Spin-down Modes Research Articles

Spin-polarized electronic, optical, and transport properties of novel Cs2XMoBr6 (X=Na, Li) HDPs: First-principles study

We explored the electronic, optical, and thermoelectric characteristics of novel Cs2XMoBr6 (X=Na, Li) halide-based double perovskites (HDPs) in their monoclinic phase using the density functional theory (DFT). The full-potential linearized augmented plane wave approach was used in the present work. The material Cs2NaMoBr6 was noticed as half-metallic as indicated by an indirect band gap semiconducting nature in the spin-down channel, and with a metallic nature in the spin-up channel. Similarly, the Cs2LiMoBr6 material also revealed a half-metallic nature with an indirect band gap semiconducting nature in spin-down mode and a metallic behavior in spin-up mode. The density of states analysis verifies the electronic behavior of these compounds. To determine the optical characteristics, the optical parameters such as the dielectric functions, the absorption coefficients, the refractivity, the energy loss function, and the reflectivity, have been described. The thermoelectric characteristics of the compounds along the temperature are calculated. It was confirmed that these materials possess extraordinary characteristics that would make them excellent options for influential electronic, optical, and thermoelectric, device applications.

Magnetic waveguides for neutron reflectometry

We show that the sensitivity and depth selectivity of neutron reflectometry can be greatly enhanced through a waveguide design that takes advantage of the spin-dependent magnetic neutron scattering potential to steer spin-up and spin-down neutrons into waveguide modes with different depth profiles. Using a bilayer of manganate and ruthenate ferromagnets, we demonstrate that a magnetic waveguide structure with sharp spin-up and spin-down modes centered in the two different layers can be generated by adding a magnetically inactive capping layer. The resulting reflectometric data allow accurate and reliable determination of a small in-plane magnetization in the ruthenate layer, despite its immediate proximity to the manganate layer with much larger magnetization. Magnetic neutron waveguides thus enable depth-sensitive measurements of small electronic spin polarizations in a large variety of magnetic multilayers and devices.

<i>Ab</i> <i>Initio</i> Investigation of Structural and Electronic Properties of ErPb<sub>3</sub> in AB<sub>3</sub> Structure

The structural properties and electronic properties of the intermetallic compound ErPb3 which crystallize in AuCu3 type structure (AB3) are studied by means of first principles total energy calculation using full potential linearized plane wave method (FP-LAPW) within the generalized gradient approximation of Perdew, Burke and Ernzrhof (PBE) and local spin density approximation (LSDA) for the exchange correlation functional and including spin magnetic calculation. The total energy is computed as a function of volume and fitted to the Birch-Murnaghan equation of state. The ground state properties of this compound such as equilibrium lattice parameter (a0), bulk modulus (B), and its pressure derivative (B’) are calculated and compared with the available experimental results. We find good agreement with the other theoretical and experimental results. For the compounds, the values of lattice constants obtained by PBE-GGA overestimates and by LSDA underestimates the available experimental values for the same, which verifies the reliability of the present calculation. The value obtained for the bulk modulus is 50.63 GPa. The analysis of electronic properties is achieved by the calculation of the band structures and the density of states in both the spin up and spin down modes, which show a metallic character of ErPB3 due to zero band gap. The values of calculated density of states are found to be 0.36 eV/states and 11.46 eV/states in spin-up and spin-down mode respectively. The calculated magnetic moment (μm) of ErPb3 is 2.06.

First Principles Based Study of Ground State and Electronic Properties of TmPb<sub>3</sub> Intermetallic Compound

The ground state behavior of rare earth intermetallic compound TmPb3, which crystallize in AuCu3 type structure, has been examined using first principles density functional theory based on full potential linearized augmented plane wave (FP-LAPW) method. Very few study on structural and electronic properties of TmPb3 compound has been available in the literature, which motivated us to perform the present study. The spin polarized calculations are carried out within the PBE-GGA and LSDA for the exchange correlation (XC) potential. Our calculated ground state properties such as lattice constant (a0), bulk modulus (B) and its pressure derivative (B’) are in good agreement with the experimental results. The value of bulk modulus of TmPb3 is found to be 44.32 GPa and 55.01GPa by PBE-GGA and LSDA respectively. The electronic band structure (BS) and density of states (DOS) verify the metallic nature of this compound. The calculated density of states at the fermi level is found to be 0.16 states/eV and 19.50 states/eV for spin-up and spin-down modes respectively. The magnetic moment of TmPb3 is found to be 0.95.

The missing magnetic morphology term in stellar rotation evolution

This study examines the relationship between magnetic field complexity and mass and angular momentum losses. Observations of open clusters have revealed a bimodal distribution of the rotation periods of solar-like stars that has proven difficult to explain under the existing rubric of magnetic braking. Recent studies suggest that magnetic complexity can play an important role in controlling stellar spin-down rates. However, magnetic morphology is still neglected in most rotation evolution models due to the difficulty of properly accounting for its effects on wind driving and angular momentum loss. Using state-of-the-art magnetohydrodynamical magnetized wind simulations we study the effect that different distributions of the magnetic flux at different levels of geometrical complexity have on mass and angular momentum loss rates. Angular momentum loss rates depend strongly on the level of complexity of the field but are independent of the way this complexity is distributed. We deduce the analytical terms representing the magnetic field morphology dependence of mass and angular momentum loss rates. We also define a parameter that best represents complexity for real stars. As a test, we use these analytical methods to estimate mass and angular momentum loss rates for $8$ stars with observed magnetograms and compare them to the simulated results. Magnetic field complexity provides a natural physical basis for stellar rotation evolution models requiring a rapid transition between weak and strong spin-down modes.

