arXiv subjects

V. V. Brazhkin

Publications and source records attributed to V. V. Brazhkin.

At least 19 recordsLinked to original sources

arXiv2026

The New High-entropy Compound RhMnFeCoGe4 with Cubic Non-centrosymmetric B20 Structure

A novel high-entropy compound, RhMnFeCoGe$_4$, with a cubic non-centrosymmetric B20 struc- ture, has been synthesized under conditions of high pressure and temperature. The electrical transport and magnetic properties of the obtained compound at both ambient and elevated pres- sures have been investigated. In addition, nuclear magnetic resonance (NMR) spectra were obtained at 4.2 K and ab initio calculations were performed. The new material exhibits ferromag- netic behavior with a critical temperature of $T_C$ = 146 K and a spontaneous moment of 2.5 $\mu_B$ per formula unit. The magnetization data obtained at the critical region yielded the critical temperature and exponents, which were found to be $T_C$ = 146(1) K, $\beta$ = 0.337(1), $\gamma$ = 1.121(1), and $\delta$ = 4.326(1). The magnetic moments of Mn and Co were determined from NMR spectra to be 2.2 $\mu_B$ and 0.5 $\mu_B$, respectively. Ab initio calculations yielded reasonable values for the lattice parameter and the magnetic moments of all constituents. The density of states and band structure are determined for both paramagnetic and ferromagnetic states. Lattice compression results in the increase in the $T_C$.

cond-mat.mtrl-sci↗

arXiv2025

Extended defects in hard disk system and melting criteria

The hard sphere model is widely used in description of fluids and solid media as a zero approximation to real systems. Despite the uniqueness of the model, few analytical results are known for it, both for the 2D and 3D cases. In present research we have investigated melting of the hard disk system by considering accumulation of extended defects of a certain type in the crystaline phase, and jamming of the disk packing. It results in formulation of melting criteria with lower and upper bounds on volume ratio at melting transition: $25/21 \le V/V_0 \le 5/4$. It was found that, in full agreement with the Berezinskii-Kosterlitz-Thouless-Halperin-Nelson-Young theory, the 2D crystal melts into anisotropic liquid. The second transition, which is the transition between anisotropic and isotropic liquid has volume ratio $5/4 \le V/V_0 \le 13/9$.

cond-mat.soft↗

arXiv2024

Density of states in liquids: quadratic or linear, and what each means?

A lot has been said about the vibrational density of states (DoS) in liquids. A more recent discussion introduces contradictions with earlier results, and here I briefly review several pieces of evidence from modeling, experiments and theory showing this. I then show that the origin of contradictions often comes from misinterpreting the nature of excitations in liquids. Classic quadratic DoS corresponds to propagating (albeit damped) phonons as in solids and applies to any medium at low frequency, whereas the linear DoS comes from overdamped modes, localised particle motions. This has implications for interpreting simulations, experiments and a theory of liquids. I also introduce a new mechanism where the exponent of quadratic DoS reduces to close to 1 due to the asymmetry of the scattering intensity, reducing the effective phonon frequency.

cond-mat.stat-mech↗

arXiv2024

Collective excitations in liquid carbon tetrachloride: a molecular dynamics study

We perform a molecular dynamic study of collective excitations of carbon tetrachloride and compare the results with experimental data from the literature. The data of simulations are in good argeement with the experimental ones. The results of the simulations confirm the presence of large positive sound dispersion (PSD) in carbon tetrachloride, which should be related to some relaxation processes which do not take place in atomic systems.

cond-mat.soft↗

arXiv2024

A rare gas mixture: From rigid to gas-like fluid by a mutual concentration change

For a number of mixtures of rate gases at high pressures, sound speed minima are experimentally observed depending on the concentration. This behavior has not yet been explained. We have studied the behavior of a mixture of argon and helium using computer simulation. Sound speed minima have been discovered at a certain concentration, which is in good agreement with experimental data. It is shown that this behavior is due to the fact that the P and T parameters for gas mixtures are near the Frenkel line, separating the states of "rigid" and quasi-gas fluid.

