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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 breathing pyrochlore and the phase diagram.(a) The breathing pyrochlore. The letter u(d) refers to the up-pointing (down-pointing) tetrahedra and J(J′) indicates the nearest-neighbour exchange couplings on the up-pointing (down-pointing) tetrahedra. (b) The phase diagram. Regions I and II have the same magnetic order and belong to the same phase, but the magnetic excitations of the two regions are topologically distinct. Region III has a different magnetic order. The details of the phase diagram are discussed in the main text.
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f1: The breathing pyrochlore and the phase diagram.(a) The breathing pyrochlore. The letter u(d) refers to the up-pointing (down-pointing) tetrahedra and J(J′) indicates the nearest-neighbour exchange couplings on the up-pointing (down-pointing) tetrahedra. (b) The phase diagram. Regions I and II have the same magnetic order and belong to the same phase, but the magnetic excitations of the two regions are topologically distinct. Region III has a different magnetic order. The details of the phase diagram are discussed in the main text.

Mentions: Since spin-orbit coupling is weak, the interaction between the local moments is primarily where we have supplemented the Heisenberg model with a local spin anisotropy17, which is generically allowed by the D3d point group symmetry at the Cr site. The anisotropic direction is the local [111] direction that points into the center of each tetrahedron and is specified for each sublattice (Methods). Here J and J′ are the exchange couplings between the nearest-neighbour spins on the up-pointing and down-pointing tetrahedra (Fig. 1), respectively. The large and negative Curie–Weiss temperatures of the Cr-based breathing pyrochlores indicate the strong atomic force microscopy interactions, hence we take J>0, J′>0. Because the up-pointing and down-pointing tetrahedra have different sizes, one thus expects J≠J′. In this work, however, we will study this model in a general parameter setting. The atomic force microscopy exchange interactions favour zero total spin on each up-pointing (down-pointing) tetrahedron, that is, . As for the regular pyrochlore lattice18, the classical ground state of the exchange part of the Hamiltonian is extensively degenerate.


Weyl magnons in breathing pyrochlore antiferromagnets
The breathing pyrochlore and the phase diagram.(a) The breathing pyrochlore. The letter u(d) refers to the up-pointing (down-pointing) tetrahedra and J(J′) indicates the nearest-neighbour exchange couplings on the up-pointing (down-pointing) tetrahedra. (b) The phase diagram. Regions I and II have the same magnetic order and belong to the same phase, but the magnetic excitations of the two regions are topologically distinct. Region III has a different magnetic order. The details of the phase diagram are discussed in the main text.
© Copyright Policy - open-access
Related In: Results  -  Collection

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

f1: The breathing pyrochlore and the phase diagram.(a) The breathing pyrochlore. The letter u(d) refers to the up-pointing (down-pointing) tetrahedra and J(J′) indicates the nearest-neighbour exchange couplings on the up-pointing (down-pointing) tetrahedra. (b) The phase diagram. Regions I and II have the same magnetic order and belong to the same phase, but the magnetic excitations of the two regions are topologically distinct. Region III has a different magnetic order. The details of the phase diagram are discussed in the main text.
Mentions: Since spin-orbit coupling is weak, the interaction between the local moments is primarily where we have supplemented the Heisenberg model with a local spin anisotropy17, which is generically allowed by the D3d point group symmetry at the Cr site. The anisotropic direction is the local [111] direction that points into the center of each tetrahedron and is specified for each sublattice (Methods). Here J and J′ are the exchange couplings between the nearest-neighbour spins on the up-pointing and down-pointing tetrahedra (Fig. 1), respectively. The large and negative Curie–Weiss temperatures of the Cr-based breathing pyrochlores indicate the strong atomic force microscopy interactions, hence we take J>0, J′>0. Because the up-pointing and down-pointing tetrahedra have different sizes, one thus expects J≠J′. In this work, however, we will study this model in a general parameter setting. The atomic force microscopy exchange interactions favour zero total spin on each up-pointing (down-pointing) tetrahedron, that is, . As for the regular pyrochlore lattice18, the classical ground state of the exchange part of the Hamiltonian is extensively degenerate.

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.