Non-magnetic Systems Research Articles

The age of cataclysmic variables: A kinematical study

Using available astrometric and radial velocity data, the space velocities of cataclysmic variables (CVs) with respect to Sun were computed and kinematical properties of various sub-groups of CVs were investigated. Although observational errors of systemic velocities ( γ) are high, propagated errors are usually less than computed dispersions. According to the analysis of propagated uncertainties of the computed space velocities, available sample was refined by removing the systems with the largest propagated uncertainties so that the reliability of the space velocity dispersions was improved. Having a dispersion of 51 ± 7 km s - 1 for the space velocities, CVs in the current refined sample (159 systems) are found to have 5 ± 1 Gyr mean kinematical age. After removing magnetic systems from the sample, it is found that non-magnetic CVs (134 systems) have a mean kinematical age of 4 ± 1 Gyr. According to 5 ± 1 and 4 ± 1 Gyr kinematical ages implied by 52 ± 8 and 45 ± 7 km s −1 dispersions for non-magnetic systems below and above the period gap, CVs below the period gap are older than systems above the gap, which is a result in agreement with the standard evolution theory of CVs. Age difference between the systems below and above the gap is smaller than that expected from the standard theory, indicating a similarity of the angular momentum loss time scales in systems with low-mass and high-mass secondary stars. Assuming an isotropic distribution, γ velocity dispersions of non-magnetic CVs below and above the period gap are calculated σ γ = 30 ± 5 km s - 1 and σ γ = 26 ± 4 km s - 1 . The small difference of γ velocity dispersions between the systems below and above the gap may imply that magnetic braking does not operate in the detached phase, during which the system evolves from the post-common envelope orbit into contact.

SIMULATIONS OF THE BOUNDARY LAYER BETWEEN A WHITE DWARF AND ITS ACCRETION DISK

Using a 2.5D time-dependent axisymmetric numerical code we recently developed, we solve the full compressible Navier-Stokes equations (including an alpha-viscosity prescription) to determine the structure of the boundary layer between the white dwarf and the accretion disk in non-magnetic cataclysmic varia ble systems. In this preliminary work, our numerical approach does not include radiation. In the energy equation, we either take the dissipation function (Phi) into account or we assumed that the energy is instantly radiated away (Phi). For a slowly rotating non magnetized accreting white dwarf, the accretion disk e xtends all the way to the stellar surface. There, the matter impacts and spread s towards the poles as new matter continuously piles up behind it. We carried out numerical simulations for different values of the alpha viscosity parameter (alpha), corresponding to different mass accretion rates. In the high viscosity cases (alpha=0.1), the spreading boundary layer sets off a gravity wave in the s urface matter. The accretion flow moves supersonically over the cusp making it s usceptible to the rapid development of gravity wave and/or Kelvin-Helmholtz shea ring instabilities. This BL is optically thick and extends more than 30 degrees to either side of the disk plane after only 3/4 of a Keplerian rotation period (19s). In the low viscosity cases (alpha=0.001), the spreading boundary layer does not set off gravity waves and it is optically thin.

Spin-glass-type state in complex nonmagnetic systems

We briefly review our works concerned with generalized models of spin glass, which describe a wide class of glasses (multipole systems, real cluster glasses, and others). We consider several new models and discuss how the scenario of glass transition depends on different factors. We propose a classification of the behaviors of complex spin glasses depending on the system symmetry properties.

Highly Enhanced Thermopower in Two-Dimensional Electron Systems at Millikelvin Temperatures

We report experimental observation of an unexpectedly large thermopower in mesoscopic two-dimensional (2D) electron systems in GaAs/AlGaAs heterostructures at sub-Kelvin temperatures and zero magnetic field. Unlike conventional nonmagnetic high-mobility 2D systems, the thermopower in our devices increases with decreasing temperature below 0.3 K, reaching values in excess of 100 microV/K, thus exceeding the free electron estimate by more than 2 orders of magnitude. With support from a parallel study of the local density of states, we suggest such a phenomenon to be linked to intrinsic localized states and many-body spin correlations in the system.

