Magnetic Field Oscillations Research Articles

Periodic Modulation of the Upper Ionosphere by ULF Waves as Observed Simultaneously by SuperDARN Radars and GPS/TEC Technique

AbstractRecent work has demonstrated that global Pc5 pulsations observed by ground‐based magnetometers may be accompanied by periodic oscillations in the total electron content (TEC) of the ionosphere measured by GPS receivers. These TEC observations may provide new insights into magnetosphere‐ionosphere coupling mechanisms, especially when combined with other ground‐based observational techniques. Presented in this paper is a large‐scale morning sector Pc5 event which was observed simultaneously by ground‐based magnetometers, two high‐frequency Super Dual Auroral Radar Network (SuperDARN) radars, and several Global Positioning System (GPS)/TEC receivers. The transient 2.6 mHz pulsations observed by the ground magnetometers and radars are accompanied by periodic fluctuations in the time rate‐of‐change of TEC (ROT) at the same frequency. To investigate possible mechanisms for the TEC modulation by ultralow frequency (ULF) waves, we determine the ratios between the spectral amplitudes of the magnetic, ionospheric Doppler velocity and ROT oscillations. The relationship between the simultaneous magnetic field and ionospheric Doppler velocity oscillations can be reasonably well interpreted using the theory of Alfven wave interaction with the thin ionospheric layer. Though the observed ratio between ROT and Doppler velocity amplitudes can be explained by the occurrence of local steep gradient of the topside ionosphere plasma at auroral latitudes, the responsible modulation mechanism cannot be considered as firmly established.

Aharonov–Bohm oscillations and equilibrium current distributions in open quantum dot and in ring interferometer

Magnetotransport in two submicron-sized devices formed on the basis of GaAs/AlGaAs structures has been simulated using nonequilibrium Green functions. The effect of a perpendicular magnetic field on quantum transport in a quasi-one-dimensional quantum dot and in an Aharonov–Bohm interferometer has been analyzed in a single-particle approximation. Magnetic field oscillations of two-terminal conductance of the devices, equilibrium (persistent) current distributions and magnetic moment generated in the devices by persistent currents have been determined using numerical methods. Correlations between the magnetic moment, magnetic field oscillations of conductance and energy resonance in a specific magnetic field have been traced. Sufficiently regular conductance oscillations similar to Aharonov–Bohm ones have been revealed for a quasi-one-dimensional quantum dot at small magnetic fields (0.05–0.4 T). For a ring interferometer the contribution to the total equilibrium current and magnetic moment at a specific energy may change abruptly both in magnitude and in sign as a result of changing magnetic field within one Aharonov–Bohm oscillation. We show that the conductance of an interferometer is determined not by the number of modes propagating in the ring but rather by the effect of triangular quantum dots at the ring entrance that produce a strong reflection. The period of the calculated Aharonov–Bohm oscillations is in agreement with the measurement results for these devices.

Numerical analysis of the influence of magnetic field waveforms on the performance of active magnetic regenerators

Magnetic cooling is an alternative to vapor compression that does not rely on the use of hazardous substances. The refrigerant is a solid material which reacts to oscillations in magnetic field by changing its temperature (the magnetocaloric effect). In active magnetic regenerators, the magnetocaloric material arranged as a porous medium is subjected to an oscillating fluid flow to allow heat transfer from a cold source to a hot sink in a thermodynamic cooling cycle. Although the literature is abundant with studies on the influence of the fluid flow waveform on magnetic refrigeration devices, the influence of the magnetic field waveform has been much less investigated. In this work, we make use of an active magnetic regenerator numerical model with different mathematically defined waveforms to determine which operating parameters yield the highest values of cooling capacity and coefficient of performance for a specific set of operating conditions. The results show that the best performance is achieved when the magnetic field is kept constant for the same time duration of the fluid flow through the magnetized material, and the transition times between the high and low levels of the magnetic field should be as short as possible.

