Field Decay Research Articles

Controlling quantum coherence and entanglement in cavity magnomechanical systems

We investigate theoretically macroscopic quantum coherence and tripartite entanglement in a three-mode magnomechanical system with a yttrium iron garnet sphere, which is simultaneously driven by a magnetic pump and a microwave field pump to establish an effective magnomechanical coupling. We discuss mainly the dependence of quantum properties on the driving power, the cavity field detuning and decay, the magnomechanical coupling strength, the bias magnetic field, and the temperature of an environment. It is found that the quantum coherence of the system can be attained easily in a large range of parameters. In contrast, tripartite photon-magnon-phonon entanglement can only be realized in a relatively small range of optical and magnetic parameters. In particular, the quantum coherence of the mechanical mode can be larger than that of the photon and magnon modes, which means that the environmental incoherence of the mechanical mode in the cavity magnomechanical system can be suppressed significantly. These results may help manipulate simultaneously the macroscopic quantum coherence and the tripartite entanglement of massive objects in coupled magnomechanical system, which may have potential applications for realizing highly tunable information processing through the flexible control of quantum properties.

Onset of Electron Captures and Shallow Heating in Magnetars

The loss of magnetic pressure accompanying the decay of the magnetic field in a magnetar may trigger exothermic electron captures by nuclei in the shallow layers of the stellar crust. Very accurate analytical formulas are obtained for the threshold density and pressure, as well as for the maximum amount of heat that can be possibly released, taking into account the Landau–Rabi quantization of electron motion. These formulas are valid for arbitrary magnetic field strengths, from the weakly quantizing regime to the most extreme situation in which electrons are all confined to the lowest level. Numerical results are also presented based on experimental nuclear data supplemented with predictions from the Brussels-Montreal model HFB-24. This same nuclear model has been already employed to calculate the equation of state in all regions of magnetars.

Long-range stray field mapping of statically magnetized nanoparticles using magnetoresistive sensor

Analyzing the spatial distribution of stray field from magnetic nanoparticles is a crucial step to design and optimize the magnetometric system for a clinical magnetic particle imaging (MPI) scanner. Here, we used a magnetoresistive (MR) sensor to probe the stray field directly from a commercial magnetic nanoparticle suspension conditioned under a static field. For a given 20 mT by a small permanent magnet, the stray field of a liquid sample with a 0.7 mgFe iron mass is in nanotesla order measured by the MR sensor at 50 mm apart from the sample, while the magnetization is comparable to several microtesla. This field decay demands picotesla sensitivity of the sensing system to record the stray field for a further distance or a smaller excitation field. Moreover, from a two-dimensional trajectory of sample and magnet movements relative to the sensor position, we confirmed that the spatial distribution of the stray field appeared to correlate with sample geometry. The distribution became broadening for low iron mass concentration of the sample. From this observation, an MR sensor proves its potential for locating the magnetic nanoparticles under a quasistatic field, which can be extensively implemented for a single-sided MPI scanner.

Gravitational wave signal from primordial magnetic fields in the Pulsar Timing Array frequency band

The NANOGrav, Parkes, European, and International Pulsar Timing Array (PTA) Collaborations have reported evidence for a common-spectrum process that can potentially correspond to a stochastic gravitational wave background (SGWB) in the 1--100 nHz frequency range. We consider the scenario in which this signal is produced by magnetohydrodynamic (MHD) turbulence in the early Universe, induced by a nonhelical primordial magnetic field at the energy scale corresponding to the quark confinement phase transition. We perform MHD simulations to study the dynamical evolution of the magnetic field and compute the resulting SGWB. We show that the SGWB output from the simulations can be very well approximated by assuming that the magnetic anisotropic stress is constant in time, over a time interval related to the eddy turnover time. The analytical spectrum that we derive under this assumption features a change of slope at a frequency corresponding to the GW source duration that we confirm with the numerical simulations. We compare the SGWB signal with the PTA data to constrain the temperature scale at which the SGWB is sourced, as well as the amplitude and characteristic scale of the initial magnetic field. We find that the generation temperature is constrained to be in the 1--200 MeV range, the magnetic field amplitude must be $>1$\% of the radiation energy density at that time, and the magnetic field characteristic scale is constrained to be $>10$\% of the horizon scale. We show that the turbulent decay of this magnetic field will lead to a field at recombination that can help to alleviate the Hubble tension and can be tested by measurements in the voids of the Large Scale Structure with gamma-ray telescopes like the Cherenkov Telescope Array.