CATACLYSMIC VARIABLE PRIMARY EFFECTIVE TEMPERATURES: CONSTRAINTS ON BINARY ANGULAR MOMENTUM LOSS

We review the most decisive currently available measurements of the surface effective temperatures, Teff, of white dwarf (WD) primaries in cataclysmic variables (CVs) during accretion quiescence, and use these as a diagnostic for their time averaged accretion rate, <Mdot>. Using time-dependent calculations of the WD envelope, we investigate the sensitivity of the quiescent Teff to long term variations in the accretion rate. We find that the quiescent Teff provides one of the best available tests of predictions for the angular momentum loss and resultant mass transfer rates which govern the evolution of CVs. While gravitational radiation is sufficient to explain the <Mdot> of strongly magnetic CVs at all Porb, faster angular momentum loss is required by the temperatures of dwarf nova primaries (non-magnetic systems). This provides evidence that a normal stellar magnetic field structure near the secondary is essential for the enhanced braking mechanism to work, supporting the well-known stellar wind braking hypothesis. The contrast in <Mdot> is most prominent for orbital periods Porb > 3 hours, above the period gap, but a modest enhancement is also present at shorter Porb. The averaging time which <Mdot> reflects is as much as 10^5 years for low-<Mdot> systems and as little as 10^3 years for high-<Mdot> systems. We discuss the security of conclusions drawn about the CV population in light of these time scales and our necessarily incomplete sample of systems. Measurements for non-magnetic systems above the period gap fall below predictions from traditional stellar wind braking prescriptions, but above more recent predictions with somewhat weaker angular momentum loss. We also discuss the apparently high Teff's found in the VY Scl stars. (abridged)

Effects of different geometries on the conductance, shot noise, and tunnel magnetoresistance of double quantum dots

The spin-polarized transport through a coherent strongly coupled double quantum dot (DQD) system is analyzed theoretically in the sequential and cotunneling regimes. Using the real-time diagrammatic technique, we analyze the current, differential conductance, shot noise, and tunnel magnetoresistance (TMR) as a function of both the bias and gate voltages for double quantum dots coupled in series, in parallel, as well as for $T$-shaped systems. For DQDs coupled in series, we find a strong dependence of the TMR on the number of electrons occupying the double dot, and super-Poissonian shot noise in the Coulomb blockade regime. In addition, for asymmetric DQDs, we analyze transport in the Pauli-spin blockade regime and explain the existence of the leakage current in terms of cotunneling and spin-flip cotunneling-assisted sequential tunneling. For DQDs coupled in parallel, we show that the transport characteristics in the weak-coupling regime are qualitatively similar to those of DQDs coupled in series. On the other hand, in the case of $T$-shaped quantum dots we predict a large super-Poissonian shot noise and TMR enhanced above the Julliere value due to increased occupation of the decoupled quantum dot. We also discuss the possibility of determining the geometry of the double dot from transport characteristics. Furthermore, where possible, we compare our results with existing experimental data on nonmagnetic systems and find qualitative agreement.

A voltage probe of the spin Hall effect

The spin Hall effect does not generally result in a transverse voltage. We predict that insystems with inhomogeneous electron density in the direction perpendicular to maincurrent flow, the spin Hall effect is instead accompanied by a transverse voltage. We findthat, unlike the ordinary Hall effect, this voltage is quadratic in the longitudinal electricfield for a wide range of parameters accessible experimentally. We also predict spinaccumulation in the bulk and sharp peaks of spin Hall induced charge accumulation nearthe edges. Our results can be readily tested experimentally, and would allow the electricalmeasurement of the spin Hall effect in non-magnetic systems and without injection ofspin-polarized electrons.

Magnetically induced dynamical stability of a Fe monolayer on W(110)

The magnetic and structural properties of the pseudomorphic iron monolayer on tungsten substrate were studied within the density functional generalized gradient approximation. The existing strains originating from significant mismatch between iron and tungsten layers were determined for both magnetic and nonmagnetic systems. By studying the spectrum of surface phonons within the direct method, a strong dependence of lattice dynamics on magnetic interactions has been revealed. In the nonmagnetic state, the in-plane surface modes soften along the [001] direction and lead to surface reconstruction. The ferromagnetic interactions were found to be the crucial element in stabilizing the iron monolayer on tungsten. The mechanism enhancing the ferromagnetic polarization of the surface and the soft mode behavior are discussed on the basis of the calculated changes in the electronic density of states.

Reflectometer for studying the surface of liquid systems

To study the free surface of liquids, a Reverans reflectometer with a vertical neutron scattering plane has been designed and is fabricated at Petersburg Nuclear Physics Institute. This reflectometer is intended to investigate surfaces and interfaces in liquid magnetic or nonmagnetic systems. The principles of reflectometer operation are described. The general view of the reflectometer with ready units is presented and its main technical parameters are given. The results of preliminary tests are reported.

