136 research outputs found
Nematic Bond Theory of Heisenberg Helimagnets
We study classical two-dimensional frustrated Heisenberg models with
generically incommensurate groundstates. A new theory for the spin-nematic
"order by disorder" transition is developed based on the self-consistent
determination of the effective exchange coupling bonds. In our approach,
fluctuations of the constraint field imposing conservation of the local
magnetic moment drive nematicity at low temperatures. The critical temperature
is found to be highly sensitive to the peak helimagnetic wavevector, and
vanishes continuously when approaching rotation symmetric Lifshitz points.
Transitions between symmetry distinct nematic orders may occur by tuning the
exchange parameters, leading to lines of bicritical points.Comment: 4 pages, 4 figure
Interplay between Magnetic and Vestigial Nematic Orders in the Layered - Classical Heisenberg Model
We study the layered - classical Heisenberg model on the square
lattice using a self-consistent bond theory. We derive the phase diagram for
fixed as a function of temperature , and interplane coupling
. Broad regions of (anti)ferromagnetic and stripe order are found, and are
separated by a first-order transition near (in units of
). Within the stripe phase the magnetic and vestigial nematic
transitions occur simultaneously in first-order fashion for strong . For
weaker there is in addition, for , an intermediate
regime of split transitions implying a finite temperature region with nematic
order but no long-range stripe magnetic order. In this split regime, the order
of the transitions depends sensitively on the deviation from and
, with split second-order transitions predominating for . We find that the value of depends weakly on the
interplane coupling and is just slightly larger than for . In contrast the value of increases quickly from at
as the interplane coupling is further reduced. In
addition, the magnetic correlation length is shown to directly depend on the
nematic order parameter and thus exhibits a sharp increase (or jump) upon
entering the nematic phase. Our results are broadly consistent with predictions
based on itinerant electron models of the iron-based superconductors in the
normal-state, and thus help substantiate a classical spin framework for
providing a phenomenological description of their magnetic properties.Comment: 13 pages, 20 figure
Comment on "Kinetic theory for a mobile impurity in a degenerate Tonks-Girardeau gas"
In a recent paper, arxiv:1402.6362, Gamayun, Lychkovskiy, and Cheianov
studied the dynamics of a mobile impurity embedded into a one-dimensional
Tonks-Girardeau gas of strongly interacting bosons. Employing the Boltzmann
equation approach, they arrived at the following main conclusions: (i) a light
impurity, being accelerated by a constant force does not exhibit Bloch
oscillations; (ii) a heavy impurity does undergo Bloch oscillations,
accompanied by a drift with the velocity proportional to the square root of
force. In this comment we argue that the result (i) is an artifact of the
classical Boltzmann approximation, which misses the formation of the (quasi)
bound-state between the impurity and a hole. Result (ii), while not valid at
asymptotically small force, indeed reflects an interesting intermediate-force
behavior. Here we clarify its limits of applicability and extend beyond the
Tonks-Girardeau limit.Comment: 2 pages, 1 figur
Composite Topological Excitations in Ferromagnet-Superconductor Heterostructures
We investigate the formation of a new type of composite topological
excitation -- the skyrmion-vortex pair (SVP) -- in hybrid systems consisting of
coupled ferromagnetic and superconducting layers. Spin-orbit interaction in the
superconductor mediates a magnetoelectric coupling between the vortex and the
skyrmion, with a sign (attractive or repulsive) that depends on the topological
indices of the constituents. We determine the conditions under which a bound
SVP is formed, and characterize the range and depth of the effective binding
potential through analytical estimates and numerical simulations. Furthermore,
we develop a semiclassical description of the coupled skyrmion-vortex dynamics
and discuss how SVPs can be controlled by applied spin currents.Comment: Final version accepted by Physical Review Letters; 9 pages, 5 figure
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