cond-mat.soft↗

arXiv2023

Local structure, thermodynamics, and melting curve of boron phosphide at high pressures by deep learning-driven ab initio simulations

Boron phosphide (BP) is a (super)hard semiconductor constituted of light elements, which is promising for high demand applications at extreme conditions. The behavior of BP at high temperatures and pressures is of special interest but is also poorly understood because both experimental and conventional ab initio methods are restricted to studying refractory covalent materials. The use of machine learning interatomic potentials is a revolutionary trend that gives a unique opportunity for high-temperature study of materials with ab initio accuracy. We develop a deep machine learning potential (DP) for accurate atomistic simulations of solid and liquid phases of BP as well as their transformations near the melting line. Our DP provides quantitative agreement with experimental and ab initio molecular dynamics data for structural and dynamic properties. DP-based simulations reveal that at ambient pressure tetrahedrally bonded cubic BP crystal melts into an open structure consisting of two interpenetrating sub-networks of boron and phosphorous with different structures. Structure transformations of BP melts under compressing are reflected by the evolution of low-pressure tetrahedral coordination to high-pressure octahedral coordination. The main contributions to structural changes at low pressures are made by the evolution of medium-range order in B-subnetwork and at high pressures by the change of short-range order in P-sub-network. Such transformations exhibit an anomalous behavior of structural characteristics in the range of 12--15 GPa. Analysis of the results obtained raise open issues in developing machine learning potentials for covalent materials and stimulate further experimental and theoretical studies of melting behavior in BP.

cond-mat.mtrl-sci↗

arXiv2023

Phenomenological model of supercooled liquid as a possible resolution of the Kauzmann paradox

The diverging relaxation time in approaching hypothetical ideal glass transition is a subject of hot debate. In the current paper we demonstrate, how diverging relaxation time and turning excess entropy to zero (which is an essence of Kauzmann's paradox) can be avoided, using as an example the model molecular glassformer, propylene carbonate. For this purpose we compare its thermodynamic and dielectric relaxation properties, both known from the literature. The agreement between two sets of data can be achieved, if we suppose, that enthalpy of supercooled liquid propylene carbonate is governed by activation law, and relaxation time follows double exponential law. We propose the generalized Adam-Gibbs law to reconcile this two dependencies, and qualitatively discuss its implications.

cond-mat.soft↗

arXiv2023

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.

cond-mat.mtrl-sci↗

arXiv2022

Solidification of the Glass-Forming Al86Ni2Co6Gd6 Melt under High Pressure

High pressures allow the synthesis of new metastable compounds that remain intact for a sufficiently long time at normal conditions. Until now, it has not been fully understood how pressure, glass-forming ability and solidification of liquids are interconnected. We have investigated the structure of the glass-forming eutectic alloy Al86Ni2Co6Gd6 obtained by rapid cooling from the melt having a temperature of 1800 K under a pressure of 10 GPa. X-ray diffraction analysis and electron microscopy show that the samples are homogeneous and dense. The structure is finely dispersed. New stable crystalline phases with cubic (cP4/2) and tetragonal (tI26/1) structures are formed in the alloy. The studies have shown that the average microhardness of the samples obtained at 10 GPa is almost 2 times higher than that of the original sample at atmospheric pressure and is about 1700 MPa. To understand the results, we used ab initio molecular dynamics and studied how the melt changes with pressure. It is shown that at a temperature of 1800 K, high pressure increases the concentration of icosahedral clusters in the melt so that at 10 GPa atoms inside the icosahedra form a percolation cluster, while at atmospheric pressure they do not. Thus, the glass-forming ability of a melt increases at high pressure strongly influencing solidification processes.

cond-mat.mtrl-sci↗

arXiv2021

Dielectric constant of disordered phases of the smallest monoalcohols : evidence for the hindered plastic crystal phase

