arXiv subjects

Kostya Trachenko

Publications and source records attributed to Kostya Trachenko.

At least 37 records · Page 2Linked to original sources

arXiv2020

Gapped momentum states

Important properties of a particle, wave or a statistical system depend on the form of a dispersion relation (DR). Two commonly-discussed dispersion relations are the gapless phonon-like DR and the DR with the energy or frequency gap. More recently, the third and intriguing type of DR has been emerging in different areas of physics: the DR with the gap in momentum, or $k$-space. It has been increasingly appreciated that gapped momentum states (GMS) have important implications for dynamical and thermodynamic properties of the system. Here, we review the origin of this phenomenon in a range of physical systems, starting from ordinary liquids to holographic models. We observe how GMS emerge in the Maxwell-Frenkel approach to liquid viscoelasticity, relate the $k$-gap to dissipation and observe how the gaps in DR can continuously change from the energy to momentum space and vice versa. We subsequently discuss how GMS emerge in the two-field description which is analogous to the quantum formulation of dissipation in the Keldysh-Schwinger approach. We discuss experimental evidence for GMS, including the direct evidence of gapped DR coming from strongly-coupled plasma. We also discuss GMS in electromagnetic waves and non-linear Sine-Gordon model. We then move on to discuss the recently developed quasihydrodynamic framework which relates the $k$-gap with the presence of a softly broken global symmetry and its applications. Finally, we review recent discussions of GMS in relativistic hydrodynamics and holographic models. Throughout the review, we point out essential physical ingredients required by GMS to emerge and make links between different areas of physics, with the view that new and deeper understanding will benefit from studying the GMS in seemingly disparate fields and from clarifying the origin of potentially similar underlying physical ideas and equations.

cond-mat.stat-mech↗

arXiv2019

Maxwell interpolation and close similarities between liquids and holographic models

We show that liquids and certain holographic models are strikingly similar in terms of several detailed and specific properties related to their energy spectra. We consider two different holographic models and ascertain their similarity with liquids on the basis of emergence of the gap in transverse momentum space and the functional form of the dispersion relation. Furthermore, we find that the gap increases with temperature, the relaxation time governing the gap decreases with temperature and, finally, the gap is inversely proportional to the relaxation time as in liquids. On this basis, we propose that Maxwell-Frenkel approach to liquids can be used to understand holographic models and their strongly-coupled field theory counterparts in a non-perturbative way.

hep-th↗

arXiv2019

Low frequency propagating shear waves in holographic liquids

Recently, it has been realized that liquids are able to support solid-like transverse modes with an interesting gap in momentum space developing in the dispersion relation. We show that this gap is also present in simple holographic bottom-up models, and it is strikingly similar to the gap in liquids in several respects. Firstly, the appropriately defined relaxation time in the holographic models decreases with temperature in the same way. More importantly, the holographic $k$-gap increases with temperature and with the inverse of the relaxation time. Our results suggest that the Maxwell-Frenkel approach to liquids, involving the additivity of liquid hydrodynamic and solid-like elastic responses, can be applicable to a much wider class of physical systems and effects than thought previously, including relativistic models and strongly-coupled quantum field theories. More precisely, the dispersion relation of the propagating shear waves is in perfect agreement with the Maxwell-Frenkel approach. On the contrary the relaxation time appearing in the holographic models considered does not match the Maxwell prediction in terms of the shear viscosity and the instantaneous elastic modulus but it shares the same temperature dependence.

hep-th↗

arXiv2015

Varying vacuum energy of a self-interacting scalar field

Understanding mechanisms capable of altering the vacuum energy is currently of interest in field theories and cosmology. We consider an interacting scalar field and show that the vacuum energy naturally takes any value between its maximum and zero because interaction affects the number of operating field modes, the assertion that involves no assumptions or postulates. The mechanism is similar to the recently discussed temperature evolution of collective modes in liquids. The cosmological implication concerns the evolution of scalar field $ϕ$ during the inflation of the Universe. $ϕ$ starts with all field modes operating and maximal vacuum energy in the early inflation-dominated epoch. As a result of inflation, $ϕ$ undergoes a dynamical crossover and arrives in the state with one long-wavelength longitudinal mode and small positive vacuum energy predicted to be asymptotically decreasing to zero in the late epoch. Accordingly, we predict that the currently observed cosmological constant will decrease in the future, and comment on the possibility of a cyclic Universe.

