472 research outputs found
Harmonic behavior of metallic glasses up to the metastable melt
In two amorphous alloys ZrTiCuNiBe and ZrAlNiCu coherent neutron scattering has been measured over five decades in energy, including measurements in the metastable melt of a metallic alloy more than 80 K above Tg. In the vibrational spectra a pronounced "boson" peak is found: Even in crystallized samples the density of states exceeds the Debye ω2 model, and in the amorphous state low-frequency vibrations are further enhanced. The peak position shows no dispersion in q, while intensities are strongly correlated with the static structure factor. Over the full energy range the temperature dependence is strictly harmonic. From high-energy resolution measurements we establish lower bounds for the temperatures at which structural α and fast β relaxation become observable
Atomic Transport in Dense, Multi-Component Metallic Liquids
Pd43Ni10Cu27P0 has been investigated in its equilibrium liquid state with
incoherent, inelastic neutron scattering. As compared to simple liquids, liquid
PdNiCuP is characterized by a dense packing with a packing fraction above 0.5.
The intermediate scattering function exhibits a fast relaxation process that
precedes structural relaxation. Structural relaxation obeys a time-temperature
superposition that extends over a temperature range of 540K. The mode-coupling
theory of the liquid to glass transition (MCT) gives a consistent description
of the dynamics which governs the mass transport in liquid PdNiCuP alloys. MCT
scaling laws extrapolate to a critical temperature Tc at about 20% below the
liquidus temperature. Diffusivities derived from the mean relaxation times
compare well with Co diffusivities from recent tracer diffusion measurements
and diffsuivities calculated from viscosity via the Stokes-Einstein relation.
In contrast to simple metallic liquids, the atomic transport in dense, liquid
PdNiCuP is characterized by a drastical slowing down of dynamics on cooling, a
q^{-2} dependence of the mean relaxation times at intermediate q and a
vanishing isotope effect as a result of a highly collective transport
mechanism. At temperatures as high as 2Tc diffusion in liquid PdNiCuP is as
fast as in simple liquids at the melting point. However, the difference in the
underlying atomic transport mechanism indicates that the diffusion mechanism in
liquids is not controlled by the value of the diffusivity but rather by that of
the packing fraction
Depinning in a Random Medium
We develop a renormalized continuum field theory for a directed polymer
interacting with a random medium and a single extended defect. The
renormalization group is based on the operator algebra of the pinning
potential; it has novel features due to the breakdown of hyperscaling in a
random system. There is a second-order transition between a localized and a
delocalized phase of the polymer; we obtain analytic results on its critical
pinning strength and scaling exponents. Our results are directly related to
spatially inhomogeneous Kardar-Parisi-Zhang surface growth.Comment: 11 pages (latex) with one figure (now printable, no other changes
Fast relaxation in a fragile liquid under pressure
The incoherent dynamic structure factor of ortho-terphenyl has been measured
by neutron time-of-flight and backscattering technique in the pressure range
from 0.1 MPa to 240 MPa for temperatures between 301 K and 335 K.
Tagged-particle correlations in the compressed liquid decay in two steps. The
alpha-relaxation lineshape is independent of pressure, and the relaxation time
proportional to viscosity. A kink in the amplitude f_Q(P) reveals the onset of
beta relaxation. The beta-relaxation regime can be described by the
mode-coupling scaling function; amplitudes and time scales allow a consistent
determination of the critical pressure P_c(T). alpha and beta relaxation depend
in the same way on the thermodynamic state; close to the mode-coupling
cross-over, this dependence can be parametrised by an effective coupling Gamma
~ n*T**{-1/4}.Comment: 4 Pages of RevTeX, 4 figures (submitted to Physical Review Letters
Molecular mode-coupling theory applied to a liquid of diatomic molecules
We study the molecular mode coupling theory for a liquid of diatomic
molecules. The equations for the critical tensorial nonergodicity parameters
and the critical amplitudes of the - relaxation
are solved up to a cut off = 2 without any
further approximations.
