Transition In A Silicon Rubber: Hypersonic Versus Calorimetric Properties
Krüger, Jan-Kristian UL; Mesquida, P.; Baller, Jörg UL

in Physical Review. B, Condensed Matter and Materials Physics (1999), 60(14), 10037-10041

Time-domain temperature-modulated differential scanning calorimetry and high-performance Brillouin spectroscopy have been used as experimental techniques in order to investigate the effect of the thermal ... [more ▼]

Time-domain temperature-modulated differential scanning calorimetry and high-performance Brillouin spectroscopy have been used as experimental techniques in order to investigate the effect of the thermal glass transition on the specific heat as well as on the acoustic properties, quantities which are intimately related by thermodynamics, but measured at completely different frequencies. Using a siloxane as a model substance, we demonstrate the efficiency of hypersonic relaxations until thermal freezing of the material. Introducing the optoacoustic dispersion function as a sensitive measure for structural relaxations, we show that the high-frequency α relaxations are truncated by the thermal glass transition instead of dying out at much higher temperatures. Moreover, the extrapolated static compliance of the rubbery state goes to zero close to but below the thermal glass transition temperature Tg, indicating the existence of an acoustic instability which is reminiscent of the so-called Kauzmann paradox. [less ▲]

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See detailSurface-induced organization of linear molecules on nanostructured polytetrafluoroethylene: Crystalline state of poly[vinylidenefluoride-trifluoroethylene].
Krüger, Jan-Kristian UL; Heydt, B.; Fischer, C. et al

in Physical Review. B, Condensed Matter and Materials Physics (1997), 55(6), 3497-3506

Highly crystalline and uniaxial-oriented polytetrafluoroethylene films, nanostructured in one dimension, were used to produce a fully crystalline, thermoreversible morphology in the ferroelectric ... [more ▼]

Highly crystalline and uniaxial-oriented polytetrafluoroethylene films, nanostructured in one dimension, were used to produce a fully crystalline, thermoreversible morphology in the ferroelectric copolymer poly[vinylidenefluoride-trifluoroethylene]. The resulting state is built up of regularly stacked lamellar crystals of about 100 nm thickness and lateral dimensions of about 6 μm. [less ▲]

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See detailNonlinear optical properties of Rb2ZnCl4 in the incommensurate and ferroelectric phases
Sanctuary, Roland UL; Jundt, D.; Baumert, J. et al

in Physical Review. B, Condensed Matter and Materials Physics (1985), 32(3), 1649-1660

Nonlinear optical properties of Rb2ZnCl4 are measured for the incommensurate and ferroelectric phases by means of second-harmonic generation experiments. In the temperature range of the ferroelectric lock ... [more ▼]

Nonlinear optical properties of Rb2ZnCl4 are measured for the incommensurate and ferroelectric phases by means of second-harmonic generation experiments. In the temperature range of the ferroelectric lock-in phase, the nonlinear optical coefficients d33, d32, and d24 and the Miller coefficients are determined using the wedge technique and are shown to be proportional to the spontaneous polarization. In the incommensurate phase of Rb2ZnCl4, the order of magnitude of the second-harmonic intensity corresponding to nonlinear optical coefficients allowed by the local point-group symmetry is estimated to be at least 107 times smaller than the second-harmonic intensity generated in quartz and thus difficult to detect. Furthermore, it is shown that quasi-phase-matching of the fundamental and second-harmonic waves in the incommensurate phase is limited to a very small temperature range of TC [less ▲]

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