arXiv subjects

M. V. Kondrin

Publications and source records attributed to M. V. Kondrin.

18 recordsLinked to original sources

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

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.

cond-mat.stat-mech↗

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

Temperature dependence of Fano resonance in nanodiamonds synthesized at high static pressures

Temperature dependence of Fano resonance, recently discovered in infra-red (IR spectra of nanodiamonds synthesized from chloroadamantane at high static pressures, is investigated. For the first time, marked variations of the resonance parameteres are observed. On heating, the shape of the Fano resonance changes considerably; the effect completely disappears above 350 C, but is recovered after cooling to ambient conditions. Such behaviour implies that assignment of the Fano effect to the surface transfer doping mechanism is not very plausible for the studied samples. The resonance shape varies due to strong temperature dependence of the difference of frequencies of IR-active "bright" and Raman-active "dark" modes of nanodiamond. The frequency of the Raman "dark" mode is only weakly temperature-dependent.

cond-mat.mes-hall↗

arXiv2022

Size-dependent thermal stability and optical properties of ultra-small nanodiamonds synthesized under high pressure

Diamond properties down to the quantum-size region are still poorly understood. High-pressure high-temperature (HPHT) synthesis from chloroadamantane molecules allows precise control of nanodiamond size. Thermal stability and optical properties of nanodiamonds with sizes spanning range from $<$1 to 8 nm are investigated. It is shown that hypothesis about enhanced thermal stability of nanodiamonds smaller than 2 nm is incorrect. The most striking feature in IR absorption of these samples is the appearance of an enhanced transmission band near the diamond Raman mode (1332 cm$^{-1}$). Following previously proposed explanation, we attribute this phenomenon to the Fano-effect caused by resonance of diamond Raman mode with continuum of conductive surface states. We assume that these surface states may be formed by reconstruction of broken bonds on the nanodiamond surfaces. This effect is also responsible for the observed asymmetry of Raman scattering peak. he mechanism of nanodiamond formation in HPHT synthesis is proposed, explaining pecularities of their structure and properties.

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 ($α$-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↗

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↗

arXiv2018

Organic crystalline polymers: structural properties and way to synthesis under high pressure

We consider different structures, which can be obtained by polymerization of aromatic organic molecules under high pressures. These 2D and 3D covalently bonded organic polymers and their functionalization can pave the way to production of energy storage and conversion devices. High-pressure synthesis might serve as a useful hint for production of these structures and their functionalized analogs by means of wet chemical synthesis.

cond-mat.mtrl-sci↗

arXiv2016

Bulk graphanes synthesized from benzene and pyridine

We report a high-pressure high-temperature synthesis of several millimeter-sized samples of bulk graphanes with the composition C-H(D) from benzene and graphene-derivative C-H-N$_{0.2}$ from pyridine. X-ray diffraction, transmission electron microscopy, and infrared spectroscopy of new materials reveal relatively large (several nanometers in size) crystalline grains of an sp3-bonded graphane lattice (3-cycle-4-step, the orthorhombic structure with Pbca space group and parameters a = 9.5-9.8, b = 8.9-9.1, c = 17.1-17.3 $Å$). The main hydrogen groups in samples are C-H groups connected by aliphatic bonds. The synthesized graphanes at atmospheric pressure are stable up to 500 C. The macroscopic density of CH samples is 1.5-1.57 g cm$^{-3} $ and the refractive index is 1.78-1.80. The absorption spectra of samples with a high degree of crystallization exhibits a weak absorption maximum at 2.8 eV, which is responsible for the yellow-orange color, large absorption maximum at 4 eV and an absorption edge associated with the width of the optical gap at 5.2 eV. The bulk modulus (30-37 GPa) and shear modulus (15-18 GPa) of the fabricated samples, as well as their hardness (1-1.5 GPa), are about twice as high as the respective values for polycrystalline graphite. The solution of metalorganic complexes in benzene and pyridine makes it possible to obtain doped graphanes, which can have extraordinary electron transport and magnetic properties.

cond-mat.mtrl-sci↗

arXiv2015

Luminescent Ge-related centre in high-pressure synthesized diamond

We report on the high-pressure synthesis of novel nano- and microcrystalline high-quality diamonds with luminescent Ge-related centers. Observation of the four-line fine structure in luminescence at 2 eV (602 nm) at temperatures below 80 K manifests a high quality of diamonds. We demonstrate germanium and carbon isotope shifts in the fine structure of luminescence at 602 nm and its vibrational sideband which allows us to unambiguously associate the center with the germanium impurity entering into the diamond lattice. We show that there are two ground-state energy levels with the separation of 0.7 meV and two excited-state levels separated by 4.6 meV in the electronic structure of the center and suggest a split-vacancy structure of this center.

