Science 1663

Magnetic Field of Dreams

Scientists use strong magnetic fields to investigate the inner workings of new, often-exotic materials. With the ability to create some of the highest magnetic fields in the world, Los Alamos is playing an important role in helping to understand the materials of tomorrow.

Ross McDonald threads a probe through the top of the 100-tesla multi-shot electromagnet, which extends downward about 7 feet. The large magnet is one of a half-dozen high-field magnets at the Magnet Lab.

With the flick of a few computer-controlled switches, a giant pulse of electricity races through the powerful electromagnet at the Los Alamos Pulsed Field Facility (the Magnet Lab). An immense magnetic field builds within the magnet's center, a field more than a million times stronger than Earth's and having the stored-energy equivalent of a few sticks of dynamite. Physicist Ross McDonald wonders whether it is strong enough to allow him to study the enigmatic electrons of a high-temperature superconductor.

Scientists at the Magnet Lab use strong ("high") magnetic fields to gain a fundamental understanding of a broad range of materials, from conducting metals to nonconducting insulators, from stringy polymers to heat-resistant ceramics. Often, these are new materials, hot off the laboratory bench and of interest to scientists because of what's going on inside them.

Consider the inner workings of metals. At the atomic scale, metals consist of positively charged atoms (ions) held together in a three-dimensional lattice by negatively charged electrons. Most of those electrons are bound to the ions, but some are free to roam through the lattice and, for example, conduct electricity.

In many of the metals brought to the Magnet Lab, those free electrons behave strangely. In some, the electrons act as if they were obese particles with masses hundreds of times greater than those of normal electrons. In others, the free electrons, which normally avoid each other, cluster at regular positions within the material, creating an unusual secondary lattice within the main one—a "charge-density wave."

Then there are superconductors. When these metals are cooled close to absolute zero temperature (–273ºC), the free electrons form "pairs," a special quantum state that allows both electrons to move through the lattice without resistance. As a result, electricity flows through a superconductor without energy loss.

Electric current sent through a wire coil creates a beautifully symmetric magnetic field that gets stronger as the current increases, the wire loops get smaller, or the number of loops increases. If the current is delivered as a pulse, a pulsed magnetic field is created.

In ordinary superconductors, the electron pairs break apart even before the temperature rises above a frigid –250ºC, and the metal becomes a normal conductor of electricity. That's not the case for the high-temperature superconductor being studied by a Los Alamos team including John Singleton Neil Harrison, Chuck Mielke, Fedor Balakirev and Ross McDonald. The superconductor's electrons remain paired for another 60 degrees or so. And researchers can't agree on why.

The scientists who study heavy-electron behavior, charge-density waves, or high-temperature superconductivity are the first to admit that the particular metals they study have few applications and are unlikely to make for better bridges or niftier iPods. But they also know that an understanding of the arcane electron behaviors in those metals will give scientists a greater chance of creating the next-generation materials that will make our buildings greener, our gadgets smaller, our power and light systems more efficient—or whatever else we can imagine.

Recently, some Magnet Lab staff took a major step toward deciphering the electron behavior in a high-temperature superconductor by measuring a distinctive and telling feature of the metal—its so-called Fermi surface. Such a measurement had eluded scientists for more than 20 years but was achieved using a cutting-edge measurement technique and the Lab's powerful new magnet, the "100-tesla multi-shot."

Higher Fields Are Better

Some of the highest pulsed magnetic fields on the planet are created at the Los Alamos Magnet Lab. The facility is part of the National High Magnetic Field Laboratory (NHMFL), a research center with headquarters at Florida State University in Tallahassee and additional facilities at the University of Florida in Gainesville. A user facility funded principally by the National Science Foundation but also by the State of Florida and the U.S. Department of Energy, the NHMFL generally affords users free access to the magnets for research in chemistry, biology, physics, geophysics, and medicine.

"Here at Los Alamos, the focus is on condensed-matter physics and materials science," says Marcelo Jaime, interim director of the Los Alamos facility. "We're also one of the few places that can conduct investigations into uranium or plutonium metals, as well as materials containing the two elements."

As an investigative tool, a magnetic field is analogous to temperature and pressure; it's something a scientist can control to change a material's properties. By mapping out how a property changes with field strength and comparing the results with theory, scientists can begin to understand the more-unusual electron interactions.

The Magnet Lab’s generator, the largest in the world, can supply second-long pulses of electricity to power the large magnets.

The changes come in part because a magnetic field interacts with the electron's spin—an intrinsic property that makes the electron act like a tiny magnet. The electron responds to the field like a subatomic compass needle, aligning its spin with or against the field direction. Depending on the alignment, the electron's energy shifts up or down by a small amount that's proportional to the field strength. The field also affects the path of a moving electron, pushing it sideways and altering its momentum.

