Tunable Magnon Weyl Points in Ferromagnetic Pyrochlores

Abstract

The dispersion relations of magnons in ferromagnetic pyrochlores with Dzyaloshinskii-Moriya interaction is shown to possess Weyl points, i. e., pairs of topological nontrivial crossings of two magnon branches with opposite topological charge. As a consequence of their topological nature, their projections onto a surface are connected by magnon arcs, thereby resembling closely Fermi arcs of electronic Weyl semimetals. On top of this, the positions of the Weyl points in reciprocal space can be tuned widely by an external magnetic field: rotated within the surface plane, the Weyl points and magnon arcs are rotated as well; tilting the magnetic field out-ofplane shifts the Weyl points toward the center Γ of the surface Brillouin zone. The theory is valid for the class of ferromagnetic pyrochlores, i. e., three-dimensional extensions of topological magnon insulators on kagome lattices. In this Letter, we focus on the (111) surface, identify candidates of established ferromagnetic pyrochlores which apply to the considered spin model, and suggest experiments for the detection of the topological features.

Topological thermal Hall effect in frustrated kagome antiferromagnets

“…This linear band crossing is different from the Goldstone modes of continuous rotational symmetry breaking. The persistence of Dirac points in the presence of SOC (in this case DMI) is the basis of Weyl magnon [15,16] and Dirac semimetal in electronic systems [48]. In this regard, the present model can be deemed a magnon analog of quasi-2D Dirac semimetal.…”

“…This suggests that the DMI is not the primary source of topological spin excitations in frustrated magnets, which sharply differs from collinear magnets [1][2][3][4][5][6][7][8][9][10][11][12][13][14][15][16][17] and triplon excitations [44] in a magnetic field with zero scalar spin chirality. We note that scalar spin chirality plays a crucial role in different areas of physical interest.…”

“…Topological phases of matter are an active research field in condensed matter physics, mostly dominated by electronic systems. Quite recently the concepts of topological matter have been extended to nonelectronic bosonic systems such as quantized spin waves (magnons) [1][2][3][4][5][6][7][8][9][10][11][12][13][14][15][16][17] and quantized lattice vibrations (phonons) [18][19][20][21][22][23]. In the former, spin-orbit coupling manifests in the form of Dzyaloshinsky-Moriya interaction [24,25] and it leads to topological spin excitations and chiral edge modes in collinear ferromagnets [5,6].…”

Topological magnetic excitations on the distorted kagomé antiferromagnets: Applications to volborthite, vesignieite, and edwardsite

“…For quantum magnets with magnetic long-range order the thermal Hall effect has been realized in the collinear kagomé ferromagnet Cu(1-3, bdc) [35] and a number of collinear pyrochlore ferromagnets [36,37]. In this case the transverse thermal Hall conductivity (κ xy ) can be explained in terms of Berry curvature induced by the DM spin-orbit interaction [38][39][40] leading to topological magnons [41][42][43][44][45][46][47][48][49] and Weyl magnons [50,51] similar to spin-orbit coupling electronic systems [52][53][54][55][56]. In a recent experiment [57], a nonzero κ xy has been observed in a frustrated distorted kagomé volborthite at a strong magnetic field of 15 T with no signs of the DM spin-orbit interaction and no discernible thermal Hall signal was observed at zero magnetic field [58].…”

Universal phase transition and band structures for spinless nodal-line and Weyl semimetals

“…Yet the purely spinless TNLSMs and the spinless WSMs have not been discovered experimentally in threedimensional electronic systems. Additionally, the topological nodal-bands are suggested in non-electronic systems which do not have spin-orbit interactions 40,[52][53][54][55][56][57][58] . Therefore, it is important to give a general framework of a phase transition between the spinless topological semimetal phases, as studied in spinful systems 1,8,59 .…”