Values Of Coupling Parameter Research Articles

Classical molecular dynamics simulations of hydrogen plasmas and development of an analytical statistical model for computational validity assessment

Classical molecular dynamics simulations of hydrogen plasmas have been performed with an emphasis on the analysis of the equilibration process. The theoretical basis of the simulation model as well as numerically relevant aspects, such as the proper choice and definition of simulation units, are discussed in detail, thus proving a thorough implementation of the computer simulation technique. Because of the lack of experimental data, molecular dynamics simulations are often considered as idealized computational experiments for benchmarking of theoretical models. However, these simulations are certainly challenging and consequently a validation procedure is also demanded. In this work we develop an analytical statistical equilibrium model for computational validity assessment of plasma particle dynamics simulations. Remarkable agreement between model and molecular dynamics results including a classical treatment of the ionization-recombination mechanism is obtained for a wide range of plasma coupling parameter values. Furthermore, the analytical model provides guidance to securely terminate simulation runs once the equilibrium stage has been reached, which in turn gives confidence in the statistics that potentially may be extracted from time histories of simulated physical quantities.

Ab initio investigation of electron-phonon interaction in LaSn3 and CaSn3

ABSTRACTWe present results for the structural, electronic, vibrational, and electron-phonon coupling properties of LaSn3 and CaSn3 adopting the simple cubic AuCu3-type structure obtained using the the generalised gradient approximation of the density functional theory and plane wave ab initio pseudopotential method. Our electronic results show that both materials display metallic character with several bands, which have mainly Sn 5p character, crossing the Fermi level. The calculated phonon spectrum of LaSn3 accords very well with reported experimental measurements. The weights of the peaks in the Eliashberg spectral function of both compounds are enhanced with the use of experimental lattice constant in our electron-phonon calculation, increasing the value of average electron phonon coupling parameter from 0.876 to 0.937 for LaSn3 (by 7%) and from 0.642 to 0.725 for CaSn3 (by 13%). The use of experimental lattice constant also improves the agreement between theoretical and experimental values of the superconducting temperature for both compounds.

Role of spin-orbit coupling in the physical properties of LaX3 ( X=In , P, Bi) superconductors

We report a comprehensive and complementary study on structural, elastic, mechanical, electronic, phonon, and electron-phonon interaction properties of $\mathrm{La}{X}_{3}$ ($X$ = In, Pb, and Bi) using first-principles density functional calculations within the local density approximation with and without the spin-orbit coupling (SOC). The calculated lattice parameters for these intermetallic compounds with and without SOC are found to differ by less than $2%$ from their experimental values. The effect of SOC on the elastic, mechanical, electronic, phonon, and electron-phonon interaction properties is more profound for ${\mathrm{LaPb}}_{3}$ and ${\mathrm{LaBi}}_{3}$ containing heavier $X$ elements rather than ${\mathrm{LaIn}}_{3}$ containing lighter $X$ element. The inclusion of SOC considerably removes the degeneracies of some bands near the Fermi level and makes some phonon branches in ${\mathrm{LaPb}}_{3}$ and ${\mathrm{LaBi}}_{3}$ softer and increases the strength of dominant peaks in their Eliashberg spectral functions. Thus the SOC related enhancement of their electron-phonon coupling parameter values can be related to both a softening of their phonon dispersion curves and an increase in their electron-phonon coupling matrix elements. The superconducting transition temperature with SOC is computed to be 0.69 K for ${\mathrm{LaIn}}_{3}$, 4.23 K for ${\mathrm{LaPb}}_{3}$, and 6.87 K for ${\mathrm{LaBi}}_{3}$, which agree very well with the respective measured values of 0.70, 4.18, and 7.30 K.

Electronic, phonon and superconducting properties of LaPtBi half-Heusler compound

In the framework of density functional theory based on plane wave pseudopotential method and linear response technique, we have studied the electronic, phonon and superconducting properties of LaPtBi half-Heusler compound. The electronic band structure and density of states show that it is gapless semiconductor which is consistent with previous results. The positive phonon frequencies confirm the stability of this compound in cubic MgAgAs phase. Superconductivity is studied in terms of Eliashberg spectral function (α2F(ω)), electron-phonon coupling constants (λ). The value of electron-phonon coupling parameter is found to be 0.41 and the superconducting transition temperature is calculated to be 0.76 K, in excellent agreement with the experimentally reported values.

