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Register a new userTwo-dimensional Dirac nodal loop magnons in collinear antiferromagnets
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Added by
Solomon Akaraka Owerre
Publication date
2018
fields
Physics
and research's language is
English
Authors
S. A. Owerre
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We study the nontrivial linear magnon band crossings in the collinear antiferromagnets on the two-dimensional (2D) CaVO lattice, also realized in some iron-based superconductors such as AFe$_{1.6+x}$Se$_2$ (A = K, Rb, Cs). It is shown that the combination of space-inversion and time-reversal symmetry ($mathcal{PT}$-symmetry) leads to doubly-degenerate eight magnon branches, which cross each other linearly along a one-dimensional loop in the 2D Brillouin zone. We show that the Dirac nodal loops (DNLs) are not present in the collinear ferromagnet on this lattice. Thus, the current 2D antiferromagnetic DNLs are symmetry-protected and they provide a novel platform to search for their analogs in 2D electronic antiferromagnetic systems.
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We study the topological properties of magnon excitations in three-dimensional antiferromagnets, where the ground state configuration is invariant under time-reversal followed by space-inversion ($PT$-symmetry). We prove that Dirac points and nodal lines, the former being the limiting case of the latter, are the generic forms of symmetry-protected band crossings between magnon branches. As a concrete example, we study a Heisenberg spin model for a spin-web compound, Cu$_3$TeO$_6$, and show the presence of the magnon Dirac points assuming a collinear magnetic structure. Upon turning on symmetry-allowed Dzyaloshinsky-Moriya interactions, which introduce a small non-collinearity in the ground state configuration, we find that the Dirac points expand into nodal lines with nontrivial $Z_2$-topological charge, a new type of nodal lines unpredicted in any materials so far.
We investigate the magnetic excitations of elemental gadolinium (Gd) using inelastic neutron scattering, showing that Gd is a Dirac magnon material with nodal lines at $K$ and nodal planes at half integer $ell$. We find an anisotropic intensity winding around the $K$-point Dirac magnon cone, which is interpreted to indicate Berry phase physics. Using linear spin wave theory calculations, we show the nodal lines have non-trivial Berry phases, and topological surface modes. Together, these results indicate a highly nontrivial topology, which is generic to hexagonal close packed ferromagnets. We discuss potential implications for other such systems.
Recent studies revealed that the electric multipole moments of insulators result in fractional electric charges localized to the hinges and corners of the sample. We here explore the magnetic analog of this relation. We show that a collinear antiferromagnet with spin $S$ defined on a $d$-dimensional cubic lattice features fractionally quantized magnetization $M_{text{c}}^z=S/2^d$ at the corners. We find that the quantization is robust even in the presence of gapless excitations originating from the spontaneous formation of the Neel order, although the localization length diverges, suggesting a power-law localization of the corner magnetization. When the spin rotational symmetry about the $z$ axis is explicitly broken, the corner magnetization is no longer sharply quantized. Even in this case, we numerically find that the deviation from the quantized value is negligibly small based on quantum Monte Carlo simulations.
We study periodically driven insulating noncollinear stacked kagome antiferromagnets with a conventional symmetry-protected three-dimensional (3D) in-plane $120^circ$ spin structure, with either positive or negative vector chirality. We show that the symmetry protection of the in-plane $120^circ$ spin structure can be broken in the presence of an off-resonant circularly or linearly polarized electric field propagating parallel to the in-plane $120^circ$ spin structure (say along the $x$ direction). Consequently, topological Floquet Weyl magnon nodes with opposite chirality are photoinduced along the $k_x$ momentum direction. They manifest as the monopoles of the photoinduced Berry curvature. We also show that the system exhibits a photoinduced magnon thermal Hall effect for circularly polarized electric field. Furthermore, we show that the photoinduced chiral spin structure is a canted 3D in-plane $120^circ$ spin structure, which was recently observed in the equilibrium noncollinear antiferromagnetic Weyl semimetals Mn$_3$Snslash Ge. Our result not only paves the way towards the experimental realization of Weyl magnons and photoinduced thermal Hall effects, but also provides a powerful mechanism for manipulating the intrinsic properties of 3D topological antiferromagnets.
Quasi-two dimensional itinerant fermions in the Anti-Ferro-Magnetic (AFM) quantum-critical region of their phase diagram, such as in the Fe-based superconductors or in some of the heavy-fermion compounds, exhibit a resistivity varying linearly with temperature and a contribution to specific heat or thermopower proportional to $T ln T$. It is shown here that a generic model of itinerant AFM can be canonically transformed such that its critical fluctuations around the AFM-vector $Q$ can be obtained from the fluctuations in the long wave-length limit of a dissipative quantum XY model. The fluctuations of the dissipative quantum XY model in 2D have been evaluated recently and in a large regime of parameters, they are determined, not by renormalized spin-fluctuations but by topological excitations. In this regime, the fluctuations are separable in their spatial and temporal dependence and have a dynamical critical exponent $z =infty.$ The time dependence gives $omega/T$-scaling at criticality. The observed resistivity and entropy then follow directly. Several predictions to test the theory are also given.
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