5,764 research outputs found
Glass transition in fullerenes: mode-coupling theory predictions
We report idealized mode-coupling theory results for the glass transition of
ensembles of model fullerenes interacting via phenomenological two-body
potentials. Transition lines are found for C60, C70 and C96 in the
temperature-density plane. We argue that the observed glass-transition behavior
is indicative of kinetic arrest that is strongly driven by the inter-particle
attraction in addition to excluded-volume repulsion. In this respect, these
systems differ from most standard glass-forming liquids. They feature arrest
that occurs at lower densities and that is stronger than would be expected for
repulsion-dominated hard-sphere-like or Lennard-Jones-like systems. The
influence of attraction increases with increasing the number of carbon atoms
per molecule. However, unrealistically large fullerenes would be needed to
yield behavior reminiscent of recently investigated model colloids with strong
short-ranged attraction (glass-glass transitions and logarithmic decay of
time-correlation functions).Comment: 10 pages, 5 figure
The Dynamics of Silica Melts under High Pressure: Mode-Coupling Theory Results
The high-pressure dynamics of a computer-modeled silica melt is studied in
the framework of the mode-coupling theory of the glass transition (MCT) using
static-structure input from molecular-dynamics (MD) computer simulation. The
theory reproduces the experimentally known viscosity minimum (diffusivity
maximum) as a function of density or pressure and explains it in terms of a
corresponding minimum in its critical temperature. This minimum arises from a
gradual change in the equilibrium static structure which shifts from being
dominated by tetrahedral ordering to showing the cageing known from
high-density liquids. The theory is in qualitative agreement with computer
simulation results.Comment: Presented at ESF EW Glassy Liquids under Pressure, to be published in
Journal of Physic
Idealized glass transitions under pressure: dynamics versus thermodynamics
The interplay of slow dynamics and thermodynamic features of dense liquids is
studied by examinining how the glass transition changes depending on the
presence or absence of Lennard-Jones-like attractions. Quite different
thermodynamic behavior leaves the dynamics unchanged, with important
consequences for high-pressure experiments on glassy liquids. Numerical results
are obtained within mode-coupling theory (MCT), but the qualitative features
are argued to hold more generally. A simple square-well model can be used to
explain generic features found in experiment.Comment: to be published in Phys. Rev. Let
Magnetic order in CaFe1-xCoxAsF (x = 0, 0.06, 0.12) superconductor compounds
A Neutron Powder Diffraction (NPD) experiment has been performed to
investigate the structural phase transition and magnetic order in CaFe1-xCoxAsF
superconductor compounds (x = 0, 0.06, 0.12). The parent compound CaFeAsF
undergoes a tetragonal to orthorhombic phase transition at 134(3) K, while the
magnetic order in form of a spin-density wave (SDW) sets in at 114(3) K. The
antiferromagnetic structure of the parent compound has been determined with a
unique propagation vector k = (1,0,1) and the Fe saturation moment of 0.49(5)uB
aligned along the long a-axis. With increasing Co doping, the long range
antiferromagnetic order has been observed to coexist with superconductivity in
the orthorhombic phase of the underdoped CaFe0.94Co0.06AsF with a reduced Fe
moment (0.15(5)uB). Magnetic order is completely suppressed in optimally doped
CaFe0.88Co0.12AsF. We argue that the coexistence of SDW and superconductivity
might be related to mesoscopic phase separation.Comment: 4pages, 4figure
Client Adaptation improves Federated Learning with Simulated Non-IID Clients
We present a federated learning approach for learning a client adaptable,
robust model when data is non-identically and non-independently distributed
(non-IID) across clients. By simulating heterogeneous clients, we show that
adding learned client-specific conditioning improves model performance, and the
approach is shown to work on balanced and imbalanced data set from both audio
and image domains. The client adaptation is implemented by a conditional gated
activation unit and is particularly beneficial when there are large differences
between the data distribution for each client, a common scenario in federated
learning.Comment: 11 pages, 11 figures. To appear at International Workshop on
Federated Learning for User Privacy and Data Confidentiality in Conjunction
with ICML 202
Role of structural relaxations and vibrational excitations in the high-frequency dynamics of liquids and glasses
We present theoretical investigation on the high-frequency collective
dynamics in liquids and glasses at microscopic length scales and terahertz
frequency region based on the mode-coupling theory for ideal liquid-glass
transition. We focus on recently investigated issues from
inelastic-X-ray-scattering and computer-simulation studies for dynamic
structure factors and longitudinal and transversal current spectra: the
anomalous dispersion of the high-frequency sound velocity and the nature of the
low-frequency excitation called the boson peak. It will be discussed how the
sound mode interferes with other low-lying modes present in the system.