With gradual temperature increase in premelting regions of solid phase of methanol and high pressure phase of ethanol, and using novel procedure of separation of electrode polarization effects, we are able to register the contribution of relaxation process to low-frequency dielectric constant. This contribution is about half the liquid's dielectric constant near temperature of solidification , and is almost an order of magnitude higher than reported earlier for ambient pressure phase of methanol. As opposed to dielectric constant of water at ambient pressure, which does not change much during crystallization, our finding indicates the hindrance of molecule rotation in orientationally disordered phases of monoalcohols. Similar dielectric responses of ambient pressure methanol and high-pressure phase of ethanol imply existence of hindered plastic crystal phase of ethanol (not observed at low pressures). We also have found some dynamic disorder in nominally fully ordered phase of these monoalcohols ($\alpha$-phase of methanol and low-pressure phase of ethanol), the contribution of this disorder being dependent on external conditions (e.g. temperature), and increasing at approaching the order-disorder transition. On the other hand, the amplitude of dielectric responce in hindered plastic crystal phases is almost independent of temperature.

cond-mat.mtrl-sci↗

arXiv2021

Glass transition in monatomic systems: smearing of the same structure vs two structure competition

In the present paper we discuss the properties of Voronoi polygons in several monatomic glass-forming systems and compare them with those of the Kob-Andersen mixture. We show that two mechanisms of glass formation are possible: smearing of Voronoi polygons or formation of polygons of two different shapes. Both mechanisms lead to disturbance of the crystalline order in the system and glass transition.

cond-mat.soft↗

arXiv2020

Universal lower bounds on energy and momentum diffusion in liquids

Thermal energy can be conducted by different mechanisms including by single particles or collective excitations. Thermal conductivity is system-specific and shows a richness of behaviors currently explored in different systems including insulators, strange metals and cuprate superconductors. Here, we show that despite the seeming complexity of thermal transport, the thermal diffusivity $\alpha$ of liquids and supercritical fluids has a lower bound which is fixed by fundamental physical constants for each system as $\alpha_m=\frac{1}{4\pi}\frac{\hbar}{\sqrt{m_em}}$, where $m_e$ and $m$ are electron and molecule masses. The newly introduced elementary thermal diffusivity has an absolute lower bound dependent on $\hbar$ and the proton-to-electron mass ratio only. We back up this result by a wide range of experimental data. We also show that theoretical minima of $\alpha$ coincide with the fundamental lower limit of kinematic viscosity $\nu_m$. Consistent with experiments, this points to a universal lower bound for two distinct properties, energy and momentum diffusion, and a surprising correlation between the two transport mechanisms at their minima. We observe that $\alpha_m$ gives the minimum on the phase diagram except in the vicinity of the critical point, whereas $\nu_m$ gives the minimum on the entire phase diagram.

cond-mat.stat-mech↗

arXiv2020

Comment on "Pentadiamond: A Hard Carbon Allotrope of a Pentagonal Network of sp2 and sp3 atoms"

In a recent Letter [1] Y. Fujii et al. proposed a new carbon allotrope and claimed its extremely high Young's and shear moduli of 1691 and 1113 GPa, respectively, surpassing those of diamond by 1.5 and 2 times! The authors also claimed unique negative averaged Poisson's ratio of -0.24, bulk modulus of 381 GPa and record value of the sound speed 28.7 km/s for this carbon modification. Here we show by two independent computations that these values were obtained as a result of errors. The new hypothetical allotrope has in fact quite moderate elastic moduli, unremarkable for a carbon structure with similar density: bulk modulus of 250 GPa, shear modulus of 170 GPa,Young's modulus of 417 GPa, positive Poisson's ratio of 0.22.

