Quantum Heisenberg antiferromagnets in a uniform magnetic field:   nonanalytic magnetic field dependence of the magnon spectrum
  We reexamine the 1/S-correction to the self-energy of the gapless magnon of a D-dimensional quantum Heisenberg antiferromagnet in a uniform magnetic field h using a hybrid approach between 1/S-expansion and non-linear sigma model, where the Holstein-Primakoff bosons are expressed in terms of Hermitian field operators representing the uniform and the staggered components of the spin-operators [N. Hasselmann and P. Kopietz, Europhys. Lett. {\bf{74}}, 1067 (2006)]. By integrating over the field associated with the uniform spin-fluctuations we obtain the effective action for the staggered spin-fluctuations on the lattice, which contains fluctuations on all length scales and does not have the cutoff ambiguities of the non-linear sigma model. We show that in dimensions D <= 3 the magnetic field dependence of the spin-wave velocity c(h) is non-analytic in h^2, with c(h) - c(0) proportional to h^2 \ln | h | in D=3, and c(h) - c(0) proportional to | h | in D=2. The frequency dependent magnon self-energy is found to exhibit an even more singular magnetic field dependence, implying a strong momentum dependence of the quasi-particle residue of the gapless magnon. We also discuss the problem of spontaneous magnon decay and show that in D > 1 dimensions the damping of magnons with momentum k is proportional to |k|^{2D -1} if spontaneous magnon decay is kinematically allowed. 
Andreas Kreisel, Francesca Sauli, Nils Hasselmann, Peter Kopietz