π-junction realization due to tunneling through a thin ferromagnetic layer

It is demonstrated that in superconductor-ferromagnet-superconductor (S/F/S) systems in the case of low interface transparency the transition into π-phase is not related with the oscillations of the superconducting order parameter in F-layer. Consequently, the π-phase may exist at very thin F-layer thickness. The crossover from π-to 0-phase results in the nonmonotonous temperature dependence of the critical current.     

Rotation in an asymmetric multidimensional periodic potential due to colored noise

We analyze the motion of an overdamped classical particle in a multidimensional periodic potential, driven by an external noise. We demonstrate that in the steady state the presence of temporal correlations in the noise and spatial asymmetry within a period of the potential could lead to particle rotation. The rotation is a direct consequence of a change in the sign of the noise-induced drift motion in each dimension. By choosing different potentials, we can generate a variety of flow patterns from laminar drifts to rotations.     

The polarization of electrons by tunneling through a spin-dependent surface potential

A model calculation is discussed which demonstrates that the spin-dependence of the surface potential can be an important factor in determining the field-emitted electron spin polarization (FEESP) from ferromagnetic metals. The significance of this model calculation for the interpretation of FEESP from Ni and Fe is discussed.     

Correlation mechanism of negative differential resistance for tunneling through a molecule

A model for electron transport through a single molecule with two electronic states, one of which is strongly hybridized with contacts and the other is localized, has been investigated. A solution for this model in the limit of strong intramolecular Coulomb interaction is found. A mechanism of negative differential resistance caused by strong intramolecular correlations is revealed.     

Dynamical tunneling of bound systems through a potential barrier: Complex way to the top

A semiclassical method of complex trajectories for the calculation of the tunneling exponent n systems with many degrees of freedom is further developed. It is supplemented with an easily implemented technique that enables one to single out the physically relevant trajectory from the whole set of complex classical trajectories. The method is applied to semiclassical transitions of a bound system through a potential barrier. We find that the properties of physically relevant complex trajectories are qualitatively different in the cases of potential tunneling at low energy and dynamical tunneling at energies exceeding the barrier height. Namely, in the case of high energies, the physically relevant complex trajectories describe tunneling via creation of a state close to the top of the barrier. The method is checked against exact solutions of the Schrödinger equation in a quantum-mechanical system of two degrees of freedom.     

A new broadening mechanism in tunneling systems

A broadening mechanism which arises through the phonon dressing factor is presented. Some of the experimental results which are more consistent with this type of broadening are mentioned. Finally, the extension to other types of tunneling systems, such as glasses, is discussed briefly.     

Massive Dirac electron tunneling through a time-periodic potential in single layer graphene

We explore the transmission through a time-periodic potential barriers in single layer graphene with an induced energy gap in this work. The analytical solutions of the transmission probabilities for the central band and the first sidebands are obtained. The transmission probabilities as the function of the induced energy gap and the incidence electron energy are discussed. We also investigate how critical angles change as the induced energy gap and incidence electron energy. The study of transmission properties will benefit applications in graphene-based nano-electronics.     

Dissipative tunneling through a parabolic potential in the Lindblad theory of open quantum systems

By using the Lindblad theory for open quantum systems, an analytical expression of the tunneling probability through an inverted parabola is obtained. This penetration probability depends on the environment coefficients. It is shown that the tunneling probability increases with the dissipation and the temperature of the thermal bath. Received 15 October 1999 and Received in final form 5 January 2000     

Slow passage through a transcritical bifurcation for Hamiltonian systems and the change in action due to a nonhyperbolic homoclinic orbit

One-degree of freedom conservative slowly varying Hamiltonian systems are analyzed in the case in which a saddle-center pair undergo a transcritical bifurcation. We analyze the case in which the method of averaging predicts the solution crosses the unperturbed homoclinic orbit at the precise time at which the transcritical bifurcation occurs. For the slow passage through the nonhyperbolic homoclinic orbit associated with a transcritical bifurcation, the solution consists of a large sequence of nonhyperbolic homoclinic orbits surrounded by autonomous nonlinear saddle approaches. The change in action is computed by matching these solutions to those obtained by averaging, valid before and after crossing the nonhyperbolic homoclinic orbit. For initial conditions near the stable manifold of the nonhyperbolic saddle point, one saddle approach has particularly small energy and instead satisfies a nonautonomous nonlinear equation, which provides a transition between nonhyperbolic homoclinic orbits, centers, and saddles. (c) 2000 American Institute of Physics.     

Fractional-power-law level statistics due to dynamical tunneling

For systems with a mixed phase space we demonstrate that dynamical tunneling universally leads to a fractional power law of the level-spacing distribution P(s) over a wide range of small spacings s. Going beyond Berry-Robnik statistics, we take into account that dynamical tunneling rates between the regular and the chaotic region vary over many orders of magnitude. This results in a prediction of P(s) which excellently describes the spectral data of the standard map. Moreover, we show that the power-law exponent is proportional to the effective Planck constant h(eff).     

An analysis of transverse fluctuations in multidimensional tunneling

We examine the asymptotic behaviour of the ground state tunnel-splitting of the multidimensional double well, with non-quadratic minima, where instanton techniques are inapplicable. We apply the recently developed path decomposition expansion for two model problems; the important effects of the transverse degrees of freedom are explored. In particular we discuss tunneling in the presences of a vacuum valley, which describes features exhibited in SU(2) gauge theories in a finite volume.     

Features of the tunneling of a system with an internal degree of freedom through a potential barrier

The tunneling of a quantum system with an internal degree of freedom through a potential barrier is considered. Based on the exact numerical solution to the nonstationary Schrödinger equation, the tunneling of a model two-particle system through a potential barrier is studied and the dependences of the tunneling transparency of the barrier on the parameters of the wave packet that describes the system at the initial moment are obtained. A sharp increase in the tunneling probability related to the formation of a long-lived quasibound state of the system in the barrier region is demonstrated. A simple analytical model of the tunneling of a system with an internal degree of freedom that allows for a qualitative interpretation of the main features of the tunneling is constructed.     

Asymmetric Landau-Zener tunneling in a periodic potential

Using a simple model for nonlinear Landau-Zener tunneling between two energy bands of a Bose-Einstein condensate in a periodic potential, we find that the tunneling rates for the two directions of tunneling are not the same. Tunneling from the ground state to the excited state is enhanced by the nonlinearity, whereas in the opposite direction it is suppressed. These findings are confirmed by numerical simulations of the condensate dynamics. Measuring the tunneling rates for a condensate of rubidium atoms in an optical lattice, we have found experimental evidence for this asymmetry.     

Parity impurities due to a static PNC potential

It is shown that unlike the σ·ϱ single-particle, velocity-dependent potential, a static parity non-conserving (PNC) potential can lead to both electric as well as magnetic irregular transitions even for a spin-independent nuclear Hamiltonian. Specific calculations are made of the circular polarization P to be expected in the case of the 480 keV γ-transition in ¹⁸¹Ta due to a static PNC potential. It is found that P ≈ −0.7 · 10⁻⁴.