Intermediate Magnetic Field Strengths Research Articles

Improvement of the Helioseismic and Magnetic Imager (HMI) Vector Magnetic Field Inversion Code

Abstract A spectral line inversion code, Very Fast Inversion of the Stokes Vector (VFISV), has been used since 2010 May to infer the solar atmospheric parameters from the spectropolarimetric observations taken by the Helioseismic and Magnetic Imager (HMI) on board the Solar Dynamics Observatory. The magnetic filling factor, the fraction of the surface with a resolution element occupied by magnetic field, is set to have a constant value of 1 in the current version of VFISV. This report describes an improved inversion strategy for the spectropolarimetric data observed with HMI for magnetic field strengths of intermediate values in areas spatially not fully resolved. The VFISV inversion code has been modified to enable inversion of the Stokes profiles with two different components: one magnetic and one nonmagnetic. In this scheme, both components share the atmospheric components except for the magnetic field vector. In order to determine whether the new strategy is useful, we evaluate the inferred parameters inverted with one magnetic component (the original version of the HMI inversion) and with two components (the improved version) using a Bayesian analysis. In pixels with intermediate magnetic field strengths (e.g., plages), the new version provides statistically significant values of filling fraction and magnetic field vector. Not only does the fitting of the Stokes profile improve, but also the inference of the magnetic parameters and line-of-sight velocity are obtained uniquely. The new strategy is also proven to be effective for mitigating the anomalous hemispheric bias in the east–west magnetic field component in moderate field regions.

Suppression of the ion drag force on dust in magnetized plasmas

Modeling the transport of “dust” particles in a magnetically confined plasma device is an area of active research and requires a detailed understanding of the forces experienced by dust immersed in a plasma. One of the most significant of these is the “ion drag force.” Dust transport codes employ a model of this force that was not specifically designed for fusion plasmas and so does not consider the relevance of strong magnetic fields. However, it is shown here that the effect of magnetic fields on the ion drag force is significant for such plasmas. In this work, the Monte Carlo code DiMPl is employed to perform the first detailed characterization of the dependence of the ion drag force on magnetic fields. A semi-empirical model of this dependence is fitted onto the simulation data, so that these magnetic effects may be straightforwardly captured by dust transport codes. The limiting behavior of the ion drag force in the case of very strong fields is derived analytically and shown to be consistent with the simulation results. The validity of the results is further motivated through a novel theoretical treatment of the ion drag force at intermediate magnetic field strengths.

The Formation and Evolution of Wide-orbit Stellar Multiples In Magnetized Clouds

Abstract Stars rarely form in isolation. Nearly half of the stars in the Milky Way have a companion, and this fraction increases in star-forming regions. However, why some dense cores and filaments form bound pairs while others form single stars remains unclear. We present a set of three-dimensional, gravo-magnetohydrodynamic simulations of turbulent star-forming clouds, aimed at understanding the formation and evolution of multiple-star systems formed through large-scale (≳103 au) turbulent fragmentation. We investigate three global magnetic field strengths, with global mass-to-flux ratios of μ ϕ = 2, 8, and 32. The initial separations of protostars in multiples depend on the global magnetic field strength, with stronger magnetic fields (e.g., μ ϕ = 2) suppressing fragmentation on smaller scales. The overall multiplicity fraction (MF) is between 0.4 and 0.6 for our strong and intermediate magnetic field strengths, which is in agreement with observations. The weak field case has a lower fraction. The MF is relatively constant throughout the simulations, even though stellar densities increase as collapse continues. While the MF rarely exceeds 60% in all three simulations, over 80% of all protostars are part of a binary system at some point. We additionally find that the distribution of binary spin misalignment angles is consistent with a randomized distribution. In all three simulations, several binaries originate with wide separations and dynamically evolve to ≲102 au separations. We show that a simple model of mass accretion and dynamical friction with the gas can explain this orbital evolution.

