1,460 research outputs found
Self-organized escape of oscillator chains in nonlinear potentials
We present the noise free escape of a chain of linearly interacting units
from a metastable state over a cubic on-site potential barrier. The underlying
dynamics is conservative and purely deterministic. The mutual interplay between
nonlinearity and harmonic interactions causes an initially uniform lattice
state to become unstable, leading to an energy redistribution with strong
localization. As a result a spontaneously emerging localized mode grows into a
critical nucleus. By surpassing this transition state, the nonlinear chain
manages a self-organized, deterministic barrier crossing. Most strikingly,
these noise-free, collective nonlinear escape events proceed generally by far
faster than transitions assisted by thermal noise when the ratio between the
average energy supplied per unit in the chain and the potential barrier energy
assumes small values
Thermal suppression of surface barrier in ultrasmall superconducting structures
In the recent experiment by Cren \textit{et al.} [Phys. Rev. Lett.
\textbf{102}, 127005 (2009)], no hysteresis for vortex penetration and
expulsion from the nano-island of Pb was observed. In the present paper, we
argue that this effect can be associated with the thermoactivated surmounting
of the surface barrier by a vortex. The typical entrance (exit) time is found
analytically from the Fokker-Planck equation, written in the form suitable for
the extreme vortex confinement. We show that this time is several orders of
magnitude smaller than 1 second under the conditions of the experiment
considered. Our results thus demonstrate a possibility for the thermal
suppression of the surface barrier in nanosized low- superconductors. We
also briefly discuss other recent experiments on vortices in related
structures.Comment: 12 pages, 2 figure
Magnetism in Closed-shell Quantum Dots: Emergence of Magnetic Bipolarons
Similar to atoms and nuclei, semiconductor quantum dots exhibit formation of
shells. Predictions of magnetic behavior of the dots are often based on the
shell occupancies. Thus, closed-shell quantum dots are assumed to be inherently
nonmagnetic. Here, we propose a possibility of magnetism in such dots doped
with magnetic impurities. On the example of the system of two interacting
fermions, the simplest embodiment of the closed-shell structure, we demonstrate
the emergence of a novel broken-symmetry ground state that is neither
spin-singlet nor spin-triplet. We propose experimental tests of our predictions
and the magnetic-dot structures to perform them.Comment: 4 pages, 4 figures;
http://link.aps.org/doi/10.1103/PhysRevLett.106.177201; minor change
Magnetization dynamics in the single-molecule magnet Fe8 under pulsed microwave irradiation
We present measurements on the single molecule magnet Fe8 in the presence of
pulsed microwave radiation at 118 GHz. The spin dynamics is studied via time
resolved magnetization experiments using a Hall probe magnetometer. We
investigate the relaxation behavior of magnetization after the microwave pulse.
The analysis of the experimental data is performed in terms of different
contributions to the magnetization after-pulse relaxation. We find that the
phonon bottleneck with a characteristic relaxation time of 10 to 100 ms
strongly affects the magnetization dynamics. In addition, the spatial effect of
spin diffusion is evidenced by using samples of different sizes and different
ways of the sample's irradiation with microwaves.Comment: 14 pages, 12 figure
Stress induced dislocation roughening -- phase transition in 1d at finite temperature
We present an example of a generically forbidden phase transition in 1d at
finite temperature -- stress induced and thermally assisted roughening of a
superclimbing dislocation in a Peierls potential. We also argue that such
roughening is behind the strong suppression of the superflow through solid \he4
in a narrow temperature range recently observed by Ray and Hallock (Phys.Rev.
Lett. {\bf 105}, 145301 (2010)).Comment: 4 revtex pages, 5 figures. Replaced with the published versio
Flux penetration and expulsion in thin superconducting disks
Using an expansion of the order parameter over the eigenfunctions of the
linearized first Ginzburg-Landau (GL) equation, we obtain numerically the
saddle points of the free energy separating the stable states with different
number of vortices. In contrast to known surface and geometrical barrier
models, we find that in a wide range of magnetic fields below the penetration
field, the saddle point state for flux penetration into a disk does not
correspond to a vortex located nearby the sample boundary, but to a region of
suppressed superconductivity at the disk edge with no winding of the current,
and which is {\it a nucleus} for the following vortex creation. The height of
this {\it nucleation barrier}, which determines the time of flux penetration,
is calculated for different disk radii and magnetic fields.Comment: Accepted for publication in Physical Review Letter
Correlated metals and the LDA+U method
While LDA+U method is well established for strongly correlated materials with
well localized orbitals, its application to weakly correlated metals is
questionable. By extending the LDA Stoner approach onto LDA+U, we show that
LDA+U enhances the Stoner factor, while reducing the density of states.
Arguably the most important correlation effects in metals, fluctuation-induced
mass renormalization and suppression of the Stoner factor, are missing from
LDA+U. On the other hand, for {\it moderately} correlated metals LDA+U may be
useful. With this in mind, we derive a new version of LDA+U that is consistent
with the Hohenberg-Kohn theorem and can be formulated as a constrained density
functional theory. We illustrate all of the above on concrete examples,
including the controversial case of magnetism in FeAl.Comment: Substantial changes. In particular, examples of application of the
proposed functional are adde
Robust Magnetic Polarons in Type-II (Zn,Mn)Te Quantum Dots
We present evidence of magnetic ordering in type-II (Zn, Mn) Te quantum dots.
This ordering is attributed to the formation of bound magnetic polarons caused
by the exchange interaction between the strongly localized holes and Mn within
the dots. In our photoluminescence studies, the magnetic polarons are detected
at temperatures up to ~ 200 K, with a binding energy of ~ 40 meV. In addition,
these dots display an unusually small Zeeman shift with applied field (2 meV at
10 T). This behavior is explained by a small and weakly temperature-dependent
magnetic susceptibility due to anti-ferromagnetic coupling of the Mn spins
Nonlinear Magneto-Optics of Fe Monolayers from first principles: Structural dependence and spin-orbit coupling strength
We calculate the nonlinear magneto-optical response of free-standing fcc
(001), (110) and (111) oriented Fe monolayers. The bandstructures are
determined from first principles using a full-potential LAPW method with the
additional implementation of spin-orbit coupling. The variation of the
spin-orbit coupling strength and the nonlinear magneto-optical spectra upon
layer orientation are investigated. We find characteristic differences which
indicate an enhanced sensitivity of nonlinear magneto-optics to surface
orientation and variation of the in-plane lattice constants. In particular the
crossover from onedimensional stripe structures to twodimensional films of
(111) layers exhibits a clean signature in the nonlinear Kerr-spectra and
demonstrates the versatility of nonlinear magneto-optics as a tool for in situ
thin-film analysis.Comment: 28 pages, RevTeX, psfig, submitted to PR
- …