Magnetic Fields In Excess Research Articles

Maximizing Orientational Order in Polymer-Stabilized Liquid Crystals Using High Magnetic Fields

Polymer-stabilized liquid crystals (PSLCs) are materials composed of a polymer mesh in a continuous phase of liquid crystal. The polymer mesh provides an anchor point for alignment of the liquid crystalline bulk. The macroscopic extent of order in such systems depends on the order parameter of the liquid crystal (given by the temperature) and the domain order parameter, induced by external stimuli, such as (rubbed) substrates or magnetic or electric fields. We studied thick PSLCs where substrate interactions cannot be employed and used magnetic fields instead. We show how the polymerization conditions, i.e., the temperature and the magnetic field, influence the overall order parameter in 4-octyl-4′-cyanobiphenyl (8CB)-based PSLCs. Optimal macroscopic alignment was obtained in samples polymerized at room temperature and at magnetic fields in excess of 5 T. The effect of mesh network can be quantified by introducing a phenomenological constant, which is correlated to the order parameter at the polymerizatio...

The Persistence of Apparent Non-Magnetostatic Equilibrium in NOAA 11035

AbstractNOAA 11035 was a highly sheared active region that appeared in December 2009 early in the new activity cycle. The leading polarity sunspot developed a highly unusual feature in its penumbra, an opposite polarity pore with a strong magnetic field in excess of 3500 G along one edge, which persisted for several days during the evolution of the region. This region was well observed by both space- and ground-based observatories, including Hinode, FIRS, TRACE, and SOHO. These observations, which span wavelength and atmospheric regimes, provide a complete picture of this unusual feature which may constitute a force-free magnetic field in the photosphere which is produced by the reconnection of magnetic loops low in the solar atmosphere.

Preparation and characterisation of Os doped MgB2

Polycrystalline samples with Mg1−xOsxB2.04 nominal stoichiometry were made by reacting elemental powders at 800°C under argon atmosphere. Based on XRD diffraction patterns, EDS analysis and magnetisation measurements, it is found that Os can replace up to about 1at.% Mg in the MgB2 lattice. Beyond this doping level, unreacted Os and Mg-rich Mg–Os impurity phases are formed. The a-axis parameter contracts upon doping while the superconducting transition temperature decreases at a rate of 2.1K/at.% Os substitution. At 10K, Os doping induces an improvement of the normalised critical current density under applied magnetic fields in excess of 0.5T, indicating a modest enhancement of flux pinning in this range.

Magnetic field measurements via visible spectroscopy on the Z machine.

Sandia's Z Machine uses its high current to magnetically implode targets relevant to inertial confinement fusion. Since target performance is highly dependent on the applied drive field, measuring magnetic field at the target is essential for accurate simulations. Recently, the magnetic field at the target was measured through splitting of the sodium 3s-3p doublet at 5890 and 5896 Å. Spectroscopic dopants were applied to the exterior of the target, and spectral lines were observed in absorption. Magnetic fields in excess of 200 T were measured, corresponding to drive currents of approximately 5 MA early in the pulse.

MAGNETIC INHIBITION OF CONVECTION AND THE FUNDAMENTAL PROPERTIES OF LOW-MASS STARS. II. FULLY CONVECTIVE MAIN-SEQUENCE STARS

We examine the hypothesis that magnetic fields are inflating the radii of fully convective main sequence stars in detached eclipsing binaries (DEBs). The magnetic Dartmouth stellar evolution code is used to analyze two systems in particular: Kepler-16 and CM Draconis. Magneto-convection is treated assuming stabilization of convection and also by assuming reductions in convective efficiency due to a turbulent dynamo. We find that magnetic stellar models are unable to reproduce the properties of inflated fully convective main sequence stars, unless strong interior magnetic fields in excess of 10 MG are present. Validation of the magnetic field hypothesis given the current generation of magnetic stellar evolution models therefore depends critically on whether the generation and maintenance of strong interior magnetic fields is physically possible. An examination of this requirement is provided. Additionally, an analysis of previous studies invoking the influence of star spots is presented to assess the suggestion that star spots are inflating stars and biasing light curve analyses toward larger radii. From our analysis, we find that there is not yet sufficient evidence to definitively support the hypothesis that magnetic fields are responsible for the observed inflation among fully convective main sequence stars in DEBs.