cond-mat.mtrl-sci↗

arXiv2015

Diamond monohydride: The most stable three-dimensional hydrocarbon

Most of hydrocarbons are either molecular structures or linear polymeric chains. Discovery of graphene and manufacturing of its monohydride -- graphane incite interest in search for three-dimensional hydrocarbon polymers. However up to now all hypothetical hydrocarbon lattices significantly lose in energy to stacked graphane sheets and solid benzene. We propose completely covalently bonded solid carbon monohydride whose density significantly exceeds one of its isomers (graphane, cubane, solid benzene). Ab-initio calculation demonstrates that the cohesion energy of this structure at least is not worse than the energy of graphane and benzene. In some aspect the crystal structure of hydrocarbon presented can be regarded as a sublattice of diamond, but with the symmetry of P-3 space group (lattice parameters a ~ 6.925 A, c ~ 12.830 A) and Z=42 formula units per unit cell. This structure (if synthesized) may turn out to be interesting to applications.

cond-mat.mtrl-sci↗

arXiv2014

Spin-polarized electron transport in the high-pressure ferromagnetic phases (GaSb)$_2$M (M=Cr,Mn)

For the first time magnetrotransport of the ferromagnetic high-pressure phases of (GaSb)$_2$M (M=Cr,Mn) was measured. It was found that the main component of magnetotransport in these phases is negative and its amplitude is increasing with temperature rising and approaching Curie temperature. Measuring of magnetoresistance in the cycling magnetic fields demonstrated that the Yosida component of magnetresistance is negligible and the main part of the magnetoresistance can be attributed to the spin-polarized electron transport.

cond-mat.mtrl-sci↗

arXiv2013

Phase transformations in methanol at high pressure measured by dielectric spectroscopy technique

Dielectric response in methanol measured in wide pressure and temperature range ($P < 6.0$ GPa; 100 K $ $ 1.2 GPa $T < 270$ K. In the intermediate P-T region $P \approx 3.4-3.7$ GPa $T \approx 260-280$ K a set of complicated structural transformations occurs involving four methanol crystalline structures. At higher pressures within a narrow range $P \approx 4.3-4.5$ GPa methanol can be obtained in the form of fragile glass ($T_g \approx 200$ K, $m_p \approx 80$ at $P= 4.5$ GPa) by relatively slow cooling.

cond-mat.mtrl-sci↗

arXiv2013

Dielectric Spectroscopy and Ultrasonic Study of Propylene Carbonate under Ultra-high Pressures

We present the high pressure dielectric spectroscopy (up to 4.2 GPa) and ultrasonic study (up to 1.7 GPa) of liquid and glassy propylene carbonate (PC). Both of the methods provide complementary pictures of the glass transition in PC under pressure. No other relaxation processes except $α$-relaxation have been found in the studied pressure interval. The propylene carbonate liquid is a glassformer where simple relaxation and the absence of $β$-relaxation are registered in the record-breaking ranges of pressures and densities. The equation of state of liquid PC was extended up to 1 GPa from ultrasonic measurements of bulk modulus and is in good accordance with the previous equations developed from volumetric data. We measured the bulk and shear moduli and Poisson's ratio of glassy PC up to 1.7 GPa. Many relaxation and elastic properties of PC can be qualitatively described by the soft-sphere or Lennard-Jones model. However, for the quantitative description of entire set of the experimental data, these models are insufficient. Moreover, the Poisson coefficient value for glassy PC indicates a significant contribution of non-central forces to the intermolecular potential. The well-known correlation between Poisson's ratio and fragility index (obtained from dielectric relaxation) is confirmed for PC at ambient pressure, but it is violated with pressure increase. This indicates that different features of the potential energy landscape are responsible for the evolution of dielectric response and elasticity with pressure increase.

cond-mat.soft↗

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↗

arXiv2011

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 CrGa$_2$Sb$_2$ phases with an orthorhombic structure $Iba2$ 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 $I4/mcm$.

cond-mat.mtrl-sci↗

arXiv2010

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.

cond-mat.dis-nn↗