Ab initiostructural and electronic investigation of magneticRNiSn(R=La,Ce,Pr,Nd)intermetallics and their hydrides

We investigated the electronic structure of the $R\mathrm{Ni}\mathrm{Sn}$ $(R=\mathrm{La},\mathrm{Ce},\mathrm{Pr},\mathrm{Nd})$ intermetallic compounds as well as their corresponding mono- and dihydrides. We computed the energy bands and the total and partial densities of states using a mixed approach; a pseudopotential scheme and ab initio full potential\char21{}all electron band-structure techniques. The crystal structure was investigated for each of the 12 compounds by performing full geometry optimization. We determine the magnetic properties of these compounds. We also analyze the details of the bonding in these systems, in particular, the effect of the hydrogen insertion in the parent intermetallic compounds. We found that, besides the nonmagnetic reference systems $\mathrm{La}\mathrm{Ni}\mathrm{Sn}{\mathrm{H}}_{x}$, the Pr- and Nd-derived systems are consistent with ${f}^{+2}$ and ${f}^{+3}$ electronic configurations. The case of the Ce compounds, as expected, showed a more complicated situation; the computed magnetic moment suggests an intermediate valence state for CeNiSnH. The decomposition enthalpies for the mono- and dihydrides have been estimated and compared with experimental values for similar compounds.

Nanowire metamaterials with extreme optical anisotropy

The authors study perspectives of nanowire metamaterials for negative-refraction waveguides, high-performance polarizers, and polarization-sensitive biosensors. They demonstrate that the behavior of these composites is strongly influenced by the concentration, distribution, and geometry of the nanowires, derive an analytical description of electromagnetism in anisotropic nanowire-based metamaterials, and explore the limitations of their approach via three-dimensional numerical simulations. Finally, they illustrate the developed approach on the examples of nanowire-based high-energy-density waveguides and nonmagnetic negative-index imaging systems with far-field resolution of one-sixth of vacuum wavelength.

Beyond spin-orbit: Probing electron correlation in the Pu 5f states using spin-resolved photoelectron spectroscopy

Experiments planned to address the issue of electron correlation in the Pu 5f states are described herein. The key is the utilization of the Fano effect: the observation of spin polarization in nonmagnetic systems, using chiral excitation such as circularly polarized X-rays.

Resistivity and AC calorimetry measurements on [formula omitted] under pressure

We present resistivity and AC calorimetry measurements of single crystalline CeNiGe 2 under high pressure and low temperature in a diamond anvil cell. CeNiGe 2 is a highly anisotropic, antiferromagnetic Kondo system at ambient pressure, located close to the boundary with non-magnetic systems. It has two ordering temperatures, T N 1 = 3.9 K and T N 2 = 3.2 K , which are suppressed under pressure. Between 0.7 and 1.4 GPa only a single transition was identified, with T N reaching 1.9 K by 1.4 GPa. At 1.9 GPa two transitions could again be seen by AC calorimetry at 2.0 and 1.3 K, accompanied by a qualitative change in the behaviour of the resistivity. The Sommerfeld coefficient showed a maximum of 755 mJ mol - 1 K - 2 at 1.4 GPa, up from 448 mJ mol - 1 K - 2 at close to ambient pressure. The residual resistivity was found to decrease sharply above 1.4 GPa after an initial increase with pressure.

Neutron standing waves in layered systems

A theory of propagation of neutron waves in multilayer magnetic and nonmagnetic systems is presented. It is shown how a system forms the wave function of a neutron, how the wave function formed can be controlled, and how this function can be used to investigate materials. The results of experimental investigations of the neutron reflection from multilayer systems with simultaneous measurement of different types of secondary radiation are reported.

The new neutron reflectometer NERO

The year 2005 saw the opening of the new NEutron ReflectOmeter (NERO) at the GKSS research centre in Geesthacht, Germany for the investigation of magnetic and non-magnetic systems as well as soft matter nano-structures. NERO operates with a monochromatic beam of neutrons of wavelength 0.433 nm with a resolution better than 2%. An angular range of −20°<2 θ<100° allows for both reflectometry and high-angle diffraction measurements to be made. NERO has both a position-sensitive detector and a pencil detector installed for flexibility when making specular and diffuse measurements. NERO has been designed to accommodate heavy-sample environments such as cryo-furnaces and various kinds of magnets. Polarization analysis is available for the investigation of magnetic nano-structures. A supermirror stack with a wide angular-acceptance range will be available in 2006 for time-efficient measurements of magnetic diffuse reflectivity. Further information and proposal forms can be obtained online at http//:genf.gkss.de.

