Coulomb Coupling Research Articles

Effects of radiation pressure and polarization force on Jeans instability in magnetized strongly coupled dusty plasma

The combined effects of radiation pressure and polarization force on the linear Jeans instability of magnetized strongly coupled dusty plasma are studied using the generalized hydrodynamic fluid model. The electron and ion fluids are assumed to be inertialess and their thermal radiation pressures including gas pressures is considered in the single fluid momentum transfer equation of strongly coupled dusty fluid. The general dispersion relation is derived using normal mode analysis and discussed for transverse and longitudinal modes in both the strongly coupled and weakly coupled plasmas. The Jeans instability criterion determines the instability of the system which is significantly modified by radiation pressure and viscoelastic effects. The stabilizing influence of the Coulomb coupling parameter, dust radiation velocity and viscoelastic coefficients on the growth rates of Jeans instability is observed. The different velocities, Jeans wavenumber and Jeans mass have been calculated in the dust molecular cloud and the astrophysical consequences of the results are discussed.

Dynamical structure factor of complex plasmas for varying wave vectors

The dynamical structure factor has been reported for three dimensional strongly coupled Yukawa liquids (SCYLs) through state-of-the-art equilibrium molecular dynamics (EMD) simulations in a microcanonical ensemble (NVE). The effects of varying wave vectors (k = 2π/L) have been computed along with different arrangements of Coulomb coupling (Γ) and the Debye screening parameter (κ) on the dynamical structure-factor S(k,ω) using EMD simulations. Our new investigations of S(k,ω) show that the amplitude of oscillation decreases and the frequency increases with increasing Γ, respectively, for the SCYLs. Our simulations show that the decreasing behavior is noted for the frequency of plasma S(k,ω) with increasing κ and system size (N). The obtained EMD results are found to be more efficient and accurate than that of various previous simulation data, and the present EMD approach gives more satisfactory results with appropriate system sizes at high values of k, for a wide range of plasma states (Γ, κ). It has been shown that the present density S(k,ω) of SCYLs fluctuates more at intermediate to high Coulomb couplings (average to lower system temperature ≡ 1/Γ) and low values of Debye screening; however, it less fluctuates at higher N and κ.

An effective Coulomb interaction in nuclear energy density functionals

It is well-known that the charge-violating interaction is usually underestimated in nuclear many-body approaches. In the framework of the Skyrme–Hartree–Fock (SHF) method, the effective two-body charge-symmetry breaking (CSB) and charge-independent breaking (CIB) interactions in nuclear medium based on Brueckner theory are included, then we constrain the effective Coulomb interaction in turn with the help of experimental Coulomb displacement energy (CDE), i.e., the binding-energy difference between mirror nuclei. Accordingly, we introduce a new (effective) Coulomb coupling constant e02=e2(1+aexcZ−2/3) to replace the original one e2 between protons (note that the original coupling constant just applies to point-like charge), where Z is proton number. This effective coupling constant e02 is phenomenologically embodying the effects of many complicated corrections beyond mean-field method or even nuclear structure physics such as core polarization, nucleon finite size and vacuum polarization. With e02=e2(1+0.45Z−2/3) for SLy4 interaction, the experimental CDE for T=1/2,1,3/2,2 isobaric multiplets, together with the excitation energy of isobaric analog states for heavy nuclei such as 208Pb, can be rather well reproduced, indicating the validity of such a treatment. Moreover, the c coefficients in the isobaric multiplet mass equation for isobaric quartets are computed, and the results based on Coulomb force turns out to be around 10%–15% lower than the experimental ones persistently, just as the Nolen–Schiffer anomaly. Yet, the introduction of both the CIB effect and the e02 systematically improves the agreement with experimental data substantially.

