RESONANT QUANTUM COHERENCE OF THE NEEL VECTOR IN A MAGNETIC-FIELD APPLIED ALONG THE MEDIUM AXIS

Based on the two-sublattice model. we study the topological phase inte rference effect in resonant macroscopic quantum coherence of the Neel vector in a single-domain antiferromagnetic nanoparticle with biaxial crystal symmetry placed in an external magnetic field a;;plied along t he medium axis. By applying the standard instanton technique in spin-c oherent-state path integral, we evaluate both the Wentzel-Kramers-Bril louin exponent and the preexponential factors in the instanton's contr ibution to the tunneling-level splitting. The Euclidean transition amp litude is obtained by making use of the dilute instanton-gas approxima tion. The low-lying tunneling-level spectrum and the thermodynamic pro perties of magnetic tunneling states are found to depend on the parity of the excess spin of the antiferromagnetic nanoparticle significantl y. possible relevance to experiments is also discussed.