Here are indices of spherical harmonics. Contrary to previous studies,
where additional approximations were applied, we find in agreement with
simulations, that all molecular degrees of freedom vitrify at a single
temperature . The theoretical results for the non ergodicity parameters
and the critical amplitudes are compared with those from simulations. The
qualitative agreement is good for all molecular degrees of freedom. To study
the influence of the cut off on the non ergodicity parameter, we also calculate
the non ergodicity parameters for an upper cut off . In addition we
also propose a new method for the calculation of the critical nonergodicity
parameterComment: 27 pages, 17 figure
Energy landscapes in random systems, driven interfaces and wetting
We discuss the zero-temperature susceptibility of elastic manifolds with
quenched randomness. It diverges with system size due to low-lying local
minima. The distribution of energy gaps is deduced to be constant in the limit
of vanishing gaps by comparing numerics with a probabilistic argument. The
typical manifold response arises from a level-crossing phenomenon and implies
that wetting in random systems begins with a discrete transition. The
associated ``jump field'' scales as and for
(1+1) and (2+1) dimensional manifolds with random bond disorder.Comment: Accepted for publication in Phys. Rev. Let
Structural Relaxation and Mode Coupling in a Simple Liquid: Depolarized Light Scattering in Benzene
We have measured depolarized light scattering in liquid benzene over the
whole accessible temperature range and over four decades in frequency. Between
40 and 180 GHz we find a susceptibility peak due to structural relaxation. This
peak shows stretching and time-temperature scaling as known from
relaxation in glass-forming materials. A simple mode-coupling model provides
consistent fits of the entire data set. We conclude that structural relaxation
in simple liquids and relaxation in glass-forming materials are
physically the same. A deeper understanding of simple liquids is reached by
applying concepts that were originally developed in the context of
glass-transition research.Comment: submitted to New J. Phy
Broadband Dielectric Spectroscopy on Glass-Forming Propylene Carbonate
Dielectric spectroscopy covering more than 18 decades of frequency has been
performed on propylene carbonate in its liquid and supercooled-liquid state.
Using quasi-optic submillimeter and far-infrared spectroscopy the dielectric
response was investigated up to frequencies well into the microscopic regime.
We discuss the alpha-process whose characteristic timescale is observed over 14
decades of frequency and the excess wing showing up at frequencies some three
decades above the peak frequency. Special attention is given to the
high-frequency response of the dielectric loss in the crossover regime between
alpha-peak and boson-peak. Similar to our previous results in other glass
forming materials we find evidence for additional processes in the crossover
regime. However, significant differences concerning the spectral form at high
frequencies are found. We compare our results to the susceptibilities obtained
from light scattering and to the predictions of various models of the glass
transition.Comment: 13 pages, 9 figures, submitted to Phys. Rev.
The mean-squared displacement of a molecule moving in a glassy system
The mean-squared displacement (MSD) of a hard sphere and of a dumbbell
molecule consisting of two fused hard spheres immersed in a dense hard-sphere
system is calculated within the mode-coupling theory for ideal liquid-glass
transitions. It is proven that the velocity correlator, which is the second
time derivative of the MSD, is the negative of a completely monotone function
for times within the structural-relaxation regime. The MSD is found to exhibit
a large time interval for structural relaxation prior to the onset of the
-process which cannot be described by the asymptotic formulas for the
mode-coupling-theory-bifurcation dynamics. The -process for molecules
with a large elongation is shown to exhibit an anomalously wide cross-over
interval between the end of the von-Schweidler decay and the beginning of
normal diffusion. The diffusivity of the molecule is predicted to vary
non-monotonically as function of its elongation.Comment: 18 pages, 12 figures, Phys. Rev. E, in prin
Mechanical Relaxation in Glasses and at the Glass Transition
The Gilroy-Phillips model of relaxational jumps in asymmetric double-well
potentials, developed for the Arrhenius-type secondary relaxations of the glass
phase, is extended to a formal description of the breakdown of the shear
modulus at the glass transition, the flow process.Comment: 13 pages, 11 figures, 49 ref
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