Weak Magnetic Field Region Research Articles

Magnetic field-induced phase transition in spinor exciton-polaritons condensate

We theoretically study the magnetic phase transition of condensed exciton-polariton microcavities in an applied magnetic field. When the magnetic field is strong, all polariton spins are polarized parallel to the magnetic field as usual. On the contrary, in the weak magnetic-field region, the polariton polarization degree is negative, namely, anti-parallel to the magnetic field. For a strong magnetic field, the magnetic phase of the polaritons arises and leads to a paramagnetic, while around a weak magnetic field, with zero exciton–photon detunings, and weak Rabi splitting the spin polarization of the polaritons leads to a diamagnetic. Thus, magneto-polariton phase transition polarization originates from the competition between the polariton Zeeman effect and polariton–polariton interactions. Moreover, the polariton polarization strongly depends on the exciton–photon detuning and Rabi splitting and has a large negative value as they are both small. At last, we compare our theoretical results with the experiments and find they match each other very well.

Magnetic Trapping of Galactic Cosmic Rays in the Outer Heliosheath and Their Preferential Entry into the Heliosphere

This paper examines the geometry of interstellar magnetic field lines close to the boundary of the heliosphere in the direction of the unperturbed local interstellar magnetic field, where the field lines are spread apart by the heliopause (HP). Such field parting establishes a region of weaker magnetic field of about 300 au in size in the northern hemisphere that acts as a giant magnetic trap affecting the propagation of galactic cosmic rays (GCRs). The choice of an analytic model of the magnetic field in the very local interstellar medium allows us to qualitatively study the resulting magnetic field draping pattern while avoiding unphysical dissipation across the HP-impeding numerical magnetohydrodynamic (MHD) models. We investigate GCR transport in the region exterior to the heliosphere, including the magnetic trap, subject to guiding center drifts, pitch angle scattering, and perpendicular diffusion. The transport coefficients were derived from Voyager 1 observations of magnetic turbulence in the VLISM. Our results predict a ring current of energetic ions drifting around the interior of the magnetic trap. It is also demonstrated that GCRs cross the HP for the first time preferentially through a crescent-shaped region between the magnetic trap and the upwind direction. The paper includes results of MHD modeling of the heliosphere that provide the coordinates of the center of the magnetic trap in ecliptic coordinates. In addition to the heliosphere, we examine several extreme field draping configurations that could describe the astrospheres of other stars.

Electron Rolling‐Pin Distributions Preceding Dipolarization Fronts

AbstractElectron rolling‐pin distribution (RPDs), characterized by triple peaks at pitch angles 0°, 90°, and 180°, have recently been discovered in the terrestrial magnetosphere. Since the RPDs' formation is typically attributed to local betatron acceleration, RPDs have been believed to appear primarily inside strong magnetic regions, such as flux pileup regions (FPRs) behind dipolarization fronts (DFs). Different from such expectation, in this study we present unique observations of RPDs made by Cluster spacecraft in the terrestrial magnetotail, showing that RPDs are present inside weak magnetic field regions ahead of the DFs but absent in the strong magnetic field region behind them. The presence of RPD prior to the fronts may be atttributed to the combined effect of global betatron acceleration, global Fermi acceleration, and frontward transport driven by magnetic gradient drift. The atypical features of RPDs are important for fully understanding electron acceleration and transport in the magnetosphere.

Quantum effects in silicon-germanium p-type heterostructures with quantum wells of different widths

The magneto-quantum and quantum interference effects in a two-dimensional gas of p-type charge carriers are studied for three quantum wells made of practically pure germanium in a Si0.6Ge0.4/Si0.2Ge0.8/Si0.6Ge0.4 heterostructure. The quantum well widths were 8 nm for sample I, 19.5 nm for sample II, and 25.6 nm for sample III. The dependences of resistance on the magnetic field for all samples exhibit Shubnikov–de Haas oscillations. Their analysis made it possible to calculate the kinetic characteristics of charge carriers for the cases of one (sample I) and two occupied subbands (samples II and III). In the region of weak magnetic fields (B < 0.1 T), the effect of weak localization of holes was revealed, which determines the negative magnetoresistance and the increase in resistance with decreasing temperature. The manifestation of the charge carriers interaction effect at various temperatures and magnetic fields is discovered and analyzed. A transition from the diffusion mode of manifestation of the quantum correction to the intermediate, and then to the ballistic mode is observed. In all regions, the behavior of the quantum correction due to the charge carriers interaction effect is in good agreement with modern theoretical predictions. The temperature dependences of the hole-phonon relaxation time are calculated. In weak magnetic fields, with an increase in the temperature of the 2D system, a transition from the “partial inelasticity” mode, characterized by the dependence τhph−1∝T2, to the small-angle scattering mode, described by the relation τhph−1∝T5, takes place. In stronger magnetic fields for samples with two occupied subbands, the dependence τhph−1∝T3was observed. Possible explanations for this dependence are presented.

