346 research outputs found
Domain walls in helical magnets
The structure of domain walls determines to a large extent the properties of
magnetic materials, in particular their hardness and switching behavior, it
represents an essential ingredient of spintronics. Common domain walls are of
Bloch and Neel types in which the magnetization rotates around a fixed axis,
giving rise to a one-dimensional magnetization profile. Domain walls in helical
magnets, most relevant in multiferroics, were never studied systematically.
Here we show that domain walls in helical magnets are fundamentally different
from Bloch and Neel walls. They are generically characterized by a
two-dimensional pattern formed by a regular lattice of vortex singularities. In
conical phases vortices carry Berry phase flux giving rise to the anomalous
Hall effect. In multiferroics vortices are charged, allowing to manipulate
magnetic domain walls by electric fields. Our theory allows the interpretation
of magnetic textures observed in helical magnetic structures
Quantum Zeno effect as a topological phase transition in full counting statistics and spin noise spectroscopy
When the interaction of a quantum system with a detector is changing from
weak to strong coupling limits, the system experiences a transition from the
regime with quantum mechanical coherent oscillations to the regime with a
frozen dynamics. In addition to this quantum Zeno transition, we show that the
full counting statistics of detector signal events experiences a topological
phase transition at the boundary between two phases at intermediate coupling of
a quantum system to the detector. We demonstrate that this transition belongs
to the class of topological phase transitions that can be classified by
elements of the braid group. We predict that this transition can be explored
experimentally by means of the optical spin noise spectroscopy.Comment: 5 pages, 2 figure
Phase Diagram for Magnon Condensate in Yttrium Iron Garnet Film
Recently, magnons, which are quasiparticles describing the collective motion
of spins, were found to undergo Bose-Einstein condensation (BEC) at room
temperature in films of Yttrium Iron Garnet (YIG). Unlike other quasiparticle
BEC systems, this system has a spectrum with two degenerate minima, which makes
it possible for the system to have two condensates in momentum space. Recent
Brillouin Light scattering studies for a microwave-pumped YIG film of thickness
d=5 m and field H=1 kOe find a low-contrast interference pattern at the
characteristic wavevector of the magnon energy minimum. In this report, we
show that this modulation pattern can be quantitatively explained as due to
non-symmetric but coherent Bose-Einstein condensation of magnons into the two
energy minima. Our theory predicts a transition from a high-contrast symmetric
phase to a low-contrast non-symmetric phase on varying the and , and a
new type of collective oscillations.Comment: 6 figures. Accepted by Nature Scientific Report
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