Spectroscopy of the enigmatic short-period cataclysmic variable IR Com in an extended low state

Abstract We report the occurrence of a deep low state in the eclipsing short-period cataclysmic variable (CV) IR Com, lasting more than two years. Spectroscopy obtained in this state shows the system as a detached white dwarf plus low-mass companion, indicating that accretion has practically ceased. The spectral type of the companion derived from the SDSS spectrum is M6–7, somewhat later than expected for the orbital period of IR Com. Its radial velocity amplitude, K2 = 419.6 ± 3.4 km s−1, together with the inclination of 75°–90° implies 0.8 < Mwd <1.0 M⊙. We estimate the white dwarf temperature to be ≃15 000 K, and the absence of Zeeman splitting in the Balmer lines rules out magnetic fields in excess of ≃5 MG. IR Com still defies an unambiguous classification, in particular the occurrence of a deep, long low state is so far unique among short-period CVs that are not strongly magnetic.

Astrophysics of Magnetically Collimated Jets Generated from Laser-Produced Plasmas

The generation of astrophysically relevant jets, from magnetically collimated, laser-produced plasmas, is investigated through three-dimensional, magnetohydrodynamic simulations. We show that for laser intensities I∼10(12)-10(14) W cm(-2), a magnetic field in excess of ∼0.1 MG, can collimate the plasma plume into a prolate cavity bounded by a shock envelope with a standing conical shock at its tip, which recollimates the flow into a supermagnetosonic jet beam. This mechanism is equivalent to astrophysical models of hydrodynamic inertial collimation, where an isotropic wind is focused into a jet by a confining circumstellar toruslike envelope. The results suggest an alternative mechanism for a large-scale magnetic field to produce jets from wide-angle winds.

A BINARY SCENARIO FOR THE FORMATION OF STRONGLY MAGNETIZED WHITE DWARFS

Since their initial discovery, the origin of isolated white dwarfs (WDs) with magnetic fields in excess of ~1 MG has remained a mystery. Recently, the formation of these high-field magnetic WDs has been observationally linked to strong binary interactions incurred during post-main-sequence evolution. Planetary, brown dwarf or stellar companions located within a few AU of main-sequence stars may become engulfed during the primary's expansion off the main sequence. Sufficiently low-mass companions in-spiral inside a common envelope until they are tidally shredded near the natal white dwarf. Formation of an accretion disk from the disrupted companion provides a source of turbulence and shear which act to amplify magnetic fields and transport them to the WD surface. We show that these disk-generated fields explain the observed range of magnetic field strengths for isolated, high-field magnetic WDs. Additionally, we discuss a high-mass binary analogue which generates a strongly-magnetized WD core inside a pre-collapse, massive star. Subsequent core-collapse to a neutron star may produce a magnetar.

The origin of the strongest magnetic fields in dwarfs

White dwarfs have frozen in magnetic fields ranging from below the measurable limit of about 3×103 to 109 G. White dwarfs with surface magnetic fields in excess of 1 MG are found as isolated single stars and relatively more often in magnetic cataclysmic variables. Some 1253 white dwarfs with a detached low-mass main-sequence companion have been identified in the Sloan Digital Sky Survey (SDSS) but none of these shows sufficient evidence for Zeeman splitting of hydrogen lines for a magnetic field in excess of 1 MG. If such high magnetic fields in white dwarfs result from the isolated evolution of a single star then there should be the same fraction of high field white dwarfs among this SDSS binary sample as among single stars. Thus, we deduce that the origin of such high magnetic fields must be intimately tied to the formation of cataclysmic variables (CVs). The formation of a CV must involve orbital shrinkage from giant star to main-sequence star dimensions. It is believed that this shrinkage occurs as the low-mass companion and the white dwarf spiral together inside a common envelope. CVs emerge as very close but detached binary stars that are then brought together by magnetic braking or gravitational radiation. We propose that the smaller the orbital separation at the end of the common envelope phase, the stronger the magnetic field. The magnetic cataclysmic variables (MCVs) originate from those common envelope systems that almost merge. Those common envelope systems that do merge are the progenitors of the single high field white dwarfs. Thus all highly magnetic white dwarfs, be they single stars or the components of MCVs, have a binary origin. This accounts for the relative dearth of single white dwarfs with fields of 104–106 G. Such intermediate-field white dwarfs are found preferentially in cataclysmic variables. The bias towards higher masses for highly magnetic white dwarfs is expected if a fraction of these form when two degenerate cores merge in a common envelope. From the space density of single highly magnetic white dwarfs we estimate that about three times as many common envelope events lead to a merged core as to a cataclysmic variable.