Parametric dependence of collisional heating of highly magnetized over-dense plasma by (far-)infrared lasers

Abstract Heating of over-dense plasma represents a long-standing quest in laser–plasma physics. When the strength of the magnetic field is above the critical value, a right-handed circularly polarized laser could propagate into and heat up the highly magnetized over-dense collisional plasma directly; the processes are dependent on the parameters of the laser, plasma and magnetic field. The parametric dependence is fully studied both qualitatively and quantitatively, resulting in scaling laws of the plasma temperature, heating depth and energy conversion efficiency. Such heating is also studied with the most powerful CO2 and strongest magnetic field in the world, where plasma with density of ${10}^{23}$ cm–3 and initial temperature of 1 keV is heated to around 10 keV within a depth of several micrometres. Several novel phenomena are also discovered and discussed, that is, local heating in the region of high density, low temperature or weak magnetic field.

Quantum interference phenomena and electron – electron interaction in topological insulator Bi2Se3 thin polycrystalline films

The temperature (T = 2.0 - 300 K) and magnetic field (B = 0 - 14 T) dependences of the transport coefficients were obtained for the topological insulator (TI) polycrystalline Bi2Se3 films with thicknesses d = 40 and 200 nm grown on glass substrates by thermal evaporation in vacuum from a single source. In the region of weak magnetic fields, a sharp increase in magnetoresistance (MR) with increasing B was observed, indicating the presence of the weak antilocalization (WAL) effect, which is typical for 3D-TIs. However, the longitudinal resistance Rxx increases logarithmically with decreasing temperature below certain temperature Tc, which is not characteristic of the WAL effect in 2D-structures and indicates possible existence of the so-called “transport paradox”. In the temperature dependences of the Hall resistance, Hall coefficient and MR we also registered clearly pronounced kinks near Tc. The Rxx(T) dependence for a 40 nm film, with decreasing temperature in addition to the Rxx minimum near Tc, exhibits a maximum near 2.1 K. Estimates of the number of independent transport channels m and the dephasing length lφ show that as d increases from 40 nm to 200 nm, m increases from 1 to 3, while lφ changes insignificantly. The nonmonotonic behavior of the T- and B- dependences of the kinetic coefficients, the occurrence of the “transport paradox” are associated with a change in the contributions of WAL and electron-electron interaction effects when the nature and magnitude of the external influence and film thickness change. It follows from the data obtained that not only in very thin monocrystalline films, but also in rather thick polycrystalline Bi2Se3 films prepared by the simple method of the thermal evaporation in vacuum on amorphous substrates, quantum interference effects and “transport paradox” can be manifested.

Local Time and Longitudinal Differences in the Occurrence Frequency of Ionospheric EMIC Waves During Magnetic Storm Periods

AbstractThe spatial and temporal distributions of ionospheric electromagnetic ion cyclotron (EMIC) waves during magnetic storms from 2014 to 2018 were studied using Swarm observations. Ionospheric EMIC waves preferably occurred during storms and their recovery phases at subauroral regions within an average magnetic latitude of 40–55°. There were more wave events during more intense and longer period storms. However, the correlation between event number and storm duration was not good. This might be due to the effect of heavy oxygen ions on EMIC wave generation and the loss of ring current high‐energy ions by the EMIC waves. There are obvious magnetic local time (MLT) differences in the peak occurrence frequency of EMIC waves during storm phases. The enhanced solar wind dynamic pressure was favorable for duskside EMIC waves. With an increased substorm activity, the wave occurrence rate peak shifted from the morning side to the dusk‐premidnight sector. During the recovery phase of a storm, EMIC waves in the 12–24 MLT sector appeared preferably in the earlier part than those in the 00–12 MLT sector. This shift in local time is related to the eastward rotation of the plasmaspheric plume toward morning during the late storm recovery phase and its overlap with the ring current. Highest occurrence frequency of the storm time EMIC waves could be found in the South Atlantic Anomaly region, which might be related to the drift shell splitting and the wave propagation effect in the weak magnetic field region.

