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  • 1
    Electronic Resource
    Electronic Resource
    Rotator phases and herringbone order in Langmuir monolayers and alkanes (2000)
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 112 (2000), S. 3897-3908 
    Details
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: Molecular ordering of a 2D array of hydrocarbon chains in alkane crystals or Langmuir monolayers arises from the anisotropic pair interaction for rotations about the molecular axes. Starting from atom–atom potentials for H and C, we determine the relevant Fourier coefficients of the molecular pair interaction and, in particular, their distance dependence. In terms of a molecular-field approximation (MFA), we find two first-order transitions between a high-temperature hexagonal phase, an intermediate orthorhombic phase, and at lower T a herringbone structure with a nonhomogeneous order parameter, in agreement with the phase diagrams of various systems. It turns out that molecular ordering is closely related to an orthorhombic distortion of the hexagonal lattice. The calculated distortion agrees well, without free parameters, with measured values. In order to account for the weak variation of the critical temperatures with the chain length, we propose a simple mean-field model for conformational defects that provides a good fit for alkanes with 10–35 carbon atoms. © 2000 American Institute of Physics.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.480537
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  • 2
    Electronic Resource
    Electronic Resource
    Mode-coupling theory for the dynamics of tunnelling systems in dielectrics (1994)
    Springer
    The European physical journal 95 (1994), S. 385-395 
    Details
    ISSN: 1434-6036
    Keywords: 63.50 ; 77.22G ; 79.43G
    Source: Springer Online Journal Archives 1860-2000
    Topics: Physics
    Notes: Abstract A mode-coupling theory (MCT) is presented for the spin-boson model with a spectral density which accounts for a heat bath made up of lattice vibrations of a dielectric solid (superohmic dissipation). A usual decoupling approximation provides a set of non-linear integral equations which are solved both numerically by iteration on a computer and analytically by means of a frequency dependent ansatz for the memory functions. There is a transition to incoherent motion at a temperatureT * where the bare two-level energy is equal to the damping rate, in contradiction to results obtained previously from a path integral formulation. The discrepancy arises since in the MCT the relevant self-energy function does not exhibit a 1/z-pole atz=0. For tunnelling systems in dielectrics this yields a new relaxation mechanism due to incoherent tunnelling: the present results might require to modify some of the basic assumptions of the standard tunnelling model for dielectric glasses.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1007/BF01343967
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  • 3
    Electronic Resource
    Electronic Resource
    Collective low-energy excitations of two-level tunnelling defects in mixed crystals (1995)
    Springer
    The European physical journal 98 (1995), S. 561-573 
    Details
    ISSN: 1434-6036
    Source: Springer Online Journal Archives 1860-2000
    Topics: Physics
    Notes: Abstract The dynamics of strongly interacting two-level tunnelling defects in mixed crystals as KCl: Li is investigated in the framework of Mori's projection method. Applying a mode-coupling scheme on the memory functions and performing the average over the random defect configuration in a mean-field type approximation, the ensueing self-consistent equations can be solved analytically. At low concentration this solution coincides with the exact dynamics of the pair model, whereas for higher concentration the motional spectrum shows several novel features. In particular we find a strong relaxation peak and a characteristic two-peak structure for the density of states; there is an additional feature at energies far below the bare tunnelling amplitude ε which accounts well for several experiments indicating the existence of such low-energy states.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1007/BF01320858
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  • 4
    Electronic Resource
    Electronic Resource
    High-temperature quantum effects in methyl rotation (1991)
    Springer
    The European physical journal 84 (1991), S. 263-267 
    Details
    ISSN: 1434-6036
    Source: Springer Online Journal Archives 1860-2000
    Topics: Physics
    Notes: Abstract Above about 50 K, methyl groups in molecular crystals are usually viewed as classical rotors. Flaws in this description are pointed out, and quantum effects are shown to persist to temperatures comparable to the first librational energy.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1007/BF01313547
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  • 5
    Electronic Resource
    Electronic Resource
    Rotational tunnelling in (CH3)2SnCl2: a neutron scattering study (1990)
    Springer
    The European physical journal 80 (1990), S. 113-118 
    Details
    ISSN: 1434-6036
    Source: Springer Online Journal Archives 1860-2000
    Topics: Physics
    Notes: Abstract The temperature dependence of the rotational tunnelling excitations in (CH3)2SnCl2 is investigated between 5 and 232K. The quasi-elasticE a −E b -transitions are found to be narrower than the inelasticA−E-transitions. At high temperatures the broadening is linear in the temperature. The results are in qualitative agreement with a recent theory.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1007/BF01390656
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  • 6
    Electronic Resource
    Electronic Resource
    Nuclear spin conversion of methyl groups (1990)
    Springer
    The European physical journal 81 (1990), S. 273-279 
    Details
    ISSN: 1434-6036
    Source: Springer Online Journal Archives 1860-2000
