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A new computer simulation developed by physicists at the University of Michigan is enabling scientists to "see" what is happening inside superconductors, which could help solve fundamental mysteries about how vortices and the electrical currents that whirl around them pass through superconducting materials.
"When vortices move they dissipate energy and destroy the material's superconductivity — the unique ability to transmit electrical currents without resistance," said Franco M. Nori, an associate professor of physics at University of Michigan, who presented his findings at a Wednesday afternoon session at the 1996 APS March Meeting. "Understanding how vortices alternately become trapped and break free as they move through superconductors is crucial to minimizing energy loss and can help us develop improved practical applications for superconducting technology — especially more powerful magnets for use in medical imaging systems and particle accelerators."
The University of Michigan simulations were developed in collaboration with experimentalists, and were based on laboratory measurements of voltage pulses and magnetic fields generated by lines of magnetic flux passing through superconducting materials. The advantage of computer simulations, according to Nori, is that they allow scientists to systematically vary the many factors that affect vortex transport phenomena — such as temperature, magnetic field strength, or the number and position of defects or pinning sites in the material — and observe how the vortices react.
According to the simulations, the magnetic field lines known as vortices flow through superconductors in streams that pool and eddy behind obstacles and merge into broad channels in open areas. If these obstacles, or "pits," are deep or strong, the vortex cannot escape and the pit remains filled. If the pits are shallow or "weak," vortices can be pushed out by the pressure of other vortices piling up behind them, producing sudden bursts of energy and a branching network of narrow meandering trails as the vortices alternately dam up and break through the pit barriers.
The forces producing these avalanches or sudden bursts of energy are the subject of intense study, not only in superconductors, but also in sand lies, water droplets, magnetic bubble arrays, earthquakes and other complex systems. "All these apparently dissimilar systems have interacting moveable objects that repel each other and are pushed toward instability by an external driving force," said Nori. "During the unstable state, particle transport occurs in the form of avalances or cascades which release accumulated strain in the system."
Nori and his colleagues are currently studying superconductors with periodic arrays of pinning sites that produce very stable vortex configurations which are unaffected by increasing currents or magnetic fields. They are using vortex transport simulations to explore basic questions about what happens when an elastic lattice is forced onto a rigid substrate, which could lead to applications in many other fields of physics.
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