Particle Of Gas Research Articles

Background gravitational waves as signature of the accelerated cosmic expansion

We propose a toy model of a spherical universe made up of an exotic dark gas with temperature T in thermal equilibrium with a black-body in adiabatic expansion. Each particle of this exotic gas mimics a kind of particle of dark energy. So, each dark particle occupies a very small area of space so-called Planck area [Formula: see text], which represents the minimum area of the whole space-time given by the spherical surface with area [Formula: see text], where [Formula: see text] is the Hubble radius. We realize that such spherical surface is the surface of the black-body for representing the dark universe as if it were the surface of an expanding balloon. Thus, we are able to derive the law of universal gravitation, thus leading us to understand the cosmological anti-gravity. We estimate the tiny order of magnitude of the cosmological constant and the acceleration of expansion of the dark sphere. In this toy model, as the dark universe can be thought of as a large black body, when we obtain its power and frequency of emission of radiation, we find very low values. We conclude that such radiation and frequency of the black body made up of dark energy is a background gravitational wave with very low frequency in the order of [Formula: see text] due to the slight stretching of the fabric of space-time.

Background gravitational waves as signature of the adiabatic expansion of a black body that represents the dark universe

We propose a toy model of a spherical universe made up of an exotic dark gas with temperature T in thermal equilibrium with a black body in adiabatic expansion. Each particle of this exotic gas mimics a kind of particle of dark energy represented by the vacuum energy, being quantized into virtual particles with extremely small masses that form such gas representing the own tissue of the expanding space–time governed by a negative pressure whose origin is the equation of state of vacuum, i.e., p = −ρ, where ρ is the vacuum energy density. So, each vacuum particle occupies a tiny area of space so-called Planck area [Formula: see text], which represents the minimum area of the whole space–time given by the spherical surface with area [Formula: see text], where RH is the Hubble radius. We realize that such spherical surface is the surface of the black body for representing the dark universe as if it were the surface of an expanding balloon. Thus, we are able to derive the law of universal gravitation, thus leading us to understand the cosmological anti-gravity. We estimate the very small order of magnitude of the cosmological constant and the acceleration of expansion of the dark sphere. In this toy model, as the dark universe can be thought of as a large black body, when we obtain its power and frequency of emission of radiation, we find very low values. We conclude that such radiation and frequency of the black body made up of dark energy is a background gravitational wave with very low frequency in the order of 10−17Hz due to the slight stretching of the fabric of space–time.

Exact result for the polaron mass in a one-dimensional Bose gas

We study the polaron quasiparticle in a one-dimensional Bose gas. In the integrable case described by the Yang-Gaudin model, we derive an exact result for the polaron mass in the thermodynamic limit. It is expressed in terms of the derivative with respect to the density of the ground-state energy per particle of the Bose gas without the polaron. This enables us to find high-order power series for the polaron mass in the regimes of weak and strong interaction.

Local Density Approximation for the Short-Range Exchange Free Energy Functional.

Analytical expressions for the exchange free energy per particle of the uniform electron gas (UEG) associated with the short-range (SR) interelectronic interaction at the low- and high-temperature limits are examined, yielding an accurate analytical parametrization for the SR exchange free energy per particle of the UEG as a function of the uniform electron density, temperature, and range-separation parameter. This parametrization constitutes the local density approximation for the SR exchange free energy functional, which can be the first step toward finding generally accurate range-separated hybrid functionals in both finite-temperature density functional theory and thermally assisted-occupation density functional theory.

Four-component relativistic range-separated density-functional theory: Short-range exchange local-density approximation.

We lay out the extension of range-separated density-functional theory to a four-component relativistic framework using a Dirac-Coulomb-Breit Hamiltonian in the no-pair approximation. This formalism combines a wave-function method for the long-range part of the electron-electron interaction with a density(-current) functional for the short-range part of the interaction. We construct for this formalism a short-range exchange local-density approximation based on calculations on a relativistic homogeneous electron gas with a modified Coulomb-Breit electron-electron interaction. More specifically, we provide the relativistic short-range Coulomb and Breit exchange energies per particle of the relativistic homogeneous electron gas in the form of Padé approximants which are systematically improvable to arbitrary accuracy. These quantities, as well as the associated effective Coulomb-Breit exchange hole, show the important impact of relativity on short-range exchange effects for high densities.