physics.gen-ph↗

arXiv2015

Electronic effects in high-energy radiation damage in tungsten

Although the effects of the electronic excitations during high-energy radiation damage processes are not currently understood, it is shown that their role in the interaction of radiation with matter is important. We perform molecular dynamics simulations of high-energy collision cascades in bcc-tungsten using the coupled two-temperature molecular dynamics (2T-MD) model that incorporates both the effects of electronic stopping and electron-phonon interaction. We compare the combination of these effects on the induced damage with only the effect of electronic stopping, and conclude in several novel insights. In the 2T-MD model, the electron-phonon coupling results in less damage production in the molten region and in faster relaxation of the damage at short times. These two effects lead to significantly smaller amount of the final damage at longer times.

cond-mat.mtrl-sci↗

arXiv2014

High-energy radiation damage in zirconia: modeling results

Zirconia is viewed as a material of exceptional resistance to amorphization by radiation damage, and consequently proposed as a candidate to immobilize nuclear waste and serve as an inert nuclear fuel matrix. Here, we perform molecular dynamics simulations of radiation damage in zirconia in the range of 0.1-0.5 MeV energies with full account of electronic energy losses. We find that the lack of amorphizability co-exists with a large number of point defects and their clusters. These, importantly, are largely isolated from each other and therefore represent a dilute damage that does not result in the loss of long-range structural coherence and amorphization. We document the nature of these defects in detail, including their sizes, distribution and morphology, and discuss practical implications of using zirconia in intense radiation environments.

cond-mat.mtrl-sci↗

arXiv2014

Electronic effects in high-energy radiation damage in iron

Electronic effects are believed to be important in high--energy radiation damage processes where high electronic temperature is expected, yet their effects are not currently understood. Here, we perform molecular dynamics simulations of high-energy collision cascades in $α$-iron using the coupled two-temperature molecular dynamics (2T-MD) model that incorporates both effects of electronic stopping and electron-ion interaction. We subsequently compare it with the model employing the electronic stopping only, and find several interesting novel insights. The 2T-MD results in both decreased damage production in the thermal spike and faster relaxation of the damage at short times. Notably, the 2T-MD model gives a similar amount of the final damage at longer times, which we interpret to be the result of two competing effects: smaller amount of short-time damage and shorter time available for damage recovery.

cond-mat.mtrl-sci↗

arXiv2013

"Liquid-gas" transition in the supercritical region: Fundamental changes in the particle dynamics

Recently, we have proposed a new dynamic line on the phase diagram in the supercritical region. Crossing this line corresponds to the radical changes of the fluid properties. Here, we focus on the dynamics of model Lennard-Jones and Soft-Sphere fluids. We show that the change of the dynamics from the liquid-like to gas-like can be established on the basis of the velocity autocorrelation function and mean-square displacement. Using the rigorous criterion, we show that the crossover of particle dynamics and key liquid properties occurs at the same line. We further show that positive sound dispersion disappears in the vicinity of this line in both kinds of systems. The dynamic line bears no relationship to the existence of the critical point. We find that the region of existence of liquid-like dynamics narrows with the increase of the exponent of the repulsive part of inter-particle potential.

cond-mat.soft↗

arXiv2012

The nature of high-energy radiation damage in iron: Modeling results

Understanding and predicting a material's performance in response to high-energy radiation damage, as well as designing future materials to be used in intense radiation environments, requires the knowledge of the structure, morphology and amount of radiation-induced structural change. We report the results of molecular dynamics simulations of high-energy radiation damage in iron in the range 0.2-0.5 MeV. We analyze and quantify the nature of collision cascades both at the global and local scale. We find that the structure of high-energy collision cascades becomes increasingly continuous as opposed to showing sub-cascade branching reported previously. At the local length scale, we find large defect clusters and novel small vacancy and interstitial clusters. These features form the basis for physical models aimed at understanding the effects of high energy radiation damage in structural materials.

