报告题目：

1.        Unexpected behavior of atoms in traps and of hydrogen in crossed fields

2.        Chaos-induced pulse trains in the ionization of hydrogen

3.        Clinical and Dynamical Significance of Oscillations of Heart Rates in Premature Infants

 

报告人：    Prof. J B Delos

Physics Department

College of William and Mary, USA

 

时间和地点：

1.      10月29日(星期三) 下午2：30，六楼报告厅

2.      10月31日(星期五) 下午2：30，六楼报告厅

3.      11月03日(星期一) 下午2：30，六楼报告厅

 

欢迎参加！

 

报告摘要：

 

 

 

 

 

 

 

 

       We say that a system exhibits monodromy if we take the system around a closed loop in its parameter space, and we find that the system does not come back to its original state. Many systems have this property: atoms in a trap, a hydrogen atom in crossed fields, and linear and quasilinear molecules. 

 

       A hydrogen atom in weak perpendicular and near-perpendicular electric and magnetic fields is one of the systems exhibiting quantum monodromy. Using perturbation theory, Sadovskii and Cushman showed predicted the presence of monodromy in perpendicular fields. It shows up as a defect in the lattice of quantum states.  When the fields are tilted from perpendicular, these lattice defects undergo a series of bifurcations.

 

       A newly discovered dynamical manifestation of monodromy can be illustrated by the behavior of atoms in a trap.  Let us imagine a collection of noninteracting classical particles moving in a two-dimensional circular box with a hard reflecting wall, and with a cylindrically-symmetric potential energy barrier.  Let us start all the particles moving on one line with angular momentum L = 0 , and with energy E < 0. Then let us impose additional smooth forces and torques on the particles so that [L(t),E(t)] move in a circle around the origin in the [L,E] plane. In other words, apply a torque to increase the angular momentum, then drive the particles to a higher energy (above the barrier), then reduce the angular momentum to a negative value, reduce the energy, and finally come back to the initial energy and angular momentum. Where in space do the particles end up? The answer is surprising.

 

 

 

 

 

We examine excitation (by a short laser pulse) of a hydrogen atom in parallel electric and magnetic fields, from an initial tightly bound state to a state above the classical ionization threshold. We predict that the atom ionizes by emitting a train of electron pulses. This prediction is based on the classical dynamics of electron escape. In particular, the pulse train is due to classical chaos, which occurs for nonvanishing magnetic field. We connect the structure of the pulse train to fractal structure in the escape dynamics.

 

 

 

 

The pacemaking system of the heart is complex; a healthy heart constantly integrates and responds to extracardiac signals, resulting in highly complex heart rate (HR) patterns.  It is impossible at present to account for all of the factors and feedback loops affecting HR under normal conditions.  In controlled laboratory situations and in some pathological or age-related states, however, dynamics can show reduced complexity that is more readily described and modeled.  Reduced complexity has both clinical and dynamical significance – it may provide warning of impending illness or clues about the dynamics of the heart’s pacemaking system.    Here we report the discovery of a reversible transition to low-dimensional, large-amplitude periodic oscillations in the cardiac rhythm of human neonates, and we propose a mathematical interpretation as a noisy hard Hopf bifurcation (a common feature of many physical systems) in the dynamics of cardiac pacemaking.  We created a new wavelet-based detector of  decelerations in heart rate, and applied it to a large database of clinical and electrocardiographic data from infants in neonatal intensive care units.  Sepsis, a bacterial infection of the bloodstream, accounted for some cases of oscillatory heart rate dynamics, but other cases spontaneously arose in the absence of any diagnosed illness.  This oscillatory behavior was observed across a large population of premature infants, so we believe that it is a general dynamical mode of the human cardiac pacemaking system near the time of birth.