Quantum statistical metastability for a finite spin

Details Quantum statistical metastability for a finite spin Quantum statistical metastability for a finite spin

2000-05-29

arxiv.org/abs/cond-mat/0005489v1

Phys. Rev. B 63, 024418 (2000)

Physics

Condensed Matter

Statistical Mechanics

7 Phys. Rev. pages, 10 figures, submitted to PRB

Scientific paper

10.1103/PhysRevB.63.024418

We study quantum-classical escape-rate transitions for uniaxial and biaxial models with finite spins S=10 (such as Mn_12Ac and Fe_8) and S=100 by a direct numerical approach. At second-order transitions the level making a dominant contribution into thermally assisted tunneling changes gradually with temperature whereas at first-order transitions a group of levels is skipped. For finite spins, the quasiclassical boundaries between first- and second-order transitions are shifted, favoring a second-order transition: For Fe_8 in zero field the transition should be first order according to a theory with S \to \infty, but we show that there are no skipped levels at the transition. Applying a field along the hard axis in Fe_8 makes transition the strongest first order. For the same model with S=100 we confirmed the existence of a region where a second-order transition is followed by a first-order transition [X. Martines Hidalgo and E. M. Chudnovsky, J. Phys.: Condensed Matter (in press)].

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