Unconventional magnetic field response of the hyperhoneycomb Kitaev magnet $β-\textbf{Li}_2\textbf{IrO}_3$

Abstract: We present a unified description of the response of the hyperhoneycomb Kitaev magnet $\beta$-$\text{Li}2\text{IrO}_3$ to applied magnetic fields along the orthorhombic directions ${\bf a}$, ${\bf b}$ and ${\bf c}$. This description is based on the minimal nearest-neighbor $J$-$K$-$\Gamma$ model and builds on the idea that the incommensurate counter-rotating order observed experimentally at zero field can be treated as a long-distance twisting of a nearby commensurate order with six spin sublattices. The results reveal that the behavior of the system for ${\bf H}\parallel{\bf a}$, ${\bf H}\parallel{\bf b}$ and ${\bf H}\parallel{\bf c}$ share a number of qualitative features, including: i) a strong intertwining of the modulated, counter-rotating order with a set of uniform orders; ii) the disappearance of the modulated order at a critical field $H^\ast$, whose value is strongly anisotropic with $H{\bf b}^\ast!<!H_{\bf c}^\ast!\ll!H_{\bf a}^\ast$; iii) the presence of a robust zigzag phase above $H^\ast$; and iv) the fulfillment of the Bragg peak intensity sum rule. It is noteworthy that the disappearance of the modulated order for ${\bf H}\parallel{\bf c}$ proceeds via a `metamagnetic' first-order transition which does not restore all broken symmetries. This implies the existence of a second finite-$T$ phase transition at higher magnetic fields. We also demonstrate that quantum fluctuations give rise to a significant reduction of the local moments for all directions of the field. The results for the total magnetization for ${\bf H}\parallel{\bf b}$ are consistent with available data and confirm a previous assertion that the system is very close to the highly-frustrated $K$-$\Gamma$ line in parameter space. Our predictions for the magnetic response for fields along ${\bf a}$ and ${\bf c}$ await experimental verification.