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Electron-skyrmion systems, in and out of equilibrium, and isolated or contacted to reservoirs

Östberg, Emil Adam Jonatan LU (2023) FYSM60 20231
Mathematical Physics
Department of Physics
Abstract
A Kondo lattice skyrmion model in contact with a macroscopic environment is simulated to explore skyrmion dynamics, which is an extension of previous work. The system is simulated using non-equilibrium Green's functions within the generalized Kadanoff-Baym ansatz and the wide band limit. To get a time-linear scaling, we present an approximation to the collision integral, which offers a good trade-off between computation time and accuracy. We find a new regime of skyrmion dynamics when the current from the macroscopic environment is sufficiently high, and the effect of impurities is explored. Furthermore, an analysis of the itinerant conduction electrons shows that a nontrivial topology is present when a skyrmion is present.
Popular Abstract
In the last two decades, a new quasiparticle was found experimentally, called a magnetic skyrmion. A quasiparticle is not a fundamental particle but rather an object which could be described as a particle since it has particle-like properties. Much like one would consider basketball a single entity, but it is really made up of many atoms. Nevertheless, a skyrmion is much smaller, typically on the order of tens of nanometers.

A skyrmion is a stable quasiparticle where the magnetic moments in atoms align in a vortex-like structure, as shown in the figure. The magnetic moment is an intrinsic property that all atoms have, and it can be thought of as an arrow which can point in any direction in three-dimensional space. It is the vortex-like... (More)
In the last two decades, a new quasiparticle was found experimentally, called a magnetic skyrmion. A quasiparticle is not a fundamental particle but rather an object which could be described as a particle since it has particle-like properties. Much like one would consider basketball a single entity, but it is really made up of many atoms. Nevertheless, a skyrmion is much smaller, typically on the order of tens of nanometers.

A skyrmion is a stable quasiparticle where the magnetic moments in atoms align in a vortex-like structure, as shown in the figure. The magnetic moment is an intrinsic property that all atoms have, and it can be thought of as an arrow which can point in any direction in three-dimensional space. It is the vortex-like alignment of the magnetic moments which we call a skyrmion.

Consider a coffee mug; if it was made out of play dough, one could deform it into a donut without removing the hole or adding a hole. If we instead consider a ball made out of play dough, then one cannot deform it into a donut without creating a hole. Similarly, in our case, the skyrmion is stabilized by topology, for example, one cannot simply force the skyrmion into a state where everything is just red (see figure) without breaking the topology.

Notice that we have a grid in the figure, which we call a lattice, and we will use a lattice model to simulate the skyrmion. A lattice model is a model that lives on a discrete set of points: instead of simulating a continuous space, one can restrict the simulation to discrete lattice sites. This is motivated by the physical scenario that one is faced with in a solid, where atoms are bound together to form a periodic structure. The discrete set of points then corresponds to the orbits of the atoms, where electrons can reside. Lattice models are used extensively in theoretical physics.

We use a much more complicated lattice model than what is common in the community to capture additional physics. The model which we use contains a further important aspect. Namely, it contains free electrons. In the compounds which can facilitate skyrmions, free electrons are typically present. These have previously been shown to have a significant effect, but they should also be addressed in the context of skyrmions.

Using such a model, we study the movement of the skyrmion when an electric current is applied. In previous studies, people have found that the velocity is proportional to the current; this is not always the case in our model. Due to our model also including free electrons, one could argue it is close to reality, and thus, our prediction could be experimentally tested. Additionally, we have found that the included free electrons behave in a topologically out-of-the-ordinary fashion. Furthermore, using our model the behaviour of the free electrons have also been characterized. (Less)
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author
Östberg, Emil Adam Jonatan LU
supervisor
organization
course
FYSM60 20231
year
type
H1 - Master's Degree (One Year)
subject
keywords
skyrmion, skyrmions, NEGF, freen's function, quantum transport, non-equlibrium, topology, topological charge, skyrmionics, current induced, open system
language
English
id
9127303
date added to LUP
2023-06-27 11:50:12
date last changed
2023-06-27 11:50:12
@misc{9127303,
  abstract     = {{A Kondo lattice skyrmion model in contact with a macroscopic environment is simulated to explore skyrmion dynamics, which is an extension of previous work. The system is simulated using non-equilibrium Green's functions within the generalized Kadanoff-Baym ansatz and the wide band limit. To get a time-linear scaling, we present an approximation to the collision integral, which offers a good trade-off between computation time and accuracy. We find a new regime of skyrmion dynamics when the current from the macroscopic environment is sufficiently high, and the effect of impurities is explored. Furthermore, an analysis of the itinerant conduction electrons shows that a nontrivial topology is present when a skyrmion is present.}},
  author       = {{Östberg, Emil Adam Jonatan}},
  language     = {{eng}},
  note         = {{Student Paper}},
  title        = {{Electron-skyrmion systems, in and out of equilibrium, and isolated or contacted to reservoirs}},
  year         = {{2023}},
}