1,367 research outputs found
Topological defects in flat nanomagnets: the magnetostatic limit
We discuss elementary topological defects in soft magnetic nanoparticles in
the thin-film geometry. In the limit dominated by magnetostatic forces the
low-energy defects are vortices (winding number n = +1), cross ties (n = -1),
and edge defects with n = -1/2. We obtain topological constraints on the
possible composition of domain walls. The simplest domain wall in this regime
is composed of two -1/2 edge defects and a vortex, in accordance with
observations and numerics.Comment: 3 pages, eps figures. Proceedings of MMM 0
The structure of Gelfand-Levitan-Marchenko type equations for Delsarte transmutation operators of linear multi-dimensional differential operators and operator pencils. Part 1
An analog of Gelfand-Levitan-Marchenko integral equations for multi-
dimensional Delsarte transmutation operators is constructed by means of
studying their differential-geometric structure based on the classical Lagrange
identity for a formally conjugated pair of differential operators. An extension
of the method for the case of affine pencils of differential operators is
suggested.Comment: 12 page
Broken parity and a chiral ground state in the frustrated magnet CdCr2O4
We present a model describing the lattice distortion and incommensurate
magnetic order in the spinel CdCr2O4, a good realization of the Heisenberg
"pyrochlore" antiferromagnet. The magnetic frustration is relieved through the
spin-Peierls distortion of the lattice involving a phonon doublet with odd
parity. The distortion stablizes a collinear magnetic order with the
propagation wavevector q=2\pi(0,0,1). The lack of inversion symmetry makes the
crystal structure chiral. The handedness is transferred to magnetic order by
the relativistic spin-orbit coupling: the collinear state is twisted into a
long spiral with the spins in the ac plane and q shifted to 2\pi(0,\delta,1).Comment: Incremental changes in response to referee report
Extension of the Poincar\'e Group and Non-Abelian Tensor Gauge Fields
In the recently proposed generalization of the Yang-Mills theory the group of
gauge transformation gets essentially enlarged. This enlargement involves an
elegant mixture of the internal and space-time symmetries. The resulting group
is an extension of the Poincar\'e group with infinitely many generators which
carry internal and space-time indices. This is similar to the super-symmetric
extension of the Poincar\'e group, where instead of an anti-commuting spinor
variable one should introduce a new vector variable. The construction of
irreducible representations of the extended Poincar\'e algebra identifies a
vector variable with the derivative of the Pauli-Lubanski vector over its
length. As a result of this identification the generators of the gauge group
have nonzero components only in the plane transversal to the momentum and are
projecting out non-Abelian tensor gauge fields into the transversal plane,
keeping only their positively definite space-like components.Comment: 21 page
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Generalized Kasha's Model: T-Dependent Spectroscopy Reveals Short-Range Structures of 2D Excitonic Systems
Carrier dynamics and coherent acoustic phonons in nitride heterostructures
We model generation and propagation of coherent acoustic phonons in
piezoelectric InGaN/GaN multi-quantum wells embedded in a \textit{pin} diode
structure and compute the time resolved reflectivity signal in simulated
pump-probe experiments. Carriers are created in the InGaN wells by ultrafast
pumping below the GaN band gap and the dynamics of the photoexcited carriers is
treated in a Boltzmann equation framework. Coherent acoustic phonons are
generated in the quantum well via both deformation potential electron-phonon
and piezoelectric electron-phonon interaction with photogenerated carriers,
with the latter mechanism being the dominant one. Coherent longitudinal
acoustic phonons propagate into the structure at the sound speed modifying the
optical properties and giving rise to a giant oscillatory differential
reflectivity signal. We demonstrate that coherent optical control of the
differential reflectivity can be achieved using a delayed control pulse.Comment: 14 pages, 11 figure
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