Electron Weibel instability induced magnetic fields in optical-field ionized plasmas

Generation and amplification of magnetic fields in plasmas is a long-standing topic that is of great interest to both plasma and space physics. The electron Weibel instability is a well-known mechanism responsible for self-generating magnetic fields in plasmas with temperature anisotropy and has been extensively investigated in both theory and simulations, yet experimental verification of this instability has been challenging. Recently, we demonstrated a new experimental platform that enables controlled initialization of highly nonthermal and/or anisotropic plasma electron velocity distributions via optical-field ionization. Using an external electron probe bunch from a linear accelerator, the onset, saturation, and decay of the self-generated magnetic fields due to electron Weibel instability were measured for the first time to our knowledge. In this paper, we will first present experimental results on time-resolved measurements of the Weibel magnetic fields in non-relativistic plasmas produced by Ti:Sapphire laser pulses (0.8 μm) and then discuss the feasibility of extending the study to a quasi-relativistic regime by using intense CO2 (e.g., 9.2 μm) lasers to produce much hotter plasmas.

Temporal behavior of hard x-ray and neutron production in plasma focus discharges

This paper concerns the correlation of hard x-ray and neutron signals, which were recorded with scintillation detectors oriented in the axial and radial directions, in a comparison with interferometric and extreme-ultraviolet radiation frames, as recorded within the plasma focus (PF)-1000 facility operated with a deuterium filling. The considered signals showed two different phases. In the initial phase, the fusion neutrons are mainly produced by deuterons moving dominantly downstream during the disruption of a pinch constriction (lasting tens nanoseconds). In the later phase (usually after about 100 ns), the fusion neutron emission reaches its maximum in the radial directions. This emission (lasting 100–200 ns) is caused by the fast deuterons moving in both the downstream and radial directions. It correlates usually with a decay of dense plasma structures in remnants of the expanding pinch column. This can be explained by a decay of internal magnetic fields. The neutron signal is usually composed of several sub-pulses of different energies. It was deduced that the primary deuterons producing the observed fusion neutrons undergo a regular and repeated temporal, directional, and energy evolution.

Multiwavelength View of the Close-by GRB 190829A Sheds Light on Gamma-Ray Burst Physics

We monitored the position of the close-by (about 370 Mpc) gamma-ray burst GRB 190829A, which originated from a massive star collapse, through very long baseline interferometry (VLBI) observations with the European VLBI Network and the Very Long Baseline Array, carrying out a total of nine observations between 9 and 117 days after the gamma-ray burst at 5 and 15 GHz, with a typical resolution of a few milliarcseconds. From a state-of-the art analysis of these data, we obtained valuable limits on the source size and expansion rate. The limits are in agreement with the size evolution entailed by a detailed modeling of the multiwavelength light curves with a forward-plus-reverse shock model, which agrees with the observations across almost 18 orders of magnitude in frequency (including the HESS data at TeV photon energies) and more than 4 orders of magnitude in time. Thanks to the multiwavelength, high-cadence coverage of the afterglow, inherent degeneracies in the afterglow model are broken to a large extent, allowing us to capture some unique physical insights; we find a low prompt emission efficiency of ≲10−3, a low fraction of relativistic electrons in the forward shock downstream χ e < 13% (90% credible level), and a rapid decay of the magnetic field in the reverse shock downstream after the shock crossing. While our model assumes an on-axis jet, our VLBI astrometry is not sufficiently tight as to exclude any off-axis viewing angle, but we can exclude the line of sight to have been more than ∼2° away from the border of the gamma-ray-producing region based on compactness arguments.

Scalar field couplings to quadratic curvature and decay into gravitons

Any field, even if it lives in a completely hidden sector, interacts with the visible sector at least via the gravitational interaction. In this paper, we show that a scalar field in such a hidden sector generically couples to the quadratic curvature via dimension five operators, i.e., ϕR2, ϕRμνRμν, ϕRμνρσRμνρσ, ϕRμνρσ {overset{sim }{R}}^{mu nu rho sigma} , because they can emerge if there exist particles heavier than it. We derive these scalar couplings to the quadratic curvature by integrating out heavy particles in a systematic manner. Such couplings are of phenomenological interest since some of them induce the scalar decay into the graviton pair. We point out that the decay of a scalar field can produce a substantial amount of the stochastic cosmic graviton background at high frequency since the suppression scale of these operators is given by the mass of heavy particles not by the Planck scale. The resultant graviton spectrum is computed in some concrete models.