Quantum oscillations in nanowires of topological insulator Bi0.83Sb0.17

Three dimensional topological insulators (TIs) are novel materials with a conducting surface covering an insulating bulk interior. We examine the topological properties of nanowires of TI Bi0.83Sb0.17 and observed that they display oscillations of the magnetoresistance (MR) that can be interpreted as topological surface electronic transport. In this work, we study the nanowires at various magnetic field orientations, at low temperatures (1.5 K ≤ T < 5 K), and for field strengths up to 14 T. In the 4 T < B < 10 T range we observe B-periodic, Aharonov–Bohm (AB) oscillations of longitudinal MR with two periods, namely, one flux quantum Φ0 and half of flux quantum Φ0/2 (ΔB1 = Φ0/S, ΔB2 = Φ0/2S, where S is the cross-sectional area of the nanowire). The periods depend on the angle α between the magnetic field and the wire axis according to a cosine law that indicates that the phenomenon is governed by the axial flux enclosed by the nanowire. In additional to the high magnetic field oscillations, in the low magnetic field range, B < 4 T, we observe a conductance maximum for zero magnetic field and the first magnetoresistance maximum for Φ = Φ0/2.

Anomalous Decay of Quantum Resistance Oscillations of 2D Helical Electrons in Magnetic Field

Shubnikov de Haas resistance oscillations of highly mobile two dimensional helical electrons propagating on a conducting surface of strained HgTe 3D topological insulator are studied in magnetic fields B tilted by angle θ from the normal to the conducting layer. Strong decrease of oscillation amplitude A is observed with the tilt: {boldsymbol{A}}sim {boldsymbol{e}}{boldsymbol{x}}{boldsymbol{p}}(,-,{boldsymbol{xi }}/{boldsymbol{c}}{boldsymbol{o}}{boldsymbol{s}}({boldsymbol{theta }})), where ξ is a constant. Evolution of the oscillations with temperature T shows that the parameter {boldsymbol{xi }} contains two terms: {boldsymbol{xi }}={{boldsymbol{xi }}}_{1}+{{boldsymbol{xi }}}_{2}{boldsymbol{T}}. The temperature independent term, {{boldsymbol{xi }}}_{{bf{1}}}, signals possible reduction of electron mean free path {l}_{q} and/or enhancement of in-homogeneous broadening of the oscillations in magnetic field B. The temperature dependent term, {{boldsymbol{xi }}}_{{bf{2}}}{boldsymbol{T}}, indicates increase of the reciprocal velocity of 2D helical electrons: delta ({v}_{F}^{-1})sim B suggesting modification of the electron spectrum in magnetic fields. Results are found in good agreement with proposed phenomenological model.

Quantum Dots in an InSb Two-Dimensional Electron Gas

Indium antimonide (InSb) two-dimensional electron gases (2DEGs) have a unique combination of material properties: high electron mobility, strong spin-orbit interaction, large Land\'{e} g-factor, and small effective mass. This makes them an attractive platform to explore a variety of mesoscopic phenomena ranging from spintronics to topological superconductivity. However, there exist limited studies of quantum confined systems in these 2DEGs, often attributed to charge instabilities and gate drifts. We overcome this by removing the $\delta$-doping layer from the heterostructure, and induce carriers electrostatically. This allows us to perform the first detailed study of stable gate-defined quantum dots in InSb 2DEGs. We demonstrate two distinct strategies for carrier confinement and study the charge stability of the dots. The small effective mass results in a relatively large single particle spacing, allowing for the observation of an even-odd variation in the addition energy. By tracking the Coulomb oscillations in a parallel magnetic field we determine the ground state spin configuration and show that the large g-factor ($\sim$30) results in a singlet-triplet transition at magnetic fields as low as 0.3 T.

Electrostatic theory of irregular conductance oscillations through a quantum constriction under quantized Hall conditions

Abstract We propose a semi-classical explanation to the conductance oscillations ( δ G ) observed at gate-controlled quantum dots under quantized Hall conditions. Our approach is based on the self-consistent screening theory of the quantum Hall effect and δ G originates either from the interference of electrons or from charging effects. In the interference regime electrons acquire an Aharonov–Bohm phase while propagating coherently along the evanescent incompressible stripes at a particular filling factor, where the stripes enclose a magnetic flux. The charging regime finds its ground in Coulomb blocked, where the transport electron generates an additional energy scale to conductance oscillations. Combining these two regimes, we found that the period of conductance oscillations in magnetic field ( Δ B ) may increase, decrease or remain constant at different filling factors, as observed in the experiments. In addition it is shown that, a gate potential can shift the incompressible stripe along the gated edge spatially, thereby changing the enclosed flux, and can induce δ G with a period Δ V g . We demonstrate that our formulation is able to generate the superimposed Δ B - Δ V g interferences in stripe or checkerboard patterns in agreement with experiments. We can account for further salient and peculiar features of experimental observations for example the surprising absence of oscillations at certain filling factors.

