37 research outputs found
Statistical coarse-graining as an approach to multiscale problems in magnetism
Multiscale phenomena which include several processes occuring simultaneously
at different length scales and exchanging energy with each other, are
widespread in magnetism. These phenomena often govern the magnetization
reversal dynamics, and their correct modeling is important. In the present
paper, we propose an approach to multiscale modeling of magnets, applying the
ideas of coarse graining. We have analyzed the choice of the weighting function
used in coarse graining, and propose an optimal form for this function. Simple
tests provide evidence that this approach may be useful for modeling of
realistic magnetic systems.Comment: 5 RevTeX pages, 2 figure
Effects of nonlinear sweep in the Landau-Zener-Stueckelberg effect
We study the Landau-Zener-Stueckelberg (LZS) effect for a two-level system
with a time-dependent nonlinear bias field (the sweep function) W(t). Our main
concern is to investigate the influence of the nonlinearity of W(t) on the
probability P to remain in the initial state. The dimensionless quantity
epsilon = pi Delta ^2/(2 hbar v) depends on the coupling Delta of both levels
and on the sweep rate v. For fast sweep rates, i.e., epsilon << l and
monotonic, analytic sweep functions linearizable in the vicinity of the
resonance we find the transition probability 1-P ~= epsilon (1+a), where a>0 is
the correction to the LSZ result due to the nonlinearity of the sweep. Further
increase of the sweep rate with nonlinearity fixed brings the system into the
nonlinear-sweep regime characterized by 1-P ~= epsilon ^gamma with gamma neq 1
depending on the type of sweep function. In case of slow sweep rates, i.e.,
epsilon >>1 an interesting interference phenomenon occurs. For analytic W(t)
the probability P=P_0 e^-eta is determined by the singularities of sqrt{Delta
^2+W^2(t)} in the upper complex plane of t. If W(t) is close to linear, there
is only one singularity, that leads to the LZS result P=e^-epsilon with
important corrections to the exponent due to nonlinearity. However, for, e.g.,
W(t) ~ t^3 there is a pair of singularities in the upper complex plane.
Interference of their contributions leads to oscillations of the prefactor P_0
that depends on the sweep rate through epsilon and turns to zero at some
epsilon. Measurements of the oscillation period and of the exponential factor
would allow to determine Delta, independently.Comment: 11 PR pages, 12 figures. To be published in PR
Destruction of the Kondo effect by a local measurement
We show that the local spin measurement which decoheres the localized spin in
a Kondo system, suppresses the Abrikosov-Suhl resonance and destroys the Kondo
effect. This happens due to elimination of the entanglement between the
localized spin and the conduction electrons, and differs essentially from
smearing of the resonance by dissipation. Considering decoherence by a spin
bath, we predict that the Kondo effect disappears when the Kondo temperature
becomes smaller than the coupling with a bath. This effect can be detected in
experiments on ``quantum corrals'' or quantum dots doped by impurities with
internal degrees of freedom.Comment: REVTeX4, 1 figure (EPS
Effect of local Coulomb interactions on the electronic structure and exchange interactions in Mn12 magnetic molecules
We have studied the effect of local Coulomb interactions on the electronic
structure of the molecular magnet Mn12-acetate within the LDA+U approach. The
account of the on-site repulsion results in a finite energy gap and an integer
value of the molecule's magnetic moment, both quantities being in a good
agreement with the experimental results. The resulting magnetic moments and
charge states of non-equivalent manganese ions agree very well with
experiments. The calculated values of the intramolecular exchange parameters
depend on the molecule's spin configuration, differing by 25-30% between the
ferrimagnetic ground state and the completely ferromagnetic configurations. The
values of the ground-state exchange coupling parameters are in reasonable
agreement with the recent data on the magnetization jumps in megagauss magnetic
fields. Simple estimates show that the obtained exchange parameters can be
applied, at least qualitatively, to the description of the spin excitations in
Mn12-acetate.Comment: RevTeX, LaTeX2e, 4 EPS figure
Feedback Effect on Landau-Zener-Stueckelberg Transitions in Magnetic Systems
We examine the effect of the dynamics of the internal magnetic field on the
staircase magnetization curves observed in large-spin molecular magnets. We
show that the size of the magnetization steps depends sensitively on the
intermolecular interactions, even if these are very small compared to the
intra-molecular couplings.Comment: 4 pages, 3 Postscript figures; paper reorganized, conclusions
modifie
Adiabatic Landau-Zener-St\"uckelberg transition with or without dissipation in low spin molecular system V15
The spin one half molecular system V15 shows no barrier against spin
reversal. This makes possible direct phonon activation between the two levels.