Finite dust clusters under strong magnetic fields

Experiments on dust clusters trapped in the sheath of a radio frequency discharge have been performed for different magnetic field strengths ranging from a few milliteslas to 5.8 T. The dynamics of the dust clusters are analyzed in terms of their normal modes. From that, various dust properties such as the kinetic temperature, the dust charge, and the screening length are derived. It is found that the kinetic temperature of the cluster rises with the magnetic field, whereas the dust charge nearly remains constant. The screening length increases slightly at intermediate magnetic field strengths. Generally, the dust properties seem to correlate with magnetization parameters of the plasma electrons and ions, however only to a small degree.

Mechanisms of canonical Kelvin-Helmholtz instability suppression in magnetohydrodynamic flows

The stabilizing influence of the streamwise magnetic field on Kelvin-Helmholtz (KH) instability is well known. We perform numerical simulations over a wide range of magnetic field strengths to clearly describe the mechanisms through which the stabilization of KH instability is achieved. KH instability evolution is known to be characterized by the stages of (i) linear precursor-vortex development; (ii) precursor-vortices merger and rollup into the primary vortex; and (iii) development of secondary-vortex bands and the nonlinear asymptotic stage. Our simulations exhibit the KH instability disruption mechanisms as a function of magnetic field strength. At strong magnetic field strengths, rapid harmonic velocity-magnetic exchange causes the precursor vortices to continually wind and unwind. Thus the perturbation development does not proceed beyond the first stage. At intermediate magnetic field strengths, the harmonic interaction permits the monotonic development of precursor vortices but prevents merger or primary vortex formation. When the magnetic field is weak, hydrodynamic mechanisms prevail at the first and second stages. The magnetic field only disrupts the nonlinear asymptotic KH growth stage due to the onset of “resistive instability” in the secondary bands.

Hot grain dynamics by electric charging and magnetic trapping in debris disks

The recent discovery of hot dust grains in the vicinity of main-sequence stars has become a hot issue among the scientific community of debris disks. Hot grains must have been enormously accumulated near their sublimation zones, but it is a mystery how such a high concentration of hot grains is sustained. The most difficult conundrum is that the size of hot dust grains is estimated to lie in the submicrometer range, while submicrometer-sized grains are instantly swept away from near-stellar environments by stellar radiation pressure. One and only mechanism proposed for prolonging the residence time of hot grains in the near-stellar environments is trapping of charged nanoparticles by stellar magnetic fields. We revisit the model of magnetic grain trapping around main-sequence stars of various spectral classes by taking into account sublimation and electric charging of the grains. The model of magnetic grain trapping predicts that hot dust grains are present in the vicinity of main-sequence stars with high rotation velocities and intermediate magnetic-field strengths. On the contrary, we find that the detection of hot dust grains has no correlation with the rotation velocities of central stars nor the magnetic field strengths of the stars. Our numerical evaluation of electric grain charging indicates that the surface potential of submicrometer-sized grains in the vicinity of main-sequence stars is typically 4– 5V, which is one order of magnitude smaller than the value assumed by the model of magnetic grain trapping. On the basis of our numerical simulation on sublimation of dust grains in the vicinity of a star, it turns out that their lives end due to sublimation in a timescale much shorter than the period of one revolution at the gyroradius. It is, therefore, infeasible to dynamically extend the dwell time of hot grains inside the sublimation zone by magnetic trapping, while we cannot completely rule out the possibility of magnetic grain trapping outside the sublimation zone where the strength of stellar magnetic field has been underestimated in the previous model. Nevertheless, the independence of hot dust detection on the stellar rotational velocity and magnetic field strength favors a scenario that some other (yet unnoticed/overlooked) ubiquitous mechanism of grain trapping is at work.

Direct Correlation between Adsorption Energetics and Nuclear Spin Relaxation in a Liquid-saturated Catalyst Material.

The ratio of NMR relaxation time constants provides a non-destructive indication of the relative surface affinities exhibited by adsorbates within liquid-saturated mesoporous catalysts. In the present work we provide supporting evidence for the existence of a quantitative relationship between such measurements and adsorption energetics. As a prototypical example with relevance to green chemical processes we examine and contrast the relaxation characteristics of primary alcohols and cyclohexane within an industrial silica catalyst support. values obtained at intermediate magnetic field strength are in good agreement with DFT adsorption energy calculations performed on single molecules interacting with an idealised silica surface. Our results demonstrate the remarkable ability of this metric to quantify surface affinities within systems of relevance to liquid-phase heterogeneous catalysis, and highlight NMR relaxation as a powerful method for the determination of adsorption phenomena within mesoporous solids.