Two-Temperature Magnetohydrodynamics Simulations of Propagation of Semi-Relativistic Jets

In astrophysical jets observed in active galactic nuclei and in microquasars, the energy exchange rate by Coulomb collision is insufficient for thermal equilibrium between ions and electrons. Therefore, it is necessary to consider the difference between the ion temperature and the electron temperature. We present the results of two-temperature magnetohydrodynamics(MHD) simulations to demonstrate the effects of Coulomb coupling. It is assumed that the thermal dissipation heats only ions. We find that the ion and electron temperatures are separated through shocks. Since the ion entropy is increased by energy dissipation at shocks and the Coulomb collisions are inefficient, electron temperature becomes about 10 times lower than the ion temperature in the hotspot ahead of the jet terminal shock. In the cocoon, electron temperature decreases by gas mixing between high temperature cocoon gas and low temperature shocked-ambient gas even when we neglect radiative cooling, but electrons can be heated through collisions with ions. Radiation intensity maps are produced by post processing numerical results. Distributions of the thermal bremsstrahlung radiation computed from electron temperature have bright filament and cavity around the jet terminal shock.

General relativistic two-temperature accretion solutions for spherical flows around black holes

Matter falling onto black holes is hot, fully ionized and has to be necessarily transonic. Since the electrons are responsible for radiative cooling via processes like synchrotron, bremsstrahlung and inverse-Compton, the electron gas and proton gas are supposed to settle into two separate temperature distribution. But the problem with two-temperature flow is that there is one more variable than the number of equations. Accretion flow in its simplest form is radial, which has two constants of motion, while the flow variables are the radial bulk three-velocity, electron and proton temperatures. Therefore, unlike single temperature flow, in the two-temperature regime, there are multiple transonic solutions, nonunique for any given set of constants of motion with a large variation in sonic points. We invoked the second law of thermodynamics to find a possible way to break the degeneracy, by showing only one of the solutions among all possible, has maximum entropy and therefore is the correct solution. By considering these correct solutions, we showed that the accretion efficiency increase with the increase in the mass accretion rate. We showed that radial flow onto super-massive black hole can radiate with efficiency more than 10%, if the accretion rate is more than 60% of the Eddington accretion rate, but accretion onto stellar-mass black hole achieve the same efficiency, when it is close to the Eddington limit. We also showed that dissipative heat quantitatively affects the two temperature solution. In the presence of explicit heat processes, the Coulomb coupling is weak.

Thermal Conductivity and Shear Viscosity of Strongly Coupled Dusty Plasma

In complex plasma, asymmetric ion flow toward the sheath gives rise to a well-known attractive oscillatory wake potential. In this paper, an investigation of effect of ion flow-induced wake potential on thermal conductivity and shear viscosity in strongly coupled complex plasmas has been reported for the first time. Green-Kubo formalism along with Langevin dynamics simulation has been used to estimate the thermal conductivity and the shear viscosity of strongly coupled dust fluid in the presence of external magnetic field and attractive wake potential superimposed with repulsive Debye-Huckel potential for a wide range of the Coulomb coupling parameter (Γ), the screening parameter (k), the neutral pressure (n n ), and the magnetic field B.

Experimental observation of a dusty plasma crystal in the cathode sheath of a DC glow discharge plasma

Dusty plasma crystals have traditionally been observed and studied in radio frequency discharge plasmas, and their formation in a DC glow discharge plasma remains experimentally challenging. We report the first ever observation of a stable dusty plasma Coulomb crystal in the cathode sheath region of a DC glow discharge plasma. The observations are made in the dusty plasma experimental (DPEx) device where crystals of mono-disperse melamine formaldehyde grains are produced in the background of an Argon plasma. The crystalline nature of the structure is confirmed through a host of characteristic parameter estimations which includes the radial pair correlation function, Voronoi diagram, Delaunay Triangulation, the structural order parameter, the dust temperature, and the Coulomb coupling parameter. The special features of the DPEx device which permit such a crystal formation are delineated, and some principal physical features of the crystal are discussed.

