By Saren Johnston
Some very complex materials exist on the borderline between different states. Precariously perched, a little nudge is all they need to push them over the edge and drastically change their properties.
Studying the behavior of novel materials, especially those that sit on the brink of becoming something else, is Robert Modler's specialty. What he finds out about how they respond to changes in their environment helps Ames Laboratory researchers better understand these materials and determine their suitability for possible future applications within fields that include computer technology, communications, medicine and the automotive industry.
Modler, an Ames Laboratory associate scientist, investigates unusual materials under simultaneous environmental extremes of very low temperature, high pressure and high magnetic field. He does his research in a new facility that he designed and set up with Iowa State University graduate student Andrew Thomas. The two researchers spent about a year putting together the equipment for the facility, which houses a helium three-helium four dilution cryostat, a high-pressure cell and a high-field superconducting solenoid to simultaneously create the three extreme environments in one instrument.
"If you want to analyze and understand the behavior of a new material, ideally you would like to know as much as possible about how it reacts in this three-dimensional parameter space," says Modler. "While quite a large number of researchers use one or two of these parameters to investigate materials, only a few really put very low temperature, high magnetic field and high pressure together in one facility. I would guess there are probably only five comparable facilities in the world."
As part of Ames Laboratory's efforts to learn more about complex materials such as exotic magnetic molecules, quasicrystalline materials, semimetals and new magnetic superconductors, Modler subjects them to severe conditions in his lab of extreme environments. Observing how the materials behave in the facility's three-parameter space often reveals unique properties that could prove beneficial to the development of new technologies.
To get a good look at the material he's studying, Modler places a sample in the dilution refrigerator, which cools it until the material reaches its ground state -- the low-temperature, lowest-energy state at which a material is almost completely free of excitations and vibrations.
The near absence of temperature disturbances in the ground state makes it easier for scientists to learn more about a material's properties. In the ground state, for example, there are fewer atomic vibrations to interfere with the behavior of the conduction electrons -- you might even say that the material has "calmed down," which Modler says allows scientists to get a very good picture of how the electrons move about in the material. Alternatively, by increasing the temperature and taking the material out of the ground state, they can see what excitations develop and how they build up.
To achieve the ground state, the cryostat takes the material under investigation through temperatures that range from 300 K (room temperature) to 0.05 K (just above absolute zero). Some intriguing "how cold is cold" facts may provide insight on how chilly 0.05K might be.
"On the Kelvin scale, absolute zero, or 0K, is unreachable," says Modler. "It would indicate the total absence of heat. As a comparison, our background universe has an average temperature of about 3K, which stems from cosmic microwave background radiation. In our lab, however, we reach 0.05K on a regular basis, which is one-twentieth of a Kelvin above absolute zero. It's an interesting thought that temperatures this low have never naturally existed in our universe."
In addition to cooling a material dramatically, Modler can further alter its environment by applying high pressure of up to 20,000 atmospheres. Subjecting a material to high pressure alters the distances between its atoms and can strongly affect or even completely change its properties.
"One atmosphere is equal to the air pressure resting on the surface of the earth," Modler explains. "Pressure of 20,000 atmospheres is equivalent to about 300,000 pounds per square inch. It can be thought of as approximately the weight of a Toyota Camry on the tip of a medium-sized Phillips-head screwdriver."
The third parameter of Modler's lab of extreme conditions is a powerful superconducting magnet for magnetic fields up to 100,000 times that of the earth's. Just like high pressure, high magnetic field can cause abrupt changes in a material. It can, for example, change how magnetic moments are arranged in a material, causing a different magnetic alignment -- a materials property perhaps most notably used in computer hard disks.
"The parameters of temperature, pressure and magnetic field are used for materials research by many scientists throughout the world. But putting all three together in one experiment might 'scare' even a tough material," says Modler. He's intrigued most by those materials that sit on the borderline between different states. These materials could stay in limbo indefinitely if their environment remained the same, but Modler's not about to let that happen. His job is to shake things up a bit, and he does it very well.
"By applying just enough pressure, magnetic field or both to a 'borderline' material, we can push it over the edge into a new state and bring about drastic changes in its physical properties," he says. "These materials are unusually complex; you might even call them 'adaptive' to their environments. By creating the right conditions, you can get a magnetic material to become superconducting or an insulated material to become metal. To improve our scientific understanding of such materials, we look very closely at how the changing states emerge from each other and how they interact at the borderline. That's something interesting and not well studied to this point. And through our new facility, we can perform this research very comprehensively."
For more information:
Robert Modler, (515) 294-6353
modler@ameslab.gov
Research funded by:
DOE Office of Basic Energy Sciences
Last revision: 9/15/00 sd
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