14,590 research outputs found
Magnetoelectric properties of [FeCl(HO)] with K, Rb, Cs
The compounds [FeCl(HO)] with K, Rb, Cs are identified as
new linear magnetoelectric materials. We present a detailed investigation of
their linear magnetoelectric properties by measurements of pyroelectric
currents, dielectric constants and magnetization. The anisotropy of the linear
magnetoelectric effect of the K-based and Rb-based compound is consistent with
the magnetic point group , already reported in literature. A symmetry
analysis of the magnetoelectric effect of the Cs-based compound allows to
determine the magnetic point group and to develop a model for its
magnetic structure. In addition, magnetic-field versus temperature phase
diagrams are derived and compared to the closely related multiferroic
(NH)[FeCl(HO)].Comment: 17 pages, 10 figures (updated to the weakly revised version that has
been accepted for publication
A simple closure approximation for slow dynamics of a multiscale system: nonlinear and multiplicative coupling
Multiscale dynamics are ubiquitous in applications of modern science. Because
of time scale separation between relatively small set of slowly evolving
variables and (typically) much larger set of rapidly changing variables, direct
numerical simulations of such systems often require relatively small time
discretization step to resolve fast dynamics, which, in turn, increases
computational expense. As a result, it became a popular approach in
applications to develop a closed approximate model for slow variables alone,
which both effectively reduces the dimension of the phase space of dynamics, as
well as allows for a longer time discretization step. In this work we develop a
new method for approximate reduced model, based on the linear
fluctuation-dissipation theorem applied to statistical states of the fast
variables. The method is suitable for situations with quadratically nonlinear
and multiplicative coupling. We show that, with complex quadratically nonlinear
and multiplicative coupling in both slow and fast variables, this method
produces comparable statistics to what is exhibited by an original multiscale
model. In contrast, it is observed that the results from the simplified closed
model with a constant coupling term parameterization are consistently less
precise
New structures in the proton-antiproton system
In the most recent measurements of the reaction
by the BABAR collaboration, new structures have been found with unknown origin.
We examine a possible relation of the most distinct peak to the recently
observed . Alternatively, we analyse possible explanations due to
the nucleon and thresholds. The latter
could explain a periodicity found in the data
Magnetoelastic Coupling in the Spin-Dimer System TlCuCl
We present high-resolution measurements of the thermal expansion and the
magnetostriction of TlCuCl which shows field-induced antiferromagnetic
order. We find pronounced anomalies in the field and temperature dependence of
different directions of the lattice signaling a large magnetoelastic coupling.
The phase boundary is extremely sensitive to pressure, e.g. the transition
field would change by about +/- 185$%/GPa under uniaxial pressure applied along
certain directions. This drastic effect can unambiguously be traced back to
changes of the intradimer coupling under uniaxial pressure. The interdimer
couplings remain essentially unchanged under pressure, but strongly change when
Tl is replaced by K.Comment: 4 pages with 4 figures include
Thermal expansion of the spin-1/2 Heisenberg-chain compound Cu(CHN)(NO)
Compounds containing magnetic subsystems representing simple model spin
systems with weak magnetic coupling constants are ideal candidates to test
theoretical predictions for the generic behavior close to quantum phase
transitions. We present measurements of the thermal expansion and
magnetostriction of the spin-1/2-chain compound copper pyrazine dinitrate
Cu(CHN)(NO). Of particular interest is the low-temperature
thermal expansion close to the saturation field ,
which defines a quantum phase transition from the gapless Luttinger liquid
state to the fully saturated state with a finite excitation gap. We observe a
sign change of the thermal expansion for the different ground states, and at
the quantum critical point the low-temperature expansion approaches a
divergence. Thus, our data agree very well with the expected
quantum critical behaviour.Comment: 4 pages, 3 figures; to appear in the proceedings of the ICM 09 held
in Karlsruhe, German
Anisotropy study of multiferroicity in the pyroxene NaFeGeO
We present a study of the anisotropy of the dielectric, magnetic and
magnetoelastic properties of the multiferroic clinopyroxene NaFeGeO.
Pyroelectric currents, dielectric constants and magnetic susceptibilities as
well as the thermal expansion and the magnetostriction were examined on large
synthetic single crystals of NaFeGeO. The spontaneous electric
polarization detected below K in an
antiferromagnetically ordered state ( K) is mainly lying
within the plane with a small component along , indicating a triclinic
symmetry of the multiferroic phase of NaFeGeO. The electric
polarization can be strongly modified by applying magnetic fields along
different directions. We derive detailed magnetic-field versus temperature
phase diagrams and identify three multiferroic low-temperature phases, which
are separated by a non-ferroelectric, antiferromagnetically ordered state from
the paramagnetic high-temperature phase.Comment: 14 pages, 8 figures. (minor modifications and corrections of the
text
On propagation failure in 1 and 2 dimensional excitable media
We present a non-perturbative technique to study pulse dynamics in excitable
media. The method is used to study propagation failure in one-dimensional and
two-dimensional excitable media. In one-dimensional media we describe the
behaviour of pulses and wave trains near the saddle node bifurcation, where
propagation fails. The generalization of our method to two dimensions captures
the point where a broken front (or finger) starts to retract. We obtain
approximate expressions for the pulse shape, pulse velocity and scaling
behavior. The results are compared with numerical simulations and show good
agreement.Comment: accepted for publication in Chao
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