"The field is like a big lever we use to induce significant changes in a material," says McDonald. "With higher fields, we can align more spins and shift the electron's energy or momentum enough to disrupt some of the stronger electron interactions, such as electron pairing in high-temperature superconductors. We couldn't have measured the Fermi surface without a really high field."

Electrons at the Fermi Surface

Transformers (red cylinders) shape the voltage pulse from the generator into customized pulses, producing fields that rise and fall as desired.

The number of electrons in most metals is astounding—in the neighborhood of a trillion trillion per cubic centimeter. Each of those electrons has a unique combination of energy, momentum, and spin alignment.

When a metal's total energy is at its theoretical minimum (at a temperature of absolute zero), its electrons assume every available energy at that temperature, from the lowest value to some very-high value known as the Fermi energy. A three-dimensional plot of the Fermi energy as a function of the electron momentum is the Fermi surface.

Because all energies below the surface are taken, most of the metal's electrons are locked in place energy-wise. They can't get enough additional energy (through collisions or whatnot) to access one of the available energies lying above the surface, so they don't respond to external influences. Only electrons on or close to the Fermi surface can change their energy and respond.

The Fermi surface is therefore a map of the metal's "important" electrons, the ones responsible for almost all of its electronic properties, and is an invaluable "reality check" for theorists trying to predict those properties. The shape of the Fermi surface determines the electrons' movement through the lattice, the metal's optical properties, and the likelihood that the metal will alter its electronic and/or crystal structure when subjected to stress. And by looking at how the measured surface differs from its predicted shape, scientists can infer new interactions that could explain strange electron behaviors.

A Mighty Magnet

A magnetic field is created when electricity runs around the circular turns of a wire coil. It takes hundreds of thousands of ampere-turns (current times the number of turns) to produce a high field, say, above 25 teslas. (A common refrigerator magnet has a strength of about 0.1 tesla.)

The Large Magnet Assembly Team surrounds the 100-tesla multi-shot magnet, seen wihtout its central insert coil.

The insert coil being loaded into the magnet.

A large coil may have hundreds of turns, and the thousands of amperes needed to produce 25 teslas generate enough heat (in a resistive, non-superconducting magnet) to melt the coil windings. That same field exerts a pressure on a piece of reinforcing steel that is more than twice what’s felt at the bottom of the ocean. Designing an electromagnet that can survive a multi-tesla field and be used over and over again is remarkably hard.

“Every high-field magnet will eventually break down because we push the limits of what the materials can withstand,” says Chuck Swenson, leader of the Pulsed Magnet Design project. “The challenge in producing ever-higher fields is to understand electromagnets well enough to create new, durable designs.”

Those designs are works of functional art, highly optimized and consisting of multiple coils placed one inside the other like a set of Russian dolls. Wherever and whenever possible, coils are wound from copper impregnated with nano-scale ribbons of niobium—an extremely strong yet excellent conductor. Reinforcing steel laced between the coils helps maintain their structural integrity.

All of the highest-field magnets are pulsed: a single swift current pulse sent through the assembly creates a field that rises, peaks, and decays (typically) within a few thousandths of a second. The short duration limits the heat and stress on the materials, so the highest fields can be contained without destroying the magnet.

More than a decade of research has culminated in the Magnet Lab’s 100-tesla multi-shot, currently the most-powerful reusable magnet in the world. Designed to operate at 100 teslas, the multi-shot has so far been kept to 89.9 teslas, still a world record.

The magnet’s commissioning late in 2006 added significant capabilities to the Magnet Lab, including the ability to break the superconducting state of a frozen cuprate.

A Super Conductivity Measurement

The Fermi surface reveals how the energy varies with momentum for the highest-energy electrons—those that have the Fermi energy. The simple Fermi surface of copper. The blue outline relates to the lattice. Every electron with a momentum that lies on the yellow surface has the Fermi energy.

The high-temperature superconductors are metallic compounds, also known as cuprates for the copper that is part of their composition. Since the cuprates' discovery in 1986, numerous theories have been proposed to explain why they remain superconducting at higher-than-usual temperatures. There was even speculation that these materials were not metals in the usual sense but were conducting electricity through some unknown mechanism. That's because all attempts to measure a cuprate's Fermi surface failed to prove it existed at all.

The unmistakable signature of the Fermi surface is the oscillation of the value of some property, such as the conductivity (a measure of the material's ability to conduct electricity), as an applied magnetic field increases. Such oscillations occur because the field shifts the energy of the electrons. At some field values, lots of electrons move onto the Fermi surface and the conductivity increases, while at others, the electrons move off the surface and the conductivity decreases.