STUDY OF THE MAGNETO-RESISTANCE SILICON P-TYPE

The study of galvanomagnetic phenomena in silicon, in strong magnetic fields, is associated with great difficulties due to the high resistance of the samples and the complexity of preparing omics contacts that work well in a wide range of temperatures and magnetic fields. Therefore, until now, all existing data on galvanomagnetic effects in silicon have been obtained only in the region of weak magnetic fields. Given the growing interest in silicon due to its large potential, it seemed appropriate to study the galvanomagnetic effects in the region of strong magnetic fields, where quantum effects play the dominant role. The study of Galvano-magnetic effects (as well as tensoeffects) in silicon under extreme conditions allows not only to identify the mechanisms of these effects, but also to identify the possibility of creating gaussmeters, infrared detectors, sensitive strain gauges, amplifiers and generators of a wide frequency range. Keywords: galvano-magnetic effects, silicon, negative magnetoresistance, uniaxial pressure.

In Situ Measurement of Curvature of Magnetic Field in Turbulent Space Plasmas: A Statistical Study

Abstract Using in situ data, accumulated in the turbulent magnetosheath by the Magnetospheric Multiscale Mission, we report a statistical study of magnetic field curvature and discuss its role in the turbulent space plasmas. Consistent with previous simulation results, the probability distribution function of the curvature is shown to have distinct power-law tails for both high and low value limits. We find that the magnetic-field-line curvature is intermittently distributed in space. High curvature values reside near weak magnetic-field regions, while low curvature values are correlated with small magnitude of the force acting normal to the field lines. A simple statistical treatment provides an explanation for the observed curvature distribution. This novel statistical characterization of magnetic curvature in space plasma provides a starting point for assessing, in a turbulence context, the applicability and impact of particle energization processes, such as curvature drift, that rely on this fundamental quantity.

Enhancing quantum annealing performance by a degenerate two-level system

Quantum annealing is an innovative idea and method for avoiding the increase of the calculation cost of the combinatorial optimization problem. Since the combinatorial optimization problems are ubiquitous, quantum annealing machine with high efficiency and scalability will give an immeasurable impact on many fields. However, the conventional quantum annealing machine may not have a high success probability for finding the solution because the energy gap closes exponentially as a function of the system size. To propose an idea for finding high success probability is one of the most important issues. Here we show that a degenerate two-level system provides the higher success probability than the conventional spin-1/2 model in a weak longitudinal magnetic field region. The physics behind this is that the quantum annealing in this model can be reduced into that in the spin-1/2 model, where the effective longitudinal magnetic field may open the energy gap, which suppresses the Landau–Zener tunneling providing leakage of the ground state. We also present the success probability of the Λ-type system, which may show the higher success probability than the conventional spin-1/2 model.

The Magnetic Flux Rope Structure of a Triangulated Solar Filament

Abstract Solar magnetic flux ropes are core structures driving solar activities. We construct a magnetic flux rope for a filament/prominence observed at 01:11 UT on 2011 June 21 in AR 11236 with a combination of state-of-the-art methods, including triangulation from multiperspective observations, the flux rope embedding method, the regularized Biot–Savart laws, and the magnetofrictional method. First, the path of the filament is reconstructed via the triangulation with 304 Å images observed by the Atmospheric Imaging Assembly on board Solar Dynamics Observatory (SDO) and by the Extreme Ultraviolet Imager on board the twin Solar Terrestrial Relations Observatory. Then, a flux rope is constructed with the regularized Biot–Savart laws using the information of its axis. Next, it is embedded into a potential magnetic field computed from the photospheric radial magnetic field observed by the Helioseismic and Magnetic Imager on board SDO. The combined magnetic field is finally relaxed by the magnetofrictional method to reach a nonlinear force-free state. It is found that both models constructed by the regularized Biot–Savart laws and after the magnetofrictional relaxation coincide with the 304 Å images. The distribution of magnetic dips coincides with part of the filament/prominence material, and the quasi-separatrix layers wrap the magnetic flux ropes, displaying hyperbolic flux tube structures. These models have the advantages of constructing magnetic flux ropes in the higher atmosphere and weak magnetic field regions, which could be used as initial conditions for magnetohydrodynamic simulations of coronal mass ejections.

The Effects of Solar Wind Dynamic Pressure on the Structure of the Topside Ionosphere of Mars

AbstractWe use Mars Atmosphere and Volatile EvolutioN observations of the upstream solar wind, and Mars Express observations of ionospheric electron densities and magnetic fields, to study how the topside ionosphere (>320 km) of Mars is affected by variations in solar wind dynamic pressure. We find that high solar wind dynamic pressures result in the topside ionosphere being depleted of plasma at all solar zenith angles, coincident with increased induced magnetic field strengths. The depletion of topside plasma in response to high solar wind dynamic pressures is observed in both weak and strong crustal magnetic field regions. Taken together, our results suggest that high solar wind dynamic pressures lead to ionospheric compression, increased ion escape, and reduced day‐to‐night plasma transport in the high‐altitude nightside ionosphere.

