arXiv subjects
Publications and source records attributed to Gwang-Hee Kim.
We have shown that the size of the magnetization step due to resonant spin tunneling in a molecular magnet can be strongly affected by sound. The transverse acoustic wave can also generate macroscopic quantum beats of the magnetization during the field sweep.
An electron transport is studied in the system which consists of scanning tunneling microscopy-single molecule magnet-metal. Due to quantum tunneling of magnetization in single-molecule magnet, linear response conductance exhibits stepwise behavior with increasing longitudinal field and each step is maximized at a certain value of field sweeping speed. The conductance at each step oscillates as a function of the additional transverse magnetic field along the hard axis. Rigorous theory is presented that combines the exchange model with the Landau-Zener model.
The level splittings in a dimer with the antiferromagnetic coupling between two single-molecule magnets are calculated perturbatively for arbitrary spin. It is found that the exchange interaction between two single-molecule magnets plays an important role in the level splitting. The results are discussed in comparison with the recent experiment.
Quantum vortex tunneling is studied for the case where the Hall and the dissipative dynamics are simultaneously present. For a given temperature, the magnetization relaxation rate is calculated as a function of the external current and the quasiparticle scattering time. The relaxation rate is solved analytically at zero temperature and obtained numerically at finite temperatures by the variational method. In the moderately clean samples, we have found that a minimum in the relaxation rate exists at zero temperature, which tends to disappear with increase in the temperature.
Quantum-classical escape-rate transition has been studied for two general forms of magnetic anisotropy in ferrimagnetic or antiferromagnetic particles. It is found that the range of the first-order transition is greatly reduced as the system becomes ferrimagnetic and there is no first-order transition in almost compensated antiferromagnetic particles. These features can be tested experimentally in nanomagnets like molecular magnets.
We have studied numerically the quantum-classical crossover in the escape-rate for an uniaxial spin system with an arbitrarily directed field. Using the simple quantum transition-state theory, we have obtained the boundary separating the first- and the second-order crossover and the escape-rate in the presence of the transverse and longitudinal field. The results apply to the molecular nanomagnet, ${\rm Mn_{12}}$.
Employing the method of mapping the spin problem onto a particle one, we have derived the particle Hamiltonian for a biaxial spin system with a transverse or longitudinal magnetic field. Using the Hamiltonian and introducing the parameter $p (\equiv (U_{max}-E)/(U_{max}-U_{min}))$ where $U_{max}$ (U_{min}) corresponds to the top (bottom) of the potential and $E$ is the energy of the particle, we have studied the first- or second-order transition around the crossover temperature between thermal and quantum regimes for the escape rate, depending on the anisotropy constant and the external magnetic field. It is shown that the phase boundary separating the first- and second-order transition and its crossover temperature are greatly influenced by the transverse anisotropy constant as well as the transverse or longitudinal magnetic field.
Within the framework of the path-integral approach we study the quantum vortex creep for the situation where both the Hall and the dissipative dynamics are simultaneously present. We calculate the relaxation rate and the crossover temperature separating the thermal activation and the quantum tunneling processes for anisotropic or multilayer superconductors. The results are compared with the available experimental data.
At finite temperature we study the quantum tunneling of magnetization for a small ferromagnetic particle with the biaxial symmetry placed in a magnetic field at an arbitrary angle. We present numerical WKB exponent below the crossover temperature in which the quantum tunneling is affected by the thermal activation, and the approximate form of the WKB exponent around the crossover region. The effect of quantum fluctuations on the thermal activation rate beyond the crossover regime are discussed.
We calculate the decay constant ratios $f_{η_c}/f_{J/ψ}$ and $f_{η_b}/f_Υ$. In the calculation we take into account the mock meson structures of the mesons, as well as the difference of the wave functions at origin of the vector and pseudoscalar mesons studied by Ahmady and Mendel. We find that the different spin structures of the mesons much affect the ratios. We incorporate our results in the prediction of the branching ratios of $B\to K η_c$.
We study the quantum nucleation in a thin ferromagnetic film placed in a magnetic field at an arbitrary angle. The dependence of the quantum nucleation and the temperature of the crossover from thermal to quantum regime on the direction and the strength of the applied field are presented. It is found that the maximal value of the rate and that of the crossover temperature are obtained at a some angle with the magnetic field, not in the direction of the applied field opposite to the initial easy axis.
The commonly used Hamiltonian of the chromomagnetic hyperfine splitting is inversely proportional to the product of the masses of two constituent quarks composing the meson. So it is expected to have $(M_{D^*}-M_{D})/(M_{D_s^*}-M_{D_s})$ $\simeq$ $(M_{B^*}-M_{B})/(M_{B_s^*}-M_{B_s})$ $\simeq$ $1.6$, when the constituent quark masses $m_{u,d}=0.33$ GeV and $m_s=0.53$ GeV are used. However, the experimental results show that the above ratios are very close to 1. We solve this puzzle by employing the Hamiltonian recently proposed by Scora and Isgur.
We calculate the quantum tunneling rate of a ferromagnetic particle of $\sim 100 Å$ diameter in a magnetic field of arbitrary angle. We consider the magnetocrystalline anisotropy with the biaxial symmetry and that with the tetragonal symmetry. Using the spin-coherent-state path integral, we obtain approximate analytic formulas of the tunneling rates in the small $ε(=1- H/H_c)$-limit for the magnetic field normal to the easy axis ($θ_H = π/2$), for the field opposite to the initial easy axis ($θ_H = π$), and for the field at an angle between these two orientations ($π/2 << θ_H << π$). In addition, we obtain numerically the tunneling rates for the biaxial symmetry in the full range of the angle $θ_H$ of the magnetic field ($π/2 < θ_H \leq π$), for the values of ε=0.01 and 0.001.
We derive formulas for the decay constants $f_P$ and $f_V$ of pseudoscalar and vector mesons in the relativistic mock meson model. Using these formulas, we obtain $f_P$ and $f_V$ of $B_s$, $B_d$, $D_s$, and $D_d$ mesons as functions of the mock meson parameter $β$. Then by using the values of $β$ which are obtained by the variational method in the relativistic quark model, we obtain the decay constants $f_P$ and $f_V$ of the heavy mesons, and the corresponding ratios $f_V/f_P$. The results are compared with other calculations and existing experimental results.
We calculate the ratio $f_{B_s}/f_{B_d}$ by following the Oakes' method which is based on chiral symmetry breaking, but we improve his calculation by performing the calculation of the factor $<0|{\bar{b}}γ_5s|B_s>$/$<0|{\bar{b}}γ_5d|B_d>$ in the mock meson model. In this calculation we improved the mock meson model by using the value of the parameter $β$ which is obtained by the variational method in the relativistic quark model. We also calculate $f_{D_s}/f_{D_d}$, and then $(f_{B_s}/f_{B_d})\, /\, (f_{D_s}/f_{D_d})$. In this method we also obtain the ratio $f_{B_s}/f_{D_s}$ and $f_{B_d}/f_{D_d}$ which are important for the knowledge of CP violation and $B$-$\bar{B}$ mixing.
We calculate the ratios of $B$ and $D$ meson decay constants by applying the variational method to the relativistic hamiltonian of the heavy meson. We adopt the Gaussian and hydrogen-type trial wave functions, and use six different potentials of the potential model. We obtain reliable results for the ratios, which are similar for different trial wave functions and different potentials. The obtained ratios show the deviation from the nonrelativistic scaling law, and they are in a pretty good agreement with the results of the Lattice calculations.