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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.


Quantum zero point energy and the magnetic order.We have chosen the representative parameters in regions I and III with D=0.2J, J′=0.6J in (a) and D=0.05J, J′=0.6J in (c), respectively. (b) The magnetic order in regions I and II with θ=π/2 and the spins pointing along the local . (d) The magnetic order in region III with θ=0 and the spins pointing along the local .
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f2: Quantum zero point energy and the magnetic order.We have chosen the representative parameters in regions I and III with D=0.2J, J′=0.6J in (a) and D=0.05J, J′=0.6J in (c), respectively. (b) The magnetic order in regions I and II with θ=π/2 and the spins pointing along the local . (d) The magnetic order in region III with θ=0 and the spins pointing along the local .

Mentions: where Ecl is the classical ground state energy, and Aμν, Bμν satisfy , and depend on the angular variable θ. Although the classical energy Ecl is independent of θ due to the U(1) degeneracy, the quantum zero point energy ΔE of the spin-wave modes depends on θ, and is given by , where ωμ(k) is the excitation energy of the μ-th spin-wave mode at momentum k and is determined for every classical spin ground state. The minimum of ΔE occurs at θ=π/6+nπ/3 (nπ/3) with in regions I and II (region III). The discrete minima and the corresponding magnetic orders are equivalent under space group symmetry operations. The U(1) degeneracy of the classical ground states is thus broken by quantum fluctuations. This is the well-known phenomenon known as quantum order by disorder19202122. The resulting optimal state is a non-collinear one in which each spin points along its local [112] ([10]) lattice direction in regions I and II (region III), see Fig. 2.


Weyl magnons in breathing pyrochlore antiferromagnets
Quantum zero point energy and the magnetic order.We have chosen the representative parameters in regions I and III with D=0.2J, J′=0.6J in (a) and D=0.05J, J′=0.6J in (c), respectively. (b) The magnetic order in regions I and II with θ=π/2 and the spins pointing along the local . (d) The magnetic order in region III with θ=0 and the spins pointing along the local .
© Copyright Policy - open-access
Related In: Results  -  Collection

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

f2: Quantum zero point energy and the magnetic order.We have chosen the representative parameters in regions I and III with D=0.2J, J′=0.6J in (a) and D=0.05J, J′=0.6J in (c), respectively. (b) The magnetic order in regions I and II with θ=π/2 and the spins pointing along the local . (d) The magnetic order in region III with θ=0 and the spins pointing along the local .
Mentions: where Ecl is the classical ground state energy, and Aμν, Bμν satisfy , and depend on the angular variable θ. Although the classical energy Ecl is independent of θ due to the U(1) degeneracy, the quantum zero point energy ΔE of the spin-wave modes depends on θ, and is given by , where ωμ(k) is the excitation energy of the μ-th spin-wave mode at momentum k and is determined for every classical spin ground state. The minimum of ΔE occurs at θ=π/6+nπ/3 (nπ/3) with in regions I and II (region III). The discrete minima and the corresponding magnetic orders are equivalent under space group symmetry operations. The U(1) degeneracy of the classical ground states is thus broken by quantum fluctuations. This is the well-known phenomenon known as quantum order by disorder19202122. The resulting optimal state is a non-collinear one in which each spin points along its local [112] ([10]) lattice direction in regions I and II (region III), see Fig. 2.

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.