"The oscillations are small," says Singleton. "Typically, there are three requirements needed to observe them: very-pure single-crystal samples, very-low temperatures, and a very-high magnetic field. For the cuprates, all three were problematic."

The strange Fermi surface (blue tubes) of a high-temperature superconductor. The inner box relates to the lattice. This surface is consistent with a model in which the spins of neighboring ions point in opposite directions.

First, the metals are far from being pure crystals. They're made by adding atoms (dopants) to the material in a disorderly manner that wreaks havoc on the crystal structure.

The low temperature was a second problem. Heat makes the ions vibrate, the vibrating ions bump electrons from their paths, and signals produced by the electrons become noisy. The noise can be reduced by making the measurements at very-low temperatures, but then the free electrons in the cuprates pair up and become superconducting. Unfortunately, pairing lowers the electron energy, so all the electrons leave the Fermi surface, making it impossible to measure its shape.

The solution to this second problem was to apply an immense magnetic field that would shift the electron energy enough to break the superconducting pairs apart while the metal was at low temperatures. The electrons would remain on the Fermi surface, and science could move forward. Producing that high field was a third problem. The Magnet Lab's solution was to develop the 100-tesla multi-shot—one of the most-powerful magnets in the world. (See "A Mighty Magnet.")

The Race Is On!

By 2007, more than two decades after high-temperature superconductivity was discovered, all the experimental obstacles had been overcome. Crystal growers had developed a technique to grow a cuprate in which the dopants were arranged in an orderly lattice. They were able to hand the experimentalists what amounted to a pure crystal. In addition, magnets of the necessary strength had been built.

Remarkably, the Los Alamos team and a Canadian group were racing to be the first to measure the Fermi surface of a cuprate metal. The Canadian group was hunkered down at the Laboratoire National des Champs Magnétiques Pulsés in Toulouse, France. The Los Alamos team was using the 100-tesla magnet and an advanced technique, contactless conductivity, to make the measurement.

Prototype insert coil for the 100-tesla multi-shot. Consisting of 16 coils layered in two sets of 8, this assembly should allow the multi-shot to reliably produce fields of 100 teslas.

"We spent a lot of time developing and refining contactless conductivity," comments Mielke, head of the Magnet Lab's user program. "A tiny electrical coil surrounds, but doesn't touch, the sample. The coil and sample each have an inductance, or magnetic resistance. When the field is off, the inductance causes a tuned circuit to resonate at a certain frequency. When the field turns on and increases, the sample's conductivity oscillates in value, which causes its inductance—and its resonance frequency—to oscillate as well. From those frequency oscillations we can deduce, with amazing precision and resolution, the shape of the Fermi surface."

Contactless conductivity did the trick. The Los Alamos team succeeded in its measurement, but not before the Canadian researchers had succeeded in theirs.

Now that the Fermi surface is known to exist, it's clear that traditional theories of metals do indeed apply to the cuprates. "The question remains as to whether the surface will yield enough information for us to tease apart the electron interactions that govern the pairing mechanism," says Singleton.

New Frontiers

The Los Alamos team is currently investigating how the Fermi surface evolves as the cuprate's composition changes. In comparing all the data (including the controversial results from an experiment conducted at Los Alamos in 1991), one sees dramatic changes in the Fermi surface as the materials get closer to the number of dopants that is optimum for the highest superconducting temperature. The Magnet Lab is continuing its quest to produce higher fields. Indeed, Mielke is spearheading a new electromagnet design, the "single-turn," named for its single loop of copper. The single-turn has already produced pulsed fields as high as 240 teslas. The field lasts but a few millionths of a second, and then—the magnet explodes! Remarkably, the magnet's design allows a sample to survive the explosion intact.

Chuck Mielke with the single-turn electromagnet. The white tube passing through the magnet’s single copper loop contains the sample and the measurement probes. A 3-million-ampere current pulse creates a 240-tesla magnetic field at the loop’s center, generating so much heat and magnetic force that the loop explodes. The tube and its contents survive intact. Inset: A successful experiment.

A successful experiment.

Mielke is planning to use the single-turn to measure the Fermi surface of plutonium and to investigate superconductivity in the heavy-electron metals, but he needs to refine his measurement techniques. "A changing magnetic field can generate an unwanted voltage—electromagnetic interference (EMI)—in the measurement probe," he explains. "It's hard enough to measure small signals in the 100-tesla magnet, where the field goes from nothing to everything in a few thousandths of a second. When the field ramps up in the single-turn's millionth of a second, the EMI is much higher, and the measurement becomes that much harder."

Mielke is patiently refining his techniques, the same way that all Magnet Lab scientists refine and advance theirs. They all recognize that tomorrow's materials will likely be discovered through an understanding of today's and that gaining such understanding is a slow process. But high magnetic fields are the quickest way.