arXiv subjects

Dal-Ho Yoon

Publications and source records attributed to Dal-Ho Yoon.

5 recordsLinked to original sources

arXiv1999

Bifurcation of Periodic Instanton in Decay-Rate Transition

We investigate a bifurcation of periodic instanton in Euclidean action-temperature diagram in quantum mechanical models. It is analytically shown that multiple zero modes of fluctuation operator should be arised at bifurcation points. This fact is used to derive a condition for the appearance of bifurcation points in action-temperature diagram. This condition enables one to compute the number of bifurcation points for a given quantum mechanical system and hence, to understand the whole behaviour of decay rate. It is explicitly shown that the previous criterion derived by nonlinear perturbation or negative-mode consideration is special limit of our case.

hep-th↗

arXiv1999

Oscillation of the tunnel splitting in nanospin systems within the particle mapping formalism

The oscillation of tunnel splitting in the biaxial spin system within magnetic field along the anisotropy axis is analyzed within the particle mapping approach, rather than in the (θ-ϕ) spin coherent-state representation. In our mapping procedure, the spin system is transformed into a particle moving in the restricted $S^1$ geometry whose wave function subjects to the boundary condition involving additional phase shift. We obtain the new topological phase that plays the same role as the Wess-Zumino action in spin coherent-state representation. Considering the interference of two possible trajectories, instanton and anti-instanton, we get the identical condition for the field at which tunneling is quenched, with the previous result within spin coherent-state representation.

cond-mat.mes-hall↗

arXiv1999

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

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 :$T_c(b_c), T_c(α_{xc}, α_{zc})$.

cond-mat.mes-hall↗

arXiv1999

Escape rate of a biaxial nanospin system in a magnetic field : first- and second-order transition between quantum and classical regimes

We investigate the escape rate of the biaxial nanospin particle with a magnetic field applied along the easy axis. The model studied here is described by the Hamiltonian ${\cal H} = -AS_z^2 - BS_x^2 - HS_z, (A>B>0)$. By reducing this Hamiltonian to a particle one, we derive, for the first time, an effective particle potential for this model and find an analytical form of the phase boundary line between first- and second-order transitions, from which a complete phase diagram can be obtained. We also derive an analytical form of the crossover temperature as a function of the applied field at the phase boundary.

cond-mat.mes-hall↗

arXiv1998

Escape rate of the nanospin system in a magnetic field: the first-order phase transition within quantum regime

We have investigated the escape rate of the nanospin particle with a magnetic field applied along the easy axis. The model studied here is described by the Hamiltonian $\hat{\cal H} = K_1 \hat{S}_z^2 + K_2 \hat{S}_y^2 + gμ_b H \hat{S}_x $, $(K_1 > K_2 > 0)$ from which the escape rate is calculated within the semiclassical approximation. We have obtained a diagram for the orders of the phase transitions depending on the anisotropy constant and the external field. For $ K_2 / K_1 > 0.85$ the present model reveals, for the first time, the existence of the first-order transition within the quantum regime.

cond-mat↗