Eccentric fractional skyrmion discovered in numerical simulations of ultra-cold superfluids
Date:
February 16, 2022
Source:
Osaka City University
Summary:
Through numerical simulations, a researcher details the discovery
of a new isolated skyrmion with a half-integer topological quantum
number in the ferromagnetic phase of the magnetic quantum fluid
Bose-Einstein condensate (BEC). The new skyrmion is generated by
applying a spin current to a magnetic domain wall and it has an
eccentric (off-center) spin singularity inside it.
FULL STORY ==========================================================================
A scientist at Osaka City University has discovered skyrmions with
half-integer topological numbers in a ferromagnetic superfluid.
========================================================================== "This may bring about a major change in the long history of research on skyrmions," states Hiromitsu Takeuchi, lecturer at the Graduate School of Science and the Nambu Yoichiro Institute of Theoretical and Experimental Physics (NITEP), Osaka City University, and sole author of the study.
Skyrmions, a type of phase defect that forms when the symmetry of a system
is spontaneously broken in a phase transition, have a dominant effect
on the macroscopic behaviour of the system wherein they occur. Thus, understanding skyrmions has been theorized as fundamental in governing
the physical properties of systems with spontaneous symmetry breaking
(SSB). To explore this, scientists have turned to ultra-cold superfluids
like Bose-Einstein condensate (BEC) as these near absolute-zero atomic
gases are free from the effects that hinder understanding of its
intrinsic properties -- essentially becoming quantum simulators. "Based
on the results of another recent study, I had predicted that an unknown asymmetric topological defect would appear in the ferromagnetic phase
of BEC," states Lecturer Takeuchi, "however I was quite shocked when I
found it was a new type of skyrmion." In a new study published in the
American Physical Society's journal Physical Review A, Lecturer Takeuchi
has shown the generation mechanism of this eccentric fractional skyrmion
to be in contrast to that of conventional skyrmions.
As spin currents are applied to a magnetic domain wall (DW), the
internal structure of a DW undergoes a transition from one phase to
another. Depending on the strength of the spin current, scientists
have observed two types of DWs to form in a BEC, antiferromagnetic
(AF)-core and broken axisymmetry (BA)-core DWs, classified according to
the local magnetization of the wall. The new skyrmion is generated from an instability that can be regarded as the magnetic quantum fluid equivalent
of the Kelvin-Helmholtz instability (KHI) known in fluid dynamics, where
the spin-upward and spin-downward domains are considered as two fluids. In
the KHI mechanism, magnetic skyrmions enclosed in the DW are released from
a BA-core DW. A conventional skyrmion has an integer topological charge, "similar to how an ordinary charged particle has only an integer amount of charge, which is a multiple of its elementary charge," points out Lecturer Takeuchi. "In the BEC system however, numerical simulations on instability
in the BA-core DW indicated the generation of an eccentric skyrmion with
a half- integer quantum number." Until now, the smallest unit of the topological quantum number of an isolated skyrmion has been recognized
as unity, but this study suggests it can be half of that. The author
observed through numerical models that this was due to the spontaneous formation of a spin singularity inside the new skyrmion. These formations
are not favored because it raises the energy of the system, "Yet it
appears this new skyrmion, which is located where magnetism and nematic
order coexist," remarks Lecturer Takeuchi, "plays a role in suppressing
the energy increase caused by these singularities." It is hoped that the vantagepoint acquired from this study will allow the realization of this
new skyrmion in other fields, such as particle physics and spintronics.
========================================================================== Story Source: Materials provided by Osaka_City_University. Note: Content
may be edited for style and length.
========================================================================== Journal Reference:
1. Hiromitsu Takeuchi. Spin-current instability at a magnetic domain
wall in
a ferromagnetic superfluid: A generation mechanism of eccentric
fractional skyrmions. Physical Review A, 2022; 105 (1) DOI:
10.1103/ PhysRevA.105.013328 ==========================================================================
Link to news story:
https://www.sciencedaily.com/releases/2022/02/220216112251.htm
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