324 research outputs found
Nonlinear Dynamics of Nuclear-Electronic Spin Processes in Ferromagnets
Spin dynamics is considered in ferromagnets consisting of electron and
nuclear subsystems interacting with each other through hyperfine forces. In
addition, the ferromagnetic sample is coupled with a resonance electric
circuit. Under these conditions, spin relaxation from a strongly nonequilibrium
initial state displays several peculiarities absent for the standard set-up in
studying spin relaxation. The main feature of the nonlinear spin dynamics
considered in this communication is the appearance of ultrafast coherent
relaxation, with characteristic relaxation times several orders shorter than
the transverse relaxation time . This type of coherent spin relaxation can
be used for extracting additional information on the intrinsic properties of
ferromagnetic materials and also can be employed for different technical
applications.Comment: 1 file, 4 pages, RevTex, no figure
Magnonic Crystal with Two-Dimensional Periodicity as a Waveguide for Spin Waves
We describe a simple method of including dissipation in the spin wave band
structure of a periodic ferromagnetic composite, by solving the Landau-Lifshitz
equation for the magnetization with the Gilbert damping term. We use this
approach to calculate the band structure of square and triangular arrays of Ni
nanocylinders embedded in an Fe host. The results show that there are certain
bands and special directions in the Brillouin zone where the spin wave lifetime
is increased by more than an order of magnitude above its average value. Thus,
it may be possible to generate spin waves in such composites decay especially
slowly, and propagate especially large distances, for certain frequencies and
directions in -space.Comment: 13 pages, 4 figures, submitted to Phys Rev
Intrinsic Energy Localization through Discrete Gap Breathers in One-Dimensional Diatomic Granular Crystals
We present a systematic study of the existence and stability of discrete
breathers that are spatially localized in the bulk of a one-dimensional chain
of compressed elastic beads that interact via Hertzian contact. The chain is
diatomic, consisting of a periodic arrangement of heavy and light spherical
particles. We examine two families of discrete gap breathers: (1) an unstable
discrete gap breather that is centered on a heavy particle and characterized by
a symmetric spatial energy profile and (2) a potentially stable discrete gap
breather that is centered on a light particle and is characterized by an
asymmetric spatial energy profile. We investigate their existence, structure,
and stability throughout the band gap of the linear spectrum and classify them
into four regimes: a regime near the lower optical band edge of the linear
spectrum, a moderately discrete regime, a strongly discrete regime that lies
deep within the band gap of the linearized version of the system, and a regime
near the upper acoustic band edge. We contrast discrete breathers in anharmonic
FPU-type diatomic chains with those in diatomic granular crystals, which have a
tensionless interaction potential between adjacent particles, and highlight in
that the asymmetric nature of the latter interaction potential may lead to a
form of hybrid bulk-surface localized solutions
Absorption features in the spectra of X-ray bursting neutron stars
The discovery of photospheric absorption lines in XMM-Newton spectra of the
X-ray bursting neutron star in EXO0748-676 by Cottam and collaborators allows
us to constrain the neutron star mass-radius ratio from the measured
gravitational redshift. A radius of R=9-12km for a plausible mass range of
M=1.4-1.8Msun was derived by these authors. It has been claimed that the
absorption features stem from gravitationally redshifted (z=0.35) n=2-3 lines
of H- and He-like iron. We investigate this identification and search for
alternatives. We compute LTE and non-LTE neutron-star model atmospheres and
detailed synthetic spectra for a wide range of effective temperatures
(effective temperatures of 1 - 20MK) and different chemical compositions.
