169 research outputs found
Barrier Softening near the onset of Non-Activated Transport in Supercooled Liquids: Implications for Establishing Detailed Connection between Thermodynamic and Kinetic Anomalies in Supercooled Liquids
According to the Random First Order Transition (RFOT) theory of glasses, the
barriers for activated dynamics in supercooled liquids vanish as the
temperature of a viscous liquid approaches the dynamical transition temperature
from below. This occurs due to a decrease of the surface tension between local
meta-stable molecular arrangements much like at a spinodal. The dynamical
transition thus represents a crossover from the low activated bevavior to a
collisional transport regime at high . This barrier softening explains the
deviation of the relaxation times, as a function of temperature, from the
simple dependence at the high viscosity to a
mode-mode coupling dominated result at lower viscosity. By calculating the
barrier softening effects, the RFOT theory provides a {\em unified} microscopic
way to interpret structural relaxation data for many distinct classes of
structural glass formers over the measured temperature range. The theory also
provides an unambiguous procedure to determine the size of dynamically
cooperative regions in the presence of barrier renormalization effects using
the experimental temperature dependence of the relaxation times and the
configurational entropy data. We use the RFOT theory framework to discuss data
for tri-naphthyl benzene, salol, propanol and silica as representative systems.Comment: Submitted to J. Chem. Phy
Stress distribution and the fragility of supercooled melts
We formulate a minimal ansatz for local stress distribution in a solid that
includes the possibility of strongly anharmonic short-length motions. We
discover a broken-symmetry metastable phase that exhibits an aperiodic,
frozen-in stress distribution. This aperiodic metastable phase is characterized
by many distinct, nearly degenerate configurations. The activated transitions
between the configurations are mapped onto the dynamics of a long range
classical Heisenberg model with 6-component spins and anisotropic couplings. We
argue the metastable phase corresponds to a deeply supercooled non-polymeric,
non-metallic liquid, and further establish an order parameter for the
glass-to-crystal transition. The spin model itself exhibits a continuous range
of behaviors between two limits corresponding to frozen-in shear and uniform
compression/dilation respectively. The two regimes are separated by a
continuous transition controlled by the anisotropy in the spin-spin
interaction, which is directly related to the Poisson ratio of the
material. The latter ratio and the ultra-violet cutoff of the theory determine
the liquid configurational entropy. Our results suggest that liquid's fragility
depends on the Poisson ratio in a non-monotonic way. The present ansatz
provides a microscopic framework for computing the configurational entropy and
relaxational spectrum of specific substances.Comment: 11 pages, 5 figures, Final version published in J Phys Chem
Electronic structure and the glass transition in pnictide and chalcogenide semiconductor alloys. Part II: The intrinsic electronic midgap states
We propose a structural model that treats in a unified fashion both the
atomic motions and electronic excitations in quenched melts of pnictide and
chalcogenide semiconductors. In Part I (submitted to J. Chem. Phys.), we argued
these quenched melts represent aperiodic -networks that are highly
stable and, at the same time, structurally degenerate. These networks are
characterized by a continuous range of coordination. Here we present a
systematic way to classify these types of coordination in terms of discrete
coordination defects in a parent structure defined on a simple cubic lattice.
We identify the lowest energy coordination defects with the intrinsic midgap
electronic states in semiconductor glasses, which were argued earlier to cause
many of the unique optoelectronic anomalies in these materials. In addition,
these coordination defects are mobile and correspond to the transition state
configurations during the activated transport above the glass transition. The
presence of the coordination defects may account for the puzzling discrepancy
between the kinetic and thermodynamic fragility in chalcogenides. Finally, the
proposed model recovers as limiting cases several popular types of bonding
patterns proposed earlier, including: valence-alternation pairs, hypervalent
configurations, and homopolar bonds in heteropolar compounds.Comment: 17 pages, 15 figures, revised version, final version to appear in J.