Symmetric bound states of Josephson vortices in BEC

A system of two parallel coupled cigar-shaped Bose–Einstein condensates is considered in an effectively one-dimensional limit. The dynamics of the system is characterized by a pair of coupled nonlinear Gross–Pitaevskii equations. In particular, the existence and stability of symmetric bound states of Josephson vortices are investigated. It is realized that the symmetric bound state Josephson vortices solution persists stably in its whole domain of existence for the coupling strength. Nevertheless, the bound states solution converts into a dark soliton at a critical value of coupling parameter.

Coupled Multiple Timescale Dynamics in Populations of Endocrine Neurons: Pulsatile and Surge Patterns of GnRH Secretion

The gonadotropin-releasing hormone (GnRH) is secreted by hypothalamic neurons into the pituitary portal blood in a pulsatile manner. The alternation between a frequency-modulated pulsatile regime and the ovulatory surge is the hallmark of the GnRH secretion pattern in ovarian cycles of female mammals. In this work, we aim at modeling additional features of the GnRH secretion pattern: the possible occurrence of a two-bump surge (``camel surge'') and an episode of partial desynchronization before the surge. We propose a six-dimensional extension of a former four-dimensional model with three timescales and introduce two mutually coupled, slightly heterogenous GnRH subpopulations (secretors) regulated by the same slow oscillator (regulator). We consider two types of coupling functions between the secretors, including dynamic state-dependent coupling, and we use numerical and analytic tools to characterize the coupling parameter values leading to the generation of a two-bump surge in both coupling cases. We reveal the impact of the slowly varying control exerted by the regulator onto the pulsatile dynamics of the secretors, which leads to dynamic bifurcations and gives rise to desynchronization. To assess the occurrence time of desynchronization during the pulsatile phase, we introduce asymptotic tools based on quasi-static and geometric approaches, as well as analytic tools based on the H-function derived from the phase equation and numerical tracking of period-doubling bifurcations. We discuss the role of coupling parameters in the two-bump surge generation and the speed of desynchronization.

Electron-phonon superconductivity in the filled skutterudites LaRu4P12, LaRu4As12 , and LaPt4Ge12

We have reported on the structural, elastic, electronic, lattice dynamical and electron-phonon interaction properties of $\text{La}{T}_{4}{X}_{12}$ ($T=\text{Ru}$, Pt and $X=\text{P}$, As, Ge) by using the generalized gradient approximation of the density functional theory and the plane-wave pseudopotential method. These lanthanum-filled skutterudites are found to be characterized with a flat band, resulting in a sharp peak in the electronic density of states, near the Fermi level. The lattice dynamical properties of these materials change considerably when the P atom is replaced by larger As or Ge atoms. The Migdal-Eliashberg approach is used to determine the Eliashberg spectral function for all the considered compounds. Using the calculated Eliashberg spectral function, the value of average electron-phonon coupling parameter is found to be 0.74 for ${\mathrm{LaRu}}_{4}{\mathrm{P}}_{12}$, 1.03 for ${\mathrm{LaRu}}_{4}{\mathrm{As}}_{12}$, and 1.08 for ${\mathrm{LaPt}}_{4}{\mathrm{Ge}}_{12}$. The superconducting critical temperature (${T}_{c}$) values for ${\mathrm{LaRu}}_{4}{\mathrm{P}}_{12}, {\mathrm{LaRu}}_{4}{\mathrm{As}}_{12}$, and ${\mathrm{LaPt}}_{4}{\mathrm{Ge}}_{12}$ are estimated to be 6.95, 11.56, and 8.32 K, respectively, which compare well enough with their experimentally measured values of 7.2, 10.45, and 8.23 K.

