arXiv subjects

K. Trachenko

Publications and source records attributed to K. Trachenko.

52 records · Page 3Linked to original sources

arXiv2014

Thermodynamics and Widom lines in supercritical carbon dioxide

Behavior of supercritical fluids attracts a lot of attention nowadays. It is important both from the point of view of fundamental science and technological applications. However, up to now the progress in the field is rather moderate. In this article we report a computational study of supercritical carbon dioxide which is one of the most important fluids for chemical industry. We study the response functions of $CO_2$ in supercritical regime and calculate the locations of their maxima (Widom lines). We also make preliminary calculations of the line of crossover of microscopic dynamics of particles (Frenkel line). The conclusions on the Frenkel line location can be applied to study of the atmosphere of Venus.

cond-mat.stat-mech↗

arXiv2014

Collective excitations and thermodynamics of disordered state: new insights into an old problem

Disorder has been long considered as a formidable foe of theoretical physicists in their attempts to understand system's behavior. Here, we review recently accumulated data and propose that from the point of view of calculating thermodynamic properties, the problem of disorder may not be as severe as has been hitherto assumed. We particularly emphasize that contrary to the long-held view, collective excitations do not decay in disordered systems. We subsequently discuss recent experimental, theoretical and modelling results related to collective excitations in disordered media, and show how these results pave the way to understanding thermodynamics of disordered systems: glasses, liquids, supercritical fluids and spin glasses. An interesting insight from the recent work is the realization that most important changes of thermodynamic properties of the disordered system are governed only by its fundamental length, the interatomic separation. We discuss how the proposed theory relates to the previous approaches based on general many-body statistical mechanics framework.

cond-mat.soft↗

arXiv2014

Dynamic transition in supercritical iron

Recent advance in understanding the supercritical state posits the existence of a new line above the critical point separating two physically distinct states of matter: rigid liquid and non-rigid gas-like fluid. The location of this line, the Frenkel line, remains unknown for important real systems. Here, we map the Frenkel line on the phase diagram of supercritical iron using molecular dynamics simulations. On the basis of our data, we propose a general recipe to locate the Frenkel line for any system, the recipe that importantly does not involve system-specific detailed calculations and relies on the knowledge of the melting line only. We further discuss the relationship between the Frenkel line and the metal-insulator transition in supercritical liquid metals. Our results enable predicting the state of supercritical iron in several conditions of interest. In particular, we predict that liquid iron in the Jupiter core is in the "rigid liquid" state and is highly conducting. We finally analyse the evolution of iron conductivity in the core of smaller planets such as Earth and Venus as well as exoplanets: as planets cool off, the supercritical core undergoes the transition to the rigid-liquid conducting state at the Frenkel line.

cond-mat.stat-mech↗

arXiv2014

Non-perturbative treatment of strongly-interacting fields: insights from liquid theory

We outline a new programme of solving the problem of treating strong interactions in field theories. The programme does not involve perturbation theories and associated problems of divergences. We apply our recent idea of treating strongly interacting liquids to field theories by showing the equivalence of Hamiltonians of liquids and interacting fields. In this approach, the motion of the field results in the disappearance of $n-1$ transverse modes with frequency smaller than the Frenkel frequency $ω_{\rm F}$, similar to the loss of two transverse modes in a liquid with frequency $ω<ω_{\rm F}$. We illustrate the proposed programme with the calculation of the energy and propagator, and show that the results can not be obtained in perturbation theory to any finite order. Importantly, the Frenkel energy gap $E_{\rm F}=\hbarω_{\rm F}$ and the associated massive Frenkel particle naturally appear in our consideration, the result that is relevant for current efforts to demonstrate a mass gap in interacting field theories such as Yang-Mills theory. Notably, our mechanism involves a physically sensible starting point in terms of real masses (frequencies) in the harmonic non-interacting field, in contrast to the Higgs effect involving the imaginary mass as a starting point. We further note that the longitudinal mode in our approach remains gapless, implying that both short-range and long-range forces with massive and massless particles naturally emerge and unify in a single interacting field, a result not hitherto anticipated. Finally, we comment on the relationship between our results and hydrodynamic description of the quark-gluon plasma.

hep-th↗

arXiv2014

Measurement of bitumen viscosity in the room-temperature drop experiment: student education, public outreach and modern science in one