Circularly polarized zero-phonon transitions of vacancies in diamond at high magnetic fields

We study the circularly polarized photoluminescence of negatively charged (${\mathrm{NV}}^{\ensuremath{-}}$) and neutral (${\mathrm{NV}}^{0}$) nitrogen-vacancy ensembles and neutral vacancies (${\mathrm{V}}^{0}$) in diamond crystals exposed to magnetic fields of up to 10 T. We determine the orbital and spin Zeeman splitting as well as the energetic ordering of their ground and first-excited states. The spin-triplet and -singlet states of the ${\mathrm{NV}}^{\ensuremath{-}}$ are described by an orbital Zeeman splitting of about 9 $\ensuremath{\mu}\mathrm{eV}/\mathrm{T}$, which corresponds to a positive orbital $g$-factor of ${g}_{L}=0.164$ under application of the magnetic field along the (001) and (111) crystallographic directions, respectively. The zero-phonon line (ZPL) of the ${\mathrm{NV}}^{\ensuremath{-}}$ singlet is defined as a transition from the ${}^{1}{E}^{\ensuremath{'}}$ states, which are split by ${g}_{L}{\ensuremath{\mu}}_{\text{B}}B$, to the ${}^{1}{A}_{1}$ state. The energies of the zero-phonon triplet transitions show a quadratic dependence on intermediate magnetic field strengths, which we attribute to a mixing of excited states with nonzero orbital angular momentum. Moreover, we identify slightly different spin Zeeman splittings in the ground (gs) and excited (es) triplet states, which can be expressed by a deviation between their spin $g$-factors: ${g}_{S,\text{es}}={g}_{S,\text{gs}}+\mathrm{\ensuremath{\Delta}}g$ with values of $\mathrm{\ensuremath{\Delta}}g=0.014$ and 0.029 in the (001) and (111) geometries, respectively. The degree of circular polarization of the ${\mathrm{NV}}^{\ensuremath{-}}$ ZPLs depends significantly on the temperature, which is explained by an efficient spin-orbit coupling of the excited states mediated through acoustic phonons. We further demonstrate that the sign of the circular polarization degree is switched under rotation of the diamond crystal. A weak Zeeman splitting similar to $\mathrm{\ensuremath{\Delta}}g{\ensuremath{\mu}}_{\text{B}}B$ measured for the ${\mathrm{NV}}^{\ensuremath{-}}$ ZPLs is also obtained for the ${\mathrm{NV}}^{0}$ zero-phonon lines, from which we conclude that the ground state is composed of two optically active states with compensated orbital contributions and opposite spin-1/2 momentum projections. The zero-phonon lines of the ${\mathrm{V}}^{0}$ show Zeeman splittings and degrees of the circular polarization with opposite signs. The magnetophotoluminescence data indicate that the electron transition from the $^{1}T_{2}$ states to the $^{1}A$ ground state defines the zero-phonon emission at 1.674 eV, while the $^{1}T_{2}\ensuremath{\rightarrow}^{1}E$ transition is responsible for the zero-phonon line at 1.666 eV. The $^{1}T_{2}$ ($^{1}E$) states are characterized by an orbital Zeeman splitting with ${g}_{L}=0.071$ (0.128).

