We predict that spin-waves in an ordered square quantum antiferromagnet in a transverse magnetic field (h) may demonstrate three modes of spin excitations. Starting from the self-consistent rotation-invariant Green's function method, a new mean-field theory is constructed for h not equal to 0. The method preserves the translational and the axial symmetries, and provides exact fulfillment of the single-site constraint for each of the three modes. We examine the dynamical structure factors S(aa)(k,w), a= x, y, z. It is shown, that the introduction of h leads to the hybridization of two degenerate spin modes due to the appearance of a nondiagonal on a, b spin-spin Green's functions. The comparison of the theory with the exact diagonalization study and with results on inelastic neutron scattering experiments is discussed at T = 0. We discuss also the correspondence of the theory to the existing theories, which allow only two spin excitations modes for the total S(k,w).

中文翻译：

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磁场中方格上反铁磁自旋1/2海森堡模型的光谱特性

我们预测，横向磁场 (h) 中有序方形量子反铁磁体中的自旋波可能表现出三种自旋激发模式。从自洽旋转不变格林函数方法出发，构建了一个新的平均场理论，h 不等于 0。该方法保留了平移和轴对称性，并提供了对每个的单点约束的精确满足三种模式中。我们检查动力结构因子 S(aa)(k,w), a= x, y, z。结果表明，由于 a, b 自旋-自旋格林函数上出现非对角线，h 的引入导致两种简并自旋模式的杂交。在 T = 0 时讨论了该理论与精确对角化研究以及非弹性中子散射实验结果的比较。