arXiv subjects

S. Zhang

Publications and source records attributed to S. Zhang.

At least 361 records · Page 20Linked to original sources

arXiv2005

Effects of spin current on ferromagnets

When a spin-polarized current flows through a ferromagnet, the local magnetization receives a spin torque. Two consequences of this spin torque are studied. First, the uniformly magnetized ferromagnet becomes unstable if a sufficiently large current is applied. The characteristics of the instability include spin wave generation and magnetization chaos. Second, the spin torque has profound effects on the structure and dynamics of the magnetic domain wall. A detail analysis on the domain wall mass, kinetic energy and wall depinning threshold is given.

cond-mat.mtrl-sci↗

arXiv2005

Discovery of new quasi-periodic oscillations in the X-ray transient source V~0332+53

We report the discovery of a new quasi-period oscillation (QPO) at 0.22 Hz, centered on the source spin frequency of the high mass X-ray binary system V~0332+53 when the source was observed during its November 2004/March 2005 outburst by {\em RXTE}. Besides this new QPO, we also detected the known 0.05 Hz QPO. Both the 0.22 and 0.05 Hz QPOs stand out clearly at a mid-flux level of the outburst within January 15--19 2005, and later at an even lower flux level as the width of 0.22 Hz QPO drops. No evolution of the centroid frequency with the flux is seen in either QPO. The rms value below 10 keV is around 4--6% for both QPOs and decreases at higher energies. We discuss our results in the context of current QPO models.

astro-ph↗

arXiv2005

COMPTEL Observations of the Gamma-ray Blazars 3C~454.3 and CTA 102 During the CGRO Mission

We have investigated the MeV behaviour of the \gray blazars 3C~454.3 and CTA 102 by analyzing all COMPTEL observations of this sky region during the complete CGRO mission. Both sources are detected by COMPTEL at the upper COMPTEL energies; although their flux estimates may be uncertain by possible minor contribution of nearby unidentified EGRET \gray sources. While CTA 102 was only detected at energies above 10 MeV during the early mission, 3C~454.3 is most significantly detected in the COMPTEL 3-10 MeV band in the sum of all data. Time-resolved analyses indicate a weak (near COMPTEL threshold) but likely steady 3-10 MeV emission over years, being independent of the observed time variability at energies above 100 MeV as observed by EGRET. This energy-dependent variability behaviour suggests different emission mechanisms at work at the two bands. Putting the COMPTEL fluxes in multifrequency perspective (radio to \grays), reveals for both sources the typical two-hump blazar spectra, with a low-energy maximum around the IR and a high-energy maximum at MeV energies. The latter one dominates the energy output across the whole electro-magnetic spectrum. The results of our analyses are discussed in the framework of current blazar modeling.

astro-ph↗

arXiv2005

Phase space methods for particles on a circle

The phase space $S\times Z$ for a particle on a circle is considered. Displacement operators in this phase space are introduced and their properties are studied. Wigner and Weyl functions in this context are also considered and their physical interpretation and properties are discussed. All results are compared and contrasted with the corresponding ones for the harmonic oscillator in the $R \times R$ phase space.

quant-ph↗

arXiv2005

Analytic Representation of Finite Quantum Systems

A transform between functions in R and functions in Zd is used to define the analogue of number and coherent states in the context of finite d-dimensional quantum systems. The coherent states are used to define an analytic representation in terms of theta functions. All states are represented by entire functions with growth of order 2, which have exactly d zeros in each cell. The analytic function of a state is constructed from its zeros. Results about the completeness of finite sets of coherent states within a cell are derived.

quant-ph↗

arXiv2005

Intrinsic Spin and Orbital-Angular-Momentum Hall Effect

A generalized definition of intrinsic and extrinsic transport coefficients is introduced. We show that transport coefficients from the intrinsic origin are solely determined by local electronic structure, and thus the intrinsic spin Hall effect is not a transport phenomenon. The intrinsic spin Hall current is always accompanied by an equal but opposite intrinsic orbital-angular-momentum Hall current. We prove that the intrinsic spin Hall effect does not induce a spin accumulation at the edge of the sample or near the interface.

cond-mat.mtrl-sci↗

arXiv2004

Domain wall dynamics driven by adiabatic spin transfer torques

In a first approximation, known as the adiabatic process, the direction of the spin polarization of currents is parallel to the local magnetization vector in a domain wall. Thus the spatial variation of the direction of the spin current inside the domain wall results in an adiabatical spin transfer torque on the magnetization. We show that domain wall motion driven by this spin torque has many unique features that do not exist in the conventional wall motion driven by a magnetic field. By analytically and numerically solving the Landau-Lifshitz-Gilbert equation along with the adiabatic spin torque in magnetic nanowires, we find the domain wall has its maximum velocity at the initial application of the current but the velocity decreases to zero as the domain wall begins to deform during its motion. We have computed domain wall displacement and domain wall deformation of nanowires, and concluded that the spin torque based on the adiabatic propagation of the spin current in the domain wall is unable to maintain wall movement. We also introduce a novel concept of domain wall inductance to characterize the capacity of the spin-torque induced magnetic energy stored in a domain wall. In the presence of domain wall pinning centers, we construct a phase diagram for the domain wall depinning by the combined action of the magnetic field and the spin current.

