What is superconductivity?

Some conductors are better than others; the key is organization. Good train conductors keep the railroads running on time -- and Arturo Toscanini kept the NBC Symphony Orchestra playing in time -- by wrangling complex elements into orderly systems.

Good electrical conductors display a similarly harmonious organization but must contend with resistance. In fact, resistance is what separates conventional conductors from their super-powered cousins.

Think of free electrons in a typical conductor as people milling about a train terminal. An applied current is like the bell announcing the train's arrival: In an instant, individual motions transform into a single, united movement toward the platforms -- or would, were it not for a few troublemakers who stumble, jostle, dither at the newsstands or refuse to make way on the escalator. Thanks to the resistance they provide, some travelers miss the train, and the current loses energy. Such is life in Conductor Terminal.

Now, replace those travelers with an undercover flash mob. At the bell, they partner up and perform a synchronized, choreographed dance across the terminal. No one misses the train, and they're all less tired when they get there. That's the wonder of travel in Superconductor Station.

Before we study the steps of this particle pas de deux, however, let's take a step back and review how resistance muddles mundane materials. We'll start simple and add complexity as we go.

Although there are exceptions, when we say electric current, we usually mean a stream of electrons running through a medium. How well a material conducts electricity relates to how easily its component atoms donate electrons. Insulators are miserly, whereas conductors spend theirs like sailors on shore leave.

The donated electrons, now known as conductance electrons, do not orbit individual atoms but instead float freely throughout the conductor, like our train commuters above. When a current is applied, they flow through the material and transmit electricity.

A conductor consists of a lattice of atoms; for electricity to flow, electrons must move through this lattice with as little interference as possible. Like a bunch of tennis balls thrown through a jungle gym, odds are good that some electrons will hit the lattice. The odds of interference go up if areas are bent out of shape. Thus, it's easy to see how material flaws constitute one cause of resistance in conductors.

In this jungle gym analogy, atoms are represented by the intersections of metal rods. In actuality, a conductor's lattice isn't stiff; its atoms vibrate, and the interactions connecting them oscillate, so it's better to think of it as a grid of springs. What causes these atoms to vibrate? The higher the temperature, the more the lattice vibrates, and the more likely our tennis balls are to run into interference. Chalk up the second major source of resistance to our old friend, heat.

This raises the question: If heat is the problem, might not cold be the answer? Just chill for a second: We'll get to that in the next section.

If heat increases resistance, then cranking down the thermostat ought to decrease it, right? Well, it does, within limits. In normal conductors, resistance falls as the thermometer drops, but it never disappears. Superconductors work a bit differently.

As a superconductor cools, it follows a similar curve of gradually dropping resistance until it reaches its particular critical temperature; then, abruptly, all resistance disappears. It's as if resistance were slowly losing a tug-of-war with conductance and then, frustrated, let go of the rope. Actually, the substance undergoes a phase transition. Like ice melting into water, the conventional material assumes a new state, one with zero resistance.

To understand what's going on here, we need to make a few modifications to our atomic jungle gym. Specifically, we need to start taking magnetism into account.

When the atoms in a conductor give up electrons, they become positively charged ions, causing a net attraction between the atomic lattice and the negatively charged electrons passing through it. In other words, as if vibrations and deformations weren't bad enough, the tennis balls we're throwing through our oscillating jungle gym are magnets. You might assume that this would increase their chances of encountering resistance while passing through our wobbly grid, and you'd be right -- for normal conductors. Superconductors, however, use it to their advantage.

Picture a pair of tennis balls thrown through the grid, one hot on the other's tail. As the first ball passes through the positively charged lattice, it attracts the surrounding atoms toward it. By bunching up, these atoms create a local area of higher positive charge, which increases the force pulling the second electron forward. Consequently, the energy spent to get through, on average, breaks even.

Like square dancers, these Cooper pairs form and break up constantly, but the overall effect perpetuates itself down the line, enabling electrons to zip through the superconductor like greased lightning.

Cooper pairs are named for physicist Leon N. Cooper who, with John Bardeen and John Robert Schrieffer, advanced the first successful model explaining superconductivity in conventional superconductors. Their achievement, known as the BCS Theory in their honor, garnered them the 1972 Nobel Prize in physics.

Superconductivity refused to remain pinned down for long, however; soon after the BCS Theory achieved traction in the field, researchers began discovering other superconductors -- such as high-temperature superconducting copper-oxides -- that broke the BCS model.

In this next section, we'll look at what sets these exotic superconductors apart from the rest.

Depending on how you slice the pie, there are either many kinds of superconductors or only two. From the perspective of how they behave in magnetic fields, however, scientists commonly classify them into two groups.

A Type I superconductor is usually made of a pure metal. When cooled below its critical temperature, such a material exhibits zero electrical resistivity and displays perfect diamagnetism, meaning magnetic fields cannot penetrate it while it is in the superconducting state.

