Field-induced decay dynamics in square-lattice antiferromagnets

Abstract

Dynamical properties of the square-lattice Heisenberg antiferromagnet in applied magnetic field are studied for arbitrary value S of the spin. Above the threshold field for two-particle decays, the standard spin-wave theory yields singular corrections to the excitation spectrum with logarithmic divergences for certain momenta. We develop a self-consistent approximation applicable for S ≥ 1, which avoids such singularities and provides regularized magnon decay rates. Results for the dynamical structure factor obtained in this approach are presented for S = 1 and S = 5/2.

Dynamical structure factor of quasi-two-dimensional antiferromagnet in high fields

“…21,22 Nevertheless, it is still important to consider them not only because the decays within the Born decay region typically remain more intense as they occur in the lower-order process, but also because such boundaries correspond to singularities in the spectrum discussed next.…”

“…These decay threshold singularities correspond to the step-like behavior in Γ k and the logarithmic divergence inε k . [20][21][22] For the k-points already inside the decay region, there is a possibility of meeting a saddle point of the continuum, in which case the real part of the spectrumε k experiences a jump and Γ k a logarithmic singularity, see the 2D data in Fig. 3.…”

“…Previous works on regularization of singularities in the spin-wave spectrum in the 2D square-lattice case 21,22 were based on partial dressings of magnon Green's functions in the one-loop diagrams of Fig. 1.…”

“…The latter problem was avoided in Ref. 22 by performing self-consistency only in the imaginary part of the magnon self-energy, applicable for larger S ≥ 1 where the real part of renormalization can be deemed small. The downside of this approach is that it remains essentially on-shell, enforcing Lorentzian shapes of the quasiparticle peaks.…”

Rotons and spin transport in square lattice antiferromagnets in magnetic fields

“…Contrary to the collinear Néel structure, sizable deviations from LSW calculations are pointed out for SLHAFs in high fields both numerically [6,7] and experimentally [8]. Moreover, nonlinear spin-wave spectra are shown to become negative at some wavevectors in high fields [3].Motivated by Ref.[3], we investigate the nonlinear spin-wave spectra within the second-order perturbation calculations based on the formalism of Zhitomirsky-Nikuni-Chernyshev [1, 2, 4]. We focus on the field region just below where we get the negative magnon spectra, near 3/4 of the saturation field, and we find a rotonlike minimum on the acoustic mode which softens rapidly to zero with small increase of fields [9].…”