Magnetic excitation spectrum of the square latticeS=1∕2Heisenberg antiferromagnetK2V

Abstract

We have explored the magnetic excitation spectrum of the S=1/2 square lattice Heisenberg antiferromagnet, K 2 V 3 O 8 , using both triple-axis and time-of-flight inelastic neutron scattering. The long-wavelength spin waves are consistent with the previously determined Hamiltonian for this material. A small energy gap of 72± 9 eV is observed at the antiferromagnetic zone center and the near-neighbor exchange constant is determined to be 1.08± 0.03 meV. A finite ferromagnetic interplanar coupling is observed along the crystallographic c axis with a magnitude of J c = −0.0036± 0.0006 meV. However, upon approaching the zone boundary, the observed excitation spectrum deviates significantly from the expectation of linear spin wave theory resulting in split modes at the ͑ /2, /2͒ zone boundary point. The effects of magnon-phonon interaction, orbital degrees of freedom, multimagnon scattering, and dilution/site randomness are considered in the context of the mode splitting. Unfortunately, no fully satisfactory explanation of this phenomenon is found and further theoretical and experimental work is needed.

Quantum effects inS= 1/2 two-dimensional Heisenberg antiferromagnet in applied magnetic field

“…In two dimensions, quantum effects are generally reduced and the ground state of S=1/2 square lattice Heisenberg antiferromagnets is long-range ordered, albeit with a reduced ordered moment and spin-waves whose energy is renormalized by quantum fluctuations [3,4]. Importantly, however, short range quantum correlations are preserved in the ground state and lead to a quantum-induced dispersion at the AF zone boundary and to a weak continuum of states at high energies [4,5,6,7,8,9,10,11,12].…”

“…In two dimensions, quantum effects are generally reduced and the ground state of S=1/2 square lattice Heisenberg antiferromagnets is long-range ordered, albeit with a reduced ordered moment and spin-waves whose energy is renormalized by quantum fluctuations [3,4]. Importantly, however, short range quantum correlations are preserved in the ground state and lead to a quantum-induced dispersion at the AF zone boundary and to a weak continuum of states at high energies [4,5,6,7,8,9,10,11,12].Competing interactions generally increase quantum fluctuations, particularly for materials close to a quantum critical point. For the 2D S=1/2 antiferromagnet, one source of competition can be next-nearest neighbor interactions that destabilize a simple nearest-neighbor antiparallel alignment and thus enhance fluctuations [13,19].…”

Coupling of spin and lattice modes in theS=1/2two-dimensional antiferromagnet K2V

“…12 of Ref. 17 ). In addition, we can estimate the exchange coupling constant between vanadium spins by resorting to the upper cut-off relation, that is, 2Z c JzS = 4.632 J, where J is the exchange integral, z is the number of nearest neighboring spins, and Z c = 1.158 is a quantum correction to a classical value.…”

“…In order to explain the observed split modes Lumsden et al 17 have discussed several possible scenarios, for example, magnon-phonon interaction, structural distortions, lattice disorders, multimagnon scatterings, and orbital degrees of freedom. However, no definite answer can be given.…”

MCViNE – An object oriented Monte Carlo neutron ray tracing simulation package

“…Such an effect is expected due to diffrent focusing conditions of the resolution ellipsoid [73,74]. The spin-wave dispersion above ∼2.1 meV (h ∼ 0.33, for example) has weaker intensity in the experimental data than expected from the simulated data, which could be attributed to mode splitting near the zone boundary [71].…”