Scientia | Laws of Attraction

Laws of Attraction

By LAURA BUENNING

At nanoscale, electrons seem to defy the known laws of physics and behave in curious and—UT scientists insist—productive ways

Run your hand along a handrail; the surface is hard. Lift a gelatin dessert; it shivers in your hand. Toss a dandelion seed into a pond and it floats, while a marble sinks to the bottom. We count on these phenomena: visible, or macroscopic, objects obey well-understood laws of physics. But things change when one deals with the infinitesimally small. At nanoscale—the scale of atoms and molecules—electrons respond to forces we could not have predicted from macroscopic observations.

For Elbio Dagotto, Adriana Moreo, and Ted Barnes, that's where the fun begins, in the realm where electrons exist both as particles and as waves, can be said to be in two places at once, and can even tunnel through seemingly impenetrable barriers. Dagotto, a Distinguished Scientist with a joint appointment at the University of Tennessee and Oak Ridge National Laboratory, operates at nanoscale and creates computer models of materials in which strong interactions among electrons generate unusual properties.

"It's fascinating to see how materials, reacting under what might be thought of naively as small changes in magnetic or electrical fields, undergo enormous changes in resistance—measuring as much as ten to the tenth power [ten billion]," Dagotto says. Resistance refers to opposition to the flow of electric current, or the flow of large numbers of electrons. Magnetoresistance is a material's capacity to lose or gain electrical resistance in the presence of a magnetic field. This is the property that allows us to store or read digital information.

Dagotto studies how we might make materials that sense the subtle magnetoresistant changes caused by the quantum nature, or wave behavior, of electrons. The materials he models on the computer have building blocks in "almost ready" atomic arrangements that individually resemble small magnets. But because the nearest neighboring building blocks do not point in the same direction, the overall system is not magnetic. "However," he says, "one small change can switch the many, many ‘almost ready' blocks into ‘ready' building blocks, causing the system to suddenly become microscopically magnetic and metallic and drastically changing its resistance."

Dagotto compares his mathematical calculations on material properties to weather-prediction models that divide the country into small grid areas. "We look inside each area to see which way the wind is blowing and from that predict how it will blow in the next area," he says. A similar pattern holds true for groups of interacting atoms. "Each electron and atom is subject to the whims of its neighbors," Dagotto says. "A computer program can look at the potential interactions and predict the way electrons and atoms will react and interact and ultimately tell us how a block of material will respond."

The Science—Simplified

Electrons in the materials that Dagotto models have simultaneously active electromagnetic properties—charge and spin are two of them—representing all the possible competing positions and directions of movement allowed by the atomic configuration. In such a configuration, one electron influences another, and another, and so on, to make what is called a "strongly correlated electronic material."

Strong correlation underpins the strange "almost ready" aspect of colossal magnetoresistance (CMR) materials, in which one tiny change in current creates a huge response in the material. "Inside hard material, electrons group into little pockets, and the properties can easily change from one pocket to the next," Dagotto says. CMR materials show promise as read–write heads for high-capacity magnetic storage systems for computers, medical instruments, and other electronic devices.

He recently began to link two widely separated areas of research, on the theory that similarities between many hard nanomaterials made of transition metal oxides (TMOs) and complex systems of soft matter—such as DNA and proteins—could lead to interesting correlations. Transition metals are distinguished from others by their electronic structure rather than their physical properties. In transition metals, the electrons available to combine with other elements are present at more than one energy level, or shell. When these elements bond with oxygen, they can form multiple stable oxides. "What we now know about transition metal oxides has dramatically challenged our view of solids," Dagotto says.

Many TMOs and soft materials display a mixed internal structure resulting from simultaneous competing interactions among their electrons and atoms. They spontaneously form unexpected nanostructures with properties that do not exist in the material's individual components, and that could be the basis for smaller, more powerful hardware for microscopic machinery, nanoscale electronic devices, and miniature chemical sensors.

As the Small World Spins

Dagotto's colleague Ted Barnes, a UT–ORNL joint faculty member in physics, is best known as a "quark model" scientist, for his studies of these elementary subatomic particles that, along with leptons (an electron is one kind of lepton), make up all matter in the universe. One of Barnes's side interests is quantum magnetism, the realm of spin and spin-chain materials.

Spin is a quantum property that refers to the magnetic orientation of electrons and atoms. Electrons spin in one of two directions, up or down, which correspond to the north and south poles of a magnetic field. A pair of neighboring upward-pointing spins requires more energy to maintain than an up-down arrangement, and electrons always seek an arrangement that requires the least amount of energy. Atoms exchange electrons continuously within a one- or two-dimensional space and, under the right conditions, they make a wave of spins, exchanging up and down positions along the length of a material. Magnetic atoms in one-dimensional space, that is, a single line of atoms, are called "spin chains," and a pair of coupled parallel spin chains forms a "spin ladder."

In 1986, while at the University of Toronto, Barnes joined forces with Dagotto and UT physics professor Adriana Moreo, who were then at the University of Illinois at Urbana–Champaign. Both Moreo and Barnes held world records in separate categories for computer modeling studies of the energies of quantum spin systems of the type found to underlie high-temperature superconductivity (high-Tc). High-Tc is possible at or above the temperature of liquid nitrogen, –196° C or –321° F, and conventional superconductivity requires temperatures a few degrees above absolute zero. Though the mechanisms of high-Tc and conventional superconductivity are different, both refer to the capacity of a material to conduct electricity without resistance.

In the mid-1980s researchers discovered high-Tc and found that it resulted from two-dimensional quantum spin systems, Barnes says. In 1992 Dagotto was part of a team that predicted that if two spin chains of copper and oxygen were distributed in a "ladder-shaped" formation, with the oxygen atoms between the copper, the material would act as a kind of high-temperature superconductor. Dagotto's teamwork with Barnes and Moreo produced the calculations that describe these superconducting spin ladders.

Loss-Free Power Transmission

What makes superconducting spin ladders so important, says Barnes, is high-Tc's potential for loss-free power transmission at ordinary temperatures. Currently, even the highest-temperature superconductivity takes place in conditions far too cold to be practical, below –150° C (–238° F).

"Superconducting materials start life as insulating magnets," Barnes explains. Though it seems counterintuitive, superconductivity results from pairs of holes—spaces devoid of electrons—arranging and rearranging themselves in positions that require the least amount of energy along the line of current flow. Through a process called "hole doping," scientists establish holes in the material's electron structure, and the voids hop from one site to the next, briefly forming pairs as they go.

The spin-spin interactions in spin-ladder arrangements, Barnes says, cause holes to pair much more easily than in more typical two-dimensional organizations, which could allow superconductivity at higher temperatures. But before loss-free power transmission at room temperature becomes a reality, he says, researchers will have to find out how to make hole binding stronger by discovering the laws that control this phenomenon.

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For more information, contact Elbio Dagotto, e-mail edagotto@utk.edu; Ted Barnes, (865) 974-3128 or (865) 574-4575, e-mail tbarnes@utk.edu; or Adriana Moreo, (865) 974-2084, e-mail amoreo@utk.edu.