Anomalous torsional tripling in the ν9 and ν10 CH3-deformation modes of ethane 12CH313CH3

We have investigated the anomalous torsional behavior in the coupled ν9 and ν10 vibrational fundamentals of 12CH313CH3, both states exhibiting a splitting into three components, instead of two, only in those rotational levels which are very close to resonance. We conclude that the intrinsic additional splitting, which occurs in the E-torsional components, for these two vibrational states is too small to be detected in the high resolution infrared spectrum, but it is substantively enhanced by their coupling. It is shown that this effect requires the simultaneous action of torsion independent operators, such as Fermi-type and z-Coriolis, not allowed in the more symmetric isotopologue 12CH312CH3, and torsion dependent operators, such as torsional-Coriolis, connecting the two vibrational states. Our conclusions lead to a simple model for the coupling of ν9 and ν10, with effective Fermi-type matrix elements W for the A-torsional components, and W±w for the two pairs of E-torsional components. This causes the additional splitting in the E-pairs.This model is consistent with the mechanism causing the Coriolis-dependent decrease of the A-E torsional splitting in degenerate vibrational states.Exploratory calculations were performed making use of results from a normal mode analysis, showing that the effects predictable by the proposed model are of the correct order of magnitude compared to the observed features, with coupling parameter values reasonably consistent with those determined by the least squares fit of the observed transition wavenumbers.

Bound states of atomic Josephson vortices

We study the existence and stability of the bound state Josephson vortices solution in two parallel quasi one-dimensional coupled Bose–Einstein condensates. The system can be elucidated by linearly coupled Gross–Pitaevskii equations. The purpose of this study is to investigate the effects of altering the strength of coupling between the two condensates over the stability of the bound-state Josephson vortices. It is found that the stability of bound-state Josephson vortices depends on the value of coupling strength. However, at a critical value of coupling parameter, the Josephson vortices solution transforms into a coupled dark soliton.

Reconstructed anisotropic models in F(T, TG) gravity

We explore the reconstruction scenario of a spatially homogenous and anisotropic universe model in the framework of F(T, TG) gravity (T represents the torsion scalar and TG is the teleparallel equivalent of the Gauss–Bonnet term). We construct F(T, TG) models by assuming different phases of the universe, like a dark energy dominated era, non-relativistic as well as relativistic matter eras, and their combinations. The graphical behavior of these models indicates a decreasing pattern for ω = −1 and its combination with ω = 0 and 1/3. Finally, we evaluate the equation of state parameter by considering the F(T, TG) model and study its evolutionary behavior for particular values of dimensionless coupling parameters.

Bistability of rotational modes in a system of coupled pendulums

The main goal of this research is to examine any peculiarities and special modes observed in the dynamics of a system of two nonlinearly coupled pendulums. In addition to steady states, an in-phase rotation limit cycle is proved to exist in the system with both damping and constant external force. This rotation mode is numerically shown to become unstable for certain values of the coupling strength. We also present an asymptotic theory developed for an infinitely small dissipation, which explains why the in-phase rotation limit cycle loses its stability. Boundaries of the instability domain mentioned above are found analytically. As a result of numerical studies, a whole range of the coupling parameter values is found for the case where the system has more than one rotation limit cycle. There exist not only a stable in-phase cycle, but also two out-of phase ones: a stable rotation limit cycle and an unstable one. Bistability of the limit periodic mode is, therefore, established for the system of two nonlinearly coupled pendulums. Bifurcations that lead to the appearance and disappearance of the out-ofphase limit regimes are discussed as well.

First-principles investigation of superconductivity in the body-centred tetragonal

We have investigated the ground state and electronic properties of in the structure using a generalised gradient approximation of the density functional theory and the ab initio pseudopotential method. We find that the calculated electronic properties of exhibit three-dimensional rather than two-dimensional characteristics in spite of the apparent two dimensionality in its atomic structure. An interesting feature of the phonon dispersion curves is the phonon anomaly in the lowest transverse acoustic branch as well as the longitudinal acoustic branch. We have shown that these phonon anomalies give rise to large electron-phonon interaction, as is evident from the calculated Eliashberg spectral function F(). By integrating this spectral function, the value of average electron-phonon coupling parameter is calculated to be 0.85, from which the superconducting critical temperature is estimated to be 3.74 K, in gratifying accord with its experimental value of 4.0 K.

