Some exact analytical results and a semi-empirical formula for single electron ionization induced by ultrarelativistic heavy ions

The delta function gauge of the electromagnetic potential allows semiclassical formulas to be obtained for the probability of exciting a single electron out of the ground state in an ultrarelativistic heavy ion reaction. Exact formulas have been obtained in the limits of zero impact parameter and large, perturbative, impact parameter. The perturbative impact parameter result can be exploited to obtain a semi-empirical cross section formula of the form, sigma = A ln(gamma) + B, for single electron ionization. A and B can be evaluated for any combination of target and projectile, and the resulting simple formula is good at all ultrarelativistic energies. The analytical form of A and B elucidates a result previously found in numerical calculations: scaled ionization cross sections decrease with increasing charge of the nucleus being ionized. The cross section values obtained from the present formula are in good agreement with recent CERN SPS data from a Pb beam on various nuclear targets.Comment: 14 pages, latex, revtex source, no figure

Two Center Light Cone Calculation of Pair Production Induced by Ultrarelativistic Heavy Ions

An exact solution of the two center time-dependent Dirac equation for pair production induced by ultrarelativistic heavy ion collisions is presented. Cross sections to specific final states approach those of perturbation theory. Multiplicity rates are reduced from perturbation theory.Comment: 22 pages, latex, revtex source, one postscript figur

Exact Z2Z^2 scaling of pair production in the high-energy limit of heavy-ion collisions

The two-center Dirac equation for an electron in the external electromagnetic field of two colliding heavy ions in the limit in which the ions are moving at the speed of light is exactly solved and nonperturbative amplitudes for free electron-positron pair production are obtained. We find the condition for the applicability of this solution for large but finite collision energy, and use it to explain recent experimental results. The observed scaling of positron yields as the square of the projectile and target charges is a result of an exact cancellation of a nonperturbative charge dependence and holds as well for large coupling. Other observables would be sensitive to nonperturbative phases.Comment: 4 pages, Revtex, no figures, submitted to PR

Magnetic domain structure and dynamics in interacting ferromagnetic stacks with perpendicular anisotropy

The time and field dependence of the magnetic domain structure at magnetization reversal were investigated by Kerr microscopy in interacting ferromagnetic Co/Pt multilayers with perpendicular anisotropy. Large local inhomogeneous magnetostatic fields favor mirroring domain structures and domain decoration by rings of opposite magnetization. The long range nature of these magnetostatic interactions gives rise to ultra-slow dynamics even in zero applied field, i.e. it affects the long time domain stability. Due to this additionnal interaction field, the magnetization reversal under short magnetic field pulses differs markedly from the well-known slow dynamic behavior. Namely, in high field, the magnetization of the coupled harder layer has been observed to reverse more rapidly by domain wall motion than the softer layer alone.Comment: 42 pages including 17 figures. submitted to JA

Dynamic binding of driven interfaces in coupled ultrathin ferromagnetic layers

We demonstrate experimentally dynamic interface binding in a system consisting of two coupled ferromagnetic layers. While domain walls in each layer have different velocity-field responses, for two broad ranges of the driving field, H, walls in the two layers are bound and move at a common velocity. The bound states have their own velocity-field response and arise when the isolated wall velocities in each layer are close, a condition which always occurs as H->0. Several features of the bound states are reproduced using a one dimensional model, illustrating their general nature.Comment: 5 pages, 4 figures, to be published in Physical Review Letter

Asymptotic channels and gauge transformations of the time-dependent Dirac equation for extremely relativistic heavy-ion collisions

We discuss the two-center, time-dependent Dirac equation describing the dynamics of an electron during a peripheral, relativistic heavy-ion collision at extreme energies. We derive a factored form, which is exact in the high-energy limit, for the asymptotic channel solutions of the Dirac equation, and elucidate their close connection with gauge transformations which transform the dynamics into a representation in which the interaction between the electron and a distant ion is of short range. We describe the implications of this relationship for solving the time-dependent Dirac equation for extremely relativistic collisions.Comment: 12 pages, RevTeX, 2 figures, submitted to PR

A light-fronts approach to electron-positron pair production in ultrarelativistic heavy-ion collisions

We perform a gauge-transformation on the time-dependent Dirac equation describing the evolution of an electron in a heavy-ion collision to remove the explicit dependence on the long-range part of the interaction. We solve, in an ultra-relativistic limit, the gauged-transformed Dirac equation using light-front variables and a light-fronts representation, obtaining non-perturbative results for the free pair-creation amplitudes in the collider frame. Our result reproduces the result of second-order perturbation theory in the small charge limit while non-perturbative effects arise for realistic charges of the ions.Comment: 39 pages, Revtex, 7 figures, submitted to PR

On the nature of Coulomb corrections to the e^+e^- pair production in ultrarelativistic heavy-ion collisions

We manifest the origin of the wrong conclusion made by several groups of authors on the absence of Coulomb corrections to the cross section of the e^+e^- pair production in ultrarelativistic heavy-ion collisions. The source of the mistake is connected with an incorrect passage to the limit in the expression for the cross section. When this error is eliminated, the Coulomb corrections do not vanish and agree with the results obtained within the Weizs\"acker-Williams approximation.Comment: 7 pages, LaTe

Coulomb corrections and multiple e+e- pair production in ultra-relativistic nuclear collisions

We consider the problem of Coulomb corrections to the inclusive cross section. We show that these corrections in the limiting case of small charge number of one of the nuclei coincide with those to the exclusive cross section. Within our approach we also obtain the Coulomb corrections for the case of large charge numbers of both nuclei.Comment: 7 pages, REVTeX

Size effects on exchange bias in sub-100 nm ferromagnetic-antiferromagnetic dots deposited on prepatterned substrates

This article may be downloaded for personal use only. Any other use requires prior permission of the author and the American Institute of Physics.Exchange bias effects have been investigated in ferromagnetic (FM)-antiferromagnetic (AFM) square dots, with lateral sizes of 90 nm, sputtered on a prepatterned Si substrate. The magnetic behavior of the dots has been compared with that of a continuous FM-AFM bilayer with the same composition. Along the unidirectional direction, the dots exhibit square hysteresis loops and preserve an exchange bias field, HE, of 70 Oe at room temperature, which is about 40% smaller than HE in the continuous film. In addition, the distribution of blocking temperatures in the nanostructures is found to be shifted toward lower values with respect to that in the continuous film. These results can be interpreted assuming that the reduced lateral dimensions of the nanostructures impose some constraints on the formation and pinning of domain walls in the AFM layer