3,128 research outputs found
Nonlinear ac stationary response and dynamic magnetic hysteresis of quantum uniaxial superparamagnets
The nonlinear ac stationary response of uniaxial paramagnets and
superparamagnets - nanoscale solids or clasters with spin number S ~ 10^0 -
10^4 - in superimposed uniform ac and dc bias magnetic fields of arbitrary
strength, each applied along the easy axis of magnetization, is determined by
solving the evolution equation for the reduced density matrix represented as a
finite set of three-term differential-recurrence relations for its diagonal
matrix elements. The various harmonic components of the magnetization, dynamic
magnetic hysteresis loops, etc. are then evaluated via matrix continued
fractions indicating a pronounced dependence of the nonlinear response on S
arising from the quantum spin dynamics. In the linear response approximation,
the results concur with existing solutions.Comment: 28 pages, 10 figures, 33 refererence
Magnetization dynamics of two interacting spins in an external magnetic field
The longitudinal relaxation time of the magnetization of a system of two
exchange coupled spins subjected to a strong magnetic field is calculated
exactly by averaging the stochastic Gilbert-Landau-Lifshitz equation for the
magnetization, i.e., the Langevin equation of the process, over its
realizations so reducing the problem to a system of linear
differential-recurrence relations for the statistical moments (averaged
spherical harmonics). The system is solved in the frequency domain by matrix
continued fractions yielding the complete solution of the two-spin problem in
external fields for all values of the damping and barrier height parameters.
The magnetization relaxation time extracted from the exact solution is compared
with the inverse relaxation rate from Langer's theory of the decay of
metastable states, which yields in the high barrier and intermediate-to-high
damping limits the asymptotic behaviour of the greatest relaxation time.Comment: 32 pages, 5 figures. The paper has been revised and new results added
(e.g., Fig. 5
Reversal time of the magnetization of magnetic nanoparticles at very low damping
The magnetization reversal time of ferromagnetic nanoparticles is
investigated in the very low damping regime. The energy-controlled diffusion
equation rooted in a generalization of the Kramers escape rate theory for point
Brownian particles in a potential to the magnetic relaxation of a macrospin,
yields the reversal time in closed integral form. The latter is calculated for
a nanomagnet with uniaxial anisotropy with a uniform field applied at an angle
to the easy axis and for a nanomagnet with biaxial anisotropy with the field
along the easy axis. The results completely agree with those yielded by
independent numerical and asymptotic methods.Comment: An extended version: 28 pages; 5 figures; Mathematica Program
Investigating Biological Matter with Theoretical Nuclear Physics Methods
The internal dynamics of strongly interacting systems and that of
biomolecules such as proteins display several important analogies, despite the
huge difference in their characteristic energy and length scales. For example,
in all such systems, collective excitations, cooperative transitions and phase
transitions emerge as the result of the interplay of strong correlations with
quantum or thermal fluctuations. In view of such an observation, some
theoretical methods initially developed in the context of theoretical nuclear
physics have been adapted to investigate the dynamics of biomolecules. In this
talk, we review some of our recent studies performed along this direction. In
particular, we discuss how the path integral formulation of the molecular
dynamics allows to overcome some of the long-standing problems and limitations
which emerge when simulating the protein folding dynamics at the atomistic
level of detail.Comment: Prepared for the proceedings of the "XII Meeting on the Problems of
Theoretical Nuclear Physics" (Cortona11
Dzyaloshinskii--Moriya interaction: How to measure its sign in weak ferromagnetics?
Three experimental techniques sensitive to the sign of the
Dzyaloshinskii--Moriya interaction are discussed: neutron diffraction,
Moessbauer gamma-ray diffraction, and resonant x-ray scattering. Classical
examples of hematite (alpha-Fe2O3) and MnCO3 crystals are considered in detailComment: 5 pages, 1 figure; to be published in JETP Letter
Accurate Results from Perturbation Theory for Strongly Frustrated Heisenberg Spin Clusters
We investigate the use of perturbation theory in finite sized frustrated spin
systems by calculating the effect of quantum fluctuations on coherent states
derived from the classical ground state. We first calculate the ground and
first excited state wavefunctions as a function of applied field for a 12-site
system and compare with the results of exact diagonalization. We then apply the
technique to a 20-site system with the same three fold site coordination as the
12-site system. Frustration results in asymptotically convergent series for
both systems which are summed with Pad\'e approximants.
We find that at zero magnetic field the different connectivity of the two
systems leads to a triplet first excited state in the 12-site system and a
singlet first excited state in the 20-site system, while the ground state is a
singlet for both. We also show how the analytic structure of the Pad\'e
approximants at evolves in the complex plane at
the values of the applied field where the ground state switches between spin
sectors and how this is connected with the non-trivial dependence of the
number on the strength of quantum fluctuations. We discuss the origin
of this difference in the energy spectra and in the analytic structures. We
also characterize the ground and first excited states according to the values
of the various spin correlation functions.Comment: Final version, accepted for publication in Physical review
Steady-State L\'evy Flights in a Confined Domain
We derive the generalized Fokker-Planck equation associated with a Langevin
equation driven by arbitrary additive white noise. We apply our result to study
the distribution of symmetric and asymmetric L\'{e}vy flights in an infinitely
deep potential well. The fractional Fokker-Planck equation for L\'{e}vy flights
is derived and solved analytically in the steady state. It is shown that
L\'{e}vy flights are distributed according to the beta distribution, whose
probability density becomes singular at the boundaries of the well. The origin
of the preferred concentration of flying objects near the boundaries in
nonequilibrium systems is clarified.Comment: 10 pages, 1 figur
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