Chem. Phy
Universality in Glassy Low-Temperature Physics
We propose a microscopic translationally invariant glass model which exhibits
two level tunneling systems with a broad range of asymmetries and barrier
heights in its glassy phase. Their distribution is qualitatively different from
what is commonly assumed in phenomenological models, in that symmetric
tunneling systems are systematically suppressed. Still, the model exhibits the
usual glassy low-temperature anomalies. Universality is due to the collective
origin of the glassy potential energy landscape. We obtain a simple explanation
also for the mysterious {\em quantitative} universality expressed in the
unusually narrow universal glassy range of values for the internal friction
plateau.Comment: 4 pages, 5 figures, uses RevTeX
Aging, jamming, and the limits of stability of amorphous solids
Apart from not having crystallized, supercooled liquids can be considered as
being properly equilibrated and thus can be described by a few thermodynamic
control variables. In contrast, glasses and other amorphous solids can be
arbitrarily far away from equilibrium and require a description of the history
of the conditions under which they formed. In this paper we describe how the
locality of interactions intrinsic to finite-dimensional systems affects the
stability of amorphous solids far off equilibrium. Our analysis encompasses
both structural glasses formed by cooling and colloidal assemblies formed by
compression. A diagram outlining regions of marginal stability can be adduced
which bears some resemblance to the quasi-equilibrium replica meanfield theory
phase diagram of hard sphere glasses in high dimensions but is distinct from
that construct in that the diagram describes not true phase transitions but
kinetic transitions that depend on the preparation protocol. The diagram
exhibits two distinct sectors. One sector corresponds to amorphous states with
relatively open structures, the other to high density, more closely-packed
ones. The former transform rapidly owing to there being motions with no free
energy barriers; these motions are string-like locally. In the dense region,
amorphous systems age via compact activated reconfigurations. The two regimes
correspond, in equilibrium, to the collisional or uniform liquid and the so
called landscape regime, respectively. These are separated by a spinodal line
of dynamical crossovers. Owing to the rigidity of the surrounding matrix in the
landscape, high-density part of the diagram, a sufficiently rapid pressure
quench adds compressive energy which also leads to an instability toward
string-like motions with near vanishing barriers. (SEE REST OF ABSTRACT IN THE
ARTICLE.)Comment: submitted to J Phys Chem
Electronic structure and the glass transition in pnictide and chalcogenide semiconductor alloys. Part I: The formation of the -network
Semiconductor glasses exhibit many unique optical and electronic anomalies.
We have put forth a semi-phenomenological scenario (J. Chem. Phys. 132, 044508
(2010)) in which several of these anomalies arise from deep midgap electronic
states residing on high-strain regions intrinsic to the activated transport
above the glass transition. Here we demonstrate at the molecular level how this
scenario is realized in an important class of semiconductor glasses, namely
chalcogen and pnictogen containing alloys. Both the glass itself and the
intrinsic electronic midgap states emerge as a result of the formation of a
network composed of -bonded atomic -orbitals that are only weakly
hybridized. Despite a large number of weak bonds, these -networks are
stable with respect to competing types of bonding, while exhibiting a high
degree of structural degeneracy. The stability is rationalized with the help of
a hereby proposed structural model, by which -networks are
symmetry-broken and distorted versions of a high symmetry structure. The latter
structure exhibits exact octahedral coordination and is fully
covalently-bonded. The present approach provides a microscopic route to a fully
consistent description of the electronic and structural excitations in vitreous
semiconductors.Comment: 22 pages, 17 figures, revised version, final version to appear in J.
Chem. Phy
What are the interactions in quantum glasses?
The form of the low-temperature interactions between defects in neutral
glasses is reconsidered. We analyse the case where the defects can be modelled
either as simple 2-level tunneling systems, or tunneling rotational impurities.