RETRACTED: Superluminal effect in a hybrid nano-electro-optomechanical system

Abstract We theoretically investigate the phenomenon of superluminosity of the transmission probe field under the effect of a strong driving field in a hybrid nano-electro-optomechnical system comprising two coupled charged mechanical end resonators connected by bias voltages. The first mechanical charged mirror is connected to the optical field, while the second charged mirror is coupled by Coulomb coupling. We find that the superluminal behavior of the transmitted probe field can be switched by adjusting the cavity field detuning and atom field detuning. The present study extends the previously developed simple optomechanical and nano-electro-optomechanical system (Ma et al., 2014)[62]. Moreover, the group delay of the pulse advancement can be controlled by adjusting the electrostatic Coulomb coupling and the power of the driving field. We explain the effect of atomic field detuning on the superluminal behavior in the regimes of anti-Stokes sidebands, i.e. Δ a = − ω 1 .

Electronic states realized by the interplay between Li diffusion and Co3+/Co4+ charge ordering in LixCoO2

Measurements of dc magnetization $(M)$ and electrical resistivity $(\ensuremath{\rho})$ have been carried out as a function of temperature $(T)$ for layered oxide ${\mathrm{Li}}_{x}{\mathrm{CoO}}_{2} (0.51\ensuremath{\le}x\ensuremath{\le}1.0)$ using single-crystal specimens. After slow cooling of the specimens down to 10 K, both of the $M(T)$ and $\ensuremath{\rho}(T)$ curves are found to exhibit a clear anomaly due to the occurrence of ${\mathrm{Co}}^{3+}/{\mathrm{Co}}^{4+}$ charge ordering (CO) at ${T}_{\mathrm{S}}\ensuremath{\sim}155\phantom{\rule{0.28em}{0ex}}\mathrm{K}$ for $0.6\ensuremath{\le}x\ensuremath{\le}0.98$ (at ${T}_{\mathrm{S}}\ensuremath{\sim}180--190\phantom{\rule{0.28em}{0ex}}\mathrm{K}$ for $0.5\ensuremath{\le}x\ensuremath{\le}0.55)$. After rapid cooling of the specimens, additional anomalies are observed related to the onset of Li diffusion at ${T}_{\mathrm{F}1}\ensuremath{\sim}370\phantom{\rule{0.28em}{0ex}}\mathrm{K}$ and/or ${T}_{\mathrm{F}2}=120--130\phantom{\rule{0.28em}{0ex}}\mathrm{K}$. Due to phase mixing with compositions of nearly ${\mathrm{LiCoO}}_{2}$ and ${\mathrm{Li}}_{2/3}{\mathrm{CoO}}_{2}$, the specimens with $0.7\ensuremath{\lesssim}x\ensuremath{\lesssim}0.9$ show anomalies both at ${T}_{\mathrm{F}1}$ and ${T}_{\mathrm{F}2}$. For $0.6\ensuremath{\lesssim}x\ensuremath{\lesssim}0.9$, the resistivity measured after rapid cooling is found to be fairly larger than that measured after slow cooling below ${T}_{\mathrm{S}}$. The enhanced resistivity can be explained by the scenario that disordered ${\mathrm{Co}}^{3+}/{\mathrm{Co}}^{4+}$ arrangements, which have been observed and revealed to have an insulating electronic structure contrasting to the regular CO state in the previous scanning tunneling microscopy measurements [K. Iwaya et al., Phys. Rev. Lett. 111, 126104 (2013)], are realized due to the formation of an amorphouslike structure of Li ions after rapid cooling via interlayer Coulomb coupling. An electronic phase diagram for $0.5\ensuremath{\le}x\ensuremath{\le}1.0$ is proposed.

On Jeff =0 Ground State Iridates(V): Tracking Residual Paramagnetism in New Bi2 NaIrO6.

