Antiferromagnetic Kitaev interaction inJ _eff = 1/2 cobalt honeycomb materials Na₃Co₂SbO₆ and Na₂Co₂TeO₆

Abstract

Finding new materials with antiferromagnetic (AFM) Kitaev interaction is an urgent issue for quantum magnetism research. We conclude that Na3Co2SbO6 and Na2Co2TeO6 are new honeycomb cobalt-based systems with AFM Kitaev interaction by carrying out inelastic neutron scattering experiments and subsequent analysis. The spin–orbit excitons observed at 20–28 meV in both compounds strongly support the idea that Co2+ ions of both compounds have a spin-orbital entangled J eff = 1/2 state. Furthermore, we found that a generalized Kitaev–Heisenberg Hamiltonian can describe the spin-wave excitations of both compounds with additional 3rd nearest-neighbor interaction. Our best-fit parameters show significant AFM Kitaev terms and off-diagonal symmetric anisotropy terms of a similar magnitude in both compounds. We also found a strong magnon-damping effect at the higher energy part of the spin waves, entirely consistent with observations in other Kitaev magnets. Our work suggests Na3Co2SbO6 and Na2Co2TeO6 as rare examples of the AFM Kitaev magnets based on the systematic studies of the spin waves and analysis.

Excitations in the ordered and paramagnetic states of honeycomb magnet Na2Co2TeO6

“…Figure 2(a) presents the lowest-energy spin wave branch along high-symmetric lines in the 2D Brillouin zone. This branch reaches its energy bottom (∼1 meV) at the M-point, consistent with previous INS results [45,47,[49][50][51]. However, a closer look along (H, 0) [Fig.…”

“…Once averaged over sample orientations (Fig. S5 in [54]), our data are fully consistent with powder INS spectra [45,47,[49][50][51], including having a concave E-Q envelope shape at small Q near the M-point, which has been taken as a key indication for zigzag order in α-RuCl 3 and Na 2 IrO 3 [6,14,63]. We believe that further theoretical work is needed to coherently account for the elusive magnetic ground state, the multiple thermal [44,48] and field-induced transitions [44,45], and the spin waves in Na 2 Co 2 TeO 6 .…”

“…Local-moment models usually have prominent nearestneighbor interactions, as has also been inferred from pow- der INS data [45,47,[49][50][51]. Surprisingly, the lowestenergy spin waves can be adequately described by an effective model with only third-nearest-neighbor AFM coupling (J 3 ) and gap-opening anisotropy (∆) terms:…”

“…Their thermal transport properties are also similar to α-RuCl 3 [42,46]. With the promising properties, Na 2 Co 2 TeO 6 has recently been intensively studied [44,45,[47][48][49][50][51]. A widely recognized goal is to establish the magnetic interaction model with inelastic neutron scattering (INS) [45,47,[49][50][51], yet most of the experiments so far were performed on powder samples and pointed to diversifying sets of parameters.…”

“…With the promising properties, Na 2 Co 2 TeO 6 has recently been intensively studied [44,45,[47][48][49][50][51]. A widely recognized goal is to establish the magnetic interaction model with inelastic neutron scattering (INS) [45,47,[49][50][51], yet most of the experiments so far were performed on powder samples and pointed to diversifying sets of parameters. In this work, we report extensive INS data taken on high-quality single crystals, which enable us to map out magnetic excitations over the two-dimensional (2D) Brillouin zone and study their temperature dependence in conjunction with thermodynamics.…”