The connection between vortex-like topological excitations and conventional excitations in quantum ferromagnetic spin systems on two-dimensional lattice and their stability

Abstract

Abstract:We present a scheme for the construction of quantum states of vortex like topological excitations corresponding to spin-½ strongly XY anisotropic nearest neighbor Heisenberg Ferromagnet on two dimensional lattice. The procedure involving Pauli spin basis states is carried out corresponding to both infinite dilute limit and finite density limit of vortex/anti-vortex. It is found that the corresponding quantum mechanical states representing charge 1 quantum vortices/ anti-vortices can be expressed as linear combinations of single magnon states, composite multi-magnon states and the ground state. Detailed calculations show that these states are quantum mechanically stable states of the Hamiltonian only when the system size exceeds certain threshold value. Our analysis indicates that the interactions between different magnon modes can very well generate these topological excitations. Possible applications of our calculations to real magnetic systems are also discussed. Magnetic measurements probing spin dynamics may be undertaken to verify the existence of the threshold size for the stability of vortices.

Semi-phenomenological analysis of neutron scattering results for quasi-two dimensional quantum anti-ferromagnet

“…At higher temperatures however, more topological excitations become free and drive a large portion of the spin dynamics of the system. At this point, it is worth mentioning that for quantum ferromagnetic systems on two dimensional square lattice it has been shown that the formation of topological excitations of vortex/ meron types from the fragile magnons and multi-magnon composites is quite plausible [62]. Moreover, some of the collective modes (i.e.…”

“…magnon and multi-magnon modes) are expected to stay intact with their damped nature and thus can provide a significant contribution to the spin dynamics. In analogy with the three dimensional systems where above the Curie temperature (T c ) the magnon-like collective excitations become fragile and damped, for pure two dimensional systems (where T c = 0) the collective excitations become fragile at any finite temperature [56,[62][63][64][65][66][67]. This process is expected to be operative too in the antiferromagnetic systems on pure two-dimensional lattices.…”