Weak Field Regime Research Articles

SPMHD simulations of structure formation

The intracluster medium of galaxy clusters is permeated by {\mu}G magnetic fields. Observations with current and future facilities have the potential to illuminate the role of these magnetic fields play in the astrophysical processes of galaxy clusters. To obtain a greater understanding of how the initial seed fields evolve to the magnetic fields in the intracluster medium requires magnetohydrodynamic simulations. We critically assess the current Smoothed Particle Magneto-Hydrodynamics (SPMHD) schemes, especially highlighting the impact of a hyperbolic divergence cleaning scheme and artificial resistivity switch on the magnetic field evolution in cosmological simulations of the formation of a galaxy cluster using the N-body/SPMHD code gcmhd++. The impact and performance of the cleaning scheme and two different schemes for the artificial resistivity switch is demonstrated via idealized test cases and cosmological simulations. We demonstrate that the hyperbolic divergence cleaning scheme is effective at suppressing the growth of the numerical divergence error of the magnetic field and should be applied to any SPMHD simulation. Although the artificial resistivity is important in the strong field regime, it can suppress the growth of the magnetic field in the weak field regime, such as galaxy clusters. With sufficient resolution, simulations with divergence cleaning can reproduce observed magnetic fields. We conclude that the cleaning scheme alone is sufficient for galaxy cluster simulations, but our results indicate that the SPMHD scheme must be carefully chosen depending on the regime of the magnetic field.

Optical Absorption in Periodic Graphene Superlattices: Perpendicular Applied Magnetic Field and Temperature Effects

AbstractA detailed study of the magneto‐optical absorption is presented for graphene superlattices (SLs) subjected to a perpendicular magnetic field. For a given temperature, this quantity exhibits a resonant peak structure whose characteristics depend on the magnetic field regime, circular polarization of light and SL barrier height. For the intermediate field regime, we demonstrated that the resonant peak structure of is directly correlated to the partial joint density of states. Specifically, the latter exhibits van Hove‐like singularities and peaks at energies where takes its maximum values. We also investigated the magnetoabsorption in the weak field regime for SLs exhibiting one and extra Dirac points in the absence of the field. It was found that for SLs with only one Dirac point, the absorption spectra consist of resonant peaks satisfying the same circular polarization dependent selection rule as that for pristine graphene, except for one of them. For SLs with extra Dirac points, the resonant peaks arise from transitions between singlet subbands or between doublet subbands and satisfy a circular polarization and peak intensity dependent selection rule. It was also found that the resonant structure of can be observed experimentally at room temperature in clean SLs.

Does electric or magnetic field affect reverse osmosis desalination?

Effect of electric or magnetic field on desalination efficiency was studied by molecular dynamics simulation. A restricted double-walled carbon nanotube (CNT) was selected, then strong and weak external field regime were applied on simulated system. Results of simulation showed that strong electric field increases water flow rate two times than untreated system. Magnetization of water decreases water flow rate because of the increase in number of hydrogen bond. In addition, it was observed that applying electric field or magnetization of water completely rejects the ions. In fact, electric field moved back Na+ ions because of the force acting on the ion opposite to the direction of reverse osmotic forces. Magnetization of water leads to a more stable hydration shell around the Na+ ion that prevents passing through the carbon nanotube. In comparison with a single-walled CNT, considering a double-walled CNT enhances the experimental agreement of simulated water flow rate.

The post-Newtonian limit of hybrid f(R)-gravity

The post-Newtonian formalism is developed for hybrid f(R)-gravity. The model is tested in the weak field regime of the Solar System. Using scalartensor representation of hybrid f(R)-gravity we impose restrictions on the parameters of the theory and show that hybrid f(R)-gravity is not ruled out by the observations in the weak field limit.

Reexamining the gluon spectrum in the boost-invariant glasma from a semianalytic approach

In high energy heavy-ion collisions, the degrees of freedom at the very early stage can be effectively represented by strong classical gluonic fields within the Color Glass Condensate framework. As the system expands, the strong gluonic fields eventually become weak such that an equivalent description using the gluonic particle degrees of freedom starts to become valid. We revisit the spectrum of these gluonic particles by solving the classical Yang-Mills equations semi-analytically with the solutions having the form of power series expansions in the proper time. We propose a different formula for the gluon spectrum which is consistent with energy density during the whole time evolution. We find that the chromo-electric fields have larger contributions to the gluon spectrum than the chromo-magnetic fields do. Furthermore, the large momentum modes take less time to reach the weak-field regime while smaller momentum modes take more time. The resulting functional form of the gluon spectrum is exponential in nature and the spectrum is close to a thermal distrubtion with effective temperatures around $0.6$ to $0.9\, Q_s$ late in the Glasma evolution. The sensitiveness of the gluon spectrum to the infrared and the ultraviolet cut-offs are discussed.