cond-mat.mtrl-sci↗

arXiv2020

Speed of sound from fundamental physical constants

Two dimensionless fundamental physical constants, the fine structure constant $\alpha$ and the proton-to-electron mass ratio $\frac{m_p}{m_e}$ are attributed a particular importance from the point of view of nuclear synthesis, formation of heavy elements, planets, and life-supporting structures. Here, we show that a combination of these two constants results in a new dimensionless constant which provides the upper bound for the speed of sound in condensed phases, $v_u$. We find that $\frac{v_u}{c}=\alpha\left(\frac{m_e}{2m_p}\right)^{\frac{1}{2}}$, where $c$ is the speed of light in vacuum. We support this result by a large set of experimental data and first principles computations for atomic hydrogen. Our result expands current understanding of how fundamental constants can impose new bounds on important physical properties.

cond-mat.mtrl-sci↗

arXiv2019

Minimal quantum viscosity from fundamental physical constants

Viscosity of fluids is strongly system-dependent, varies across many orders of magnitude and depends on molecular interactions and structure in a complex way not amenable to first-principles theories. Despite the variations and theoretical difficulties, we find a new quantity setting the minimal kinematic viscosity of fluids: $\nu_m=\frac{1}{4\pi}\frac{\hbar}{\sqrt{m_em}}$, where $m_e$ and $m$ are electron and molecule masses. We subsequently introduce a new property, the "elementary" viscosity $\iota$ with the lower bound set by fundamental physical constants and notably involving the proton-to-electron mass ratio: $\iota_m=\frac{\hbar}{4\pi}\left({\frac{m_p}{m_e}}\right)^{\frac{1}{2}}$, where $m_p$ is the proton mass. We discuss the connection of our result to the bound found by Kovtun, Son and Starinets in strongly-interacting field theories.

cond-mat.soft↗

arXiv2019

Mechanism of Universal Conductance Fluctuations

Universal conductance fluctuations are usually observed in the form of aperiodic oscillations in the magnetoresistance of thin wires as a function of the magnetic field B. If such oscillations are completely random at scales exceeding \xi_B, their Fourier analysis should reveal a white noise spectrum at frequencies below \xi_B^{-1}. Comparison with the results for 1D systems suggests another scenario: according to it, such oscillations are due to the superposition of incommensurate harmonics and their spectrum should contain discrete frequencies. An accurate Fourier analysis of the classical experiment by Washburn and Webb reveals a purely discrete spectrum in agreement with the latter scenario. However, this spectrum is close in shape to the discrete white noise spectrum whose properties are similar to a continuous one.

cond-mat.dis-nn↗

arXiv2019

Anomalous behavior of dispersion of longitudinal and transverse collective excitations in water

We study the dependence of the excitation frequency of water along an isochore and an isotherm crossing the region of density anomaly. We have shown that the frequency of the longitudinal excitations demonstrated anomalous dependence on temperature along the isochore. At the same time the dependence for both longitudinal and transverse excitation frequencies on density along the isotherm are very modest or even negligible in rather wide range of densities. This kind of behavior also seems anomalous in comparison with the ordinary liquids.

cond-mat.soft↗

arXiv2019

Thermodynamic heterogeneity and crossover in the supercritical state of matter

A hallmark of a thermodynamic phase transition is the qualitative change of system thermodynamic properties such as energy and heat capacity. On the other hand, no phase transition is thought to operate in the supercritical state of matter and, for this reason, it was believed that supercritical thermodynamic properties vary smoothly and without any qualitative changes. Here, we perform extensive molecular dynamics simulations in a wide temperature range and find that a deeply supercritical state is thermodynamically heterogeneous, as witnessed by different temperature dependence of energy, heat capacity and its derivatives at low and high temperature. The evidence comes from three different methods of analysis, two of which are model-independent. We propose a new definition of the relative width of the thermodynamic crossover and calculate it to be in the fairly narrow relative range of 13-20\%. On the basis of our results, we relate the crossover to the supercritical Frenkel line.

cond-mat.stat-mech↗