Entropy Signatures of Topological Phase Transitions

We review the behavior of the entropy per particle in various two-dimensional electronic systems. The entropy per particle is an important characteristic of any many body system that tells how the entropy of the ensemble of electrons changes if one adds one more electron. Recently, it has been demonstrated how the entropy per particle of a two-dimensional electron gas can be extracted from the recharging current dynamics in a planar capacitor geometry. These experiments pave the way to the systematic studies of entropy in various crystal systems including novel two-dimensional crystals such as gapped graphene, germanene and silicene. Theoretically, the entropy per particle is linked to the temperature derivative of the chemical potential of the electron gas by the Maxwell relation. Using this relation, we calculate the entropy per particle in the vicinity of topological transitions in various two-dimensional electronic systems. We show that the entropy experiences quantized steps at the points of Lifshitz transitions in a two-dimensional electronic gas with a parabolic energy spectrum. In contrast, in doubled-gapped Dirac materials, the entropy per particles demonstrates characteristic spikes once the chemical potential passes through the band edges. The transition from a topological to trivial insulator phase in germanene is manifested by the disappearance of a strong zero-energy resonance in the entropy per particle dependence on the chemical potential. We conclude that studies of the entropy per particle shed light on multiple otherwise hidden peculiarities of the electronic band structure of novel two-dimensional crystals.

Energy fluctuation and discontinuity of specific heat

The specific heat per particle (cv) of an ideal gas, on many occasions, is interpreted as energy fluctuation per particle (▵ϵ2) of the ideal gas through the relation ▵ϵ2 = kT2cv, where k is the Boltzmann constant and T is the temperature. This relationship is true only in the classical limit and deviates significantly in the quantum degenerate regime. We have analytically explored quantum to classical crossover of this relationship, in particular, for 3D free Bose and Fermi gases. We have explored the same for harmonically trapped cases. We have obtained a hump of around its condensation point for 3D harmonically trapped Bose gas. We have discussed the possibility of an occurring phase transition with the discontinuity of heat capacity from the existence of such a hump for other Bose and Fermi systems.

Acoustic pressure losses in woven screen regenerators

In thermoacoustic travelling-wave engines and other Stirling cycle devices, good performance depends on the material of a regenerator being in intimate contact with the gas inside it, so that each particle of gas oscillates in temperature following the adjacent material as it is acoustically displaced. This requires that the passages are small enough for temperature waves to penetrate across the gas path with the frequencies of interest. One type of ‘regenerator’ that is commonly used for this purpose is composed of multiple layers of woven stainless steel mesh, laid on top of one another in random registration. Associated with the thermal penetration is a viscous loss of pressure and this must be quantified if efficient engines are to be designed.In the literature, reliance has been placed on the correlation of steady-flow loss data for these meshes, but for the coarser ones operating at frequencies greater than 28Hz, the assumption of quasi steady-flow is dubious and direct acoustic measurements must be made. This paper reports acoustic pressure loss data for meshes with 34 and 75 wires per inch taken in two configurations of impedance tube, and finds that the dependence on velocity is the same as in steady-flow, but that there is indeed some enhancement of loss for frequencies above 40Hz. (Separation of the mesh layers is probably responsible for the anomalously low loss coefficients that were recorded in one set of data.) It is shown that the acoustic pressure losses can be correlated in terms that give the acoustic impedance more directly than the friction factor correlations.

Orbital evolution under the action of fast interstellar gas flow with a non-constant drag coefficient

The acceleration of a spherical dust particle caused by an interstellar gas flow depends on the drag coefficient which is, for the given particle and flow of interstellar gas, a specific function of the relative speed of the dust particle with respect to the interstellar gas. We investigate the motion of a dust particle in the case when the acceleration caused by the interstellar gas flow represent a small perturbation to the gravity of a central star. We present the secular time derivatives of the Keplerian orbital elements of the dust particle under the action of the acceleration from the interstellar gas flow for arbitrary orbit orientation. The semimajor axis of the dust particle is a decreasing function of time for an interstellar gas flow acceleration with constant drag coefficient and also for such an acceleration with the linearly variable drag coefficient. The decrease of the semimajor axis is slower for the interstellar gas flow acceleration with the variable drag coefficient. The minimal and maximal values of the decrease of the semimajor axis are determined. In the planar case, when the interstellar gas flow velocity lies in the orbital plane of the particle, the orbit always approaches the position with the maximal value of the transversal component of the interstellar gas flow velocity vector measured at perihelion. The properties of the orbital evolution derived from the secular time derivatives are consistent with numerical integrations of the equation of motion. If the interstellar gas flow speed is much larger than the speed of the dust particle, then the linear approximation of dependence of the drag coefficient on the relative speed of the dust particle with respect to the interstellar gas is usable for practically arbitrary (no close to zero) values of the molecular speed ratios (Mach numbers).

