Some models allowing explicit calculation of periodic instantons and evaluation of their action are studied with regard to transitions from classical to quantum behaviour as the temperature is lowered and tunneling sets in. It is shown that (beyond a critical value of a coupling) the spin system considered acquires a first order transition as a result of the field dependence of its effective mass, whereas models with constant mass exhibit only second order transitions.Comment: 10 pages, 3 figure

A formula suitable for a quantitative evaluation of the tunneling effect in a ferromagnetic particle is derived with the help of the instanton method.The tunneling between n-th degenerate states of neighboring wells is dominated by a periodic pseudoparticle configuration. The low-lying level-splitting previously obtained with the LSZ method in field theory in which the tunneling is viewed as the transition of n bosons induced by the usual (vacuum) instanton is recovered.The observation made with our new result is that the tunneling effect increases at excited states. The results should be useful in analyzing results of experimental tests of macroscopic quantum coherence in ferromagnetic particles.

Transitions from classical to quantum behaviour in a spin system with two degenerate ground states separated by twin energy barriers which are asymmetric due to an applied magnetic field are investigated. It is shown that these transitions can be interpreted as first-or second-order phase transitions depending on the anisotropy and magnetic parameters defining the system in an effective Lagrangian description.

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The Kalb-Ramond monopole, as discussed by Nepomechie, is identical with the (singular) Dirac monopole in d = 3 dimensions. The latter can be described by the (regular) 't Hooft-Polyakov monopole, via the 't Hooft tensor construction. This construction is extended to arbitrary odd dimensions by performing the d = 5 case explicitly, exploiting the (regular) 'monopoles' of generalised Georgi-Glashow models and identifying their 't Hooft tensors as the Kalb-Ramond fields. The relevant 'magnetic charges' are expressed as topological invariants.