Abstract
We have investigated the field-angle behaviors of magnetic excitations under an in-plane magnetic field for proximate Kitaev systems. By employing the exact diagonalization method in conjunction with the linear spin wave theory, we have demonstrated that the magnetic excitation gap in the polarized phase is determined by the magnon excitation at $M$ points and has a strong anisotropy with respect to the field direction in the vicinity of the critical field limit. The specific heat from this magnon excitation bears qualitatively the same anisotropic behaviors as expected one for the non-Abelian spin liquid phase in the Kitaev model and experimentally observed one of the intermediate phases in $\alpha$-RuCl$_3$.
Highlights
Quantum fluctuation in frustrated spin systems can prevent any classical magnetic orders and induce exotic quantum phases such as a quantum spin liquid (QSL)
Using the exact diagonalization (ED) method and linear spin wave theory (LSWT), we demonstrated that the low-energy excitation features in the polarized phase for various models relevant to α-RuCl3 can be interpreted in terms of the field-angle anisotropy of the magnon gap, determined at one of three M points depending on the field direction [see Fig. 1(c)]
Based on the numerical ED calculation and LSWT analysis, we have explored the field-angle anisotropy of proximate Kitaev systems under an in-plane magnetic field
Summary
Quantum fluctuation in frustrated spin systems can prevent any classical magnetic orders and induce exotic quantum phases such as a quantum spin liquid (QSL). Majorana fermions in the Kitaev model acquire a mass gap under the magnetic field. We investigated the field-angle dependence of magnetic excitation and magnetic specific heat for proximate Kitaev systems under an in-plane magnetic field. Using the exact diagonalization (ED) method and linear spin wave theory (LSWT), we demonstrated that the low-energy excitation features in the polarized phase for various models relevant to α-RuCl3 can be interpreted in terms of the field-angle anisotropy of the magnon gap, determined at one of three M points depending on the field direction [see Fig. 1(c)]. The magnetic specific heat dictated by this magnon dynamics shows qualitatively the same anisotropic behaviors as those in the Kitaev model. Behaviors in thermodynamic quantities alone are not a smoking gun of the intermediate NASL phase in α-RuCl3 under the magnetic field, but require further considerations
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