마이크로 반구 쉘 형상의 화학증착 탄화규소 TRISO 코팅층의 파괴강도 직접평가

CVD-SiC coating has been introduced as a protective layer in TRISO nuclear fuel particle of high temperature gas cooled reactor (HTGR) due to its excellent mechanical stability at high temperature. In order to prevent the failure of the TRISO particles, it is important to evaluate the fracture strength of the SiC coating layer. It is needed to develop a new simple characterization technique to evaluate the mechanical properties of the coating layer as a pre-irradiation step. In present work, direct strength measurement method with the specimen of hemi-spherical shell configuration was suggested. The indentation experiment on a hemisphere shell with a plate indenter was conducted. The fracture strength of the coating layer is related with the critical load for radial cracking of the shell. The finite element analysis was used to drive the semi-empirical equation for the strength measurement. The SiC hemispherical shells were successfully recovered from the section-grinding of TRISO coated particle and successive heat treatment in air. The strength of CVDSiC coating layer was evaluated from the experimentally measured critical load during the indentation on SiC hemisphere shell. Weibull diagram of fracture strength was also constructed. This study suggested a new strength equation and experimental method to measure the fracture strength of CVD-SiC coating of TRISO coated fuel particles.

Large Scatter in X-Ray Emission among Elliptical Galaxies: Correlations with Mass of Host Halo

Optically similar elliptical galaxies have an enormous range of X-ray luminosities. We show that this range can be attributed to large variations in the dark halo mass Mvir determined from X-ray observations. The K-band luminosity of ellipticals varies with virial mass, LK ∝ M, but with considerable scatter, probably due to the stochastic incidence of massive satellite galaxies that merge by dynamical friction to form group-centered ellipticals. Both the observed X-ray luminosity LX ∝ M and LX/LK ∝ M are sufficiently sensitive to the virial mass to explain the wide variation observed in LX among galaxies of similar LK. The central galaxy supernova energy per particle of diffuse gas increases dramatically with decreasing virial mass, and elliptical galaxies with the lowest X-ray luminosities (and Mvir) are easily explained by supernova-driven outflows.

화학증착법에 의하여 제조된 탄화규소 코팅층의 기계적 특성

SiC coating has been introduced as protective layer in TRISO nuclear fuel particle of High Temperature Gas cooled Reactor (HTGR) due to excellent mechanical stability at high temperature. In order to inhibit the failure of the TRISO particles, it is important to evaluate the fracture strength of the SiC coating layer. In present work, thin silicon carbide coating was fabricated using chemical vapor deposition process with different microstructures and thicknesses. Processing condition and surface status of substrate affect on the microstructure of SiC coating layer. Sphere indentation method on trilayer configuration was conducted to measure the fracture strength of the SiC film. The fracture strength of SiC film with different microstructure and thickness were characterized by trilayer strength measurement method nanoindentation technique was also used to characterize the elastic modulus and the hardness of the SiC film. Relationships between microstructure and mechanical properties of CVD SiC thin film were discussed.

Effect of spontaneous spin depopulation on the ground-state energy of a two-dimensional spintronic system

We calculate the ${r}_{s}$ expansion of the ground-state energy per particle of a two-dimensional electron gas with spin-orbit interaction induced by the Rashba coupling. At high areal electron density, we obtain the energy for noninteracting electrons, their exchange energy, and the lowest-order approximation for the correlation energy. A closed-form expression is obtained for the energy of noninteracting electrons. However, we must calculate the exchange and correlation energy numerically. As the density is increased and ${r}_{s}$ decreases, the ground-state energy changes rapidly at some value ${r}_{s}={r}_{s}^{*}$ and the Fermi energy ${E}_{F}$ changes from negative to positive. When ${E}_{F}l0$, only the lower spin subband is occupied. An interesting effect occurs in the presence of electron-electron interaction as ${E}_{F}$ increases through $E=0$ and the upper spin subband suddenly gets a finite population rather than increasing gradually.