An efficient approach for superconducting joint of YBCO coated conductors

Superconducting joints are crucial for second generation high-temperature superconducting (2G HTS) closed-loop coils that work in the persistent current mode (PCM) operation. Here, we report an efficient approach for superconducting joints of YBa2Cu3O7-σ (YBCO) coated conductors (CCs). The YBCO layer is etched to and from some microchannels, which serve as oxygen diffusion paths during oxygenation annealing (OA), to accelerate the recovery of critical current (I c) of the joined two YBCO layers. The I c of the superconducting joint is 118 A at 77 K with an optimized joining temperature and a short OA time (10 h), which is about 82% I c of the joined YBCO CCs. The persistent field decay of the closed-loop coil shows a joint resistance (R j) of less than 2.2 × 10−13 Ω at 77 K. Two YBCO films epitaxially grown along the c-axis diffuse into each other at the interface and form a dense joining, enabling superconducting current path. This superconducting joint technique can promote the achievement of the PCM operation in 2G HTS magnet applications, such as magnetic resonance imaging and nuclear magnetic resonance.

Critical behavior of quantum Fisher information in finite-size open Dicke model

We explore the steady-state critical behavior of the finite-size open Dicke model—a model that incorporates spontaneous emission decay of the collective atomic spin states and decay of the cavity field. From the perspective of quantum information theory, we can often better characterize the quantum phase transition. In this paper, we characterize the super-radiant phase transition of the steady state of the open Dicke model by numerically calculating the quantum Fisher information (QFI). We calculate the QFI for the atomic state and the cavity field state, as well as their derivatives. We find that the QFI of the cavity field state is more sensitive to atomic decay, and is suppressed more severely in the presence of atomic decay. In contrast, the QFI of the atomic state is less sensitive to the photon loss of the cavity field.

Timing and evolution of PSR B0950+08

ABSTRACT We present timing solutions of PSR B0950+08, using 14 years of observations from the Nanshan 26-m Radio Telescope of Xinjiang Astronomical Observatory. The braking index of PSR B0950+08 varies from –367 392 to 168 883, which shows an oscillation with large amplitude (∼105) and uncertainty. Considering the variation of braking indices and the most probable kinematic age of PSR B0950+08, a model with long-term magnetic field decay modulated by short-term oscillations is proposed to explain the timing data. With this magnetic field decay model, we discuss the spin and thermal evolution of PSR B0950+08. The uncertainties of its age are also considered. The results show that three-component oscillations are the more reasonable for the spin-frequency derivative distributions of PSR B0950+08, and the initial spin period of PSR B0950+08 must be shorter than $97\rm \ ms$ when the age is equal to the lower bound of its kinematic age. The standard cooling model could explain the surface temperature of PSR B0950+08 with its most probable kinematic age. Vortex creep heating with a long-term magnetic field decay could maintain a relatively high temperature at the later stages of evolution and explain the thermal emission data of old and warm pulsars. Coupling with the long-term magnetic field decay, an explanation of the temperature of PSR B0950+08 with roto-chemical heating needs an implausibly short initial rotation period ($P_0 \lesssim 17\rm { ms}$). The spin and thermal evolution of pulsars should be studied simultaneously. Future timing, ultraviolet or X-ray observations are essential for studying the evolution and interior properties of pulsars.

Instability of dwell-time constrained switched nonlinear systems

Analyzing the stability of switched nonlinear systems under dwell-time constraints, this article investigates different scenarios where all the subsystems have a common globally asymptotically stable (GAS) equilibrium, but for the switched system, the equilibrium is not uniformly GAS for arbitrarily large values of dwell-time. We motivate our study with the help of examples showing that, if near the origin all the vector fields decay at a rate slower than the linear vector fields, then the trajectories are ultimately bounded for large enough dwell-time. On the other hand, if away from the origin, the vector fields do not grow as fast as the linear vector fields, then we can only guarantee local asymptotic stability for large enough dwell-times, with region of attraction depending on the dwell-time itself. We formalize our observations for homogeneous systems, and show that, even if the origin is not uniformly GAS with dwell-time switching for nonlinear systems, it still holds that the trajectories starting from a bounded set converge to a neighborhood of the origin if the dwell-time is large enough.

Effect of equilibrium radial position on horizontal instability of middle single null divertor configuration in the iron core tokamak

The unique high-field side middle single null divertor configuration in the Joint-Texas Experimental Tokamak is attractive in studying detachment, impurity control, L-H transition, and negative triangularity. Its equilibrium configuration with a horizontally elongated (or flattened) poloidal cross section causes position instability and difficulty in equilibrium control. This work disassembles the vertical magnetic field of each group of poloidal field windings and calculates the field decay index (n-index) at different radial positions. It is found that the instability growth time τg of horizontal displacement increases as the equilibrium radial position of plasma moves outward, which is beneficial to the position control. The simulation and experiments verified the effect of equilibrium position on horizontal instability. The equilibrium position slightly outward from the geometric center of the vacuum chamber effectively decreases the horizontal displacement oscillation amplitude and guarantees the controllable horizontal displacement.