Characterization of Freeze-Cast Micro-Channel Monoliths as Active and Passive Regenerators

The efficiency of the magnetic refrigeration process strongly depends on the heat transfer performance of the regenerator. As a potential way to improve the heat transfer performance of a regenerator, the design of sub-millimeter hydraulic diameter porous structures is realized by freeze-cast structures. Four freeze-cast regenerators with different pore widths are characterized experimentally and numerically. Empirical parameters are determined for the correlations of heat transfer and flow resistance via a 1D model. Thermal effectiveness and pressure drop are measured for thermal-hydraulic evaluations. Temperature span and specific cooling capacity are obtained to compare the magnetocaloric potential based on the material La0.66Ca0.27Sr0.06Mn1.05O3. The stability of freeze-cast regenerators is validated by comparing the performance during, before and after oscillatory flow and periodic magnetic field tests. Smaller pore design obtain the better heat transfer performance and required mechanical strength, while pore design with significant dendrites provides the worst tradeoff between heat transfer performance and flow resistance.

A double cantilever beam incorporating cohesive crack modeling for superconductors

In this paper, a double cantilever beam (DCB) specimen incorporating cohesive crack is developed for superconductors which have potential applications in high temperature superconducting cables in space solar power station. The cohesive interface is introduced along the crack front of the DCB model under electromagnetic force. The load-separation relation (i.e. the crack opening displacement) is used as the fracture mechanics parameter and the corresponding curves during fracture process are obtained and verified by the finite element numerical method. Results show that the presence of tensile electromagnetic force makes crack propagate easily. Superconductors with small cracks have good adaptability to the oscillation of magnetic fields while that with large cracks are easier to fracture during the descent of the magnetic field. In addition, the ductility ratio of the cohesive interface can significantly increase the fracture strength. The length of fracture zone decreases as the crack length increases.

Discovery of long-period magnetic field oscillations and motions in isolated sunspots

We analyse the temporal evolution of the inclination component of the magnetic field vector for the penumbral area of 25 isolated sunspots. Compared to previous works, the use of data from the HMI instrument aboard the SDO observatory facilitates the study of a very long time series (≈1 week) with a good spatial and temporal resolution. We used the wavelet technique and we found some filamentary-shaped events with large wavelet power. Their distribution of periods is broad, ranging from the lower limit for this study of 48 min up to 63 h. An interesting property of these events is that they do not appear homogeneously all around the penumbra but they seem to concentrate at particular locations. The cross-comparison of these wavelet maps with AIA data shows that the regions where these events appear are visually related to the coronal loops that connect the outer penumbra to one or more neighbouring opposite polarity flux patches.

The Flapping Motion of Mercury's Magnetotail Current Sheet: MESSENGER Observations

AbstractThe flapping motion of Mercury's magnetotail current sheet is investigated based on the observations of MErcury Surface, Space ENvironment, GEochemistry, and Ranging (MESSENGER). The typical flapping period in Mercury's magnetotail is much shorter (~13 s), comparing with ~10–20 min at Earth's magnetotail. The magnetic field oscillation amplitude induced by flapping motion is larger near both flanks than that around tail center. Similar to Earth's magnetotail, there are two flapping types existent in Mercury's magnetotail, one is the kink‐like flapping that can propagate as traveling waves, and the other one is the steady flapping that does not propagate. Both flapping types distribute widely in magnetotail. The kink‐like flapping waves propagating either dawnward or duskward can be observed across the entire magnetotail current sheet, which suggests that the energy sources for triggering the kink‐like flapping waves should locate around both tail flanks instead of tail center as is for the Earth's magnetotail.