    Topics: Physics
    Notes: Abstract A theory is presented for nuclear spin conversion of methyl groups, where the total spin of the three protons changes fromI=1/2 toI=3/2. The transition may be mediated by the magnetic dipolar interaction of the protons among themselves or with a nearby unpaired electron. In general the excess energy, i.e. the tunnelling energy Δ, is transferred from the spin system to the lattice via the rotor-phonon coupling; for the case of an almost free rotor in the magnetic field of an unpaired electron spin, the direct coupling of the electron-proton interaction to the lattice motion may be the more efficient mechanism. AtT=0 the rate is found to be finite, at high temperatures it shows an Arrhenius behaviour. In the intermediate range, two different power laws may govern the temperature dependence, namely 1/τ∝T or 1/τ∝T 7; the latter is due to two-phonon scattering.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1007/BF01309359
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  • 7
    Electronic Resource
    Electronic Resource
    Mode-coupling theory for relaxation of coupled two-level systems (1994)
    Springer
    The European physical journal 94 (1994), S. 173-186 
    Details
    ISSN: 1434-6036
    Keywords: 63.50 ; 61.72.S ; 77.22.G
    Source: Springer Online Journal Archives 1860-2000
    Topics: Physics
    Notes: Abstract A Mori approach to the dynamics of coupled tunnelling units in defect crystals is presented. Transverse and longitudinal correlation functions of a two-level system are given as continued fractions; the memory kernels are evaluated in a usual decoupling approximation. Due to the random configuration of the defects on the host lattice both two-level splitting and relaxation rates show a broad distribution; the corresponding distribution function is derived for the case of a dipolar interaction of the defects. The theory covers both weak and strong coupling; the average interaction energy turns out to be the essential parameter. Dependence on frequency, temperature and concentration of the dynamical susceptibility is discussed. When passing from weak to strong coupling, the zero-temperature susceptibility shows a crossover from a constant value to a decrease with the third power of inverse defect concentration; there is quite a strong relaxational peak in the susceptibility. The theory accounts for several features observed in a recent low-frequency experiment on KCl:Li.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1007/BF01307669
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  • 8
    Electronic Resource
    Electronic Resource
    Rotational tunnelling: The width of the quasi-elastic line (1990)
    Springer
    The European physical journal 78 (1990), S. 479-484 
    Details
    ISSN: 1434-6036
    Source: Springer Online Journal Archives 1860-2000
    Topics: Physics
    Notes: Abstract The tunnelling spectrum of methyl groups or amine molecules in crystals can be measured by high-resolution neutron scattering. With rising temperatures the lines broaden considerably. The broadening mechanism is quite different for the inelastic and quasi-elastic components. The latter mechanism has recently been identified and the quasi-elastic line broadening has been calculated in a perturbative approach. In this paper we examine the same mechanism for the latter, but the perturbation expansion is avoided. This is achieved by a transformation to a surrogate harmonic Hamiltonian for which the relevant correlation function can be calculated for arbitrarily strong coupling to the lattice. The quasi-elastic line is found to be much narrower than the inelastic ones, in agreement with recent experiments.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1007/BF01313330
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  • 9
    Electronic Resource
    Electronic Resource
    Ultrasound propagation in dielectric glasses (1991)
    Springer
    The European physical journal 85 (1991), S. 93-98 
    Details
    ISSN: 1434-6036
    Source: Springer Online Journal Archives 1860-2000
    Topics: Physics
    Notes: Abstract The susceptibility of a two-level system coupled to a phonon heat bath is calculated by setting up its equation of motion and expanding the memory function (or self energy) and the inhomogeneity to second order in the coupling potential. Averaging over disorder yields attenuation and variation with temperature of sound velocity, which are compared to previous results obtained in the framework of the Bloch equations. The relaxation time approximation is avoided; there are new terms in both relaxation and resonant contributions.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1007/BF01387792
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  • 10
    Electronic Resource
    Electronic Resource
    Non-linear dynamics of a driven two-level tunnelling defect (1993)
    Springer
    The European physical journal 93 (1993), S. 109-118 
    Details
    ISSN: 1434-6036
    Keywords: 43.25 ; 42.65
    Source: Springer Online Journal Archives 1860-2000
    Topics: Physics
    Notes: Abstract With increasing external-field amplitude the dynamics of a driven two-level system shows a cross-over from the weak-coupling or linear-response behaviour to a non-linear regime which is characterised by the appearance of Rabi oscillations, a field-dependent dynamical susceptibility, saturation of energy dissipation and additional frequencies in the correlation function. The system any initial distribution will relax towards a stationary state with a time-dependent polarisation vector. These features are derived from the propagator of the two-level system which is calculated using a perturbative scheme with a well-defined small parameter; the roatating-wave approximation of the Bloch equations is avoided. the dynamical behaviour is discussed by means of correlation and response functions.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1007/BF01308814
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