High-accuracy evaluation of nonthermal magnetic states beyond spin-wave theory: Applications to higher-energy states

We present an approximation scheme based on selective Hilbert space truncation for characterizing nonthermal states of magnetic systems beyond spin-wave theory. We study applications to states that are inaccessible through linear spin-wave theory, such as multimagnon states and higher-energy states. Our approach is based on the existence of an exact representation of spin operators in terms of finite-order polynomials of bosonic operators---crucially representations used do not couple physical and physical parts of bosonic Fock space. It can be applied to approximate systems with and without a magnetically ordered ground state. The approximation exactly diagonalizes the bosonic Hamiltonian restricted to particular boson occupation subspaces, improving the conventional linear spin-wave approach and exponentially reducing the computing time relative to exact diagonalization schemes. As a test case, we apply the approach to a prototypical one-dimensional model---an XXZ spin chain with an applied magnetic field and antisymmetric exchange coupling. Here, the antisymmetric coupling introduces a continuous parameter to tune the system away from its exactly solvable limit. We find excellent agreement between numerically exact eigenstates and eigenvalues and those found via the approximation scheme. Our approach applies not just to higher lying states but also to boson bound states, which could make them more accessible to theoretical predictions for comparison with experiment.