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Quantum-classical phase transition of escape rate in biaxial spin system with an arbitrarily directed magnetic field

Chang-Soo Park·Sahng-Kyoon Yoo·Dal-Ho Yoon
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Abstract

We investigate the escape rate of a biaxial spin particle with an arbitrarily dierected magnetic field in the easy plane, described by Hamiltonian ${\cal H} = -AS_z^2 - BS_x^2 -H_x S_x -H_z S_z, (A>B>0)$. We derive an effective particle potential by using the method of particle mapping. With the help of the criterion for the presence of a first-order quantum-classical transition of the escape rate we obtained various phase boundary curves depending on the anisotropy parameter $b \equiv B/A$ and the field parameters $α_{x,z} \equiv H_{x,z}/AS$ : $α_{zc}(b_c)'s, α_{xc}(b_c)'s$, and $α_{zc} = α_{zc}(α_{xc})$. It is found from $α_{zc}(b_c)'s$ and $α_{xc}(b_c)'s$ that the-first-order region decreases as $b$ and $α_x $ (or $α_z$) increase. The phase boundary line $α_{zc} = α_{zc}(α_{xc}) shows that compared with the uniaxial system, both the first- and second-oredr regions are diminished due to the transverse anisotropy. Moreover, it is observed that, in the limit $α_{xc} \to 0$, $α_{zc}$ does not coinsides with the coercive field line, which yields more reduction in the first-order region. We have also computed the crossover temperatures at the phase boundary :$T_c(b_c), T_c(α_{xc}, α_{zc})$.

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Chang-Soo Park, Sahng-Kyoon Yoo, Dal-Ho Yoon. 1999-09-15. Quantum-classical phase transition of escape rate in biaxial spin system with an arbitrarily directed magnetic field. https://doi.org/10.1103/physrevb.61.11618

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