Ab initio investigation of superconductivity in orthorhombic MgPtSi

We have performed an ab initio study of electronic, vibrational and superconducting properties of the orthorhombic MgPtSi by employing the density functional theory, a linear-response formalism, and the plane-wave pseudopotential method. Our electronic results suggest that the density of states at the Fermi level is primarily contributed by Pt 5d and Si 3p states with much smaller contribution from Mg electronic states. Phonon anomalies have been found for all three acoustic branches. Due to these phonon anomalies, the acoustic branches make large contributions to the average electron-phonon coupling parameter. From the Eliashberg spectral function, the value of average electron-phonon coupling parameter is found to 0.707. Using this value, the superconducting critical temperature is obtained to be 2.4 K, in excellent accordance with its experimental value of 2.5 K.

Kähler potential braneworld inflation in supergravity after Planck 2015

We analyze the slow-roll mechanism in Randall–Sundrum II braneworld with a supersymmetric hybrid inflation model. We are interested in studying various inflationary spectrum perturbation parameters in two cases. The first is the minimal Kähler potential where the inflationary parameters depend on the number of [Formula: see text]-foldings [Formula: see text] as well as the brane tension [Formula: see text], such that we produce the central value of the spectral index [Formula: see text]. The second case concerns the nonminimal Kähler potential, where the inflationary parameters depend only on the brane tension. Also, the [Formula: see text]-problem is solved for natural coupling parameter values which is not realized in the standard case. In addition, for a suitable choice of brane tension value [Formula: see text], the perturbation spectrum is widely consistent with Planck data.

Thermodynamic study in modified f ( T ) $f(T)$ gravity with cosmological constant regime

This study is conducted to examine the validity of thermodynamical laws in a modified $f(T)$ gravity involving a direct coupling of torsion scalar with matter contents. For this purpose, we consider spatially flat FRW geometry with matter contents as perfect fluid and formulate the first thermodynamical law in this gravity at apparent horizon. It is found that equilibrium description of thermodynamics exists in this modified gravity in a similar way to Einstein and other gravities. Further we discuss generalized second law of thermodynamics at apparent horizon of FRW universe for three different $f(T)$ models using Gibbs law as well as the assumption that temperature of matter within apparent horizon is similar to that of horizon. It is found that for some particular cosmologically consistent values of coupling parameters, GSLT remains valid in observationally consistent cosmic eras.

Motion and high energy collision of magnetized particles around a Hořava-Lifshitz black hole

We have studied the orbits of magnetized particles around Hořava-Lifshitz black hole with mass $M$ immersed in an asymptotically uniform magnetic field in the infrared approximation when $\omega M^{2}\gg 1$ . It is shown that magnetized particle’s orbit in Hořava-Lifshitz spacetime is different with compare to one in Schwarzschild spacetime due to the presence of additional terms related to the Kehagias-Sfetsos (KS) parameter $\omega$ . Using the Hamilton-Jacobi formalism, we have found the dependence of the area of stable circular orbits of the magnetized particle on dimensionless KS parameter $\tilde{\omega}$ and have plotted them for several values of magnetic coupling parameter $\beta$ as well as obtained the equations of motion of the magnetized particle. Moreover, we have studied the dependence of the collision of (magnetized, charged, non-charged) particles on KS parameter $\omega$ for some fixed values of the magnetic coupling parameter $\beta$ .

Kolmogorov flow in two dimensional strongly coupled Yukawa liquid: A molecular dynamics study

The transition from laminar to turbulent flows in liquids remains a problem of great interest despite decades of intensive research. Here, we report an atomistic study of this transition in a model Yukawa liquid using molecular dynamics simulations. Starting from an thermally equilibrated Yukawa liquid, for a given value of coupling parameter Γ (defined as ratio of potential energy to kinetic energy per particle) and screening length κ, a subsonic flow of magnitude U0 is superposed and transition to an unstable regime is observed eventually leading to turbulent flow at sufficiently high Reynolds numbers. We have performed a parametric study for a range of Reynolds number R and found that the flow is neutrally stable for R<Rc(Γ), while a transition from laminar to turbulent flow occurs for R>Rc(Γ), where Rc is the critical value of Reynolds number. Strong molecular shear heating is observed in all cases studied here. It is found that the coupling parameter Γ decreases because of molecular shear heating on a time scale comparable to the instability time scale. Irrespective of the initial value of coupling parameter Γ, the average heating rate is found to be sensitive to the ratio of equilibrium flow speed to the thermal speed, say, α=U0vth, where vth=2Γ. Our results reported here are expected to be generic and should apply to a wide variety of strongly coupled systems such as laboratory dusty plasma, molten salts, and charged colloidal systems.

