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Numerical study on tunneling splitting in biaxial spin systems is done by performing diagonalization of the Hamilton operator. It is found that the calculated energy splitting agrees quantitatively with theoretical prediction of instanton method. Our result shows that both the instanton method and the large spin limit work well for the total spin around 10. By including the fourth-order term in Hamiltonian, experimental observation can be re-covered quantitatively. 相似文献
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The coherent quantum tunneling effects in antiferromagnets in the presence of a strong external magnetic field parallel to the easy axis have been investigated using the instanton formalism. In a wide field range including the region of the phase spin-flop transition, the tunneling is described by 180° instantons for which the Euclidean action is real and destructive interference is absent. At the transition point, 90° instantons describing the tunneling between the collinear and spin-flop states appear. The Euclidean action decreases, whereas the tunneling probability and tunneling level splitting in both phases increase significantly in the immediate vicinity of the spin-flop transition point. The possibility of observing the coherent tunneling effects for artificial small particles (magnetic dots) made of antiferromagnets is discussed. 相似文献
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The quantum tunneling effect in a ferromagnetic particle which can be evaluated only for the ground state previously is extended to excited states within the framework of instanton method. The tunneling between n-th degenerate states of neighboring wells is dominated by a periodic pseudoparticle configuration with which a formula of level-splitting valid for entire energy region is derived. In low energy region periodic instanton results coincide exactly with those derived through the LSZ reduction procedure. The tunneling effect increases at excited states. These results should be useful in further investigation of phase transition problem in ferromagnetic particles. 相似文献
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Comparison of calculation methods for the tunnel splitting at excited states of biaxial spin models
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We present the calculation and comparison of tunnel splitting at excited levels of biaxial spin models by various methods, including the generalized instanton method, the generalized path integral method for coherent spin states, the perturbation method, and the exact method by numerical diagonalization of the Hamiltonian. It is found that, for integer spin with spin number around 10, tunnel splitting predicted by the generalized path integral for coherent spin states is about 10^{-n} times of the exact numerical result for the nth excited level, while the ratio of the results of the perturbation method and the exact numerical method diverges in the large spin limit. We thus conclude that the generalized instanton method is the best approximate way for calculating tunnel splitting in spin models. 相似文献
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A. I. Popov A. F. Popkov N. E. Kulagin 《Journal of Experimental and Theoretical Physics》2004,99(4):834-849
The paper presents a theoretical analysis of macroscopic quantum tunneling phenomena in small particles of a cubic ferromagnet of the HoxY3?xFe5O12 type with strongly anisotropic (Ising) impurity ions present in a low concentration x ? 1 in the region of strong magnetic fields, at which many orientation phase transitions related to the competition of external and exchange field actions on the spin subsystem are observed. The theory of path integrals for the magnetic subsystem was used to calculate the instanton contributions to interphase tunneling amplitudes in the vicinity of first-order transitions for three principal orientations of an external magnetic field in a cubic crystal. It was shown that low-energy barriers separating angular phases could result in anomalously large mesoscopic volumes at which macroscopic spin tunneling effects could appear in the energy spectra of particles. The special features of spectral splitting caused by the mixing of azimuthally degenerate angular phases and phases with different polar angles of magnetization orientation were revealed. 相似文献
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Dae-Yup Song 《Annals of Physics》2008,323(12):2991-2999
An asymmetric double-well potential is considered, assuming that the minima of the wells are quadratic with a frequency ω and the difference of the minima is close to a multiple of ?ω. A WKB wave function is constructed on both sides of the local maximum between the wells, by matching the WKB function to the exact wave functions near the classical turning points. The continuities of the wave function and its first derivative at the local maximum then give the energy-level splitting formula, which not only reproduces the instanton result for a symmetric potential, but also elucidates the appearance of resonances of tunneling in the asymmetric potential. 相似文献
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We consider the process of quantum tunneling between the superconducting and paramagnetic states of a nanometer-scale superconducting grain placed in a magnetic field. The grain is supposed to be weakly coupled to a normal metallic contact that plays the role of the spin reservoir. Using the instanton method, we find the probability of the quantum tunneling process and express it in terms of the applied magnetic field, order parameter of the superconducting grain, and conductance of the tunneling junction between the grain and metallic contact. 相似文献
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The low-energy-level macroscopic wave functions of the Bose-Einstein
condensate (BEC) trapped in a symmetric double-well and a periodic potential
are obtained by solving the Gross-Pitaevskii equation numerically. The
ground state tunnel splitting is evaluated in terms of the even and odd wave
functions corresponding to the global ground and excited states
respectively. We show that the numerical result is in good agreement with
the analytic level splitting obtained by means of the periodic instanton
method. 相似文献
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Y.-B. Zhang H.J.W. Müller-Kirsten 《The European Physical Journal D - Atomic, Molecular, Optical and Plasma Physics》2001,17(3):351-363
An instanton method is proposed to investigate the quantum tunneling between two weakly-linked Bose-Einstein condensates confined
in double-well potential traps. We point out some intrinsic pathologies in the earlier treatments of other authors and make
an effort to go beyond these very simple zero order models. The tunneling amplitude may be calculated in the Thomas-Fermi
approximation and beyond it; we find it depends on the number of the trapped atoms, through the chemical potential. Some suggestions
are given for the observation of the Josephson oscillation and the MQST.
