268 research outputs found
Phenomenology of current-skyrmion interactions in thin films with perpendicular magnetic anisotropy
We study skyrmions in magnetic thin films with structural inversion asymmetry
perpendicular to the film plane. We determine the magnetization texture of a
single skyrmion and its dependence on the strength of the Dzyaloshinskii-Moriya
interaction relative to the magnetostatic energy. Furthermore, we construct a
phenomenological model that describes the interaction between the motion of
skyrmions and electric currents to lowest order in spin-orbit coupling. We
estimate the experimental verifiable velocities for current-driven motion of
skyrmion textures based on available results obtained from domain walls
dynamics
Roughness correction to the Casimir force at short separations: Contact distance and extreme value statistics
So far there has been no reliable method to calculate the Casimir force at
separations comparable to the root-mean-square of the height fluctuations of
the surfaces. Statistical analysis of rough gold samples has revealed the
presence of peaks considerably higher than the root-mean-square roughness.
These peaks redefine the minimum separation distance between the bodies and can
be described by extreme value statistics. Here we show that the contribution of
the high peaks to the Casimir force can be calculated with a pairwise additive
summation, while the contribution of asperities with normal height can be
evaluated perturbatively. This method provides a reliable estimate of the
Casimir force at short distances, and it solves the significant, so far
unexplained discrepancy between measurements of the Casimir force between rough
surfaces and the results of perturbation theory. Furthermore, we illustrate the
importance of our results in a technologically relevant situation.Comment: 29 pages, 11 figures, to appear in Phys. Rev.
Signature of Anomalous Exciton Localization in the Optical Response of Self-Assembled Organic Nanotubes
We show that the disorder scaling of the low-temperature optical absorption linewidth of tubular molecular assemblies sharply contrasts with that known for one-dimensional aggregates. The difference can be explained by an anomalous localization of excitons, which arises from the combination of long-range intermolecular interactions and the tube's higher-dimensional geometry. As a result, the exciton density of states near the band bottom drops to zero, leading to a strong suppression of exciton localization. Our results explain the strong linear dichroism and weak exciton-exciton scattering in tubular J aggregates observed in experiments and suggest that for nanoscale wirelike applications a tubular shape is to be preferred over a truly one-dimensional chain
Temperature dependent fluorescence in disordered Frenkel chains: interplay of equilibration and local band-edge level structure
We model the optical dynamics in linear Frenkel exciton systems governed by
scattering on static disorder and lattice vibrations, and calculate the
temperature dependent fluorescence spectrum and lifetime. The fluorescence
Stokes shift shows a nonmonotonic behavior with temperature, which derives from
the interplay of the local band-edge level structure and thermal equilibration.
The model yields excellent fits to experiments performed on linear dye
aggregates.Comment: 4 pages, 3 figure
Superradiance of low density Frenkel excitons in a crystal slab of three-level atoms: Quantum interference effect
We systematically study the fluorescence of low density Frenkel excitons in a
crystal slab containing V-type three-level atoms. Based on symmetric
quasi-spin realization of SU(3) in large limit, the two-mode exciton
operators are invoked to depict various collective excitations of the
collection of these V-type atoms starting from their ground state. By making
use of the rotating wave approximation, the light intensity of radiation for
the single lattice layer is investigated in detail. As a quantum coherence
effect, the quantum beat phenomenon is discussed in detail for different
initial excitonic states. We also test the above results analytically without
the consideration of the rotating wave approximation and the self-interaction
of radiance field is also included.Comment: 18pages, 17 figures. Resubmit to Phys. Rev.
Drastic effects of damping mechanisms on the third-order optical nonlinearity
We have investigated the optical response of superradiant atoms, which
undergoes three different damping mechanisms: radiative dissipation
(), dephasing (), and nonradiative dissipation
(). Whereas the roles of and are equivalent in
the linear susceptibility, the third-order nonlinear susceptibility drastically
depends on the ratio of and : When , the third-order susceptibility is essentially that of a single atom.
Contrarily, in the opposite case of , the third-order
susceptibility suffers the size-enhancement effect and becomes proportional to
the system size.Comment: 5pages, 2figure
Critical temperature and density of spin-flips in the anisotropic random field Ising model
We present analytical results for the strongly anisotropic random field Ising
model, consisting of weakly interacting spin chains. We combine the mean-field
treatment of interchain interactions with an analytical calculation of the
average chain free energy (``chain mean-field'' approach). The free energy is
found using a mapping on a Brownian motion model. We calculate the order
parameter and give expressions for the critical random magnetic field strength
below which the ground state exhibits long range order and for the critical
temperature as a function of the random magnetic field strength. In the limit
of vanishing interchain interactions, we obtain corrections to the
zero-temperature estimate by Imry and Ma [Phys. Rev. Lett. 35, 1399 (1975)] of
the ground state density of domain walls (spin-flips) in the one-dimensional
random field Ising model. One of the problems to which our model has direct
relevance is the lattice dimerization in disordered quasi-one-dimensional
Peierls materials, such as the conjugated polymer trans-polyacetylene.Comment: 28 pages, revtex, 4 postscript figures, to appear in Phys. Rev.
Density of Neutral Solitons in Weakly Disordered Peierls Chains
We study the effects of weak off-diagonal disorder on Peierls systems with a
doubly degenerate ground state. We show that for these systems disorder in the
electron hopping amplitudes induces a finite density of solitons in the
minimal-energy lattice configuration of a single chain. These disorder-induced
dimerization kinks are neutral and have spin 1/2. Using a continuum model for
the Peierls chain and treating the lattice classically, we analytically
calculate the average free energy and density of kinks. We compare these
results to numerical calculations for a discrete model and discuss the
implications of the kinks for the optical and magnetic properties of the
conjugated polymer trans-polyacetylene.Comment: 28 pages, revtex, 5 Postscript figures, to appear in Phys. Rev.
Spontaneous emission and level shifts in absorbing disordered dielectrics and dense atomic gases: A Green's function approach
Spontaneous emission and Lamb shift of atoms in absorbing dielectrics are
discussed. A Green's-function approach is used based on the multipolar
interaction Hamiltonian of a collection of atomic dipoles with the quantised
radiation field. The rate of decay and level shifts are determined by the
retarded Green's-function of the interacting electric displacement field, which
is calculated from a Dyson equation describing multiple scattering. The
positions of the atomic dipoles forming the dielectrics are assumed to be
uncorrelated and a continuum approximation is used. The associated unphysical
interactions between different atoms at the same location is eliminated by
removing the point-interaction term from the free-space Green's-function (local
field correction). For the case of an atom in a purely dispersive medium the
spontaneous emission rate is altered by the well-known Lorentz local-field
factor. In the presence of absorption a result different from previously
suggested expressions is found and nearest-neighbour interactions are shown to
be important.Comment: 6 pages no figure
Decay rate and renormalized frequency shift of a quantum wire Wannier exciton in a planar microcavity
The superradiant decay rate and frequency shift of a Wannier exciton in a
one-dimensional quantum wire are studied. It is shown that the dark mode
exciton can be examined experimentally when the quantum wire is embedded in a
planar microcavity. It is also found that the decay rate is greatly enhanced as
the cavity length is equal to the multiple wavelength of the emitted
photon. Similar to its decay rate counterpart, the frequency shift also shows
discontinuities at resonant modes.Comment: 12 pages, 2 figures. To appear in P. R. B. September 200
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