math-ph↗

arXiv2004

Roles of non-equilibrium conduction electrons on magnetization dynamics of ferromagnets

The mutual dependence of spin-dependent conduction and magnetization dynamics of ferromagnets provides the key mechanisms in various spin-dependent phenomena. We compute the response of the conduction electron spins to a spatial and time varying magnetization ${\bf M} ({\bf r},t)$ within the time-dependent semiclassical transport theory. We show that the induced non-equilibrium conduction spin density in turn generates four spin torques acting on the magnetization--with each torque playing different roles in magnetization dynamics. By comparing with recent theoretical models, we find that one of these torques that has not been previously identified is crucial to consistently interpret experimental data on domain wall motion.

cond-mat.mtrl-sci↗

arXiv2004

Thermally assisted magnetization reversal in the presence of a spin-transfer torque

We propose a generalized stochastic Landau-Lifshitz equation and its corresponding Fokker-Planck equation for the magnetization dynamics in the presence of spin transfer torques. Since the spin transfer torque can pump a magnetic energy into the magnetic system, the equilibrium temperature of the magnetic system is ill-defined. We introduce an effective temperature based on a stationary solution of the Fokker-Planck equation. In the limit of high energy barriers, the law of thermal agitation is derived. We find that the Néel-Brown relaxation formula remains valid as long as we replace the temperature by an effective one that is linearly dependent of the spin torque. We carry out the numerical integration of the stochastic Landau-Lifshitz equation to support our theory. Our results agree with existing experimental data.

cond-mat.mtrl-sci↗

arXiv2004

Spectral constraints on unidentified EGRET gamma-ray sources from COMPTEL MeV observations

We investigated the MeV properties of 173 unidentified or only tentatively identified EGRET sources listed in the third EGRET catalogue, by analyzing the simultaneously collected COMPTEL MeV data for each individual source. The sources can generally be divided into 4 groups. In this paper we focus on one of these, a group of 22 EGRET sources for which we can provide additional constraining information: their spectral extrapolations from the energy range above 100 MeV towards lower energies overshoot the fluxes or upper limits derived simultaneously at MeV energies. This means that for these sources a spectral turnover/break between 1 MeV and 100 MeV is required. At least two of these sources, but most likely the majority of this sample, have the maxima of their gamma-ray luminosities in this energy band. The sources have rather soft EGRET spectra (average photon index: 2.72), and seem to spatially cluster in the inner Galaxy. Variability analyses revealed 11 out of the 22 sources to be significantly variable. Object classes proposed as possible counterparts for the unidentified EGRET sources are discussed in the light of these additional constraints.

astro-ph↗

arXiv2003

Astrophysical constraints on hypothetical variability of fundamental constants

(Abridged) Many-multiplet (MM) method applied to three inhomogeneous samples of Keck/HIRES quasar absorption spectra gives a shift in the value of the fine-structure constant of Delta alpha/alpha = (-5.4+/-1.2) 10^{-6} in the redshift range 0.2 < z < 3.7 (Murphy et al. 2003).The 1sigma error is, however, much too small and cannot be maintained by current observations of quasars. We present a modified MM method to set an upper limit on Delta alpha/alpha from a homogeneous sample of FeII lines identified in the up-to-date best quality VLT/UVES spectrum of HE 0515-4414. Our result is Delta alpha/alpha = (1.1+/-1.1) 10^{-5} at z = 1.149. Theoretical models of the fundamental physical interactions predict that the proton-to-electron mass ratio (mu = m_p/m_e) may relate to the shift in Delta alpha/alpha as Delta mu/mu = R Delta alpha/alpha. We use VLT/UVES high-resolution observations of molecular hydrogen H2 ultraviolet absorption lines at z = 3.025 toward Q 0347-3818 to bound the value of R. The obtained constraints on Delta ln mu = (2.1+/-3.6) 10^{-5} and on Delta ln alpha = (1.1+/-1.1) 10^{-5} rule out vary large values of |R| > 6.

astro-ph↗

arXiv2003

Magnetization dynamics with a spin-transfer torque

The magnetization reversal and dynamics of a spin valve pillar, whose lateral size is 64$\times$64 nm$^2$, are studied by using micromagnetic simulation in the presence of spin transfer torque. Spin torques display both characteristics of magnetic damping (or anti-damping) and of an effective magnetic field. For a steady-state current, both M-I and M-H hysteresis loops show unique features, including multiple jumps, unusual plateaus and precessional states. These states originate from the competition between the energy dissipation due to Gilbert damping and the energy accumulation due to the spin torque supplied by the spin current. The magnetic energy oscillates as a function of time even for a steady-state current. For a pulsed current, the minimum width and amplitude of the spin torque for achieving current-driven magnetization reversal are quantitatively determined. The spin torque also shows very interesting thermal activation that is fundamentally different from an ordinary damping effect.