Magnetic renormalons in a scalar self-interacting λϕ4 theory

We present an analysis about the influence of an external magnetic field on renormalons in a self interacting theory $\lambda \phi ^{4}$. In the weak magnetic field region, using an appropriate expansion for the Schwinger propagator's, we find new renormalons as poles on the real axis of the Borel plane, whose position do not depend on the magnetic field, but where the residues acquire in fact a magnetic dependence. In the strong magnetic limit, working in the lowest Landau level approximation (LLLA), these new poles are not longer present. We compare the magnetic scenario with previous results in the literature concerning thermal effects on renormalons in this theory.

Analysis and design method of a combined radial–axial magnetic bearing based on asymmetric factor

Combined radial–axial magnetic bearings (MBs) use a common bias magnetic circuit to provide radial and axial bias magnetic flux. Different from separate MBs, there is a constraint relation between the radial and axial bias flux. Meanwhile, there is also a constraint relationship between the radial and axial maximum bearing capacity. Therefore, the design method for separated MB is not suitable for a combined radial-axial MB (CRAMB). In this study, a design method of the CRAMB based on an asymmetric factor was proposed. The definition of the asymmetric factor was proposed. In order to avoid the radial and axial magnetic flux into the weak magnetic field and saturation region, the optimum range of the asymmetric factor is analysed. The influence of the asymmetry factor on stiffness is analysed; the prototype designed by this method can run stably. The validity and accuracy of the design method are verified by experimental results.

Weak Magnetic Field Resonance Effects in Diamond with Nitrogen-Vacancy Centers

The photoluminescence intensity of the ensemble of NV centers in diamond at weak magnetic field was studied. Two resonances were revealed in the region of weak magnetic fields: the first resonance with a width of the order of 20 G, and the second resonance with a width of the order of 180 G. The presence of these optical resonances makes it possible to propose a new concept for creating magnetic field sensors based on a diamond with an ensemble of NV centers without the use of a microwave field.

The immediate environment of an astrophysical black hole

Abstract In view of the upcoming observations with the Event Horizon Telescope (EHT), we present our thoughts on the immediate environment of an astrophysical black hole. We are concerned that two approximations used in general relativistic magnetohydrodynamic numerical simulations, namely numerical density floors implemented near the base of the black hole jet, and a magnetic field that comes from large distances, may mislead our interpretation of the observations. We predict that three physical processes will manifest themselves in EHT observations, namely dynamic pair formation just above the horizon, electromagnetic energy dissipation along the boundary of the black hole jet, and a region of weak magnetic field separating the black hole jet from the disc wind.

Anderson transition of two-dimensional spinful electrons in the Gaussian unitary ensemble

We study the Anderson localization of disordered two-dimensional electron gases (2DEGs) on a square lattice subject to a perpendicular magnetic field $B$, random scalar potentials, and Rashba spin-orbit interactions. Our focus is on the weak magnetic field region, motivated by the intriguing question of how those extended states, existing in the absence of the magnetic field $(B=0)$ when the 2DEGs are the Gaussian symplectic ensemble, change with $B$. Using highly accurate numerical procedures based on the transfer matrix technique and the level statistics, we found that a metallic phase exists at weak magnetic fields, in contrast to the predictions of the one-parameter scaling theory that all states are localized at weak fields except at zero field, and the metallic phase evolves continuously into those at strong magnetic fields. A schematic phase diagram drawn in the field-energy plane elucidates the occurrence and evolution of extended states.

Structure formation through self-gravitational instability in degenerate and non-degenerate anisotropic magnetized plasma