Nonlinearity of the zigzag graphene nanoribbons with antidots via the f-deformed Dirac oscillator in (2+1)-dimensions

We study the nonlinearity for the zigzag graphene nanoribbons (ZGNRs) with zigzag triangular holes (ZTHs). We show that in the presence of an external uniform magnetic field, a two-dimensional f-deformed Dirac oscillator can be used to describe the dynamics of the electrons in the ZGNRs with ZTHs. It is shown for the first time that the magnetic field direction has effect on the chirality of charge carriers in the ZGNRs punched with triangular holes. We also obtain the Landau-level spectrum in the weak and strong magnetic field regimes. Additionally, we compare Landau-level spectrum of this graphene-based device in the f-deformed scenario and original one. Our results provide a general viewpoint for the development of the zigzag graphene nanoribbons.

Field-effect transistors with high mobility and small hysteresis of transfer characteristics based on CH3NH3PbBr3 films

Field-effect transistor (FET) structures based on soluble organometallic perovskites, CH3NH3PbBr3, were obtained and their electrical properties were studied. FETs made of CH3NH3PbBr3 films possess current- voltage characteristics (IVs) typical for ambipolar FETs with saturation regime. The transfer characteristics of FETs based on CH3NH3PbBr3 have an insignificant hysteresis and slightly depend on voltage at the source-drain. Mobilities of charge carriers (holes) calculated from IVs of FETs based on CH3NH3PbBr3 at 300 K in saturation and weak field regimes were ~5 and ~2 cm2/V s, respectively, whereas electron mobility is ~3 cm2/V s, which exceeds the mobility value ~1 cm2/V s obtained earlier for FETs based on CH3NH3PbI3.

Rotation curves of LSBGs and dwarf galaxies in a nearly Newtonian solution

The observed motion of stars close to galaxy nuclei shows that the resulting velocities are small of the order of a few hundred of km s−1. In these regions of strong gravity, the Newtonian gravitational field or even a post-Newtonian approximation may not be adequate to describe the motion of stars. In this paper, we study the possibility that the rotation curves problem may be explained mostly in the realm of Einstein's general relativity in a nearly weak field regime. By using the geodesic equations to obtain a gravitational potential generated from a point-like source, we end up in the concept of a nearly Newtonian gravity, and we show that its resulting potential responds to the dark halo mostly attributed to dark matter. We show that it comes essentially from the propagation of the non-linear effects of the obtained effective velocity field. As a test, we study a sample of 27 low surface brightness galaxies (LSBGs) and nine dwarf galaxies obtaining rotation curve shapes in nearly agreement with observations.

Testing quantum gravity

The search for a theory of quantum gravity is the most fundamental problem in all of theoretical physics, but there are as yet no experimental results at all to guide this endeavor. What seems to be needed is a pragmatic way to test if gravitation really occurs between quantum objects or not. In this paper, we suggest such a potential way out of this deadlock, utilizing macroscopic quantum systems; superfluid helium, gaseous Bose–Einstein condensates and “macroscopic” molecules. It turns out that true quantum gravity effects — here defined as observable gravitational interactions between truly quantum objects — could and should be seen (if they occur in nature) using existing technology. A falsification of the low-energy limit in the accessible weak-field regime would also falsify the full theory of quantum gravity, making it enter the realm of testable, potentially falsifiable theories, i.e. becoming real physics after almost a century of pure theorizing. If weak-field gravity between quantum objects is shown to be absent (in the regime where the approximation should apply), we know that gravity then is a strictly classical phenomenon absent at the quantum level.

Nonequilibrium transport in the pseudospin-1 Dirac-Weyl system

Solid state materials hosting pseudospin-1 quasiparticles have attracted a great deal of recent attention. In these materials, the energy band contains of a pair of Dirac cones and a flat band through the connecting point of the cones. As the "caging" of carriers with a zero group velocity, the flat band itself has zero conductivity. However, in a non-equilibrium situation where a constant electric field is suddenly switched on, the flat band can enhance the resulting current in both the linear and nonlinear response regimes through distinct physical mechanisms. Using the ($2+1$) dimensional pseudospin-$1$ Dirac-Weyl system as a concrete setting, we demonstrate that, in the weak field regime, the interband current is about twice larger than that for pseudospin-1/2 system due to the interplay between the flat band and the negative band, with the scaling behavior determined by the Kubo formula. In the strong field regime, the intraband current is $\sqrt{2}$ times larger than that in the pseudospin-1/2 system, due to the additional contribution from particles residing in the flat band. In this case, the current and field follows the scaling law associated with Landau-Zener tunneling. These results provide a better understanding of the role of the flat band in non-equilibrium transport and are experimentally testable using electronic or photonic systems.