DUAL SPIN-DOWN STATES OF THE PULSAR J1001-5507

Quasi-regular timing observations obtained, using the 26 m Hartebeesthoek Radio Astronomy Observatory radiotelescope near center frequencies of 1.668 and 2.272 GHz, between 1985 July and 2005 August have given a further insight into the long-term spin-down behavior of the pulsar J1001–5507 (B0959–54). Here, we show that the spin-down of the pulsar during ~20 years is dominated by dual spin-down modes characterized by two discrete and nearly stable spin-down rates ( s–2 and s–2). Our analysis demonstrates that the switching of the pulsar's spin-down rate from to modes occurred over about 800 days (between about MJD 48500 and MJD 49300) at a steady rate of ~–4.6 × 10–24 s–3, after which the pulsar was found to be slowing down 1.3% faster. It is observed that the extreme pulse profile shapes at the two spin-down modes are notably different, while the interval of steady switching between the spin-down modes is coincident with evidence for some correlated pulse shape variations. We discuss these results in light of the current understanding of various mechanisms for radio pulsar spin-down modulation.

Simulations of spin filtering effects in a quantum point contact

In this paper, we review the results of transport simulations that we havedone in recent years, of quantum point contacts (QPCs) which incorporatespin-density-functional theory. Besides the usual plateaus at integer multiples ofG0 = (2e2/h) our calculationsyield the type of ∼0.7G0 features that have now become familiar in experiment, as well as anomalies at othernon-integer values. These features appear to be correlated with the formation of anenergy barrier structure within the QPC that can be very strongly spin dependent.In particular, these can be strong enough that there are circumstances wheretwo spin-down modes may pass through the QPC before any current from theother spin orientation is allowed through. The barriers can be tuned to producedifferent spin filtering effects and can lead to the formation of a local magneticmoment in the QPC region, which however is only weakly confined in our case.We can also account for the absence of plateaus at higher integer values ofG0.

X-ray timing and spectral measurements of X Persei

The results of an X-ray timing and spectral study of the Be star X-ray binary system X Per are reported, based on two series of Exosat observations. The timing measurements indicate that the neutron star in the X Per system was in a net spin-down mode during the 1984-1986 period and that this trend may have been in progress since 1978. The phase-averaged X-ray spectrum is consistent with a power law of photon spectral index 1.2 with a high-energy cutoff which becomes pronounced above 5 keV. The spectral form remains fairly constant with pulse phase, except for a brief episode of spectral hardening near pulse minimum. The results are discussed in terms of current models for wide-orbit binaries containing a Be star primary and an accreting neutron star. 33 refs.

Initialization and data assimilation experiments with a primitive equation model

Abstract The process of combining models of the ocean circulation with large data sets is known in meteorology as model initialization and data assimilation. This process is new to oceanographers, who only now are on the verge of having available world-wide synoptic maps of dynamic variables. In this paper we carry out a series of idealized initialization/assimilation experiments with a primitive equation (PE) model, which constitute a first step in developing a realistic process model and data assimilation techniques for the Gulf Stream system. The PE model is used in a spin-down mode and initialized with an analytic jet profile with geostrophically balanced fields. Two major questions are addressed in the present study. The first concerns the initialization process of a PE model during which internal/inertial gravity wave noise is produced. We ask: are the initialization shocks equally crucial for ocean models as they have been for their atmospheric counterparts? The results of an extensive series of balanced versus unbalanced initializations indicate that, for a PE model with a rigid lid, a brutally unbalanced initialization is required to produce strong internal gravity wave shocks. A geostrophically balanced initialization is sufficient to ensure smooth jet evolutions, with no apparent gravity waves, over long time durations in the spin-down mode. No sophisticated initialization procedures seem, therefore, to be required. The second question addressed is: which component of the flow is the most important in data assimilation to drive the model response towards a baseline reference ocean? We specifically compare the knowledge of the depth-integrated flow only, corresponding to measurements of the total transport, with the knowledge of the density field only, or equivalently the velocity shear. The knowledge of the interior density field is much more effective in decreasing the root-mean-square (r.m.s.) errors relative to the reference ocean. If the baroclinic structure is known, coarse horizontal resolutions of data insertion can be reached before significantly worsening the model estimates. If only the depth-averaged flow is known, a decrease in the horizontal resolution of data assimilation has an immediate effect: the r.m.s. errors sharply increase and the assimilation run diverges from the reference ocean. In the assimilation of the barotropic flow alone, even with dense resolution, the errors in the deep layers always show an increasing trend. The relative effectiveness of baroclinic versus barotropic data insertion can be rationalized in the context of geostrophic adjustment theory.