Nonisothermal Brownian motion: Thermophoresis as the macroscopic manifestation of thermally biased molecular motion

A quiescent single-component gravity-free gas subject to a small steady uniform temperature gradient T, despite being at rest, is shown to experience a drift velocity UD=-D* gradient ln T, where D* is the gas's nonisothermal self-diffusion coefficient. D* is identified as being the gas's thermometric diffusivity alpha. The latter differs from the gas's isothermal isotopic self-diffusion coefficient D, albeit only slightly. Two independent derivations are given of this drift velocity formula, one kinematical and the other dynamical, both derivations being strictly macroscopic in nature. Within modest experimental and theoretical uncertainties, this virtual drift velocity UD=-alpha gradient ln T is shown to be constitutively and phenomenologically indistinguishable from the well-known experimental and theoretical formulas for the thermophoretic velocity U of a macroscopic (i.e., non-Brownian) non-heat-conducting particle moving under the influence of a uniform temperature gradient through an otherwise quiescent single-component rarefied gas continuum at small Knudsen numbers. Coupled with the size independence of the particle's thermophoretic velocity, the empirically observed equality, U=UD, leads naturally to the hypothesis that these two velocities, the former real and the latter virtual, are, in fact, simply manifestations of the same underlying molecular phenomenon, namely the gas's Brownian movement, albeit biased by the temperature gradient. This purely hydrodynamic continuum-mechanical equality is confirmed by theoretical calculations effected at the kinetic-molecular level on the basis of an existing solution of the Boltzmann equation for a quasi-Lorentzian gas, modulo small uncertainties pertaining to the choice of collision model. Explicitly, this asymptotically valid molecular model allows the virtual drift velocity UD of the light gas and the thermophoretic velocity U of the massive, effectively non-Brownian, particle, now regarded as the tracer particle of the light gas's drift velocity, to each be identified with the Chapman-Enskog "thermal diffusion velocity" of the quasi-Lorentzian gas, here designated by the symbol UM/M, as calculated by de la Mora and Mercer. It is further pointed out that, modulo the collective uncertainties cited above, the common velocities UD,U, and UM/M are identical to the single-component gas's diffuse volume current jv, the latter representing yet another, independent, strictly continuum-mechanical concept. Finally, comments are offered on the extension of the single-component drift velocity notion to liquids, and its application towards rationalizing Soret thermal-diffusion separation phenomena in quasi-Lorentzian liquid-phase binary mixtures composed of disparately sized solute and solvent molecules, with the massive Brownian solute molecules (e.g., colloidal particles) present in disproportionately small amounts relative to that of the solvent.

Three-dimensional hydrodynamical modeling of the formation of the accretion disk in the SS 433 binary system

Three-dimensional hydrodynamical modeling of the formation of the accretion disk in the SS 433 binary system is carried out with various types of cooling and numerical grids. These computations show that a thick accretion disk with a height of 0.25–0.30 (in units of the component separation) is formed around the compact object, from a flow with a large radius (0.2–0.3 in the same units) that forms in the vicinity of the inner Lagrangian point. This disk has the form of a flattened torus. The number of orbits of a particle of gas in the disk is 100–150, testifying to a minimal influence of numerical viscosity in these computations. The computations also show that the stream flowing from L1 is nearly conservative, and spirals in the disk are not formed due to the influence of the donor gravitation.

Spin resolution of the electron-gas correlation energy: Positive same spin contributions

The negative correlation energy ${\ensuremath{\epsilon}}_{c}{(r}_{s},\ensuremath{\zeta})$ per particle of a uniform electron gas of density parameter ${r}_{s}$ and spin polarization $\ensuremath{\zeta}$ is well known, but its spin resolution into $\ensuremath{\uparrow}\ensuremath{\downarrow},$ $\ensuremath{\uparrow}\ensuremath{\uparrow},$ and $\ensuremath{\downarrow}\ensuremath{\downarrow}$ contributions is not. Widely used estimates are incorrect, and hamper the development of reliable density functionals and pair distribution functions. For the spin resolution, we present interpolations between high- and low-density limits that agree with available quantum Monte Carlo data. In the low-density limit for $\ensuremath{\zeta}=0,$ we find that the same-spin correlation energy is unexpectedly positive, and we explain why. We also estimate the $\ensuremath{\uparrow}$ and $\ensuremath{\downarrow}$ contributions to the kinetic energy of correlation.