Spin-resolved electronic structure studies of non-magnetic systems: Possible observation of the Fano Effect in polycrystal Ce

Using circularly polarized X-rays and true spin detection via a MiniMott Detector, preliminary evidence of strong spin specific effects has been observed in the valence bands of ultrathin films of non-magnetic Ce.

Slow light and 3D imaging with non-magnetic negative index systems

We demonstrate that strongly anisotropic planar dielectric systems can be used to create waveguides supporting modes with extremely slow group velocity. Furthermore, we show that such systems can be used for 3D imaging, with a potential for subwavelength resolution.

Why Are the Secondary Stars in Polars So Normal?

We have used NIRSPEC on Keck II to obtain $K$-band spectroscopy of several magnetic cataclysmic variables. These data reveal that the secondary stars in these binary systems have spectra that are consistent with normal, late-type dwarfs in both their atomic and molecular line strengths, as well as in the slopes of their continuua. This result is in stark contrast to the infrared spectra of their non-magnetic cousins, nearly all of which show peculiar abundances, especially of CNO species and their isotopes. It appears that the evolutionary path taken by the secondary stars in magnetic systems differs from that for the non-magnetic systems. We discuss the implications of this result.

Features of the magnetic state of ƒ electrons in the stabilized δ phase of the Pu0.95Ga0.05 alloy

For the Pu0.95Ga0.05 alloy belonging to the stabilized δ phase of the Pu-Ga system, the 69Ga NMR spectra are measured in the temperature interval 10–650 K and the static magnetic susceptibility χ is measured in the interval T = 20–350 K. In the region T > T* = 235 K, the temperature-dependent magnetic part of the shift K(T) of the Ga NMR line reproduces the χ(T) dependence and follows the Curie-Weiss law K(T) ∼ (T + Θ)−1 with Θ = 280(40) K, which is typical of incoherent spin fluctuations of localized ƒ electrons in concentrated nonmagnetic Kondo systems. The estimate of the effective magnetic moment μeff, 5ƒ(ge = 2) = 0.15(5)μB per Pu atom testifies to a strong suppression of the spin magnetism in the alloy, where the configuration of the ƒ shell of a Pu ion is close to atomic-like ƒ6. The difference between the K(T) and χ(T) dependences observed for the alloy in the temperature range T < T* is analyzed in terms of the two-fluid description of Kondo lattices developed by Pines et al. [15, 16] in application to the coherent state of a heavy-fermion liquid. Possible causes for the anomalous increase in long-wave contributions in the spatial dispersion of the localized spin susceptibility component of ƒ electrons χƒƒ at temperatures below 50 K are discussed.

Spin-resolved magnetic studies of focused ion beam etched nano-sized magnetic structures

Scanning ion microscopy with polarization analysis (SIMPA) is used to study the spin-resolved surface magnetic structure of nano-sized magnetic systems. SIMPA is utilized for in situ topographic and spin-resolved magnetic domain imaging as well as for focused ion beam (FIB) etching of desired structures in magnetic or non-magnetic systems. Ultra-thin Co films are deposited on surfaces of Si(100) substrates, and ultra-thin, tri-layered, bct Fe(100)/Mn/bct Fe(100) wedged magnetic structures are deposited on fcc Pd(100) substrates. SIMPA experiments clearly show that ion-induced electrons emitted from magnetic surfaces exhibit non-zero electron spin polarization (ESP), whereas electrons emitted from non-magnetic surfaces such as Si and Pd exhibit zero ESP, which can be used to calibrate sputtering rates in situ. We report on new, spin-resolved magnetic microstructures, such as magnetic “C” states and magnetic vortices, found at surfaces of FIB patterned magnetic elements. It is found that FIB milling has a negligible effect on surface magnetic domain and domain wall structures. It is demonstrated that SIMPA can evolve into an important and efficient tool to study magnetic domain, domain wall and other structures as well as to perform magnetic depth profiling of magnetic nano-systems to be used in ultra-high density magnetic recording and in magnetic sensors.