Slow flow of the viscous liquid is a thought-provoking experiment that challenges students, academics and public to think about some fundamental questions in modern science. In the Queensland demonstration, the world-longest running experiment earning the Ig Nobel prize, one drop of pitch takes about 10 years to fall, leading to problems of actually observing the drops. Here, we describe our recent demonstration of slowly-flowing bitumen where appreciable flow is observed on the time scale of months. The experiment is free from dissipative heating effects and has the potential to improve the accuracy of measurement. Bitumen viscosity was calculated by undergraduate students during the summer project. The worldwide access to the running experiment is provided by webcams uploading the images to a dedicated website, enhancing student education experience and promotion of science. This demonstration serves as an attractive student education exercise and stimulates the discussion of fundamental concepts and hotly debated ideas in modern physics research: difference between solids and liquids, the nature of liquid-glass transition, emergence of long time scales in a physical process, and the conflict between human intuition and physical reality.

cond-mat.soft↗

arXiv2014

Dynamic transition of supercritical hydrogen in gas giants: defining the boundary between interior and atmosphere

Understanding physics of gas giants requires the knowledge about the behavior of hydrogen at extreme pressures and temperatures. Molecular hydrogen in these planets is supercritical, and has been considered as a physically homogeneous state where no differences can be made between a liquid and a gas and where all properties undergo no marked or distinct changes with pressure and temperature, the picture believed to hold below the dissociation and metallization transition. Here, we show that in Jupiter and Saturn, supercritical molecular hydrogen undergoes a dynamic transition around 10 GPa and 3000 K from the "rigid" liquid state to the "non-rigid" gas-like fluid state at the Frenkel line recently proposed, with accompanying qualitative changes of all major physical properties. The consequences of this finding are discussed, including a physically justified way to demarcate the interior and the atmosphere in gas giants.

astro-ph.EP↗

arXiv2013

Evidence for structural crossover in the supercritical state

The state of matter above the critical point is terra incognita, and is loosely discussed as a physically homogeneous flowing state where no differences can be made between a liquid and a gas and where properties undergo no marked or distinct changes with pressure and temperature. In particular, the structure of supercritical state is currently viewed to be the same everywhere on the phase diagram, and to change only gradually and in a featureless way while moving along any temperature and pressure path above the critical point. Here, we demonstrate that this is not the case, but that there is a well-defined structural crossover instead. Evidenced by the qualitative changes of distribution functions of interatomic distances and angles, the crossover demarcates liquid-like and gas-like configurations and the presence of medium-range structural correlations. Importantly, the discovered structural crossover is closely related to both dynamic and thermodynamic crossovers operating in the supercritical state, providing new unexpected fundamental interlinks between the supercritical structure, dynamics and thermodynamics.

cond-mat.stat-mech↗

arXiv2013

Symmetry breaking gives rise to energy spectra of three states of matter

A fundamental task of statistical physics is to start with a microscopic Hamiltonian, predict the system's statistical properties and compare them with observable data. A notable current fundamental challenge is to tell whether and how an interacting Hamiltonian predicts different energy spectra, including solid, liquid and gas phases. Here, we propose a new idea that enables a unified description of all three states of matter. We introduce a generic form of an interacting phonon Hamiltonian with ground state configurations minimising the potential. Symmetry breaking, from the group of rotations in reciprocal space to its subgroup, leads to emergence of energy gaps of shear excitations as a consequence of the Goldstone theorem, and readily results in the emergence of energy spectra of solid, liquid and gas phases.

cond-mat.str-el↗

arXiv2013

Structural Origin of Light Emission in Germanium Quantum Dots

The origin of visible light emission from nanostructures has been a subject of an intense debate since the early work by L. E. Brus and A. P. Alivisatos in 1980s. The intense research that followed has paved the way towards applications of quantum structures in optoelectronics and in bio-sensing and contributed to the development of nanotechnology. The major new challenge is in accessing the structural, electronic and optical properties of quantum dots on a nanoparticle scale in order to understand complex relationships between structural motifs and their contributions to the relevant physical (e. g. optical and electronic) properties. Here we demonstrate that a combination of molecular dynamics simulations and optically-detected x-ray absorption spectroscopy shows sufficient sensitivity to distinguish between regions contributing to the luminescence signal in oxygen and hydrogen terminated Ge quantum dots, thus potentially providing a sub-nanoparticle resolution.

cond-mat.mes-hall↗

arXiv2013

Thermodynamic behavior of supercritical matter

Since their discovery in 1822, supercritical fluids have been of enduring interest, and have started to be deployed in many important applications. Theoretical understanding of the supercritical state is lacking, and is seen to limit further industrial deployment. Here, we study thermodynamic properties of the supercritical state, and discover that specific heat shows a crossover between two different regimes, an unexpected result in view of currently perceived homogeneity of supercritical state in terms of physical properties. We subsequently formulate a theory of system thermodynamics above the crossover, and find good agreement between calculated and experimental specific heat with no free fitting parameters. We derive a power law and analyze supercritical scaling exponents in the system above the Frenkel line. In this theory, energy and heat capacity are governed by the minimal length of the longitudinal mode in the system only, and do not explicitly depend on system-specific structure and interactions.