cond-mat.mtrl-sci↗

arXiv2002

A Search for Variations of Fundamental Constants using Atomic Fountain Clocks

Over five years we have compared the hyperfine frequencies of 133Cs and 87Rb atoms in their electronic ground state using several laser cooled 133Cs and 87Rb atomic fountains with an accuracy of ~10^{-15}. These measurements set a stringent upper bound to a possible fractional time variation of the ratio between the two frequencies : (d/dt)ln(nu_Rb/nu_Cs)=(0.2 +/- 7.0)*10^{-16} yr^{-1} (1 sigma uncertainty). The same limit applies to a possible variation of the quantity (mu_Rb/mu_Cs)*alpha^{-0.44}, which involves the ratio of nuclear magnetic moments and the fine structure constant.

physics.atom-ph↗

arXiv2002

Spin diffusion at finite electric and magnetic fields

Spin transport properties at finite electric and magnetic fields are studied by using the generalized semiclassical Boltzmann equation. It is found that the spin diffusion equation for non-equilibrium spin density and spin currents involves a number of length scales that explicitly depend on the electric and magnetic fields. The set of macroscopic equations can be used to address a broad range of the spin transport problems in magnetic multilayers as well as in semiconductor heterostructure. A specific example of spin injection into semiconductors at arbitrary electric and magnetic fields is illustrated.

cond-mat.mtrl-sci↗

arXiv2002

An Unidentified Variable Gamma-Ray Source near the Galactic Plane Detected by COMPTEL

We report the detection of an unidentified gamma-ray source near the Galactic plane by the COMPTEL experiment aboard the Compton Gamma-Ray Observatory. The source is detected at a significance level of ~ 7.2 sigma in the energy range 1-3 MeV and at ~ 4.6 sigma in the lower 0.75-1 MeV band in the time period March to July 1995. At energies above 3 MeV are only marginal hints or upper limits obtained. The MeV spectrum has a soft shape. Strong flux variability is found within one year at energies below 3 MeV. Possible counterparts of galactic and extragalactic nature are discussed.

astro-ph↗

arXiv2002

COMPTEL Observations of the Gamma-Ray Blazar PKS 1622-297

We report results of observations and analyses on the gamma-ray blazar PKS 1622-297, with emphasis on the COMPTEL data (0.75 - 30 MeV) collected between April 1991 and November 1997. PKS 1622-297 was detected as a source of gamma-rays by the EGRET experiment aboard CGRO in 1995 during a gamma-ray outburst at energies above 100 MeV lasting for five weeks. In this time period the blazar was significantly (~ 5.9 sigma) detected by COMPTEL at 10-30 MeV. At lower COMPTEL energies the detection is marginal, resulting in a hard MeV spectrum. The combined COMPTEL/EGRET energy spectrum shows a break at MeV energies. The broad-band spectrum (radio - gamma-rays) shows that the gamma-ray emission dominates the overall power output. On top of the 5-week gamma-ray outburst, EGRET detected a huge flare lasting for > 1 day. Enhanced MeV emission (10 - 30 MeV) is found near the time of this flare, suggesting a possible time delay with respect to the emission above 100 MeV. Outside the 5-week flaring period in 1995, we do not detect MeV emission from PKS 1622-297.

astro-ph↗

arXiv2002

Mechanisms of spin-polarized current-driven magnetization switching

The mechanisms of the magnetization switching of magnetic multilayers driven by a current are studied by including exchange interaction between local moments and spin accumulation of conduction electrons. It is found that this exchange interaction leads to two additional terms in the Landau-Lifshitz-Gilbert equation: an effective field and a spin torque. Both terms are proportional to the transverse spin accumulation and have comparable magnitudes.

cond-mat.mtrl-sci↗

arXiv2001

COMPTEL Observations of the Blazars 3C 454.3 and CTA 102

We have analyzed the two blazars of 3C 454.3 and CTA 102 using all available COMPTEL data from 1991 to 1999. In the 10-30 MeV band, emission from the general direction of the sources is found at the 4$σ$-level, being consistent with contributions from both sources. Below 10 MeV only 3C 454.3 is significantly detected, with the strongest evidence (5.6 $σ$) in the 3-10 MeV band. Significant flux variability is not observed for both sources, while a low emission is seen most of the years in the 3-10 MeV light curve for 3C 454.3. Its time-averaged MeV spectrum suggests a power maximum between 3 to 10 MeV.

astro-ph↗