Correlations in the Kosterlitz-Thouless phase of the two-dimensional Coulomb gas

The particle and charge correlations of the two-dimensional Coulomb gas are studied in the dielectric phase. A term-by-term analysis of the low-fugacity expansions suggests that the large-distance behaviors of the particle correlations are governed by multipolar interactions, similar to what happens in a system of permanent dipoles. These behaviors are compatible with the asymptotic structure of the BGY hierarchy equations; on the other hand, a new identity for the dielectric constant ɛ is used to show that the four-particle correlations decay as the dipole-dipole potential 1/r2 when two neutral pairs are separated by a large distancer. Near the zero-density critical point of the Kosterlitz-Thouless transition, we resum the low-fugacity expansions of both 1/ɛ and the charge correlation C(r). We thus retrieve the coupling constant flow equations of the renormalization group as well as the effective interaction energy of the iterated mean-field theory by Kosterlitz and Thouless. The coupling constant at the RG fixed point is then identified with 1/ɛ. The nonanalyticity of 1/ɛ at the transition turns out to coincide with the divergence of the low-fugacity series for this quantity. The leading term in the large-distance behavior of C(r) is found to be the same as for external charges. Moreover, we exhibit the subleading terms which also contribute to 1/ɛ.

Stochastic model of a dense spinless hard-sphere gas

The motion of a spinless quantum particle subjected to random hard-sphere scatterings is studied in the framework of stochastic mechanics. The result is applied to the case of a generic particle of a dense spinless hard-sphere gas leading to a description that displays the competition between the thermal and the quantum noise. Finally, the model is discussed in connection with the superfluidity phenomenon.

Correlated hopping in honeycomb lattice: tracer diffusion coefficient at arbitrary lattice gas concentration

The tracer diffusion coefficient is investigated for a correlated diffusion of a tracer, which is a particle of the lattice gas. The lattice gas consists of particles of arbitrary concentration hopping on a two-dimensional honeycomb lattice, which interact only as repulsing point hard cores. Exchange between different particles is excluded, and any particle can jump only to the nearest-neighbour empty sites. The tracer diffusion coefficient is studied in a wide range of concentrations by the Monte Carlo simulations and compared with different theoretical predictions. It is shown that the Nakazato and Kitahara theory (1980) as well as the Tahir-Kheli and Elliot (1982) one give a good description of the simulation data. Moreover, it is proved that the traditional tracer diffusion coefficient that results from direct correlations over two consecutive jumps of a tracer is in distinct disagreement with the simulation data. Therefore a generalised expression for the tracer diffusion coefficient is derived; it includes direct correlations over an arbitrary number of jumps expressed in terms of integrals over 'waiting-time distributions'. The description of the data is much improved already by taking into account direct correlations over four jumps. Hence, it is concluded that the Nakazato and Kitahara theory as well as that of Tahir-Kheli and Elliott must include direct correlations over several consecutive jumps of a tracer-particle, which is not evident from the formalisms of these authors.

Accurate spin-dependent electron liquid correlation energies for local spin density calculations: a critical analysis

We assess various approximate forms for the correlation energy per particle of the spin-polarized homogeneous electron gas that have frequently been used in applications of the local spin density approximation to the exchange-correlation energy functional. By accurately recalculating the RPA correlation energy as a function of electron density and spin polarization we demonstrate the inadequacies of the usual approximation for interpolating between the para- and ferro-magnetic states and present an accurate new interpolation formula. A Padé approximant technique is used to accurately interpolate the recent Monte Carlo results (para and ferro) of Ceperley and Alder into the important range of densities for atoms, molecules, and metals. These results can be combined with the RPA spin-dependence so as to produce a correlation energy for a spin-polarized homogeneous electron gas with an estimated maximum error of 1 mRy and thus should reliably determine the magnitude of non-local corrections to the local spin density approximation in real systems.