cond-mat.mtrl-sci↗

arXiv2012

The phonon theory of liquid thermodynamics

Heat capacity of matter is considered to be its most important property because it holds information about system's degrees of freedom as well as the regime in which the system operates, classical or quantum. Heat capacity is well understood in gases and solids but not in the third state of matter, liquids, and is not discussed in physics textbooks as a result. The perceived difficulty is that interactions in a liquid are both strong and system-specific, implying that the energy strongly depends on the liquid type and that, therefore, liquid energy can not be calculated in general form. Here, we develop a phonon theory of liquids where this problem is avoided. The theory covers both classical and quantum regimes. We demonstrate good agreement of calculated and experimental heat capacity of 21 liquids, including noble, metallic, molecular and hydrogen-bonded network liquids in a wide range of temperature and pressure.

cond-mat.stat-mech↗

arXiv2011

Understanding spin glass transition as a dynamic phenomenon

Existing theories explain spin glass transition in terms of a phase transition and order parameters, and assume the existence of a distinct spin glass phase. In addition to problems related to clarifying the nature of this phase, the common challenge is to explain profound dynamic effects. Here, we propose that the main experimental results of spin glass transition can be understood in an entirely dynamic picture, without a reference to a distinct spin glass phase, phase transition and order parameters. In this theory, the susceptibility cusp at the glass transition temperature is due to the dynamic crossover between the high-temperature relaxational and low-temperature spin wave, or elastic, regime. The crossover takes place when $t=τ$, where $t$ is observation time and $τ$ is relaxation time. Time-dependent effects, inconsistent with the phase transition approach, and the logarithmic increase of $T_g$ with field frequency in particular, originate as the immediate consequence of the proposed picture. We comment on the behavior of non-linear susceptibility. In our discussion, we explore similarities between the spin and structural glass transitions.

cond-mat.dis-nn↗

arXiv2011

Liquid heat capacity in the approach from the solid state: anharmonic theory

Calculating liquid energy and heat capacity in general form is an open problem in condensed matter physics. We develop a recent approach to liquids from the solid state by accounting for the contribution of anharmonicity and thermal expansion to liquid energy and heat capacity. We subsequently compare theoretical predictions to the experiments results of 5 commonly discussed liquids, and find a good agreement with no free fitting parameters. We discuss and compare the proposed theory to previous approaches.

cond-mat.stat-mech↗

arXiv2011

Two liquid states of matter: A new dynamic line on a phase diagram

It is generally agreed that the supercritical region of a liquid consists of one single state (supercritical fluid). On the other hand, we show here that liquids in this region exist in two qualitatively different states: "rigid" and "non-rigid" liquid. Rigid to non-rigid transition corresponds to the condition τ ~ τ0, where τis liquid relaxation time and τ0 is the minimal period of transverse quasi-harmonic waves. This condition defines a new dynamic line on the phase diagram, and corresponds to the loss of shear stiffness of a liquid at all available frequencies, and consequently to the qualitative change of many important liquid properties. We analyze the dynamic line theoretically as well as in real and model liquids, and show that the transition corresponds to the disappearance of high-frequency sound, qualitative changes of diffusion and viscous flow, increase of particle thermal speed to half of the speed of sound and reduction of the constant volume specific heat to 2kB per particle. In contrast to the Widom line that exists near the critical point only, the new dynamic line is universal: it separates two liquid states at arbitrarily high pressure and temperature, and exists in systems where liquid - gas transition and the critical point are absent overall.