Received 29 June 2001 and Received in final form 17 September 2001 相似文献
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It is shown that the problem of instantons in ferromagnetic materials in a large-spin model is reduced to an exactly integrable dynamical system with a finite number of variables. For a rather wide class of models, there exists a continuum of instanton paths that form a one-parameter family of paths with essentially different shapes but with the same value of the Euclidean action. On the basis of the formalism developed, exact instanton solutions are constructed that describe macroscopic quantum tunneling for a small ferromagnetic particle with uniaxial or biaxial quadratic anisotropy in the presence of a magnetic field applied perpendicularly to the easy axis. These solutions are valid for any relations between the anisotropy parameters and for any magnitude of the magnetic field and its direction in the base plane. Based on the solutions obtained, the principles of macroscopic quantum tunneling in high-spin-molecule-type magnetic particles are described. Tunneling regimes of two types are obtained: (1) regimes that are characterized by destructive interference of instanton trajectories and oscillatory dependence of the transition probability on the magnitude of the magnetic field and (2) regimes in which all instantons have the same purely real value of the Euclidean action and there is no destructive interference. 相似文献
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《Physics Reports》1999,322(5):347-417
In light of the fundamental importance and renewed interest of the tunnel phenomena, we review the recent development of semiclassical tunneling theory, particularly from the view point of “tunneling path”, beginning from a simple one-dimensional formula to semiclassical theories making use of the analytic continuation, in time, coordinates, or momentum, which are the stationary solutions of semiclassical approximations to the Feynman path integrals. We also pay special attention to the instanton path and introduce various conventional and/or intuitive ideas to generate tunneling paths, to which one-dimensional tunneling theory is applied. Then, we review the recent progress in generalized classical mechanics based on the Hamilton–Jacobi equation, in which both the ordinary Newtonian solutions and the instanton paths are regarded as just special cases. Those new complex-valued solutions are generated along real-valued paths in configuration space. Such non-Newtonian mechanics is introduced in terms of a quantity called “parity of motion”. As many-body effects in tunneling, illustrative numerical examples are presented mainly in the context of the Hamilton chaos and chemical reaction dynamics, showing how the multidimensional tunneling is affected by the system parameters such as mass combination and anisotropy of potential functions. 相似文献
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Rong Lü Hui Hu Jia-Lin Zhu Xiao-Bing Wang Lee Chang Bing-Lin Gu 《The European Physical Journal B - Condensed Matter and Complex Systems》2000,14(2):349-361
Resonant quantum tunneling of the Néel vector between nonequivalent magnetic wells is investigated theoretically for a nanometer-scale
single-domain antiferromagnet with biaxial crystal symmetry in the presence of an external magnetic field applied along the
easy anisotropy axis, based on the two-sublattice model. Both the Wentzel-Kramers-Brillouin exponent and the preexponential
factors are evaluated in the instanton contribution to the tunneling rate for finite and zero magnetic fields by applying
the instanton technique in the spin-coherent-state path-integral representation, respectively. The quantum interference or
spin-parity effects induced by the topological phase term in the Euclidean action are discussed in the rate of quantum tunneling
of the Néel vector. In the absence of an external applied magnetic field, the effect of destructive phase interference or
topological quenching on resonant quantum tunneling of the Néel vector is evident for the half-integer excess spin antiferromagnetic
nanoparticle. In the weak field limit, the tunneling rates are found to oscillate with the external applied magnetic field
for both integer and half-integer excess spins. We discuss the experimental condition on the applied magnetic field which
may allow one to observe the topological quenching effect for nanometer-scale single-domain antiferromagnets with half-integer
excess spins. Tunneling behavior in resonant quantum tunneling of the magnetization vector between nonequivalent magnetic
wells is also studied for a nanometer-scale single-domain ferromagnet by applying the similar technique, but in the large
noncompensation limit.
Received 4 June 1999 相似文献
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Daniel Arteaga Esteban Calzetta Albert Roura Enric Verdaguer 《International Journal of Theoretical Physics》2003,42(6):1257-1280
We examine the possibility that a metastable quantum state could experiment a phenomenon similar to thermal activation but at zero temperature. To do that we study the real-time dynamics of the reduced Wigner function in a simple open quantum system: an anharmonic oscillator with a cubic potential linearly interacting with an environment of harmonic oscillators. Our results suggest that this activation-like phenomenon exists indeed as a consequence of the fluctuations induced by the environment and that its associated decay rate is comparable to the tunneling rate as computed by the instanton method, at least for the particular potential of the system and the distribution of frequencies for the environment considered in this paper. However, we are not able to properly deal with the term which leads to tunneling in closed quantum systems, and a definite conclusion cannot be reached until tunneling and activation-like effects are considered simultaneously. 相似文献
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A new method to compute effects of tunneling in one-dimensional multiple well is developed. A tunneling parameter built with physical quantities is introduced to measure the splitting between eigenvalues due to tunneling. These splittings are given by convergent series in term of this tunneling parameter for a wide class of double well. 相似文献
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本文对双势阱中凝聚的冷原子通过约瑟夫森结隧穿时所形成的亚稳态进行研究.通过介观自旋算符建立了体系的精确量子相位模型,利用对量子自旋的势场描述给出了在两阱中振荡的原子之间相位差为时,即亚稳态时所要满足的物理条件,并用瞬子方法计算了该亚稳态存在的寿命. 相似文献