The coupling to strain fields is determined up to 2nd order in the displacement
field. It is shown that the linear coupling generates not only the usual
Ising-like interaction between the rotational tunneling defect modes,
which cause them to freeze around a temperature , but also a random field
term. At lower temperatures the inversion symmetric tunneling modes are still
active - however the coupling of these to the frozen rotational modes, now via
the 2nd-order coupling to phonons, generates another random field term acting
on the inversion symmetric modes (as well as shorter-range interactions
between them). Detailed expressions for all these couplings are given.Comment: 12 pages, 2 figures. Minor modifications, published versio
Лазерные фазовые дальномеры: пути повышения точности
Scientific and technological progress in the field of geodetic and industrial measurements in terms of the use of laser rangefinders operating in ranges up to 5000 meters has led to a reduction in the error of such measuring instruments over the past ten years by two or more times. Such rapid development of high-precision rangefinder technologies has led to a significant revision of the requirements for their metrological support, as well as to the need to develop a new generation of length standards, the stock of metrological accuracy of which would provide an assessment of the metrological characteristics of all types of existing and promising length measuring instruments with a laser rangefinder. To solve this problem, the Institute’s staff conducted research within the framework of a number of thematic research and development works in terms of developing the appearance of a new generation of length standards operating in the range up to 5000 meters in an open atmosphere. Within the framework of this article, one of the developed models of a high-precision complex of measuring instruments for length and coordinate increments is considered, which is a serial high-precision laser phase light meter, modified by the institute’s staff in terms of the system for receiving and processing measuring signals. At the same time, in order to increase the accuracy of length measurements using the developed range finder layout, it is proposed to investigate ways to reduce the errors of the model components of the boundaries of its error. To ensure the smallest error in determining the hardware correction of the rangefinder layout, it is proposed to use funds from the state primary special standard of the unit of length. As promising ways to reduce the error in determining the phase difference of signals, it is proposed to use digital recording and signal processing devices that implement a method for calculating the phase difference of signals by mathematically processing the recorded data using a specially developed computational algorithm based on Fourier analysis. For the most accurate determination of the values of the pulse repetition frequency of signals and the values of the speed of light on the measured track, it is proposed to improve the means of determining these indicators. The use of the proposed methods to improve the accuracy of measuring the length of laser phase rangefinders allows you to provide the necessary margin of metrological accuracy. Научно-технический прогресс в сфере геодезических и промышленных измерений в части использования лазерных дальномеров, работающих в диапазонах до 5 000 метров, привел к уменьшению погрешности выше перечисленных средств измерений за последние десять лет в два и более раза. Следствием такого стремительного развития технологий высокоточной дальнометрии стал значительный пересмотр требований по их метрологическому обеспечению, а также необходимость разработки нового поколения эталонов длины, запас метрологической точности которых обеспечивал бы оценку метрологических характеристик всех типов существующих и перспективных средств измерений длины, имеющих в своем составе лазерный светодальномер. Для решения этой задачи авторами в рамках ряда тематических научно-исследовательских опытно-конструкторских работ проводились исследования с целью разработки нового поколения эталонов длины, работающих в диапазоне до 5 000 метров, в условиях открытой атмосферы. В данной статье рассмотрен один из разработанных макетов высокоточного комплекса средств измерений длины и приращений координат. Макет представляет собой высокоточный лазерный фазовый дальномер с доработанной системой приема и обработки измерительных сигналов. С помощью данного макета дальномера предлагается исследовать пути уменьшения составляющих его погрешности с целью повышения точности измерений длины. Для обеспечения наименьшей погрешности определения аппаратурной поправки макета дальномера могут служить средства из состава Государственного первичного специального эталона единицы длины. В качестве перспективных путей уменьшения погрешности определения разности фаз сигналов представляется возможным использовать устройства цифровой регистрации и обработки сигналов, в которых реализован метод расчета разности фаз сигналов путем математической обработки зарегистрированных данных с помощью специально разработанного вычислительного алгоритма на основе Фурье-анализа. Наиболее точное определение значений частоты следования импульсных сигналов и значений скорости света на измеряемой трассе может быть получено благодаря использованию высокоточных средств определения данных показателей. Реализация предложенных авторами методов повышения точности измерений длины лазерных фазовых дальномеров позволяет обеспечивать необходимый запас метрологической точности
On the Strength of Glasses
The remarkable strength of glasses is examined using the random first order
transition theory of the glass transition. The theory predicts that strength
depends on elastic modulus but also on the configurational energy frozen in
when the glass is prepared. The stress catalysis of cooperative rearrangements
of the type responsible for the supercooled liquid's high viscosity account
quantitatively for the measured strength of a range of metallic glasses, silica
and a polymer glass.Comment: 22 pages, 6 figure
The Ultimate Fate of Supercooled Liquids
In recent years it has become widely accepted that a dynamical length scale
{\xi}_{\alpha} plays an important role in supercooled liquids near the glass
transition. We examine the implications of the interplay between the growing
{\xi}_{\alpha} and the size of the crystal nucleus, {\xi}_M, which shrinks on
cooling. We argue that at low temperatures where {\xi}_{\alpha} > {\xi}_M a new
crystallization mechanism emerges enabling rapid development of a large scale
web of sparsely connected crystallinity. Though we predict this web percolates
the system at too low a temperature to be easily seen in the laboratory, there
are noticeable residual effects near the glass transition that can account for
several previously observed unexplained phenomena of deeply supercooled liquids
including Fischer clusters, and anomalous crystal growth near T_g
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