Stiff, porous scaffolds from magnetized alumina particles aligned by magnetic freeze casting

Bone consists of a hard mineral phase and a compliant biopolymer phase resulting in a composite material that is both lightweight and strong. Osteoporosis that degrades spongy bone preferentially over time leads to bone brittleness in the elderly. A porous ceramic material that can mimic spongy bone for a one-time implant provides a potential solution for the future needs of an aging population. Scaffolds made by magnetic freeze casting resemble the aligned porosity of spongy bone. A magnetic field applied throughout freezing induces particle chaining and alignment of lamellae structures between growing ice crystals. After freeze drying to extract the ice and sintering to strengthen the scaffold, cubes from the scaffold center are mechanically compressed along longitudinal (z-axis, ice growth direction) and transverse (y-axis, magnetic field direction) axes. The best alignment of lamellar walls in the scaffold center occurs when applying magnetic freeze casting with the largest particles (350nm) at an intermediate magnetic field strength (75mT), which also agrees with stiffness enhancement results in both z and y-axes. Magnetic moments of different sized magnetized alumina particles help determine the ideal magnetic field strength needed to induce alignment in the scaffold center rather than just at the poles.

Magnetic droplet solitons in orthogonal spin valves

We review the recent experimental advancements in the realization and understanding of magnetic droplet solitons generated by spin transfer torque in orthogonal nanocontact based spin torque nanooscillators (STNOs) fabricated on extended spin valves and spin valve nanowires. The magnetic droplets are detected and studied using the STNO microwave signal and its resistance, the latter both quasistatically and time-resolved. The droplet nucleation current is found to have a minimum at intermediate magnetic field strengths and the nature of the nucleation changes gradually from a single sharp step well above this field, mode-hopping around the minimum, and continuous at low fields. The mode-hopping and continuous transitions are ascribed to droplet drift instability and re-nucleation at different time scales, which is corroborated by time-resolved measurements. We argue that the use of tilted anisotropy fixed layers could reduce the nucleation current further, move the nucleation current minimum to lower fields, and potentially remove the need for an applied magnetic field altogether. Finally, evidence of an edge mode droplet in a nanowire is presented.

Enhanced sources of acoustic power surrounding AR 11429

Multi-frequency power maps of the local acoustic oscillations show acoustic enhancements ("acoustic-power halos") at high frequencies surrounding large active region. Computational seismic holography reveals a high-frequency "acoustic-emission halo", or "seismic glory" surrounding large active regions. In this study, we have applied computational seismic holography to map the seismic seismic source density surrounding AR 11429. Studies of HMI/SDO Doppler data, shows that the "acoustic halos" and the "seismic glories" are prominent at high frequencies 5–8 mHz. We investigate morphological properties of acoustic-power and acoustic emission halos around an active region to see if they are spatially correlated. Details about the local magnetic field from vectormagnetograms of AR 11429 are included.We identify a 15" region of seismic deficit power (dark moat) shielding the white-light boundary of the active region. The size of the seismic moat is related to region of intermediate magnetic field strength. The acoustic moat is circled by the halo of enhanced seismic amplitude as well as enhanced seismic emission. Overall, the results suggest that features are related. However, if we narrow the frequency band to 5.5 – 6.5 mHz, we find that the seismic source density dominates over the local acoustic power, suggesting the existence of sources that emit more energy downward into the solar interior than upward toward the solar surface.

Identifying H–N proximities in solid-state NMR using 14N overtone irradiation under fast MAS

Two NMR methods for the identification of 1H–14N proximities in solid powder samples are considered. Both approaches involve the irradiation of the 14N overtone transition (Δm = 2) and observation of 1H signals under fast MAS and at an intermediate magnetic field strength (11.7 T). First, the recently reported 1H–14NOT HMQC experiment is developed into a generally-applicable, broadband version using frequency-swept WURST pulses capable of exciting the full range of 14NOT signals. We demonstrate the validity of this approach on various samples, also showing that standard pulses can be used to selectively excite individual nitrogen sites and provide more accurate indirect 14NOT powder patterns from which 14N NMR parameters can be extracted if required. As a second approach, we present some preliminary results demonstrating the feasibility of 1H–14NOT overtone rotary recoupling (ORR) under fast MAS. Dephasing fractions of up to 0.3 have been obtained using a basic TRAPDOR-style pulse sequence, a simple approach that shows much potential for optimisation. WURST pulses are also shown to provide improvements in the recoupling bandwidth. Crucially, for both of these methods our experimental results and exact numerical simulations confirm that the +2ωr overtone sideband should be irradiated for optimal experimental efficiency.