Electric field decay without pair production: lattice, bosonization and novel worldline instantons

Electric fields can spontaneously decay via the Schwinger effect, the nucleation of a charged particle-anti particle pair separated by a critical distance d. What happens if the available distance is smaller than d? Previous work on this question has produced contradictory results. Here, we study the quantum evolution of electric fields when the field points in a compact direction with circumference L < d using the massive Schwinger model, quantum electrodynamics in one space dimension with massive charged fermions. We uncover a new and previously unknown set of instantons that result in novel physics that disagrees with all previous estimates. In parameter regimes where the field value can be well-defined in the quantum theory, generic initial fields E are in fact stable and do not decay, while initial values that are quantized in half-integer units of the charge E = (k/2)g with k ∈ ℤ oscillate in time from +(k/2)g to −(k/2)g, with exponentially small probability of ever taking any other value. We verify our results with four distinct techniques: numerically by measuring the decay directly in Lorentzian time on the lattice, numerically using the spectrum of the Hamiltonian, numerically and semi-analytically using the bosonized description of the Schwinger model, and analytically via our instanton estimate.

Variable thermal applications of radiative micropolar nanofluid solutal boundary conditions

The nanomaterials are the most effective thermal approach which reflects many applications in the sectors of nanotechnology, engineering and industries. This theoretical investigation presents thermal effectives of micropolar nanomaterials in presence of variable thermal features and activation energy. The micropolar nanofluid containing microorganisms in order to improves the stability of nanomaterials. The analysis is performed by using solutal boundary constraints. The problem is switched into system of ordinary ones with the aid of appropriate transformation. The solution procedure is followed through shooting algorithm. The salient behavior of flow controlling parameters via subjective profiles of fluid is comprehensively discussed. The impacts of prominent parameters on engineering quantities namely skin friction, heat transfer rate, mass transfer rate and local density numbers of microorganisms are elaborated through tabular data. The results reflect that velocity field decay for buoyancy ratio parameter and rotation parameter. The induced velocity components are boosted for material parameter. The larger values of spin gradient viscosity parameter reduce the microrotation velocity field. The temperature and concentration field is boosted via thermal and solutal Biot numbers. Moreover, the consideration of temperature dependent thermal conductivity is more effective to improve the heat and mass transportation system. The obtained results convey applications in heat exchangers, heat transfer equipments, biomedicine, biotechnology, biofuels cells, nanotechnology, cooling of electronic devises medical, microfluidic, microelectronics double windowpane, food, transportation etc.

Evolution of Highly Magnetic White Dwarfs by Field Decay and Cooling: Theory and Simulations

Abstract We investigate the luminosity suppression and its effect on the mass–radius relation and cooling evolution of highly magnetized white dwarfs. Based on the effect of magnetic field relative to gravitational energy, we suitably modify our treatment of the radiative opacity, magnetostatic equilibrium, and degenerate core equation of state to obtain the structural properties of these stars. Although the Chandrasekhar mass limit is retained in the absence of magnetic field and irrespective of the luminosity, strong central fields of about 1014 G can yield super-Chandrasekhar white dwarfs with masses ∼2.0 M ⊙. Smaller white dwarfs tend to remain super-Chandrasekhar for sufficiently strong central fields even when their luminosity is significantly suppressed to 10−16 L ⊙. Nevertheless, owing to the cooling evolution and simultaneous field decay over 10 Gyr, the limiting masses of small magnetized white dwarfs can fall to 1.5 M ⊙ over time. However, the majority of these systems still remain practically hidden throughout their cooling evolution because of their high fields and correspondingly low luminosities. Utilizing the stellar evolution code stars, we obtain close agreement with the analytical mass limit estimates, which suggests that our analytical formalism is physically motivated. Our results argue that super-Chandrasekhar white dwarfs born as a result of strong-field effects may not remain so forever. This explains their apparent scarcity, in addition to making them hard to detect because of their suppressed luminosities.