DISCOVERY OF NEW GAMMA-RAY PULSARS WITH AGILE

Using gamma-ray data collected by the Astro-rivelatore Gamma ad Immagini LEggero (AGILE) satellite over a period of almost one year (from 2007 July to 2008 June), we searched for pulsed signals from 35 potentially interesting radio pulsars, ordered according to and for which contemporary or recent radio data were available. AGILE detected three new top-ranking nearby and Vela-like pulsars with good confidence both through timing and spatial analysis. Among the newcomers we find pulsars with very high rotational energy losses, such as the remarkable PSR B1509 − 58 with a magnetic field in excess of 1013 Gauss, and PSR J2229 + 6114 providing a reliable identification for the previously unidentified EGRET source 3EG 2227 + 6122. Moreover, the powerful millisecond pulsar B1821 − 24, in the globular cluster M28, is detected during a fraction of the observations. Four other promising gamma-ray pulsar candidates, among which is the notable J2043 + 2740 with an age in excess of 1 million years, show a possible detection in the timing analysis only and deserve confirmation.

Binary star origin of high field magnetic white dwarfs

White dwarfs with surface magnetic fields in excess of $1 $MG are found as isolated single stars and relatively more often in magnetic cataclysmic variables. Some 1,253 white dwarfs with a detached low-mass main-sequence companion are identified in the Sloan Digital Sky Survey but none of these is observed to show evidence for Zeeman splitting of hydrogen lines associated with a magnetic field in excess of 1MG. If such high magnetic fields on white dwarfs result from the isolated evolution of a single star then there should be the same fraction of high field white dwarfs among this SDSS binary sample as among single stars. Thus we deduce that the origin of such high magnetic fields must be intimately tied to the formation of cataclysmic variables. CVs emerge from common envelope evolution as very close but detached binary stars that are then brought together by magnetic braking or gravitational radiation. We propose that the smaller the orbital separation at the end of the common envelope phase, the stronger the magnetic field. The magnetic cataclysmic variables originate from those common envelope systems that almost merge. We propose further that those common envelope systems that do merge are the progenitors of the single high field white dwarfs. Thus all highly magnetic white dwarfs, be they single stars or the components of MCVs, have a binary origin. This hypothesis also accounts for the relative dearth of single white dwarfs with fields of 10,000 - 1,000,000G. Such intermediate-field white dwarfs are found preferentially in cataclysmic variables. In addition the bias towards higher masses for highly magnetic white dwarfs is expected if a fraction of these form when two degenerate cores merge in a common envelope. Similar scenarios may account for very high field neutron stars.

Status of the US program in magneto-inertial fusion

A status report of the current U.S. program in magneto-inertial fusion (MIF) conducted by the Office of Fusion Energy Sciences (OFES) of the U.S. Department of Energy is given. Magneto-inertial fusion is an emerging concept for inertial fusion and a pathway to the study of dense plasmas in ultrahigh magnetic fields (magnetic fields in excess of 500 T). The presence of magnetic field in an inertial fusion target suppresses cross-field thermal transport and potentially could enable more attractive inertial fusion energy systems. The program is part of the OFES program in high energy density laboratory plasmas (HED-LP).

Instability of current and N-shaped current-voltage characteristic in a silicon p-i-n diode subjected to a magnetic field

Experimental results of studying the p-i-n structures based on n-Si with the resistivity ρ = 120 Ω cm are reported. The dynamic S-shaped current-voltage characteristics were studied at various values of the applied magnetic field. Initiation of the current and voltage large-amplitude oscillations was observed at a magnetic field of 2 kOe. A further increase in the magnetic field leads to an increase in the amplitude of oscillations and to the appearance of a high-frequency oscillation mode with the frequency of ∼ 3 MHz; this mode modulates the low-frequency mode. A magnetic field in excess of 5 kOe reduces the amplitude of both modes of current oscillations and completely suppresses oscillations in the circuit so that the S-shaped form of the current-volt-age characteristic disappears. In accordance with the theory, experimental current-voltage characteristics were observed for the first time to feature portions with negative differential N-type conductivity in the presence of a high magnetic field and with the chosen sample configuration and relevant values of electric field.

