3,621 research outputs found
Landau-Zener quantum tunneling in disordered nanomagnets
We study Landau-Zener macroscopic quantum transitions in ferromagnetic metal
nanoparticles containing on the order of 100 atoms. The model that we consider
is described by an effective giant-spin Hamiltonian, with a coupling to a
random transverse magnetic field mimicking the effect of quasiparticle
excitations and structural disorder on the gap structure of the spin collective
modes. We find different types of time evolutions depending on the interplay
between the disorder in the transverse field and the initial conditions of the
system. In the absence of disorder, if the system starts from a low-energy
state, there is one main coherent quantum tunneling event where the
initial-state amplitude is completely depleted in favor of a few discrete
states, with nearby spin quantum numbers; when starting from the highest
excited state, we observe complete inversion of the magnetization through a
peculiar ``backward cascade evolution''. In the random case, the
disorder-averaged transition probability for a low-energy initial state becomes
a smooth distribution, which is nevertheless still sharply peaked around one of
the transitions present in the disorder-free case. On the other hand, the
coherent backward cascade phenomenon turns into a damped cascade with
frustrated magnetic inversion.Comment: 21 pages, 7 figures, to be published in Phys.Rev.
Magnetic moment non-conservation in magnetohydrodynamic turbulence models
The fundamental assumptions of the adiabatic theory do not apply in presence
of sharp field gradients as well as in presence of well developed
magnetohydrodynamic turbulence. For this reason in such conditions the magnetic
moment is no longer expected to be constant. This can influence particle
acceleration and have considerable implications in many astrophysical problems.
Starting with the resonant interaction between ions and a single parallel
propagating electromagnetic wave, we derive expressions for the magnetic moment
trapping width (defined as the half peak-to-peak difference in the
particle magnetic moment) and the bounce frequency . We perform
test-particle simulations to investigate magnetic moment behavior when
resonances overlapping occurs and during the interaction of a ring-beam
particle distribution with a broad-band slab spectrum.
We find that magnetic moment dynamics is strictly related to pitch angle
for a low level of magnetic fluctuation, , where is the constant and uniform background magnetic field.
Stochasticity arises for intermediate fluctuation values and its effect on
pitch angle is the isotropization of the distribution function .
This is a transient regime during which magnetic moment distribution
exhibits a characteristic one-sided long tail and starts to be influenced by
the onset of spatial parallel diffusion, i.e., the variance
grows linearly in time as in normal diffusion. With strong fluctuations
isotropizes completely, spatial diffusion sets in and
behavior is closely related to the sampling of the varying magnetic field
associated with that spatial diffusion.Comment: 13 pages, 10 figures, submitted to PR
Slow dynamics in glassy soft matter
Measuring, characterizing and modelling the slow dynamics of glassy soft
matter is a great challenge, with an impact that ranges from industrial
applications to fundamental issues in modern statistical physics, such as the
glass transition and the description of out-of-equilibrium systems. Although
our understanding of these phenomena is still far from complete, recent
simulations and novel theoretical approaches and experimental methods have shed
new light on the dynamics of soft glassy materials. In this paper, we review
the work of the last few years, with an emphasis on experiments in four
distinct and yet related areas: the existence of two different glass states
(attractive and repulsive), the dynamics of systems very far from equilibrium,
the effect of an external perturbation on glassy materials, and dynamical
heterogeneity
Considering Fluctuation Energy as a Measure of Gyrokinetic Turbulence
In gyrokinetic theory there are two quadratic measures of fluctuation energy,
left invariant under nonlinear interactions, that constrain the turbulence. The
recent work of Plunk and Tatsuno [Phys. Rev. Lett. 106, 165003 (2011)] reported
on the novel consequences that this constraint has on the direction and
locality of spectral energy transfer. This paper builds on that work. We
provide detailed analysis in support of the results of Plunk and Tatsuno but
also significantly broaden the scope and use additional methods to address the
problem of energy transfer. The perspective taken here is that the fluctuation
energies are not merely formal invariants of an idealized model
(two-dimensional gyrokinetics) but are general measures of gyrokinetic
turbulence, i.e. quantities that can be used to predict the behavior of the
turbulence. Though many open questions remain, this paper collects evidence in
favor of this perspective by demonstrating in several contexts that constrained
spectral energy transfer governs the dynamics.Comment: Final version as published. Some cosmetic changes and update of
reference
Turbulence patterns and neutrino flavor transitions in high-resolution supernova models
During the shock-wave propagation in a core-collapse supernova (SN), matter
turbulence may affect neutrino flavor conversion probabilities. Such effects
have been usually studied by adding parametrized small-scale random
fluctuations (with arbitrary amplitude) on top of coarse, spherically symmetric
matter density profiles. Recently, however, two-dimensional (2D) SN models have
reached a space resolution high enough to directly trace anisotropic density
profiles, down to scales smaller than the typical neutrino oscillation length.
In this context, we analyze the statistical properties of a large set of SN
matter density profiles obtained in a high-resolution 2D simulation, focusing
on a post-bounce time (2 s) suited to study shock-wave effects on neutrino
propagation on scales as small as O(100) km and possibly below. We clearly find
the imprint of a broken (Kolmogorov-Kraichnan) power-law structure, as
generically expected in 2D turbulence spectra. We then compute the flavor
evolution of SN neutrinos along representative realizations of the turbulent
matter density profiles, and observe no or modest damping of the neutrino
crossing probabilities on their way through the shock wave. In order to check
the effect of possibly unresolved fluctuations at scales below O(100) km, we
also apply a randomization procedure anchored to the power spectrum calculated
from the simulation, and find consistent results within \pm 1 sigma
fluctuations. These results show the importance of anchoring turbulence effects
on SN neutrinos to realistic, fine-grained SN models.Comment: 19 pages, 8 figures (Major changes in the text, references added,
analysis and figures improved, main results unchanged. To appear in JCAP.
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