488 research outputs found
Magnetic field control of cycloidal domains and electric polarization in multiferroic BiFeO
The magnetic field induced rearrangement of the cycloidal spin structure in
ferroelectric mono-domain single crystals of the room-temperature multiferroic
BiFeO is studied using small-angle neutron scattering (SANS). The cycloid
propagation vectors are observed to rotate when magnetic fields applied
perpendicular to the rhombohedral (polar) axis exceed a pinning threshold value
of 5\,T. In light of these experimental results, a phenomenological model
is proposed that captures the rearrangement of the cycloidal domains, and we
revisit the microscopic origin of the magnetoelectric effect. A new coupling
between the magnetic anisotropy and the polarization is proposed that explains
the recently discovered magnetoelectric polarization to the rhombohedral axis
Unpinning the skyrmion lattice in MnSi: Effect of substitutional disorder
By employing magnetization and small angle neutron scattering measurements, we have investigated the behavior of the skyrmion lattice (SKL) and the helical order in MnS i 0 . 992 G a 0 . 008 Our results indicate that the order of the SKL is sensitive to the orientation of an applied magnetic field with respect to the crystal lattice and to variations in the sequence of small temperature and applied magnetic field changes. The disorder caused by the substitution of the heavier element Ga for Si is sufficient to reduce the pinning of the SKL to the underlying crystalline lattice, reducing the propensity for the SKL to be aligned with the crystal lattice. This tendency is most evident when the applied field is not well oriented with respect to the high symmetry axes of the crystal resulting in disorder in the long range SKL while maintaining sharp short range (radial) order. We have also investigated the effect of substituting heavier elements into MnSi on the reorientation process of the helical domains with field cycling in MnS i 0 . 992 G a 0 . 008 and M n 0 . 985 I r 0 . 015 Si A comparison of the reorientation process in these materials with field reduction indicates that the substitution of heavier elements on either Mn or Si sites creates a higher energy barrier for the reorientation of the helical order and for the formation of domains
Structural Transition Kinetics and Activated Behavior in the Superconducting Vortex Lattice
Using small-angle neutron scattering, we investigated the behavior of a
metastable vortex lattice state in MgB2 as it is driven towards equilibrium by
an AC magnetic field. This shows an activated behavior, where the AC field
amplitude and cycle count are equivalent to, respectively, an effective
"temperature" and "time". The activation barrier increases as the metastable
state is suppressed, corresponding to an aging of the vortex lattice.
Furthermore, we find a cross-over from a partial to a complete suppression of
metastable domains depending on the AC field amplitude, which may empirically
be described by a single free parameter. This represents a novel kind of
collective vortex behavior, most likely governed by the nucleation and growth
of equilibrium vortex lattice domains.Comment: 5 pages plus 3 pages of supplemental materia
A Hybrid Lagrangian Variation Method for Bose-Einstein Condensates in Optical Lattices
Solving the Gross--Pitaevskii (GP) equation describing a Bose--Einstein
condensate (BEC) immersed in an optical lattice potential can be a numerically
demanding task. We present a variational technique for providing fast, accurate
solutions of the GP equation for systems where the external potential exhibits
rapid varation along one spatial direction. Examples of such systems include a
BEC subjected to a one--dimensional optical lattice or a Bragg pulse. This
variational method is a hybrid form of the Lagrangian Variational Method for
the GP equation in which a hybrid trial wavefunction assumes a gaussian form in
two coordinates while being totally unspecified in the third coordinate. The
resulting equations of motion consist of a quasi--one--dimensional GP equation
coupled to ordinary differential equations for the widths of the transverse
gaussians. We use this method to investigate how an optical lattice can be used
to move a condensate non--adiabatically.Comment: 16 pages and 1 figur
Complete loss of heparin-releasable triacylglycerol lipase activity after collagenase treatment of the rat liver
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