The Galactic Center as a point source of neutrons at EeV energies

The central region of our Galaxy is a very peculiar environment, containing magnetic fields in excess of 100 mG and gas densities reaching ~ 10^4cm^-3. This region was observed as a strong source of GeV and TeVs gammas, what suggests that a mechanism of proton-neutron conversion could be taking place therein. We propose that the Galactic Center must also be a source of EeV neutrons due to the conversion of ultra high energy cosmic ray protons into neutrons via p-p interactions inside this region. This scenario should be falsifiable by the Pierre Auger Observatory after a few years of full exposure.

High magnetic field induced changes of gene expression in arabidopsis

BackgroundHigh magnetic fields are becoming increasingly prevalent components of non-invasive, biomedical imaging tools (such as MRI), thus, an understanding of the molecular impacts associated with these field strengths in biological systems is of central importance. The biological impact of magnetic field strengths up to 30 Tesla were investigated in this study through the use of transgenic Arabidopsis plants engineered with a stress response gene consisting of the alcohol dehydrogenase (Adh) gene promoter driving the β-glucuronidase (GUS) gene reporter.MethodsMagnetic field induced Adh/GUS activity was evaluated with histochemical staining to assess tissue specific expression and distribution, and with quantitative, spectrofluometric assays to measure degree of activation. The evaluation of global changes in the Arabidopsis genome in response to exposure to high magnetic fields was facilitated with Affymetrix Gene Chip microarrays. Quantitative analyses of gene expression were performed with quantitative real-time polymerase-chain-reaction (qRT-PCR).ResultsField strengths in excess of about 15 Tesla induce expression of the Adh/GUS transgene in the roots and leaves. From the microarray analyses that surveyed 8000 genes, 114 genes were differentially expressed to a degree greater than 2.5 fold over the control. These results were quantitatively corroborated by qRT-PCR examination of 4 of the 114 genes.ConclusionThe data suggest that magnetic fields in excess of 15 Tesla have far-reaching effect on the genome. The wide-spread induction of stress-related genes and transcription factors, and a depression of genes associated with cell wall metabolism, are prominent examples. The roles of magnetic field orientation of macromolecules and magnetophoretic effects are discussed as possible factors that contribute to the mounting of this response.

The Toulouse pulsed magnet facility

The ‘Laboratoire National des Champs Magnétiques Pulsés’ (LNCMP) is an international user facility providing access to pulsed magnetic fields up to and beyond 60 T. The laboratory disposes of 10 magnet stations equipped with long-pulse magnets operating in the 35–60 T range and a short-pulse system reaching magnetic fields in excess of 70 T. The experimental infrastructure includes various high and low-temperature systems ranging from ordinary flow-type cryostats to dilution refrigerators reaching 50 mK, as well as different types of high-pressure cells. Experimental techniques include magnetization, transport, luminescence, IR-spectroscopy and polarimetry. The LNCMP pursues an extensive in-house research program focussing on all technological and scientific aspects of pulsed magnetic fields. Recent technical developments include the implementation of 60 T rapid-cooling coils, an 80 T prototype, a pulsed dipole magnet for optical investigations of dilute matter and a transportable horizontal access magnet for small angle x-ray scattering experiments. Scientific activities cover a variety of domains, including correlated electron systems, magnetism, semiconductors and nanoscience.

Progress towards single spin optoelectronics using quantum dot nanostructures

We summarise recent progress in our understanding of the physics of fundamental charge and spin excitations in quantum dot semiconductor nanostructures. Many novel potential applications of these nanostructures have arisen from the strong optical non-linearities that exist in the few-particle quantum dot absorption spectrum. By comparison, the interaction of the electron spin with other localised charges in the dot and surroundings and its coupling to environmental degrees of freedom in zero-dimensional electronic systems is still comparatively poorly understood. Here, we present recent investigations that demonstrate that the electron spin is a very stable quantum number in zero dimensional heterostructures. It couples only very weakly to the solid-state environment via spin orbit interaction due to the discrete quantum dot electronic structure. Using specially designed ‘spin memory’ devices we optically generate and orientate individual electron spins in quantum dots with the frequency selectivity provided by optical excitation. Time resolved spectroscopy is used to probe their relaxation dynamics as a function of magnetic field, lattice temperature and quantum dot size. The results of these measurements show that spin relaxation in quantum dot nanostructures is strongly suppressed when compared with systems of higher dimensionality and, moreover, demonstrate that the longitudinal spin relaxation occurs over millisecond timescales at low temperatures and large static magnetic fields. With static magnetic fields in excess of B=4 T, the spin relaxation is shown to proceed via spin orbit interaction mediated by single phonon scattering processes.