Current‐density functional study of the HeH+ molecular ion under a strong ultrashort magnetic field

AbstractThe HeH+ molecular ion under an ultrashort magnetic field on the order of 109 G is investigated through quantum fluid dynamics and a current‐density functional theory (CDFT) based approach, employing a vector exchange–correlation (XC) potential which depends on the electronic charge‐density as well as on the current‐density. The behavior of the exchange and correlation energies of the HeH+ ion is analyzed and compared with those obtained using an approach based on the time‐dependent density functional theory (TD‐DFT) under similar computational constraints but employing a scalar XC potential dependent only on the electronic charge‐density. The CDFT‐based approach yields exchange and correlation energies as well as TD electronic charge‐densities drastically different from those obtained using the TD‐DFT‐based approach particularly, at typical TD magnetic field strengths. This is attributed to the nonadiabatic effects induced by the vector XC potential of the CDFT in the oscillating charge‐density of the HeH+ ion, which are further explained in the terminology of quantum fluid dynamics. The vector XC potential of the CDFT‐based approach is observed to augment the magnetic interactions in the H2 molecule and in the He ion, whereas it opposes the magnetic interactions in the HeH+ ion particularly, at the intermediate magnetic field strengths. © 2012 Wiley Periodicals, Inc.

Heavy-fermion instability in double-degenerate plasmas

In this work, we study the propagations of normal frequency modes for quantum hydrodynamic waves in the linear limit and introduce a new kind of instability in a double-degenerate plasma. Three different regimes, namely, low, intermediate, and high magnetic field strengths are considered which span the applicability of the work to a wide variety of environments. Distinct behavior is observed for different regimes, for instance, in the laboratory-scale field regime no frequency-mode instability occurs unlike those of intermediate and high magnetic-field strength regimes. It is also found that the instability of this kind is due to the heavy-fermions which appear below a critical effective-mass parameter (μcr=3) and that the responses of the two (lower and upper frequency) modes to fractional effective-mass change in different effective-mass parameter ranges (below and above the critical value) are quite opposite to each other. It is shown that the heavy-fermion instability due to extremely high magnetic field such as that encountered for a neutron-star crust can lead to confinement of stable propagations in both lower and upper frequency modes to the magnetic poles. Current study can have important implications for linear wave dynamics in both laboratory and astrophysical environments possessing high magnetic fields.

Signal of quark deconfinement in thermal evolution neutron stars with deconfinement heating

Abstract As neutron stars spin-down and contract, the deconfinement phase transition can continue to occur, resulting in energy release (so-called deconfinement heating) in case of the first-order phase transition. The thermal evolution of neutron stars is investigated to combine phase transition and the related energy release self-consistently. We find that the appearance of deconfinement heating during spin-down result in not only the cooling delay but also the increase of surface temperature of stars. For stars characterized by intermediate and weak magnetic field strength, a period of increasing surface temperature could exist. Especially, a sharp jump in surface temperature can be produced as soon as quark matter appears in the core of stars with a weak magnetic field. We think that this may serve as evidence for the existence of deconfinement quark matter. The results show that deconfinement heating facilitates the emergence of such characteristic signature during the thermal evolution process of neutron stars.

Signal of quark deconfinement in thermal evolution neutron stars with deconfinement heating

As neutron stars spin-down and contract, the deconfinement phase transition can continue to occur, resulting in energy release (so-called deconfinement heating) in case of the first-order phase transition. The thermal evolution of neutron stars is investigated to combine phase transition and the related energy release self-consistently. We find that the appearance of deconfinement heating during spin-down result in not only the cooling delay but also the increase of surface temperature of stars. For stars characterized by intermediate and weak magnetic field strength, a period of increasing surface temperature could exist. Especially, a sharp jump in surface temperature can be produced as soon as quark matter appears in the core of stars with a weak magnetic field. We think that this may serve as evidence for the existence of deconfinement quark matter. The results show that deconfinement heating facilitates the emergence of such characteristic signature during the thermal evolution process of neutron stars.

