Towards observing the intercommutation of flux tubes in superconductors

Abstract

We propose a simple experiment to investigate the intercommutation of flux tubes in type II superconductors. Using this method the intercommutation of strings can be observed directly and the dependence of intercommutation on the angle of crossing of strings can also be analyzed.Interaction of topological defects has been a subject of great interest. Interaction of flux tubes in type II superconductors is not only of theoretical interest but also has technological implications. In many particle theory models, strings in the early Universe play crucial role in generating density fluctuations responsible for the structure formation. In such models interaction properties of strings are of utmost importance. An essential property of cosmic strings is the intercommutativity; that is when two strings cross each other, they exchange partners at the crossing point as shown in Fig.1. Due to this property, large loops of cosmic strings chop themselves into smaller ones which rapidly decay through oscillations and emission of radiation. If this was not so then strings would have dominated the energy density of the Universe very early and would be observationally ruled out.There have been many theoretical studies of string intercommutativity in relativistic field theory models. Shellard investigated string interactions numerically and calculated the probability of intercommutation as a function of the crossing angle and the relative velocity of strings for global U(1) strings [1]. He showed that strings almost always intercommute even for velocities very close to the speed of light. For gauge strings intercommutativity was demonstrated in numerical simulations by Moriarty, Myers and Rebbi [2]. Some qualitative understanding of the intercommutativity of global strings is provided in [3]. Intercommutativity for global strings has been seen to occur in liquid crystal experiments where one clearly sees strings chopping in smaller loops which shrink down rapidly [4]. Fluid vortices also show intercommutativity [5]. There is some indirect evidence for intercommutation of flux tubes in type II superconductors [6]. [Assuming that flux tubes intercommute, it has been suggested in [7] that the density of strings produced in a superconducting transition can be experimentally investigated.] However, a systematic experimental study of the intercommutation of strings has not been possible, primarily due to the difficulties in controlling the crossing of the strings.In this letter we propose a simple experiment where the intercommutativity of flux tubes can be directly investigated. The essential idea of the experiment is that flux tubes in type II superconductors, which arise due to an external magnet, tend to follow the motion of the magnet if that magnet is gradually moved along the surface of the superconductor [8]. This provides us with a way of effectively holding the ends of the flux tubes. All that is needed after this is to take a crossed configuration of flux tubes and move magnets to force one bunch of flux tubes to go ...