Nano-Diamond Synthesis in Strong Magnetic Field

Diamond is one of the five known allotropes of carbon. Diamonds are important due to their esthetic beauty and excellently remarkable properties. Single crystalline diamond has been synthesized by catalytic processes under high temperature and high pressure. Polycrystalline diamond has been prepared by various activated hot filament and plasma chemical vapor deposition methods. Industrial synthesis of diamonds has been limited by the extreme synthetic conditions, purity, crystallinity, size and high cost. Synthetic advancement for future industrial production will require better understanding and exploitation of the mechanism of formation. Current mechanistic uncertainty arises out of limited consideration of atomic scale dynamics. A new comprehensive model by Little addresses atomic scale electronic processes with nonclassical significance of the intermediary states. This new nonclassical, electronic mechanistic perspective identifies the creation, stabilization and condensation of carbon and metal intermediates for pathways from carbon precursors thru solvated carbon and carbides to sp3 carbon condensation as diamond. In order to test these mechanistic ideas and exploit the impact for better diamond synthesis, catalytic carbon condensation has been explored in strong magnetic field in an effort to influence various radicals and high spin metal and carbon intermediates along the pathway for more efficient diamond condensation. In this effort, static magnetic fields in excess of 15 T are observed to direct these carbon and metal intermediary microstates to promote nano-diamond nucleation and growth at atmospheric pressure and temperature of 900°C. The observed nano-diamond formation is consistent with the predicted faster kinetics due to lower potential energies of intermediates along pathways to diamond. The predicted stability of nano-diamond relative to nano-graphite also accounts for the observed nano-diamond in this work. This novel use of static magnetic field for diamond synthesis is compared with advancements since the first synthetics bulk diamond formation by scientists at both ASEA (Sweden) and GE Research Laboratory, Schenectady, NY, 50 years ago. On the basis of this predicted, invented, and intrinsic magnetic influence on the dynamics of carbon condensation and the promise for faster, feasible single crystal diamond formation, this discovery of the use of strong magnetic field for diamond production is asserted a major advancement during this 50-year period since the first successful synthesis.

On the Central Engine of Short Gamma-ray Bursts

SummaryWe assess the ability of neutron star binary coalescence to produce short gamma-ray bursts (GRBs). We find that the neutrino annihilation above the merged remnant will drive bipolar, relativistic jets along the initial binary rotation axis. This outflow can be collimated by the energetic, neutrino-driven baryonic wind that is blown off the remnant. Despite the narrow neutron star mass distribution the apparent luminosities will be spread over a broad range from ~ 1049 to ~ 1052erg s−1, typical jet opening half-angles are around 5 degrees. If the central core of the merger remnant does not collapse immediately convective dynamo action will set in and the available kinetic energy can be transformed into magnetic fields in excess of 1017 G. The corresponding spin-down time scale is ~ 0.2 s, just about the duration of a short GRB.

The physics of neutron stars.

Neutron stars are some of the densest manifestations of massive objects in the universe. They are ideal astrophysical laboratories for testing theories of dense matter physics and provide connections among nuclear physics, particle physics, and astrophysics. Neutron stars may exhibit conditions and phenomena not observed elsewhere, such as hyperon-dominated matter, deconfined quark matter, superfluidity and superconductivity with critical temperatures near 10(10) kelvin, opaqueness to neutrinos, and magnetic fields in excess of 10(13) Gauss. Here, we describe the formation, structure, internal composition, and evolution of neutron stars. Observations that include studies of pulsars in binary systems, thermal emission from isolated neutron stars, glitches from pulsars, and quasi-periodic oscillations from accreting neutron stars provide information about neutron star masses, radii, temperatures, ages, and internal compositions.