Oscillatory Reconnection of a 2D X-point in a Hot Coronal Plasma

Abstract Oscillatory reconnection (a relaxation mechanism with periodic changes in connectivity) has been proposed as a potential physical mechanism underpinning several periodic phenomena in the solar atmosphere, including, but not limited to, quasi-periodic pulsations (QPPs). Despite its importance, however, the mechanism has never been studied within a hot, coronal plasma. We investigate oscillatory reconnection in a one million Kelvin plasma by solving the fully-compressive, resistive MHD equations for a 2D magnetic X-point under coronal conditions using the PLUTO code. We report on the resulting oscillatory reconnection including its periodicity and decay rate. We observe a more complicated oscillating profile for the current density compared to that found for a cold plasma, due to mode-conversion at the equipartition layer. We also consider, for the first time, the effect of adding anisotropic thermal conduction to the oscillatory reconnection mechanism, and we find this simplifies the spectrum of the oscillation profile and increases the decay rate. Crucially, the addition of thermal conduction does not prevent the oscillatory reconnection mechanism from manifesting. Finally, we reveal a relationship between the equilibrium magnetic field strength, decay rate, and period of oscillatory reconnection, which opens the tantalising possibility of utilizing oscillatory reconnection as a seismological tool.

Microwave Soil Heating with Evanescent Fields from Slow-Wave Comb and Ceramic Applicators

Microwave soil heating deactivates weed seeds; however, in many modern agricultural settings, weed seeds are mostly found in the top 1–2 cm of the soil profile. Until recently, microwave soil heating has been achieved using various antennas, which project the microwave energy deeply into the soil. The aim of this research was to develop new microwave applicators that provide shallow heating (less than 50 mm). This paper presents two applicator designs, one based on a comb slow-wave structure and the other on the frustrated total internal reflection (FTIR) principle, which utilise evanescent microwave fields to restrict the depth of microwave heating. The background theory to their performance is presented, followed by experimental evidence of their constrained heating performance under different soil moisture scenarios. Experimental measurements of the heating performance of these applicators, in soils of varying moisture content, demonstrate that the evanescent microwave fields restrict the depth of heating, so that most of the energy is manifested in the top 50 mm of soil. The evanescent field decay rate for the FTIR applicator changes from 44.0 ± 0.7 m−1 to 30 ± 1.2 m−1 as the soil moisture changes from 32% to 174% (dry weight basis). This is higher than the evanescent field decay rate for the comb slow-wave applicator (17.6 ± 0.7 m−1 to 19.9 ± 1.5 m−1). The FTIR applicator has a wider and shallower heating pattern than the comb slow-wave applicator. Because of the double heating lobes of the FTIR applicator, the effective half temperature heating width is approximately 150 mm. This is wider than the half temperature heating width of the comb slow-wave applicator (95 mm).

Combined effects of emitter-emitter and emitter-plasmonic surface separations dictate photoluminescence enhancement in a plasmonic field.

The brightness of an emitter can be enhanced by metal-enhanced fluorescence, wherein the excitonic dipole couples with the electromagnetic field of the surface plasmon. Herein, we experimentally map the landscape of photoluminescence enhancement (EFexp) of emitters in a plasmonic field as a function of the emitter-emitter separation, s, and the emitter-plasmon distance, t. We use Au nanoparticles overcoated with inert spacers as plasmonic systems and CdSe/ZnS quantum dots (QDs) as an emitter bearing opposite surface charges. The t and s are varied by changing the spacer thickness and number density of QDs on the plasmonic surface, respectively. The electrostatic binding of emitters on the plasmonic surface and their number density are established by following the variation of zeta-potential. EFexp is high, when t is short and s is large; nevertheless, it decreases when the emitter-emitter interaction dominates due to plasmon assisted nonradiative processes. In the absence of a plasmonic field, the enhancement observed is attributed to environmental effects and is independent of s, confirming the role of the electric field. Indeed, the distance dependence of EFexp closely follows the decay of the plasmonic field upon dilution of the emitter concentration on nanoparticles' surface (s = 18 nm). The QD-plasmon system is visualized in the framework of the Thomson problem, and classical electrodynamics calculations give the trends in t and s dependence of the photoluminescence. Being the first report on the simultaneous dependence of t and s on plasmon-enhanced photoluminescence, the results presented herein will open newer opportunities in the design of hybrid systems with a high brightness.

Local Well-Posedness to the 2d Cauchy Problem of Full Compressible Magnetohydrodynamic Equations with Vacuum at Infinity ∗

This paper concerns the Cauchy problem of two-dimensional (2D) full compressible magnetohydrodynamic (MHD) equations in the whole plane $\mathbb{R}^2$ with zero density at infinity. By spatial weighted energy method, we derive the local existence and uniqueness of strong solutions provided that the initial density and the initial magnetic field decay not too slowly at infinity. Note that the initial temperature does not need to decay slowly at infinity. In particular, vacuum states at both the interior domain and the far field are allowed.