Conductance through single biphenyl molecules: symmetric and asymmetric coupling to electrodes.

The contacts and the chemical bonds formed between metallic electrodes and molecules determine to a large extent the conductive properties of single molecular junctions, which represent the smallest possible active elements in an electronic circuit. We therefore investigated in a comparative study, using the break junction technique (MCBJ), the conductive properties of [1,1’-biphenyl]-4,4’-dithiol (M1) and of 4’-mercapto-[1,1’-biphenyl]-4-carbonitrile (M2) between gold electrodes. As a function of electrode separation, characterized by the conductance close to 0 V, we found several plateaus of relative stability, with those close to 0.01G0 being the most pronounced. The overall conductance of symmetric and asymmetric molecules were surprisingly similar, only the range of stability was smaller for M2. While M1 yielded symmetric I–V-curves, only small asymmetries were detected for M2. These are also reflected in the comparable values for coupling parameters using the single level resonance model. The high conductance for the asymmetric molecule is interpreted as a result of coherent coupling of electronic states through the whole molecule, so that the outcome cannot be predicted just by adding conductive properties of individual molecular groups.

Dynamic nuclear polarization properties of nitroxyl radical in high viscous liquid using Overhauser-enhanced Magnetic Resonance Imaging (OMRI)

The dynamic nuclear polarization (DNP) studies were carried out for 15N labeled carbamoyl-PROXYL in pure water and pure water/glycerol mixtures of different viscosities (1.8cP, 7cP and 14cP). The dependence of DNP parameters was demonstrated over a range of agent concentration, viscosities, RF power levels and ESR irradiation time. DNP spectra were also recorded for 2mM concentration of 15N labeled carbamoyl-PROXYL in pure water and pure water/glycerol mixtures of different viscosities. The DNP factors were measured as a function of ESR irradiation time, which increases linearly up to 2mM agent concentration in pure water and pure water/glycerol mixtures of different viscosities. The DNP factor started declining in the higher concentration region (∼3mM), which is due to the ESR line width broadening. The water proton spin–lattice relaxation time was measured at very low Zeeman field (14.529mT). The increased DNP factor (35%) was observed for solvent 2 (η=1.8cP) compared with solvent 1 (η=1cP). The increase in the DNP factor was brought about by the shortening of water proton spin–lattice relaxation time of solvent 2. The decreased DNP factors (30% and 53%) were observed for solvent 3 (η=7cP) and solvent 4 (η=14cP) compared with solvent 2, which is mainly due to the low value of coupling parameter in high viscous liquid samples. The longitudinal relaxivity, leakage factor and coupling parameter were estimated. The coupling parameter values reveal that the dipolar interaction as the major mechanism. The longitudinal relaxivity increases with the increasing viscosity of pure water/glycerol mixtures. The leakage factor showed an asymptotic increase with the increasing agent concentration. It is envisaged that the results reported here may provide guidelines for the design of new viscosity prone nitroxyl radicals, suited to the biological applications of DNP.

Exponential approximation for one-component Yukawa plasma.

A theory based on the exponential approximation of the liquid-state theory is applied to study properties of several models of one-component Yukawa plasma characterized by different values of the screening parameter z. The results of the new theory are compared to the results of a conventional theory, which is based on the first-order mean spherical approximation, and to the results of a Monte Carlo simulation. The new theory shows improvements in the predictions for the thermodynamic and structural properties of Yukawa plasmas with high and intermediate values of the screening parameter, z, and coupling parameter, Γ. For low values of z and Γ, the new theory is comparable in accuracy to the conventional theory, which in turn agrees well with the results of the Monte Carlo simulation.