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Weyl magnons in breathing pyrochlore antiferromagnets

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ABSTRACT

Frustrated quantum magnets not only provide exotic ground states and unusual magnetic structures, but also support unconventional excitations in many cases. Using a physically relevant spin model for a breathing pyrochlore lattice, we discuss the presence of topological linear band crossings of magnons in antiferromagnets. These are the analogues of Weyl fermions in electronic systems, which we dub Weyl magnons. The bulk Weyl magnon implies the presence of chiral magnon surface states forming arcs at finite energy. We argue that such antiferromagnets present a unique example, in which Weyl points can be manipulated in situ in the laboratory by applied fields. We discuss their appearance specifically in the breathing pyrochlore lattice, and give some general discussion of conditions to find Weyl magnons, and how they may be probed experimentally. Our work may inspire a re-examination of the magnetic excitations in many magnetically ordered systems.

No MeSH data available.


The evolution of Weyl nodes under the magnetic field.Applying a magnetic field along the global z direction, , Weyl nodes are shifted but still in kz=0 plane. They are annihilated at Γ when magnetic field is strong enough. Red and blue indicate the opposite chirality. (a,f): B=0, 0.1J, 0.5J, 0.9J, 1.0J, 1.1J. We have set D=0.2J, J′=0.6J and θ=π/2.
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f5: The evolution of Weyl nodes under the magnetic field.Applying a magnetic field along the global z direction, , Weyl nodes are shifted but still in kz=0 plane. They are annihilated at Γ when magnetic field is strong enough. Red and blue indicate the opposite chirality. (a,f): B=0, 0.1J, 0.5J, 0.9J, 1.0J, 1.1J. We have set D=0.2J, J′=0.6J and θ=π/2.

Mentions: When we apply an external magnetic field to the system, the spin only couples to the field via a Zeeman coupling. This is quite different from the case of electronic systems, in which a magnetic field also has an orbital effect, which leads to cyclotron motion of electrons and a transformation from ordinary bands into Landau ones. In the latter case, the meaning of quasi-momentum is irrevocably changed by an applied field, and one cannot follow the Weyl point evolution with field. By contrast, since magnons are neutral, there is no orbital effect, and quasi-momentum and the Weyl points themselves remain well-defined even for strong fields. Therefore, a magnetic field can be used to manipulate the Weyl nodes. To demonstrate this explicitly, we focus on one specific classical order in region I and apply a magnetic field along the global z direction. The magnetic field perturbs the classical ground state and indirectly changes the spin-wave Hamiltonian. As we show in Fig. 5, the Weyl nodes are shifted gradually and finally annihilated when the magnetic field is increased.


Weyl magnons in breathing pyrochlore antiferromagnets
The evolution of Weyl nodes under the magnetic field.Applying a magnetic field along the global z direction, , Weyl nodes are shifted but still in kz=0 plane. They are annihilated at Γ when magnetic field is strong enough. Red and blue indicate the opposite chirality. (a,f): B=0, 0.1J, 0.5J, 0.9J, 1.0J, 1.1J. We have set D=0.2J, J′=0.6J and θ=π/2.
© Copyright Policy - open-access
Related In: Results  -  Collection

License
Show All Figures
getmorefigures.php?uid=PMC5036007&req=5

f5: The evolution of Weyl nodes under the magnetic field.Applying a magnetic field along the global z direction, , Weyl nodes are shifted but still in kz=0 plane. They are annihilated at Γ when magnetic field is strong enough. Red and blue indicate the opposite chirality. (a,f): B=0, 0.1J, 0.5J, 0.9J, 1.0J, 1.1J. We have set D=0.2J, J′=0.6J and θ=π/2.
Mentions: When we apply an external magnetic field to the system, the spin only couples to the field via a Zeeman coupling. This is quite different from the case of electronic systems, in which a magnetic field also has an orbital effect, which leads to cyclotron motion of electrons and a transformation from ordinary bands into Landau ones. In the latter case, the meaning of quasi-momentum is irrevocably changed by an applied field, and one cannot follow the Weyl point evolution with field. By contrast, since magnons are neutral, there is no orbital effect, and quasi-momentum and the Weyl points themselves remain well-defined even for strong fields. Therefore, a magnetic field can be used to manipulate the Weyl nodes. To demonstrate this explicitly, we focus on one specific classical order in region I and apply a magnetic field along the global z direction. The magnetic field perturbs the classical ground state and indirectly changes the spin-wave Hamiltonian. As we show in Fig. 5, the Weyl nodes are shifted gradually and finally annihilated when the magnetic field is increased.

View Article: PubMed Central - PubMed

ABSTRACT

Frustrated quantum magnets not only provide exotic ground states and unusual magnetic structures, but also support unconventional excitations in many cases. Using a physically relevant spin model for a breathing pyrochlore lattice, we discuss the presence of topological linear band crossings of magnons in antiferromagnets. These are the analogues of Weyl fermions in electronic systems, which we dub Weyl magnons. The bulk Weyl magnon implies the presence of chiral magnon surface states forming arcs at finite energy. We argue that such antiferromagnets present a unique example, in which Weyl points can be manipulated in situ in the laboratory by applied fields. We discuss their appearance specifically in the breathing pyrochlore lattice, and give some general discussion of conditions to find Weyl magnons, and how they may be probed experimentally. Our work may inspire a re-examination of the magnetic excitations in many magnetically ordered systems.

No MeSH data available.