Topological spin excitations in a three-dimensional antiferromagnet

Abstract

Band topology, or global wave-function structure that enforces novel properties in the bulk and on the surface of crystalline materials, is currently under intense investigations for both fundamental interest and its technological promises [1-4]. While band crossing of non-trivial topological nature was first studied in three dimensions for electrons [4-10], the underlying physical idea is not restricted to fermionic excitations [11-15]. In fact, experiments have confirmed the possibility to have topological band crossing of electromagnetic waves in artificial structures [16]. Fundamental bosonic excitations in real crystals, however, have not been observed to exhibit the counterpart under ambient pressure and magnetic field, where the difficulty is in part because natural materials cannot be precisely engineered like artificial structures. Here, we use inelastic neutron scattering to reveal the presence of topological spin excitations (magnons) in a three-dimensional antiferromagnet, Cu3TeO6, which features a unique lattice of magnetic spin-1/2 Cu 2+ ions [17]. Beyond previous understanding [17,18], we find that the material's spin lattice possesses a variety of exchange interactions, with the interaction between the ninth-nearest neighbours being as strong as that between the nearest neighbours. Although theoretical analysis indicates that the presence of topological magnon band crossing is independent of model details [15], Cu3TeO6 turns out to be highly favourable for the experimental observation, as its optical magnons are spectrally sharp and intense due

Synthesis and Structural Determination of the Disordered Bixbyite Cu3‐xSb1+xO5.5+3x/2 with Spin‐Glass Behaviour

“…Studies concluded that this exchange interaction must be significantly weaker than the nearest neighbour exchange and should only produce a slight tilting of the spin orientation . Inelastic neutron scattering studies showed that super‐super exchange (Cu‐O‐O‐Cu) through the 9 th nearest neighbour contributed significantly to the stability of the magnetic structure with an exchange energy comparable to the first nearest neighbour exchange . It was postulated that if the spin tilt was not present, the unit cell would distort away from the cubic system due to the resultant magneto‐elastic strain .…”

“…Two FM‐coupled layers are out of plane from each other, magnetic moments of this given AFM domain are aligned perpendicular to the plane. Magnetic moment vectors shown are arbitrary in size …”

“…As of the last decade, the copper tellurate Cu 3 TeO 6 with a bixbyite structure a= 9.537 Å has been the focus of intense studies due to its unique magnetic spin‐web structure driven by the ordering of spin of 3d 9 copper and the proof of topological magnons for the first time in a 3D antiferromagnet . The nearest neighbour copper ions (≈3.18 Å) form a 3D network of 8 corner sharing hexagons centered around Te 6+ in the lattice.…”

Dirac Magnons in a Honeycomb Lattice Quantum XY Magnet CoTiO3

“…Remarkably, the concept of Dirac particles is not limited to electrons or other fermionic quasiparticles, prompting a search for analogues in photonic crystals [7,8], acoustic metamaterials [9], and quantum magnets [10][11][12][13]. In particular, Dirac magnons, or more broadly defined topological magnons [14][15][16][17][18][19], have attracted much attention as platforms to investigate the effect of inter-particle interaction or external perturbations on Dirac bosons, and are proposed to be of potential interest in spintronic applications.In contrast to light and sound, the symmetry broken states and emergent bosonic excitations of quantum magnets depend crucially on dimensionality and spin symmetry, which provides a fertile playground for examining the physics of topological bosons. To date, gapped topological magnons in Ising-like ferromagnets have been reported in a kagome lattice material Cu(1,3bdc) [16] and in a layered honeycomb magnet CrI 3 [17].…”

Discovering Topological Surface States of Dirac Points

“…Playing a central role in 3D band topology, Dirac points can, upon symmetry breaking, transition into Weyl points, line nodes or topological bandgaps with gapless surface states. Although 3D Dirac points have been experimentally discovered in electron [2][3][4][5][6][7], magnon [8,9] and photonic [10] systems along with a variety of other theoretical proposals [11][12][13][14][15][16][17][18][19][20][21][22][23], none of the surface states are topological. Specifically, there have been no robust gapless surface bands associated with the bulk Dirac points [24][25][26].…”