Type II superconductors are usually alloys, and their diamagnetism is more complex. To understand why, we need to look at how superconductors respond to magnetism.

Just as every superconductor has a critical temperature that makes or breaks its superconducting state, each is also subject to a critical magnetic field. A Type I superconductor enters and leaves the superconducting state at one such threshold, but a Type II material changes states twice, at two different magnetic field thresholds.

The distinction between Type I and Type II materials resembles the difference between dry ice (solid carbon dioxide) and water ice. Both solids cool well, but they handle heat differently: Water ice melts into a mixed state, ice water, whereas dry ice sublimates: At normal pressure, it transitions directly from solid to gas.

With respect to magnetism, a Type I superconductor is like dry ice: When exposed to its critical field, its superconductivity burns off instantly. A Type II is more versatile.

While within a weak field, a Type II material exhibits behavior similar to a Type I, just as H₂O and CO₂ both cool effectively while in their solid states. Raise the magnetic field above a certain threshold, however, and the material reorganizes into a mixed state -- a vortex state in which small whirlpools of superconducting current flow around islands of normal material. Like ice water, it still does its job pretty well. If the magnetic field strength rises, however, the islands of normalcy grow together, thus destroying the surrounding whirlpools of superconductivity.

What does this mixed state mean for magnetism? We've discussed what happens when a superconductor gets warm. Now, let's look at it from the other direction.

In their normal, warm states, both Type I and Type II materials allow magnetic fields to flow through them, but as they cool toward their critical temperatures, they increasingly expel these fields; electrons in the material set up eddy currents that produce a counter-field, a phenomenon known as the Meissner effect.

When they reach their critical temperature, Type I superconductors evict any remaining magnetic field like so many deadbeat roommates. Depending on the strength of the magnetic field in which they exist, Type II fields might do the same -- or they might get a little clingy. If they're in a vortex state, the magnetic field that still flows through the islands of normal material in their superconducting streams can become stuck, a phenomenon known as flux pinning (see sidebar) Magnetic flux is a measure of the amount of magnetic field passing through a given surface.

Because they can remain superconductors in this stronger magnetic field, Type II materials like niobium-titanium (NbTi) make good candidates for the type of superconducting magnets found in, say, Fermilab's proton accelerator or in MRI machines.

The industrial and scientific applications of superconductors are limited by the special temperature conditions they require to work their electromagnetic mojo, so it makes sense to classify materials based on their critical temperatures and pressure requirements.

Hundreds of substances, including 27 metallic elements -- such as aluminum, lead, mercury and tin -- become superconductors at low temperatures and pressures. Another 11 chemical elements -- including selenium, silicon and uranium -- transition to a superconductive state at low temperatures and high pressures [source: Encyclopaedia Britannica].

Until 1986, when IBM researchers Karl Alexander Mulller and Johannes Georg Bednorz ushered in the age of high-temperature superconductors with a barium-lanthanum-copper oxide that achieved zero resistance at 35 K (minus 238 C, minus 397 F), the highest critical temperature achieved by a superconductor measured 23 K (minus 250 C, minus 418 F). Such low-temperature superconductors required cooling by liquid helium, which was difficult to produce and tended to break budgets [source: Haldar and Abetti]. High-temperature superconductors bring the temperature range up to around 130 K (minus 143 C, minus 226 F), meaning they can be cooled using liquid nitrogen made cheaply from air [source: Mehta].

Although physicists understand the mechanisms governing low-temperature superconductors, which follow the BCS model, high-temperature superconductors remain enigmatic [source: CERN]. The holy grail would be to achieve a material with zero resistance at room temperature, but thus far that dream remains elusive. Perhaps it cannot be done or, perhaps, like other scientific revolutions, it lies just over the horizon, awaiting the necessary technological or theoretical innovation to make the dream a reality.

In the meantime, the powerful advantages that superconductors offer suggest a wide array of present and future applications in the areas of electric power, transportation, medical imaging and diagnostics, nuclear magnetic resonance (NMR), industrial processing, high energy physics, wireless communications, instrumentation, sensors, radar, high-end computing and even cryogenics [source: CCAS].

In addition to the maglev, MRI and particle accelerator applications we mentioned earlier, superconductors are currently used commercially in NMR spectroscopy, a key tool for biotechnology, genomics, pharmaceutical research and materials science work. Industry also applies them in a magnetic process for separating kaolin clay, a common filler in paper and ceramic products.

As for the future, if researchers and manufacturers can overcome superconductors' limitations of cost, refrigeration, reliability and acceptance, the sky's the limit. Some see green technologies, such as windmills, as the next step in a more widespread acceptance and application of the technology, but larger possibilities loom.

Who knows? Perhaps a future reader will peruse this very article on a computer equipped with near-light-speed processors, hooked to a grid powered by fusion reactors -- all thanks to superconductivity.