By tuning the field sweeping rate and the thermal coupling between sample and
thermal reservoir we have control over the phonon-bottleneck phenomena
previously reported in this system. We demonstrate adiabatic motion of molecule
spins in time dependent magnetic fields and with different thermal coupling to
the cryostat bath. We also discuss the origin of the zero-field tunneling
splitting for a half-integer spin.Comment: to appear in Phys. Rev. B - Rapid Communication
Dipolar-controlled spin tunneling and relaxation in molecular magnets
Spin tunneling in molecular magnets controlled by dipole-dipole interactions
(DDI) in the disordered state has been considered numerically on the basis of
the microscopic model using the quantum mean-field approximation. In the actual
case of a strong DDI spin coherence is completely lost and there is a slow
relaxation of magnetization, described by t^{3/4} at short times. Fast
precessing nuclear spins, included in the model microscopically, only
moderately speed up the relaxation.Comment: 10 pages, 9 figures, to be published in EPJ
Dislocation-induced spin tunneling in Mn-12 acetate
Comprehensive theory of quantum spin relaxation in Mn-12 acetate crystals is
developed, that takes into account imperfections of the crystal structure and
is based upon the generalization of the Landau-Zener effect for incoherent
tunneling from excited energy levels. It is shown that linear dislocations at
plausible concentrations provide the transverse anisotropy which is the main
source of tunneling in Mn-12. Local rotations of the easy axis due to
dislocations result in a transverse magnetic field generated by the field
applied along the c-axis of the crystal, which explains the presence of odd
tunneling resonances. Long-range deformations due to dislocations produce a
broad distribution of tunnel splittings. The theory predicts that at subkelvin
temperatures the relaxation curves for different tunneling resonances can be
scaled onto a single master curve. The magnetic relaxation in the thermally
activated regime follows the stretched-exponential law with the exponent
depending on the field, temperature, and concentration of defects.Comment: 17 pages, 14 figures, 1 table, submitted to PR
Electronic structure of a Mn12 molecular magnet: Theory and experiment
金沢大学大学院自然科学研究科物質情報解析We used site-selective and element-specific resonant inelastic x-ray scattering (RIXS) to study the electronic structure and the electron interaction effects in the molecular magnet [Mn12 O12 (C H3 COO)16 (H2 O)4] 2C H3 COOH 4 H2 O, and compared the experimental data with the results of local spin density approximation +U electron structure calculations which include the on-site Coulomb interactions. We found a good agreement between theory and experiment for the Coulomb repulsion parameter U=4 eV. In particular, the p-d band separation of 1.8 eV has been found from the RIXS spectra, which is in accordance with the calculations. Similarly, the positions of the peaks in the XPS spectra agree with the calculated densities of p and d states. Using the results of the electronic structure calculations, we determined the intramolecular exchange parameters, and used them for diagonalization of the Mn12 spin Hamiltonian. The calculated exchanges gave the correct ground state with the total spin S=10. © 2007 The American Physical Society
Finite-time destruction of entanglement and non-locality by environmental influences
Entanglement and non-locality are non-classical global characteristics of
quantum states important to the foundations of quantum mechanics. Recent
investigations have shown that environmental noise, even when it is entirely
local in influence, can destroy both of these properties in finite time despite
giving rise to full quantum state decoherence only in the infinite time limit.
These investigations, which have been carried out in a range of theoretical and
experimental situations, are reviewed here.Comment: 27 pages, 6 figures, review article to appear in Foundations of
Physic