Instanton paths and coherent quantum tunneling in antiferromagnetic spin clusters subject to a strong magnetic field

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.     

Macroscopic quantum tunneling in small magnetic particles

Macroscopic quantum tunneling of magnetization in small antiferromagnetic clusters is studied for the spin-1/2 anisotropic (J _z ≡ J, J _x =J _y ≡J _xy ) Heisenberg model with the use of both perturbation theory and an exact numerical diagonalization technique. Splitting of the two lowest levels due to the lifting of the degeneracy between them (in the case J _xy =0) by the in-plane exchange (J _xy ≠ 0) is found to occur in the N/2th order of perturbation theory. Tunneling is absent in systems with an odd number of sites, in which the levels are doubly degenerate in zero magnetic field. Pis’ma Zh. éksp. Teor. Fiz. 63, No. 9, 688–693 (10 May 1996) Published in English in the original Russian journal. Edited by Steve Torstveit.     

Linear mobility for coherent quantum tunneling in a periodic potential

The dynamics of a quantum particle moving in a tight binding lattice and coupled to an ohmic heat bath is investigated. Imaginary-time functional integral methods are utilized to determine the self-energy of the velocity-velocity correlation function for weak damping and arbitrary temperatures. Recent results for the linear mobility atT=0 and high temperatures are confirmed and extended to intermediate temperatures. The dominant corrections to the zero temperature mobility are given in terms of a power law.On leave from Dipartimento di Fisica, CISM, Università di Genova Italy     

Interference of instanton trajectories during quantum tunneling in small particles of real antiferromagnets

For a two-sublattice antiferromagnet, the Lagrangian is constructed taking into account Berry’s phase whose form is matched with the quantum-mechanical Heisenberg Hamiltonian. Tunnel effects are analyzed taking into account the crystallographic symmetry and possible types of Dzyaloshinski interaction. It is shown that, when the real magnetic symmetry and the Dzyaloshinski interaction are taken into consideration, the effects of destructive instanton interference and the suppression of macroscopic quantum tunneling may come into play. This may lead to a periodic dependence of the ground-state level splitting on the Dzyaloshinski interaction constant; the magnitude of this splitting is calculated.     

铁磁金属量子阱态的共振隧穿

非铁磁金属层中的量子阱态在磁输运过程中的重要性已被广泛认识．铁磁金属层中自旋极化的量子阱态以前并没有详尽的理论研究；实验上也没有清晰地观测到自旋极化量子阱态的隧穿．文章介绍了最近由卢仲毅、张晓光和Pantelides预言的Fe／MgO／FeO／Fe／Cr和其他铁磁量子阱隧道结中的共振隧穿，并解释铁、钴、铬的△1能带的对称性在这种共振隧穿中的作用．     

铁磁金属量子阱态的共振隧穿

非铁磁金属层中的量子阱态在磁输运过程中的重要性已被广泛认识.铁磁金属层中自旋极化的量子阱态以前并没有详尽的理论研究;实验上也没有清晰地观测到自旋极化量子阱态的隧穿.文章介绍了最近由卢仲毅、张晓光和Pantelides［1］预言的Fe/MgO/FeO/Fe/Cr和其他铁磁量子阱隧道结中的共振隧穿,并解释铁、钴、铬的Δ1能带的对称性在这种共振隧穿中的作用.     

Resonant spin tunneling in small magnetic particles

A new macroscopic test of spin tunneling in single-domain magnetic particles is proposed. It is shown that quantization of spin, even in particles of a considerable size, may lead to a detectable zero-field maximum in the dependence of the rate of the magnetic relaxation on the magnetic field. This effect must be especially pronounced in antiferromagnetic particles. It is argued that recent observation by several groups of the non-monotonic field dependence of the blocking temperature in ferritin may, in fact, present the experimental evidence of this effect.     