Open-shell transition metal oxides are capable of developing a rich diversity of electronic phases. The specific features evolving crucially depend on an intricate interplay of various local and long-range electronic interactions. Recently, the 5d transition elements have come into sharp focus because for these elements spin-orbit coupling (SOC) and onsite Coulomb repulsion (U) are on a comparable energy scale. For Ir4+ the t2g level associated to an octahedral crystal field (CF) is split by SOC, giving rise to a Jeff = spin state and rendering respective oxides like Sr2 IrO4 as Mott insulators. Transferring this scenario to iridium(V) oxides would lead to a diamagnetic ground state, Jeff =0. However, reported experimental results do not lend unambiguous support for such an electronic state. Theoretical explanations for the breakdown of the J=0 magnetic state suffer from conspicuous discrepancies. In an attempt to empirically contribute to resolving the puzzle, Bi2 NaIrO6 was synthesized in high purity by precipitation from homogeneous solution; it represents an iridium(V) oxide where long range band structure effects and magnetic superexchange are minimized, and the t2g degeneracy is lifted geometrically. We managed to reduce the strength of paramagnetic response, lending support to a Jeff =0 ground state of Bi2 NaIrO6 , exhibiting van Vleck type behavior.

Low threshold and efficient multiple exciton generation in halide perovskite nanocrystals

Multiple exciton generation (MEG) or carrier multiplication, a process that spawns two or more electron–hole pairs from an absorbed high-energy photon (larger than two times bandgap energy Eg), is a promising way to augment the photocurrent and overcome the Shockley–Queisser limit. Conventional semiconductor nanocrystals, the forerunners, face severe challenges from fast hot-carrier cooling. Perovskite nanocrystals possess an intrinsic phonon bottleneck that prolongs slow hot-carrier cooling, transcending these limitations. Herein, we demonstrate enhanced MEG with 2.25Eg threshold and 75% slope efficiency in intermediate-confined colloidal formamidinium lead iodide nanocrystals, surpassing those in strongly confined lead sulfide or lead selenide incumbents. Efficient MEG occurs via inverse Auger process within 90 fs, afforded by the slow cooling of energetic hot carriers. These nanocrystals circumvent the conundrum over enhanced Coulombic coupling and reduced density of states in strongly confined nanocrystals. These insights may lead to the realization of next generation of solar cells and efficient optoelectronic devices.

Controllable non-ideal plasmas from photoionized compressed gases

Based on a suite of molecular dynamics simulations, we propose a strategy for producing non-ideal plasmas with controllable properties over a wide range of densities between those of ultracold neutral plasmas and those of solid-density plasmas. We simulated the formation of non-equilibrium plasmas from photoionized, cool gases that are spatially precorrelated through neutral–neutral interactions that are important at moderate-to-high pressures. A wide range of physical properties, including Coulomb collisional rates, partial pressures, screening strengths, continuum lowering, interspecies Coulomb coupling, electron degeneracy and ionization states, were characterized across more than an order of magnitude variation in the initial gas pressure. A wide range of plasma properties are also found to vary when the initial pressure of a precorrelated gas is varied. Thus, we propose that non-ideal plasmas with tunable properties can be generated by photo-ionizing a dense, precorrelated gas. We find that the optimal initial density range for the gas is near a Kirkwood/Widom–Fisher line in the neutral-gas phase diagram. This strategy for generating non-ideal plasmas suggests experiments that have significant advantages over both ultracold and solid-density plasma experiments because the collisional, collective and recombination timescales can be tuned across many orders of magnitude, potentially allowing for a wider range of diagnostics. Moreover, the added costs of cooling ultracold plasmas and diagnosing dense matter with x-rays are eliminated.