SIGNATURE OF DECONFINEMENT WITH SPIN-DOWN COMPRESSION IN COOLING HYBRID STARS

The thermal evolution of neutron stars is coupled to their spin down and the resulting changes in structure and chemical composition. This coupling correlates stellar surface temperatures with rotational state as well as time. We report an extensive investigation of the coupling between spin down and cooling for hybrid stars which undergo a phase transition to deconfined quark matter at the high densities present in stars at low rotation frequencies. The thermal balance of neutron stars is re-analyzed to incorporate phase transitions and the related latent heat self-consistently, and numerical calculations are undertaken to simultaneously evolve the stellar structure and temperature distribution. We find that the changes in stellar structure and chemical composition with the introduction of a pure quark matter phase in the core delay the cooling and produce a period of increasing surface temperature for strongly superfluid stars of strong and intermediate magnetic field strength. The latent heat of deconfinement is found to reinforce this signature if quark matter is superfluid and it can dominate the thermal balance during the formation of a pure quark matter core. At other times it is less important and does not significantly change the thermal evolution.

Thermomagnetic history effects inSmMn2Ge2

The intermetallic compound SmMn$_2$Ge$_2$, displaying multiple magnetic phase transitions, is being investigated in detail for its magnetization behavior near the 145 K first order ferromagnetic to antiferromagnetic transition occuring on cooling, in particular for thermomagnetic history effects in the magnetization data. The most unusual finding is that the thermomagnetic irreversibility, [= M$^{FCW}$(T)-M$^{ZFC}$(T)] at 135 K is higher in intermediate magnetic field strengths. By studying the response of the sample (i.e., thermomagnetic irreversibility and thermal hysteresis) to different histories of application of magnetic field and temperature, we demonstrate how the supercooling and superheating of the metastable magnetic phases across the first order transition at 145 K contribute to overall thermomagnetic irreversibility.

Angular-momentum spatial distribution symmetry breaking in Rb by an external magnetic field

Excited state angular momentum alignment -- orientation conversion for atoms with hyperfine structure in presence of an external magnetic field is investigated. Transversal orientation in these conditions is reported for the first time. This phenomenon occurs under Paschen Back conditions at intermediate magnetic field strength. Weak radiation from a linearly polarized diode laser is used to excite Rb atoms in a cell. The laser beam is polarized at an angle of pi/4 with respect to the external magnetic field direction. Ground state hyperfine levels of the 5S_1/2 state are resolved using laser-induced fluorescence spectroscopy under conditions for which all excited 5P_3/2 state hyperfine components are excited simultaneously. Circularly polarized fluorescence is observed to be emitted in the direction perpendicular to both to the direction of the magnetic field B and direction of the light polarization E. The obtained circularity is shown to be in quantitative agreement with theoretical predictions.

Magnetic field effects on cylindrical magnetron reactive ion etching of Si/SiO2 in CF4 and CF4/H2 plasmas

Etching of Si and SiO2 has been investigated as a function of magnetic field strength in a radio-frequency cylindrical magnetron discharge using CF4 and CF4/H2 gas mixtures. Si and SiO2 etch rates increased with increasing magnetic field strength by a factor as large as 5 at a given power density. Futher increases in magnetic field strength, though, decreased etch rates in both CF4 and CF4/H2 plasmas. The density of CF, CF2, and F radicals, inferred from optical emission intensities, increased continuously as a function of magnetic field strength. Ion densities, measured by electrical probes, increased by a factor of ≊15 as the magnetic field strength was increased from 0 to 250 G in a CF4 discharge (0.45 W/cm2). However, the self-bias voltage on the powered electrode decreased rapidly with increasing magnetic field, from ∼1 kV at 0 G to almost 0 V at 250 G. The maximum etch rates occurred at an intermediate magnetic field strength where the self-bias voltage was between 25 and 50 V, which corresponds to conditions which maximize both the effectiveness of ion bombardment and ion density. With these conditions, the polymer thickness formed on the Si was a minimum and C/F ratio on the surface was a maximum. Highly anisotropic etching was also obtained near these conditions.