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Vortices in the quantum stream of potential-well scattering

Ming-Houng Hsu, Keh-Ning Huang

Published Jul 8, 2004• 0 citations
Publisher

Abstract

Vortices around nodes where the wave function vanishes are studied in the potential scattering. They can be visualized in quantum streamlines, which follow the directions of current densities in space. We consider an incident particle with mass m and total energy E is scattered by a spherical potential-well. We shall study the vortex formation and its evolution at low incident energies.

DOI
10.1109/cleopr.2003.1274695

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Vortices around nodes where the wave function vanishes are studied in the potential scattering. They can be visualized in quantum streamlines, which follow the directions of current densities in space. We consider an incident particle with mass m and total energy E is scattered by a spherical potential-well. We shall study the vortex formation and its evolution at low incident energies.
Quantum mechanical streamlines and probability density contours help in the understanding of collision dynamics by showing what happens during the collision. This is illustrated by considering the elastic scattering of two particles which interact with a spherically symmetric square potential, V (r) =0 when r≳a and V (r) =C when r<a, where the constant C is negative for a potential well and positive for a barrier. The streamlines and density contours completely determine the wavefunction (which is separable in r and ϑ and independent of φ) since the pre-exponential factor is the square root of the density and the phase is everywhere perpendicular to the streamlines. The streamlines often form quantized vortices surrounding wavefunction nodes. Since the nodal regions are circular rings, the vortices are toroidal (like smoke rings). If one or both of the particles is charged, the vortex ring corresponds to a collisional magnetic moment. The distortion of the streamlines provides a visual explanation of the large collision cross sections. In S-wave resonances 4πr2=σ0, where r is the distance from the origin to the center of the outermost vortex and σ0 is the cross section of the S-wave component. Thus, r is essentially the ’’scattering length.’’ In P-wave resonances (where again r is the radial distance to the outermost vortex), 1.85πr2?σ1 and in D-wave resonances, 1.45πr2?σ2. In the square well resonances, the density inside of the interaction sphere has the shape of the dominant wave and often has maxima hundreds to thousands of times greater than the density of the incident wave. By happenstance, whenever the potential well has the proper depth to form a newly bound D state, there is a Ramsauer–Townsend effect so that (for small energy of the incident wave) the total scattering cross section is very small;outside the interaction zone, the streamlines are only slightly deflected by the potential well;inside, the streamlines are deflected towards the symmetry axis and the probability density is concentrated near this axis
Vortices are usually associated with systems containing large numbers of particles. Of particular topical interest though are those formed within atomic-scale wave functions and observed in macroscopic systems such as superfluids and quantum condensates. We uncover them here in one of the most fundamental quantum systems consisting of just one electron and two protons. Moreover, the results of novel simulations of the dynamics of this system reveal previously unknown mechanisms of angular momentum transfer and new ways to image atomic-scale quantized vortices at macroscopic distances. Probing of vortices and vortex-driven dynamics in quantum systems is thereby illustrated.
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We develop scattering theory in a noncommutative space defined by an su(2)su(2)su(2) coordinate algebra. By introducing a positive operator valued measure as a replacement for strong position measurements, we are able to derive explicit expressions for the probability current, differential and total cross sections. We show that at low incident energies the kinematics of these expressions is identical to that of commutative scattering theory. The consequences of spatial noncommutativity are found to be more pronounced at the dynamical level where, even at low incident energies, the phase shifts of the partial waves can deviate strongly from commutative results. This is demonstrated for scattering from a spherical well. The impact of noncommutativity on the well's spectrum and on the properties of its bound and scattering states are considered in detail. It is found that for sufficiently large well depths the potential effectively becomes repulsive and that the cross section tends towards that of hard sphere scattering. This can occur even at low incident energies when the particle's wavelength inside the well becomes comparable to the noncommutative length scale.