Hybrid Perovskites: Ultrafast Carrier Transfer Promoted by Interlayer Coulomb Coupling in 2D/3D Perovskite Heterostructures (Laser Photonics Rev. 12(10)/2018)

Hybrid Perovskites: Ultrafast Carrier Transfer Promoted by Interlayer Coulomb Coupling in 2D/3D Perovskite Heterostructures (Laser Photonics Rev. 12(10)/2018)

Double optomechanically induced transparency and absorption in parity-time-symmetric optomechanical systems

We explore the optomechanically induced transparency (OMIT) in a parity-time-symmetric ($\mathcal{PT}$-symmetric) optomechanical system (OMS) in which the mechanical oscillator is coupled to another one via the Coulomb interaction. Calculating the transmission rate of the probe field, we find that double optomechanically induced absorption (OMIA) can be observed by modulating the Coulomb coupling strength with zero gain rate of the active cavity. The absorption window for the system in the $\mathcal{PT}$-symmetry-broken phase is deeper than that in other phases due to the strong field localization. When the gain rate is not zero, the output field of the system exhibits double inverted OMIT. Both splits of the double OMIA and double OMIT depend linearly on the strength of the Coulomb interaction. The inverted OMIT can also be changed into an absorption window and the system can achieve perfect absorption by modulating the Coulomb coupling strength. The transmission light at the double inverted OMIT is always fast light and the transmission light at $\ensuremath{\delta}={\ensuremath{\omega}}_{m}$ is always of slow light, independent of the phase the system in (the $\mathcal{PT}$-symmetry phase or the $\mathcal{PT}$-symmetry-broken phase). The absolute value of the group delay at $\ensuremath{\delta}={\ensuremath{\omega}}_{m}$ can be manipulated by controlling the strength of the Coulomb interaction. These observations provide us with a tool to control light propagation which might have potential application in quantum optical devices and quantum information networks.

Ultrafast Carrier Transfer Promoted by Interlayer Coulomb Coupling in 2D/3D Perovskite Heterostructures

AbstractEngineering 2D‐structured lead halide perovskites leads to both excellent optoelectronic performance and intriguing photophysics. Here, the underlying mechanisms of exciton interaction and carrier transfer in 2D/3D hybrid perovskites are studied. The investigated perovskites are in the form of self‐assembled microplatelets, which are naturally composed of multiple perovskite phases, with n being 1, 2, 3, or 4 or nanocrystals in bulk phase (n ≈ ∞). Excitonic energy transfer and charge separation between different phases are found to coexist in this hybrid system, which occur at an ultrafast timescale (<100 fs) followed by a relatively slow channel (2–15 ps). The experimental results reveal that this hybrid perovskite naturally forms a series of “heterostructures,” with excitons generated in different phases, showing Coulomb interactions across the interface. This interlayer Coulomb coupling should account for the aforementioned ultrafast carrier transfer. This work provides an accurate and thorough explanation for the remarkable charge transfer rate, which is extremely beneficial for their applications in photovoltaic and optoelectronic devices, even with the apparent interfacial scattering, defect trapping, and disorder‐induced exciton localization in 2D/3D hybrid perovskites.

Coherent Energy Transfer Dynamics and Photoluminescence of Coupled Quantum Dots: Fully Analytical Approach

We theoretically studied coherent excitation energy transfer between self-growth semiconductor quantum dots (QDs) by solving Heisenberg’s equations of motion for density matrix elements in second quantization regime. In a local excitation condition where only one QD electron is optically excited by the pump laser field, coherent excitation energy transfer to the other QD electron can be achieved through Coulomb (Forster) and electron-photon (radiation field) interactions. We calculated three diagonal and one off-diagonal Coulomb coupling constants, which are responsible for the biexcitonic frequency renormalization and the coherent energy transfer between QDs, respectively, and radiation field coupling coefficients by using electron and hole wave functions derived from eight-band kp-theorem, whose validity has already been tested by comparison with experiment. In linear optical regime where the occupation densities of electrons at higher energy level are negligibly small, we could successfully derive fully analytical behaviors of temporal dynamics of the interband polarizations and level occupation densities of both QDs by using Hartree-Fock approximation (HFA), in eventual, the stationary photoluminescence of the coupled QDs in an analytical form. Additionally, the validity of the HFA was examined by comparing the numerical results with those obtained from the exact correlation expansion model for different values of the pump field intensity.