Thereby, we provide a systematic explanation of the anomalous sound-velocity
dispersion in systems -- ranging from high temperature liquid down to deep
inside the glass state -- in terms of the contributions from the
structural-relaxation processes and from vibrational excitations called the
anomalous-oscillation peak (AOP). A possibility of observing negative
dispersion -- the {\em decrease} of the sound velocity upon increase of the
wave number -- is argued when the sound-velocity dispersion is dominated by the
contribution from the vibrational dynamics. We also show that the low-frequency
excitation, observable in both of the glass-state longitudinal and transversal
current spectra at the same resonance frequency, is the manifestation of the
AOP. As a consequence of the presence of the AOP in the transversal current
spectra, it is predicted that the transversal sound velocity also exhibits the
anomalous dispersion. These results of the theory are demonstrated for a model
of the Lennard-Jones system.Comment: 25 pages, 22 figure
Spitzer Space Telescope evidence in NGC 6791: no super-mass-loss at super-solar metallicity to explain helium white dwarfs?
We use archival Spitzer Space Telescope photometry of the old, super-solar
metallicity massive open cluster NGC 6791 to look for evidence of enhanced mass
loss, which has been postulated to explain the optical luminosity function and
low white dwarf masses in this benchmark cluster. We find a conspicuous lack of
evidence for prolificacy of circumstellar dust production that would have been
expected to accompany such mass loss. We also construct the optical and
infrared luminosity functions, and demonstrate that these fully agree with
theoretical expectations. We thus conclude that there is no evidence for the
mass loss of super-solar metallicity red giants to be sufficiently high that
they can avoid the helium flash at the tip of the red giant branch.Comment: accepted for publication in ApJ Letter
Neutron diffraction study on phase transition and thermal expansion of SrFeAsF
The magnetic ordering and crystal structure of iron pnictide SrFeAsF was
investigated by using neutron powder diffraction method. With decreasing
temperature, the tetragonal to orthorhombic phase transition is found at 180 K,
while the paramagnetic to antiferromagnetic phase transition set in at 133 K.
Similar to the parent compound of other iron pnictide system, the striped Fe
magnetism is confirmed in antiferromagnetic phase and the Fe moment of 0.58(6)
uB aligned along long a axis. The thermal expansion of orthorhombic phase of
SrFeAsF is also investigated. Based on the Grueneisen approximation and Debye
approximation for internal energy, the volume of SrFeAsF can be well fitted
with Debye temperature of 347(5) K. The experimental atomic displacement
parameters for different crystallographic sites in SrFeAsF are analyzed with
Debye model. The results suggested that the expansion of FeAs layers plays an
important role in determining the thermal expansion coefficient.Comment: 4 pages, 3 figure
Colloidal gelation and non-ergodicity transitions
Within the framework of the mode coupling theory (MCT) of structural
relaxation, mechanisms and properties of non-ergodicity transitions in rather
dilute suspensions of colloidal particles characterized by strong short-ranged
attractions are studied. Results building on the virial expansion for particles
with hard cores and interacting via an attractive square well potential are
presented, and their relevance to colloidal gelation is discussed.Comment: 10 pages, 4 figures; Talk at the Conference: "Unifying Concepts in
Glass Physics" ICTP Trieste, September 1999; to be published in J. Phys.:
Condens. Matte
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