The (2+1)-dimensional f-deformed Dirac oscillator in the presence of an external field

We consider a two-dimensional f-deformed Dirac oscillator in the presence of an external uniform static magnetic field. We show that the two-dimensional f-deformed Dirac oscillator maps exactly onto the anti-Jaynes–Cummings (AJC) and Jaynes–Cummings (JC) models. We also obtain the energy spectrum and corresponding eigenstates in the weak and strong magnetic field regimes. We show how the change in chirality is associated with the magnitude of the magnetic field. We investigate the two-dimensional f-deformed Dirac oscillator in an external (isospin) field and find its energy spectrum.

Detecting spin fractionalization in a spinon Fermi surface spin liquid

Motivated by the recent proposal of the spinon Fermi surface spin liquids for several candidate materials such as YbMgGaO4, we explore the experimental consequences of the external magnetic fields on this exotic state. Specifically, we focus on the weak field regime where the spin liquid state is well preserved and the spinon remain to be a good description of the magnetic excitations. From the spin-1/2 nature of the spinon excitation, we predict the unique features of the spinon continuum when the weak magnetic field is applied to the system. Due to the small energy scale of the exchange interactions between the local moments in the spin liquid candidate like YbMgGaO4, our proposal for the spectral weight shifts and spectral crossing in the magnetic fields can be immediately tested by inelastic neutron scattering experiments. Several other experimental aspects about the spinon Fermi surface and the spinon excitations are discussed and proposed. Our work provides an experimental scheme to examine the fractionalized spinon excitation and the candidate spin liquid states in YbMgGaO4, the 6H-B phase of Ba3NiSb2O9 and other relevant materials.

Born-Infeld gravity with a Brans-Dicke scalar

Recently proposed Born-Infeld (BI) theories of gravity assume a constant BI parameter ($\kappa$). However, no clear consensus exists on the sign and value of $\kappa$. Recalling the Brans-Dicke (BD) approach, where a scalar field was used to generate the gravitational constant $G$, we suggest an extension of Born-Infeld gravity with a similar Brans-Dicke flavor. Thus, a new action, with $\kappa$ elevated to a spacetime dependent real scalar field, is proposed. We illustrate this new theory in a cosmological setting with pressureless dust and radiation as matter. Assuming a functional form of $\kappa(t)$, we numerically obtain the scale factor evolution and other details of the background cosmology. It is known that BI gravity differs from general relativity (GR) in the strong-field regime but reduces to GR for intermediate and weak fields. Our studies in cosmology demonstrate how, with this new, scalar-tensor BI gravity, deviations from GR as well as usual BI gravity, may arise in the weak-field regime too. For example, we note a late-time acceleration without any dark energy contribution. Apart from such qualitative differences, we note that fixing the sign and value of $\kappa$ is no longer a necessity in this theory, though the origin of the BD scalar does remain an open question.

Polarized Line Formation in Arbitrary Strength Magnetic Fields Angle-averaged and Angle-dependent Partial Frequency Redistribution

Abstract Magnetic fields in the solar atmosphere leave their fingerprints in the polarized spectrum of the Sun via the Hanle and Zeeman effects. While the Hanle and Zeeman effects dominate, respectively, in the weak and strong field regimes, both these effects jointly operate in the intermediate field strength regime. Therefore, it is necessary to solve the polarized line transfer equation, including the combined influence of Hanle and Zeeman effects. Furthermore, it is required to take into account the effects of partial frequency redistribution (PRD) in scattering when dealing with strong chromospheric lines with broad damping wings. In this paper, we present a numerical method to solve the problem of polarized PRD line formation in magnetic fields of arbitrary strength and orientation. This numerical method is based on the concept of operator perturbation. For our studies, we consider a two-level atom model without hyperfine structure and lower-level polarization. We compare the PRD idealization of angle-averaged Hanle–Zeeman redistribution matrices with the full treatment of angle-dependent PRD, to indicate when the idealized treatment is inadequate and what kind of polarization effects are specific to angle-dependent PRD. Because the angle-dependent treatment is presently computationally prohibitive when applied to realistic model atmospheres, we present the computed emergent Stokes profiles for a range of magnetic fields, with the assumption of an isothermal one-dimensional medium.

