Electronic and magnetic phase diagrams of Kitaev quantum spin liquid candidate Na₂Co₂TeO₆

We present a comprehensive temperature-magnetic field phase diagrams of the Kitaev spin liquid candidate Na₂Co₂TeO₆ (NCTO) up to 14 T along both a- and a*-axes constructed from the DC/AC magnetization, specific heat, dielectric constant, magnetostriction, and magnetocaloric effect measurements. Three successive field-induced phases are observed below the antiferromagnetic phase transition. Magnetization data up to 60 T revealed no additional transitions at higher fields. Hence, we argue that the magnetic phase diagram of Na₂Co₂TeO₆ is qualitatively different from that of RuCl₃. The matching phase boundaries between magnetic and dielectric measurements and peaks in the dielectric constant strongly suggest a magnetoelectric coupling existing in the system. One major debate on NCTO is whether the spin structure of the low-field ground state is zig-zag or triple-Q. We found that the critical fields observed along the a- and a*-axes are significantly different. This is more consistent with the zigzag scenario as it naturally has drastically different spin configurations along the two directions whereas the difference is rather small in triple-Q scenario. In addition, the lack of any change in net electric polarization with temperature or field across the magnetic phase transitions is consistent with a no mirror-breaking symmetry, and thus with the zigzag spin structure. Electric polarization was measured for single-crystals along a*-axis with H || a-axis and in a large polycrystal.