Первые наблюдения альфвеновских каскадов на токамаке Глобус-М2 и их применение для анализа минимума запаса устойчивости

On the spherical tokamak Globus-M2 in discharges with toroidal magnetic field B= 0.7 T by means of magnetic probes and a number of other diagnostics during neutral beam injection at the current ramp up stage, magnetic field oscillations in the range of 100-300 kHz having their frequency increasing in time, had been observed. Eigenmodes with wavenumbers $n = 1-3$ (toroidal number) and $m = 2-4$ (poloidal number) were being registered. These oscillations were identified as Alfven cascades. By means of multi-channel fluctuation reflectometer, observed eigenmodes were found to be localized near magnetic shear reversal radius. Application of MHD-spectroscopy technique allowed us to determine safety factor $q_{min}$ temporal evolution and experimental values are in well agreement with modelling results, provided by ASTRA transport code.

Neutrino quantum decoherence due to entanglement with magnetic field

Neutrino oscillations in a constant magnetic field are considered. The backward influence of neutrinos on the environment is accounted for using the density matrix formalism. The entanglement of neutrinos with the magnetic field can destroy quantum coherent superposition of different neutrino states and thus lead to the suppression of neutrino oscillations. The master equation for the neutrino density matrix accounting for the effects of quantum decoherence due to entanglement with the magnetic field is derived.

Спиновая динамика отрицательно заряженных экситонов в квантовых точках InP/(In,Ga)P в магнитном поле

The dynamics of the photoluminescence negative circular polarization of the InP/(In,Ga)P quantum dots ensemble was studied. We find that in the time-resolved dependences of the polarization there are no oscillations in Voigt magnetic field. Also, with increasing field the polarization declines to zero. Such behavior is attributed to the peculiarities of the negatively charged exciton spin dynamics, particularly, to the fact that in the negatively charged exciton ground state the spin dynamics is governed by the heavy hole. We show that magnetic field depolarization of the photoluminescence occurs once the field of dynamically polarized nuclear spins acting on electron spins is surpassed.

The influence of hydrodynamic effects on the complex susceptibility response of magnetic fluids undergoing oscillatory fields: New insights for magnetic hyperthermia

In this work, we perform Langevin dynamics simulations to examine microstructure-macroscopic related properties of magnetic fluids in an attempt to understand the influence of the long range viscous hydrodynamic and dipolar interparticle interactions on the complex susceptibility response of a magnetic suspension undergoing an oscillatory magnetic field. The simulations use periodic boundary conditions in order to properly compute particle interactions through the Ewald summation technique. The imaginary part of the complex susceptibility predicted by the simulations is presented in terms of the frequency, particle volume fraction, and Péclet number. This property is used to investigate the process of magnetic hyperthermia. A detailed comparison between our simulations and the prediction of an asymptotic theory for a small Péclet number in the absence of hydrodynamic interactions shows an excellent agreement. The influence of the hydrodynamic and dipolar interactions on the average rate of temperature rise is investigated here. The coupling between the particle relaxation time and the forcing frequency of the applied field is also discussed. The simulations exhibit inhomogeneous chainlike structures in the numerical box induced by interparticle dipolar interactions. We find that the presence of these structures enhances magnetic heating production, whereas hydrodynamic interactions weaken this effect. Our results also suggest that the way of combining and controlling physical parameters at moderate frequencies of the applied oscillatory field can improve the heating performance of magnetic hyperthermia.

Many-Body Dynamics and Decoherence of the XXZ Central Spin Model in External Magnetic Field.

The many-body dynamics of an electron spin−1/2 qubit coupled to a bath of nuclear spins by hyperfine interactions, as described by the central spin model in two kinds of external field, are studied in this paper. In a completely polarized bath, we use the state recurrence method to obtain the exact solution of the central spin model in a constant magnetic field and numerically analyze the influence of the disorder strength of the magnetic field on fidelity and entanglement entropy. For a constant magnetic field, the fidelity presents non-attenuating oscillations. The anisotropic parameter and the magnetic field strength B significantly affect the dynamic behaviour of the central spin. Unlike the periodic oscillation in the constant magnetic field, the decoherence dynamics of the central spin act like a damping oscillation in a disordered field, where the central spin undergoes a relaxation process and eventually reaches a stable state. The relaxation time of this process is affected by the disorder strength and the anisotropic parameter, where a larger anisotropic parameter or disorder strength can speed up the relaxation process. Compared with the constant magnetic field, the disordered field can regulate the decoherence over a large range, independent of the anisotropic parameter.

