We consider the frequency dependence of the neutron scattering amplitude from a two-dimensional quantum antiferromagnet. It is well known that the long-range order disappears at any finite temperature and hence the elastic neutron scattering Bragg peak is transformed into the quasielastic neutron scattering spectrum proportional to dω/ω. We show that the widely known formula for the spectrum of an isotropic antiferromagnet derived by Auerbach and Arovas should be supplemented by a logarithmic term that changes the integrated intensity by two times. The analogous formula for an easy-plane magnet is very different because of the Berezinsky-Kosterlitz-Thouless physics. An external uniform magnetic field switches the isotropic magnet smoothly to the easy-plane magnet. We demonstrate that the quasielastic neutron scattering spectrum in the crossover regime combines properties of both limiting cases. We also consider a quantum antiferromagnet close to the O(3) quantum critical point and show that in an external uniform magnetic field the intensity of the elastic (quasielastic) neutron scattering peak depends linearly and significantly on the applied field.
|Journal||Physical Review B - Condensed Matter and Materials Physics|
|Publication status||Published - 28 Mar 2011|