physics.chem-ph↗

arXiv2013

Duality of liquids

Liquids flow, making them remarkably distinct from solids and close to gases. At the same time, interactions in liquids are strong as in solids. The combination of these two properties is believed to be the ultimate obstacle to constructing a general theory of liquids. Here, we adopt a new approach to liquids: instead of focusing on the problem of strong interactions, we zero in on the relative contributions of vibrational and diffusional motion in liquids. We subsequently show that from the point of view of thermodynamics, liquid energy and specific heat are given, to a very good approximation, by their vibrational contributions as in solids, for relaxation times spanning 15 orders of magnitude. We therefore find that liquids show an interesting {\it duality} not hitherto known: they are close to solids from the thermodynamical point of view and to gases from the point of view of flow. We discuss the experimental implications of this approach.

cond-mat.soft↗

arXiv2013

Heat capacity of matter beyond the Dulong-Petit value

We propose a new simple way to evaluate the effect of anharmonicity on a system's thermodynamic functions such as heat capacity. In this approach, the contribution of all potentially complicated anharmonic effects to constant-volume heat capacity is evaluated by one parameter only, the coefficient of thermal expansion. Importantly, this approach is applicable not only to crystals but also to glasses and viscous liquids. To support this proposal, we perform molecular dynamics simulations of several crystalline and amorphous solids as well as liquids, and find a good agreement between results from theory and simulations. We observe an interesting non-monotonic behavior of liquid heat capacity with a maximum, and explain this effect as a result of competition between anharmonicity at low temperature and decreasing number of transverse modes at high temperature.

cond-mat.stat-mech↗

arXiv2013

Helium at elevated pressures: Quantum liquid with non-static shear rigidity

The properties of liquid helium have always been a fascinating subject to scientists. The phonon theory of liquids taking into account liquid non-static shear rigidity is employed here for studying internal energy and heat capacity of compressed liquid 4-He. We demonstrate good agreement of calculated and experimental heat capacity of liquid helium at elevated pressures and supercritical temperatures. Unexpectedly helium remains a quantum liquid at elevated pressures for a wide range of temperature supporting both longitudinal and transverse-like phonon excitations. We have found that in the very wide pressure range 5 MPa-500 MPa liquid helium near melting temperature is both solid-like and quantum.

cond-mat.stat-mech↗

arXiv2011

Observation of non-local dielectric relaxation in glycerol

Since its introduction, liquid viscosity and relaxation time $τ$ 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, $τ$ 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.

cond-mat.soft↗

arXiv2008

Nature of Structural Transformations in the B2O3 Glass under High Pressure

We report the results of the X-ray diffraction study of B2O3 glass in the pressure interval up to 10 GPa in the 300-700 K temperature range, the results of in-situ volumetric measurements of the glass at pressures up to 9 GPa at room temperature, and first-principles simulations data at pressures up to 250GPa. The behavior of B2O3 glass under pressure can be described as two broad pressure-overlapping transitions. The first transition starts at P > 1 GPa and proceeds without any changes in thecoordination number of the boron atoms; the other one starts at P > 5 GPa and is accompanied by a gradual increase of coordination number. The second transition is completely reversible; the residual densification of the B2O3 glass after decompression is associated with the incomplete reversibility of the first transformation. The fraction of boron atoms transferred to the 4-coordination state in the glass at P < 10 GPa is much smaller than was assumed from indirect experimental data [4], but is considerably larger (by tens of times) than was previously suggested by the classical molecular dynamics simulations [5,6]. The observed transformations under both compression and decompression are quite broad, contrary to the assumption in Ref. [7]. On the basis of ab-initio results, we also predict the one more transformation under higher pressures to a super-dense phase, in which B atoms become 5- and 6-fold coordinated.

cond-mat.dis-nn↗

arXiv2007

Relationship between non-exponentiality of relaxation and relaxation time at the glass transition

By analyzing the experimental data for various glass-forming liquids and polymers, we find that non-exponentiality $β$ and the relaxation time $τ$ are uniquely related: $\log(τ)$ is an approximately linear function of $1/β$, followed by a crossover to a higher linear slope. We rationalize the observed relationship using a recently developed approach, in which the problem of the glass transition is discussed as the elasticity problem.

cond-mat.dis-nn↗