Self-consistent calculations of coherent tunneling oscillations in asymmetric double quantum wells

Coherent carrier tunneling in asymmetric double quantum well heterostructures is analytically investigated through a self-consistent method of calculation. Longitudinal electric fields are applied to reach the interwell resonance conditions. Exact wave functions in asymmetric double quantum well systems (modified Airy functions) are used to include the electron-hole interaction in the coherent tunneling process. Results are in good agreement with the available previous works.     

Decoherence and entanglement in coherent quantum tunneling of magnetization with dissipation

We study the coherent quantum tunneling of magnetization, for example, in a biaxial molecular magnet with dissipation of the environment which results in the suppression of the tunneling and therefore the decoherence of superposition of macroscopic quantum states in terms of the general spin-boson model. The degree of entanglement between the magnet and the environment is evaluated explicitly with the help of reduced density matrix. We show an interesting relation that the degree of entanglement approaches maximum value when the coherent tunneling is suppressed completely.     

Instanton contributions in reggeon quantum mechanics

The full semiclassical approximation to reggeon field theory without transverse dimensions is derived. By using Polyakov's method in Lagrangian form and paying due attention to the quantum terms of the potential it is shown that instanton contributions are able to explain the tunnel-like energy gap for α(0) ? 1 > > λ.     

On the problem of quantum tunneling of a singular potential

The problem of quantum tunneling through the singular potential barrier \(V(x) = \left\{ {\begin{array}{*{20}c} {V_0 (b/x - x/a)^2 ,} & {0 < \left| x \right| \leqslant \sqrt {ab} } \\ {0,} & {\left| x \right| > \sqrt {ab} } \\ \end{array} } \right.\) is discussed on the subject of the possibility to replace the singular behavior of the problem at the point x = 0 by a limiting process at the top of the truncated potential. The validity of such a replacement and, on this basis, the zero transparency of the quantum potential barrier are shown.     

Magnetic vortices in small ferromagnetic particles with the strong dipolar interaction

The ground state of cylindrical magnetic samples of different sizes at nonzero applied magnetic field is studied theoretically taking into account the exchange and dipole-dipole interactions at an arbitrary ratio of coupling constants. In addition to the weakly inhomogeneous and standard vortex states well known for the case of the weak dipole-dipole interaction, the vortex states with the complicated structure of the vortex core have been found. The state diagram for these particles has been constructed and analyzed in terms of scaling consideration.     

Renormalization problem in the quantum electrodynamics of non-point particles

The main postulates of the quantum electrodynamics of bilocal fields are formulated, the theory containing three universal constants c, h, and k. The fermion-photon form factor is determined, and the new theory is used to calculate radiative corrections in first-order perturbation theory for the fine-structure constant, fermion mass and magnetic moment, Coulomb potential, and fermion and photon Green's functions. The value = 1/137.0345 is obtained for the fine-structure constant.Kharkov Physico-Technical Institute of the Ukrainian Academy of Sciences. Translated from Izvestiya Vysshikh Uchebnykh Zavedenii, Fizika, No. 6, pp. 76–89, June, 1994.     

Semianalytic solution of the Kramers exit problem for a small ferromagnetic particle

The distribution Q(t) of magnetization reversal times in a small uniaxial particle is computed here directly from Brown's Fokker-Planck equation. Constant applied field and axial symmetry are assumed. The Laplace transform of Q(t) has the form Q(z)=F(1)(z)/F(2)(z) where the regular functions F(i)(z) are defined by a solution of a Volterra integral equation. A separate integral equation is derived for the function dF(2)(z)/dz, and the poles and residues of Q(z) may then be found numerically with arbitary precision.     

Controlled dephasing of a quantum dot: from coherent to sequential tunneling

Resonant tunneling through two identical potential barriers renders them transparent, as particle trajectories interfere coherently. Here we realize resonant tunneling in a quantum dot (QD), and show that detection of electron trajectories renders the dot nearly insulating. Measurements were made in the integer quantum Hall regime, with the tunneling electrons in an inner edge channel coupled to detector electrons in a neighboring outer channel, which was partitioned. Quantitative analysis indicates that just a few detector electrons completely dephase the QD.