The self-gravitational instability is examined for non-degenerate and degenerate magnetized plasma. In the case of non-degenerate collisionless magnetized plasma the pressure is considered as anisotropic while in the case of degenerate situations it is taken as isotropic. The effect of finite Larmor radius correction of non-degenerate ions and viscous dissipation is taken into account in both the cases. Firstly in non-degenerate anisotropic plasma the conventional magnetohydrodynamic model is used to construct basic set of equations within the framework of modified Chew–Goldberger and Low theory. Secondly, in the case of degenerate isotropic plasma, which is considered to be composed of degenerate electrons and non-degenerate ions, the model equations are constructed using quantum magneto hydrodynamic model. The dynamics of degenerate particles are governed by Bohm and exchange potentials. The general dispersion relations are derived for both degenerate and non-degenerate situations separately using linearized perturbation equations. The results are discussed analytically and numerically for various modes of propagation. In case of non degenerate strongly magnetized plasma the effects of stress tensor anisotropy dominate over the influence of FLR effects while the FLR effects prevail in the weak magnetic field region. In case of isotropic degenerate plasma the implications of exchange parameter on the Jeans mass have been estimated and it is found that the increase in exchange parameter increases the limit of Jeans mass. The Jeans length and Jeans mass have been estimated for the white dwarf stars as $L_{J} \approx 2.1 \times 10^{11}~\mbox{m}$ and $M_{J} \approx 5 \times 10^{39}~\mbox{kg}$ respectively assist the existence of super Chandrasekhar white dwarfs.

BRIGHT HOT IMPACTS BY ERUPTED FRAGMENTS FALLING BACK ON THE SUN: MAGNETIC CHANNELLING

ABSTRACT Dense plasma fragments were observed to fall back on the solar surface by the Solar Dynamics Observatory after an eruption on 2011 June 7, producing strong extreme-ultraviolet brightenings. Previous studies investigated impacts in regions of weak magnetic field. Here we model the km s−1 impact of fragments channelled by the magnetic field close to active regions. In the observations, the magnetic channel brightens before the fragment impact. We use a 3D-MHD model of spherical blobs downfalling in a magnetized atmosphere. The blob parameters are constrained from the observation. We run numerical simulations with different ambient densitie and magnetic field intensities. We compare the model emission in the 171 Å channel of the Atmospheric Imaging Assembly with the observed one. We find that a model of downfall channelled in an ∼1 MK coronal loop confined by a magnetic field of ∼10–20 G, best explains qualitatively and quantitatively the observed evolution. The blobs are highly deformed and further fragmented when the ram pressure becomes comparable to the local magnetic pressure, and they are deviated to be channelled by the field because of the differential stress applied by the perturbed magnetic field. Ahead of them, in the relatively dense coronal medium, shock fronts propagate, heat, and brighten the channel between the cold falling plasma and the solar surface. This study shows a new mechanism that brightens downflows channelled by the magnetic field, such as in accreting young stars, and also works as a probe of the ambient atmosphere, providing information about the local plasma density and magnetic field.

Magnetothermoelectric transport properties of multiterminal graphene nanoribbons

The Peltier effect and the Ettingshausen effect are investigated in graphene nanoribbons, where charge current produces heat current along the longitudinal direction in the former case, and longitudinal charge current generates transverse heat current in the latter case. With the aid of the nonequilibrium Green's function and the Landauer-B\"uttiker formalism, the Peltier coefficient ${\mathrm{\ensuremath{\Pi}}}_{c}$ and the Ettingshausen coefficient ${\mathrm{E}}_{c}$ are obtained. We found that the Kelvin relation is always valid for the longitudinal thermoelectric transport, i.e., ${\mathrm{\ensuremath{\Pi}}}_{c}=\mathcal{T}{S}_{c}$, with $\mathcal{T}$ the temperature and ${S}_{c}$ the Seebeck coefficient. In contrast, for transverse magnetothermoelectric transport, the Kelvin relation breaks down and ${\mathrm{E}}_{c}\ensuremath{\ne}\mathcal{T}{N}_{c}$ usually, with ${N}_{c}$ the Nernst coefficient. In the region of weak magnetic field, the Ettingshausen effect depends strongly on device parameters. When the Fermi energy ${E}_{F}$ is close to the Dirac point, the Ettingshausen effect of the semiconducting armchair graphene nanoribbon is much stronger than that of the metallic one. When ${E}_{F}$ is far away from the Dirac point, the Ettingshausen coefficient ${\mathrm{E}}_{c}$ oscillates around zero. When under a strong magnetic field, ${\mathrm{E}}_{c}$ is independent of the device parameters and swells only near the Dirac point. Further, the dependence of ${\mathrm{E}}_{c}$ on ${E}_{F}$ can be scaled by ${E}_{F}/{k}_{B}\mathcal{T}$, with a peak value of $(2ln2){k}_{B}\mathcal{T}/e$ for the three-terminal system and $(\frac{4}{3}ln2){k}_{B}\mathcal{T}/e$ for the four-terminal system. We also study the impact of disorder on the Ettingshausen effect. Regardless of the magnetic field strength, ${\mathrm{E}}_{c}$ is robust against moderate disorder scattering. In addition, in the strong magnetic field, ${\mathrm{E}}_{c}$ with additional regular oscillating structure can be caused by disorder.