cond-mat.stat-mech↗

arXiv2010

Heat capacity at the glass transition

A fundamental problem of glass transition is to explain the jump of heat capacity at the glass transition temperature $T_g$ without asserting the existence of a distinct solid glass phase. This problem is also common to other disordered systems, including spin glasses. We propose that if $T_g$ is defined as the temperature at which the liquid stops relaxing at the experimental time scale, the jump of heat capacity at $T_g$ follows as a necessary consequence due to the change of system's elastic, vibrational and thermal properties. In this picture, we discuss time-dependent effects of glass transition, and identify three distinct regimes of relaxation. Our approach explains widely observed logarithmic increase of $T_g$ with the quench rate and the correlation of heat capacity jump with liquid fragility.

cond-mat.soft↗

arXiv2009

Understanding the problem of glass transition on the basis of elastic waves in a liquid

We propose that the properties of glass transition can be understood on the basis of elastic waves. Elastic waves originating from atomic jumps in a liquid propagate local expansion due to the anharmonicity of interatomic potential. This creates dynamic compressive stress, which increases the activation barrier for other events in a liquid. The non-trivial point is that the range of propagation of high-frequency elastic waves, $d_{\rm el}$, increases with liquid relaxation time $τ$. A self-consistent calculation shows that this increase gives the Vogel-Fulcher-Tammann (VFT) law. In the proposed theory, we discuss the origin of two dynamic crossovers in a liquid: 1) the crossover from exponential to non-exponential and from Arrhenius to VFT relaxation at high temperature and 2) the crossover from the VFT to a more Arrhenius-like relaxation at low temperature. The corresponding values of $τ$ at the two crossovers are in quantitative parameter-free agreement with experiments. The origin of the second crossover allows us to reconcile the ongoing controversy surrounding the possible divergence of $τ$. The crossover to Arrhenius relaxation universally takes place when $d_{\rm el}$ reaches system size, thus avoiding divergence and associated theoretical complications such as identifying the nature of the phase transition and the second phase itself. Finally, we discuss the effect of volume on $τ$ and the origin of liquid fragility.

cond-mat.dis-nn↗

arXiv2008

Heat capacity of liquids: an approach from the solid phase

We calculate the energy and heat capacity of a liquid on the basis of its elastic properties and vibrational states. The experimental decrease of liquid heat capacity with temperature is attributed to the increasing loss of two transverse modes with frequency $ω<1/τ$, where $τ$ is liquid relaxation time. In a simple model, liquid heat capacity is related to viscosity and is compared with the experimental data of mercury. We also calculate the vibrational energy of a quantum liquid, and show that transverse phonons can not be excited in the low-temperature limit. Finally, we discuss the implications of the proposed approach to liquids for the problem of glass transition.

cond-mat.soft↗

arXiv2008

Understanding the problem of glass transition on the basis of elastic interactions in a liquid

We review the recently proposed elastic approach to glass transition. This approach is based on a simple and a physically transparent idea of elastic interactions between local relaxation events in a liquid. Central to this picture is the range of this interaction. Its increase on lowering the temperature explains several important open questions in the area of glass transition, including universal relaxation laws and dynamic crossovers. In particular, we show how the proposed theory explains (1) the physical origin of cooperativity of relaxation; (2) the origin of the crossover from exponential to non-exponential relaxation at $τ=$1 ps, where $τ$ is liquid relaxation time; (3) the origin of the Vogel-Fulcher-Tammann law; (4) the origin of stretched-exponential relaxation; (5) the absence of divergence of $τ$ at the VFT temperature $T_0$ and the crossover to a more Arrhenius relaxation at $τ\approx 10^{-6}$ sec; (6) the origin of liquid ``fragility''; and (7) the relationship between non-exponentiality of relaxation and relaxation time.

cond-mat.soft↗

arXiv2008

The Vogel-Fulcher-Tamman law in the elastic theory of glass transition

We propose that the origin of the Vogel-Fulcher-Tammann law is the increase of the range of elastic interaction between local relaxation events in a liquid. In this picture, we discuss the origin of cooperativity of relaxation, the absence of divergence of relaxation time at a finite temperature and the crossover to a more Arrhenius behaviour at low temperature.

cond-mat.dis-nn↗