Entropic effects counteract the topological protection of magnetic skyrmions, leading to faster decay rates than expected.
The chiral magnet Cu2OSeO3 hosts a skyrmion lattice, that may be equivalently described as a superposition of plane waves or lattice of particle-like topological objects. A thermal gradient may break up the skyrmion lattice and induce rotating domains raising the question which of these scenarios better describes the violent dynamics at the domain boundaries. Here we show that in an inhomogeneous temperature gradient caused by illumination in a Lorentz Transmission Electron Microscope different parts of the skyrmion lattice can be set into motion with different angular velocities. Tracking the time dependence we show that the constant rearrangement of domain walls is governed by dynamic 5-7 defects arranging into lines. An analysis of the associated defect density is described by Frank's equation and agrees well with classical 2D-Monte Carlo simulations. Fluctuations of boundaries show surge-like rearrangement of skyrmion clusters driven by defect rearrangement consistent with simulations treating skyrmions as point particles. Our findings underline the particle character of the skyrmion.In the last decade a non collinear topological spin texture, the skyrmion, has attracted great attention representing a new type of topological soliton in magnetic materials. Skyrmion lattices are periodic arrangements of a kind of magnetic whirls that may be found in a great variety of chiral magnets [1][2][3][4][5][6][7][8][9][10][11][12], as well as thin magnetic (multi-) layers [13][14][15][16]. The topology of skyrmions is encoded in a quantized winding number of the spin orientation. Emergent magnetic and electric fields describe the efficient coupling of the topological spin texture to electrons and magnons [17][18][19][20]. Skyrmion lattices may be described by two distinct approaches. In a wave-like picture as suggested e.g. by Small-Angle-Neutron Scattering (SANS) measurements, the skyrmionic crystal may be accounted for by a superposition of three spin helices with their propagation vector rotated by 120• with respect to each other. From this point of view, the individual (solitonic) character of single skyrmions vanishes in the collective. Note that in most materials in the skyrmion lattice phase, higher order scattering is basically absent; for MnSi it is of the order of 10 −4 suggesting a rather smooth spin texture [21]. In contrast, in a particle-like picture skyrmions are viewed as individual solitonic particles. Indeed, individual skyrmions have been observed early on [6, 13] but the non-linear character as well as the degree of the particlecharacter in the skyrmion phase remained unresolved. In fact, recent studies reveal strong deformation of the precise shape of skyrmions under large strain [22].The validity of either approach may be tested critically in studies of imperfect skyrmion lattices, alluding to similarities with well-known atomic lattices which also allows to verify particle conservation. In fact, the existence of defects and domains in skyrmion lattices has been reported [23][24][25][26], however,...
The polarization fields in wurtzite group III-nitrides strongly influence the optical properties of InAlGaN-based light emitters, e.g., the electron and hole wave function overlap in quantum wells. In this paper, we propose a new approach to determine these fields by capacitance-voltage measurements (CVM). Sheet charges generated by a change of the microscopic polarization at heterointerfaces influence the charge distribution in PIN junctions and therefore the depletion width and the capacitance. We show that it is possible to determine the strength and direction of the internal fields by comparing the depletion widths of two PIN junctions, one influenced by internal polarization fields and one without as a reference. For comparison, we conducted coupled Poisson/carrier transport simulations on the CVM of the polarization-influenced sample. We also demonstrate the feasibility and limits of the method by determining the fields in GaN/InGaN and GaN/AlGaN double heterostructures on (0001) c-plane grown by metal organic vapor phase epitaxy and compare both evaluation methods. The method yields (−0.50 ± 0.07) MV/cm for In0.08Ga0.92N/GaN, (0.90 ± 0.13) MV/cm for Al0.18Ga0.82N/GaN, and (2.0 ± 0.3) MV/cm for Al0.31Ga0.69N/GaN heterostructures.
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