We are unable to confirm the identification of the absorption features in the
X-ray spectrum of EXO0748-676 as n=2-3 lines of H- and He-like iron (Fe XXVI
and Fe XXV). These are subordinate lines that are predicted by our models to be
too weak at any effective temperature. It is more likely that the strongest
feature is from the n=2-3 resonance transition in Fe XXIV with a redshift of
z=0.24. Adopting this value yields a larger neutron star radius, namely
R=12-15km for the mass range M=1.4-1.8Msun, favoring a stiff equation-of-state
and excluding mass-radius relations based on exotic matter. Combined with an
estimate of the stellar radius R>12.5km from the work of Oezel and
collaborators, the z=0.24 value provides a minimum neutron-star mass of
M>1.48Msun, instead of M>1.9Msun, when assuming z=0.35.Comment: 8 pages, 17 figure
Nonlinear spin relaxation in strongly nonequilibrium magnets
A general theory is developed for describing the nonlinear relaxation of spin
systems from a strongly nonequilibrium initial state, when, in addition, the
sample is coupled to a resonator. Such processes are characterized by nonlinear
stochastic differential equations. This makes these strongly nonequilibrium
processes principally different from the spin relaxation close to an
equilibrium state, which is represented by linear differential equations. The
consideration is based on a realistic microscopic Hamiltonian including the
Zeeman terms, dipole interactions, exchange interactions, and a single-site
anisotropy. The influence of cross correlations between several spin species is
investigated. The critically important function of coupling between the spin
system and a resonant electric circuit is emphasized. The role of all main
relaxation rates is analyzed. The phenomenon of self-organization of transition
coherence in spin motion, from the quantum chaotic stage of incoherent
fluctuations, is thoroughly described. Local spin fluctuations are found to be
the triggering cause for starting the spin relaxation from an incoherent
nonequilibrium state. The basic regimes of collective coherent spin relaxation
are studied.Comment: Latex file, 31 page
Spin gap behavior and charge ordering in \alpha^{\prime}-NaV_2O_5 probed by light scattering
We present a detailed analysis of light scattering experiments performed on
the quarter-filled spin ladder compound -NaVO for
the temperature range 5 KT300 K. This system undergoes a phase
transition into a singlet ground state at T=34 K accompanied by the formation
of a super structure. For T34 K several new modes were detected. Three of
these modes are identified as magnetic bound states. Experimental evidence for
charge ordering on the V sites is detected as an anomalous shift and splitting
of a V-O vibration at 422 cm for temperatures above 34 K. The smooth and
crossover-like onset of this ordering at T= 80 K is accompanied by
pretransitional fluctuations both in magnetic and phononic Raman scattering. It
resembles the effect of stripe order on the super structure intensities in
LaNiO.Comment: 36 pages, 11 figures, accepted for publication in PRB (sept.99
Resonant two-magnon Raman scattering in parent compounds of high-T superconductors.
We propose a theory of two-magnon Raman scattering from the insulating parent
compounds of high-T superconductors, which contains information not only on
magnetism, but also on the electronic properties in these materials. We use
spin density wave formalism for the Hubbard model, and study diagrammatically
the profile of the two-magnon scattering and its intensity dependence on the
incoming photon frequency both for and in the
resonant regime, in which the energy of the incident photon is close to the gap
between conduction and valence bands. In the nonresonant case, we identify the
diagrams which contribute to the conventional Loudon-Fleury Hamiltonian. In the
resonant regime, where most of the experiments have been done, we find that the
dominant contribution to Raman intensity comes from a different diagram, one
which allows for a simultaneous vanishing of all three of its denominators
(i.e., a triple resonance). We study this diagram in detail and show that the
triple resonance, combined with the spin-density-wave dispersion relation for
the carriers, explains the unusual features found in the two-magnon profile and
in the two-magnon peak intensity dependence on the incoming photon frequency.
In particular, our theory predicts a maximum of the two-magnon peak intensity
right at the upper edge of the features in the optical data, which has been one
of the key experimental puzzles.Comment: Revtex, 12 postscript figures (uuencoded
Resonant Raman Scattering in Antiferromagnets
Two-magnon Raman scattering provides important information about electronic
correlations in the insulating parent compounds of high- materials. Recent
experiments have shown a strong dependence of the Raman signal in
geometry on the frequency of the incoming photon. We present an analytical and
numerical study of the Raman intensity in the resonant regime. It has been
previously argued by one of us (A.Ch) and D. Frenkel that the most relevant
contribution to the Raman vertex at resonance is given by the triple resonance
diagram. We derive an expression for the Raman intensity in which we
simultaneously include the enhancement due to the triple resonance and a final
state interaction. We compute the two-magnon peak height (TMPH) as a function
of incident frequency and find two maxima at and . We argue that the
high-frequency maximum is cut only by a quasiparticle damping, while the
low-frequency maximum has a finite amplitude even in the absence of damping. We
also obtain an evolution of the Raman profile from an asymmetric form around
to a symmetric form around . We
further show that the TMPH depends on the fermionic quasiparticle damping, the
next-nearest neighbor hopping term and the corrections to the
interaction vertex between light and the fermionic current. We discuss our
results in the context of recent experiments by Blumberg et al. on
and and R\"{u}bhausen et al. on
and show that the triple resonance theory yields a qualitative
and to some extent also quantitative understanding of the experimental data.Comment: 19 pages, RevTeX, 16 figures embedded in the text, ps-file is also
available at http://lifshitz.physics.wisc.edu/www/morr/morr_homepage.htm
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