Electrodynamics and spacetime geometry: Astrophysical applications

After a brief review of the foundations of (pre-metric) electromagnetism, we explore some physical consequences of electrodynamics in curved spacetime. In general, new electromagnetic couplings and related phenomena are induced by the spacetime curvature. The applications of astrophysical interest considered here correspond essentially to the following geometries: the Schwarzschild spacetime and the spacetime around a rotating spherical mass in the weak field and slow rotation regime. In the latter, we use the Parameterised Post-Newtonian (PPN) formalism. We also explore the hypothesis that the electric and magnetic properties of vacuum reflect the spacetime isometries. Therefore, the permittivity and permeability tensors should not be considered homogeneous and isotropic a priori. For spherical geometries we consider the effect of relaxing the homogeneity assumption in the constitutive relations between the fields and excitations. This affects the generalized Gauss and Maxwell-Amp\`{e}re laws where the electric permittivity and magnetic permeability in vacuum depend on the radial coordinate in accordance with the local isometries of space. For the axially symmetric geometries we relax both the assumptions of homogeneity and isotropy. We explore simple solutions and discuss the physical implications related to different phenomena such as: the decay of electromagnetic fields in the presence of gravity, magnetic terms in Gauss law due to the gravitomagnetism of the spacetime around rotating objects, a frame-dragging effect on electric fields and the possibility of a spatial (radial) variability of the velocity of light in vacuum around spherical astrophysical objects for strong gravitational fields.

Conformal Flow on S3 and Weak Field Integrability in AdS4

We consider the conformally invariant cubic wave equation on the Einstein cylinder {mathbb{R} times mathbb{S}^3} for small rotationally symmetric initial data. This simple equation captures many key challenges of nonlinear wave dynamics in confining geometries, while a conformal transformation relates it to a self-interacting conformally coupled scalar in four-dimensional anti-de Sitter spacetime (AdS4) and connects it to various questions of AdS stability. We construct an effective infinite-dimensional time-averaged dynamical system accurately approximating the original equation in the weak field regime. It turns out that this effective system, which we call the conformal flow, exhibits some remarkable features, such as low-dimensional invariant subspaces, a wealth of stationary states (for which energy does not flow between the modes), as well as solutions with nontrivial exactly periodic energy flows. Based on these observations and close parallels to the cubic Szegő equation, which was shown by Gérard and Grellier to be Lax-integrable, it is tempting to conjecture that the conformal flow and the corresponding weak field dynamics in AdS4 are integrable as well.

Anisotropies of gravitational wave backgrounds: A line of sight approach

In the weak field regime, gravitational waves can be considered as being made up of collisionless, relativistic tensor modes that travel along null geodesics of the perturbed background metric. We work in this geometric optics picture to calculate the anisotropies in gravitational wave backgrounds resulting from astrophysical and cosmological sources. Our formalism yields expressions for the angular power spectrum of the anisotropies. We show how the anisotropies are sourced by intrinsic, Doppler, Sachs–Wolfe, and Integrated Sachs–Wolfe terms in analogy with Cosmic Microwave Background photons.

Theoretical aspects of femtosecond double-pump single-molecule spectroscopy. I. Weak-field regime.

We present a theoretical description of double-pump femtosecond single-molecule signals with fluorescence detection. We simulate these signals in the weak-field regime for a model mimicking a chromophore with a Franck-Condon-active vibrational mode. We establish several signatures of these signals which are characteristic for the weak-field regime. The signatures include the quenching of vibrational beatings by electronic dephasing and a pronounced tilt of the phase-time profiles in the two-dimensional (2D) maps. We study how environment-induced slow modulations of the electronic dephasing and relevant chromophore parameters (electronic energy, orientation, vibrational frequency and relative shift of the potential energy surfaces) affect the signals.

Scalar field self-force effects on a particle orbiting a Reissner-Nordström black hole

Scalar field self-force effects on a scalar charge orbiting a Reissner-Nordstr\"om black hole are investigated. The scalar wave equation is solved analytically in a post-Newtonian framework, and the solution is used to compute the self-field as well as the components of the self-force at the particle's location up to 7.5 post-Newtonian order. The energy fluxes radiated to infinity and down the hole are also evaluated. Comparison with previous numerical results in the Schwarzschild case shows a good agreement in both strong-field and weak-field regimes.

Minicharged particles search by strong laser pulse-induced vacuum polarization effects

Laser-based searches of the yet unobserved vacuum birefringence might be sensitive for very light hypothetical particles carrying a tiny fraction of the electron charge. We show that, with the help of contemporary techniques, polarimetric investigations driven by an optical laser pulse of moderate intensity might allow for excluding regions of the parameter space of these particle candidates which have not been discarded so far by laboratory measurement data. Particular attention is paid to the role of a Gaussian wave profile. It is argued that, at energy regimes in which the vacuum becomes dichroic due to these minicharges, the transmission probability of a probe beam through an analyzer set crossed to the initial polarization direction will depend on both the induced ellipticity as well as the rotation of the initial polarization plane. The weak and strong field regimes, relative to the attributes of these minicharged particles, and the relevance of the polarization of the strong field are investigated.