Coherent Transport of Electron Excitations in Organic Solar Cells

The mechanisms of formation and coherent transport of free and bound electron excited states in organic solar cells are considered. In the model of a photocell (one-dimensional chain of photosensitive molecules in a uniform electric field of the p–n junction), the energy eigenvalues and the eigenfunctions of molecular excitons, charge-transfer excitons (CTE), and electron–hole pairs are determined. It is assumed that processes of transport between adjacent sites dominate in the case of the Coulomb interaction between the electron and the hole constituting a CTE. With decreasing Coulomb coupling energy, the CTE wavefunctions become superpositions of localized functions of the increasing number of sites. The decay time determined by independent transitions of the electron and the hole in this case becomes shorter than the transport time of the CTE as a whole. It is shown that autoionization of molecular excitons and small-radius CTEs in a strong electric field of a nanosize chain induces the mixing of states of these excitons as well as of electron–hole pairs, which substantially increases the quantum yield of the photoeffect.

All-DFTB Approach to the Parametrization of the System-Bath Hamiltonian Describing Exciton-Vibrational Dynamics of Molecular Assemblies.

Spectral density functions are central to the simulation of complex many body systems. Their determination requires making approximations not only to the dynamics but also to the underlying electronic structure theory. Here, blending different methods bears the danger of an inconsistent description. To solve this issue we propose an all-DFTB approach to determine spectral densities for the description of Frenkel excitons in molecular assemblies. The protocol is illustrated for a model of a PTCDI crystal, which involves the calculation of monomeric excitation energies and Coulomb couplings between monomer transitions, as well as their spectral distributions due to thermal fluctuations of the nuclei. Using dynamically defined normal modes, a mapping onto the standard harmonic oscillator spectral densities is achieved.

Jeans Instability of Self Gravitating Dust Cloud in Presence of Effective Electrostatic Pressure

The role of viscosity coefficient (η′), coulomb coupling parameter (Γ) and dust mass on the growth of jeans mode is investigated in strongly coupled dusty plasma using equations of Generalized Hydrodynamics (GH) Model. The novel aspect of this work is that the force arising due to electrostatic pressure caused by grain grain interaction has been included in the dynamics of dust particles. This force is found to play a significant role in counter balancing the self gravity effect, thereby reducing the growth rate of jeans instability. The present work may provide more physical insight in understanding the mechanisms behind formation of planetesimals, stars etc.

Spectroscopy of Plasmon-Excitons in Semiconductor-Metal Nanostructures

The results of the theory considering mixed plasmon-excitonic modes and their spectroscopy are presented. The plasmon-excitons are formed owing to strong Coulomb coupling between quasi-two-dimensional excitons of a quantum well and dipole plasmons of nanoparticles. The effective polarizability associated with a nanoparticle is calculated in a self-consistent approximation taking into account the local field determined by in-layer dipole plasmons and their image charges due to the excitonic polarization of a near quantum well. The spectra of elastic scattering and specular reflection of light are investigated in cases of a single silver nanoparticle and a monolayer of such particles situated in close proximity to a quantum well GaAs/AlGaAs. The optical spectra show a two-peak structure with a deep and narrow dip in the resonant range of plasmon-excitons. Propagation of plasmon-excitonic polaritons is discussed for periodic superlattices whose unit cell consists of a quantum well and a layer of metal nanoparticles. The superradiance regime originating in the Bragg diffraction of plasmon-excitonic polaritons by the superlattice is investigated. It is shown that the broad spectrum of plasmonic reflection depending on the number of unit cells in a superlattice also has a narrow dip at the exciton frequency.