44 research outputs found
The precision of molecular dynamics simulations and what we can learn from it?
We have investigated by molecular dynamics method the influence of a finite
number of particles used in computer simulations on fluctuations of
thermodynamic properties. As a case study, we used the two-dimensional
Lennard-Jones system. 2D Lennard-Jones system, besides being an archetypal one,
is a subject of long debate, as to whether it has continuous (infinite-order)
or discontinuous (first-order) melting transition. We have found that anomalies
on the equation of state (the van-der-Waals or Myer-Wood loops) previously
considered a hallmark of the first order phase transition, are at best at the
level of noise, since they have the same magnitude as the amplitude of pressure
fluctuations. So, they could be regarded as statistically unsignificant effect.
Also, we estimated inherent statistical noise, present in computer simulations,
and came to conclusion, that it is larger, than predicted by statistical
physics, and the difference between them (called algorithmic fluctuations) is
possibly due to the computer-related issues. It was demonstrated that these
fluctuations in principle could be observed in real-life physical experiments
which would lead to practical resolution of The Matrix hypothesis
Electrotransport and magnetic properies of Cr-GaSb spintronic materials synthesized under high pressure
Electrotarnsport and magnetic properties of new phases in the system Cr-GaSb
were studied. The samples were prepared by high-pressure (P=6-8 GPa)
high-temperature treatment and identified by x-ray diffraction and scanning
electron microscopy (SEM). One of the CrGaSb phases with an
orthorhombic structure has a combination of ferromagnetic and
semiconductor properties and is potentially promising for spintronic
applications. Another high-temperature phase is paramagnetic and identified as
tetragonal
Planar defects as a way to account for explicit anharmonicity in high temperature thermodynamic properties of silicon
Silicon is indispensable in semiconductor industry. Understanding its
high-temperature thermodynamic properties is essential both for theory and
applications. However, first-principle description of high-temperature
thermodynamic properties of silicon (thermal expansion coefficient and specific
heat) is still incomplete. Strong deviation of its specific heat at high
temperatures from the Dulong-Petit law suggests substantial contribution of
anharmonicity effects. We demonstrate, that anharmonicity is mostly due to two
transverse phonon modes, propagating in (111) and (100) directions, and can be
quantitatively described with formation of the certain type of nanostructured
planar defects of the crystal structure. Calculation of these defects'
formation energy enabled us to determine their input into the specific heat and
thermal expansion coefficient. This contribution turns out to be significantly
greater than the one calculated in quasi-harmonic approximation
Glassy Dynamics Under Superhigh Pressure
Nearly all glass-forming liquids feature, along with the structural
alpha-relaxation process, a faster secondary process (beta-relaxation), whose
nature belongs to the great mysteries of glass physics. However, for some of
these liquids, no well-pronounced secondary relaxation is observed. A prominent
example is the archetypical glass-forming liquid glycerol. In the present work,
by performing dielectric spectroscopy under superhigh pressures up to 6 GPa, we
show that in glycerol a significant secondary relaxation peak appears in the
dielectric loss at P > 3 GPa. We identify this beta-relaxation to be of
Johari-Goldstein type and discuss its relation to the excess wing. We provide
evidence for a smooth but significant increase of glass-transition temperature
and fragility on increasing pressure.Comment: 5 pages, 5 figures, final version with minor changes according to
referee demands and corrected Figs 1 and
Observation of non-local dielectric relaxation in glycerol
Since its introduction, liquid viscosity and relaxation time have been
considered to be an intrinsic property of the system that is essentially local
in nature and therefore independent of system size. We perform dielectric
relaxation experiments in glycerol, and find that this is the case at high
temperature only. At low temperature, increases with system size and
becomes non-local. We discuss the origin of this effect in a picture based on
liquid elasticity length, the length over which local relaxation events in a
liquid interact via induced elastic waves, and find good agreement between
experiment and theory