Gate-tunable h/e -period magnetoresistance oscillations in Bi2O2Se nanowires

We report on the successful synthesis and low-temperature electron transport investigations of a new form of material - Bi2O2Se semiconducting nanowires. Gate-tunable 0- and $\pi$-h/e (h is the Planck constant and e the elementary charge) periodic resistance oscillations in longitudinal magnetic field were observed unexpectedly, demonstrating novel quasi-ballistic, phase-coherent surface states in Bi2O2Se nanowires. By reaching a very good agreement between the calculated density of states and the experimental data, we clarified the mechanism to be the one dimensional subbands formed along the circumference of the nanowire rather than the usually considered Aharonov-Bohm interference. A qualitative physical picture based on downward band bending associated with the complex band structure is proposed to describe the formation of the surface states.

SDO Data Artifacts and Anticorrelation of Oscillations in the Sunspot Magnetic Field and Umbral Area

The paper considers a special class of artifacts that appears in the useful signal when the source moves along a discrete matrix of the receiver. A parasitic component forms in the signal due to the displacement of the extreme value of the distributed field of the physical quantity from pixel to pixel (p2p effect). It is shown that a stepwise discretization of Δt < T (p2p) is a necessary but not sufficient condition for the formation of the p2p-artifact. For small field gradients (in particular, with good spatial resolution), the parasitic signal in the time series of the studied variable is either weakened or does not form at all. This assumption has been qualitatively verified by comparative analysis of sunspot data obtained with SOHO/MDI and SDO/HMI. In addition, the Y-p2p effect, which is associated with sunspot displacement along the meridian and influences the process of long-period sunspot oscillations, is considered in detail based on SDO/HMI data. It was found that there is an inverse relationship between changes in the umbral area of a spot and variations of its magnetic field strength. This property of long-period sunspot oscillations as a single magnetic structure indicates the magnetic flux in the sunspot umbra is preserved, and it plays an extremely important role for our understanding of the physical nature of these oscillations.

Multi-resonance cyclotron-undulator electron acceleration

In this work, we propose a method of particle acceleration by the electromagnetic wave pulse propagating in the waveguide. The method is based on a “superposition” the electron cyclotron oscillations in the uniform longitudinal magnetic field and their bounce-oscillations in the periodical transverse magnetic field (undulator). Correspondingly, two mechanisms of the electron accelerations by the wave are provided simultaneously, namely, the well-known cyclotron acceleration and the undulator acceleration in the regime of the inversed free-electron laser. The multi-resonance character of this system leads to a chaotic behaviour of the electron motion in the field of the wave. In the process of this motion, electrons can get a very high energy (as compared to the case of the purely cyclotron acceleration by the same wave pulse.

The Structure of Planetary Period Oscillations in Saturn's Equatorial Magnetosphere: Results From the Cassini Mission

AbstractSaturn's magnetospheric magnetic field, planetary radio emissions, plasma populations, and magnetospheric structure are all known to be modulated at periods close to the assumed rotation period of the planetary interior. These oscillations are readily apparent despite the high degree of axisymmetry in the internally produced magnetic field of the planet and have different rotation periods in the northern and southern hemispheres. In this paper we study the spatial structure of (near‐)planetary period magnetic field oscillations in Saturn's equatorial magnetosphere. Extending previous analyses of these phenomena, we include all suitable data from the entire Cassini mission during its orbital tour of the planet so as to be able to quantify both the amplitude and phase of these field oscillations throughout Saturn's equatorial plane, to distances of 30 planetary radii. We study the structure of these field oscillations in view of both independently rotating northern and southern systems, finding spatial variations in both magnetic fields and inferred currents flowing north‐south that are common to both systems. With the greatly expanded coverage of the equatorial plane achieved during the latter years of the mission, we are able to present a complete survey of dawn‐dusk and day‐night asymmetries in the structure of the oscillating fields and currents. We show that the general structure of the rotating currents is simpler than previously reported and that the relatively enhanced nightside equatorial fields and currents are due in part to related periodic vertical motion of Saturn's magnetotail current sheet.