Direct ab initio calculation of magnons in altermagnets: method, spin-space symmetry aspects, and application to MnTe

Abstract

We suggest the method for direct ab initio calculation of magnons in complex collinear magnets. The method is based on the density-functional-theory calculation under two different constraints: one constraint governs the change of the magnetization with respect to the ground state, and the other is the symmetry constraint responsible for the value of the magnon wave vector. The performance of the method is demonstrated by the application to an altermagnet MnTe. An important role in both the formulation and the application of the method play the aspects of generalized symmetry described by the spin-space groups. The symmetry analysis connects in one coherent picture the following three parts of the consideration: (i) the generalized translational symmetry of the magnons as a crucial condition for their efficient ab-initio calculation, (ii) altermagnetic spin-splitting of the electron states in the ground magnetic state, and (iii) chirality splitting of the magnon excitations. It is demonstrated that both the spin splitting of the electron states and the chirality splitting of the magnons have identical patterns in the corresponding wave vector spaces. Since the altermagnetism of MnTe is the consequence of the presence of the Te atoms, an adequate attention is devoted to the symmetry analysis and calculation results for the Te moments induced in the magnon states. The knowledge of the symmetry properties of the Te moments allows to accelerate the numerical convergence of the magnon states and serves as a test for the accuracy of the calculations. To expose the connection between electron band structures of the magnon states of the system and the chirality properties of these states we investigate the transformation of the electron structure in the transition from the collinear ground state to a noncollinear magnon state.

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