. Frustrated magnetics were named so because they are not magnetized at low temperatures. Princeton University scientists conducted an experiment to reveal unusual behavior in frustrated magnets. This discovery is believed to answer a long-debated issue about the nature and properties of discontented, quantum materials. N. Phuan Ong (Princeton’s Eugene Higgins professor of Physics) and his colleagues consider their discovery to be a remarkable one that could lead to new directions in advanced electronic research. The paper, published in Science (citation below), could also explain high-temperature superconductivity and the frictionless transmissions of electricity. Tb2Ti2O7 crystal of frustrated magnet. Image Credit: Jason Krizan. Testing for the Hall Effect. Scientists tested frustrated magnets in order to determine if they exhibit the Hall Effect. A magnetic field can be applied to an electrical current flowing in conductors like a copper ribbon. The current will deflect to the one side of the ribbon. This deflection was first noticed by Edwin Herbert Hall (1855-1938), an American physicist. This effect is used to detect devices like computer printers or vehicle antilock brake systems. Most scientists believe that because the Hall Effect is only observed in charged particles, it cannot be seen in non-charged particles such as those found in frustrated magnets. Professor Ong stated that it was absurd to talk about the Hall Effect in neutral particles. However, theoretical physicists have speculated that neutrons in frustrated magnets could be subject to the Hall Rule in near absolute zero (super-cold), in which particles follow quantum mechanics rather than the physical laws in our world. The potential for some amazing innovations in computing and electronic devices could be achieved by harnessing quantum behavior. Is there a Hall Effect in frustrated magnets Along with their graduate students Jason Krizan, Max Hirschberger and Chemistry Professor Robert Cava they set out to resolve the issue and prove that there is a Hall Effect for frustrated magnets. The scientists used pyrochlores to accomplish this. Pyrochlores are a type of magnet that contains magnetic moments. At extremely low temperatures, near absolute zero they should align in an ordered manner so all their “spins” (a quantum-mechanical property), point in the same direction. Scientists have discovered that spins can point in any direction. These materials, also known as “quantum spin-ice”, are frustrated. Professor Ong stated that these materials were very intriguing because they have the potential for spins aligning. However, due to what is called “geometric frustration”, the spins become entangled, not ordered. This is a key property that quantum systems possess that researchers hope to use to build a quantum computer that could address problems currently unable to be solved by current computers. Professor Ong met Cava while chatting down a corridor. He had both the experimental infrastructure and know-how to create such materials. Krizan, a graduate student in chemistry, was assigned to grow the crystals while Hirschberger, a graduate student in physics, set up the necessary experiments for looking at the Hall Effect. Hirschberger stated that the main problem was to determine the Hall Effect’s temperature at which the materials are quantum. The experiment took place at temperatures as low as 0.5 degrees Kelvin. Hirschberger was challenged to find solutions for temperature variations as small as 1/10th of degrees between crystal edges. Krizan had to first synthesize titanium oxide and Terbium oxide in an oven-like kiln before he could grow crystals. He formed the pyrochlore into a cylindrical suitable for crystal growth. Then he blasted the chamber with focused light from four 1000-Watt Halogen lamps. These were thin, transparent, orange-colored slabs that measured no more than a sesame seeds. Hirschberger used microheaters and tiny gold electrodes on each side of each slab to test the crystal. This heat current works in a similar way to an ordinary Hall Effect experiment, even at such low temperatures. Hall Effect in non-magnetic materials He applied magnetic fields in the opposite direction to the heat current. Surprised, he discovered that the heat current had been deflected towards one side of his crystal. He had seen the Hall Effect in non-magnetic materials. Professor Ong was so impressed by these results that he asked Hirschberger to do the same experiment in reverse – by changing the direction of heat current. Hirschberger should have observed the Hall Effect. The current would deflect towards the opposite side of crystal if he had. It was not easy to reconfigure the experiment at low temperatures. Hirschberger eventually proved that the Hall Effect was correct when the signal reversed. Professor Ong stated that “all of us were surprised because work and play are in the non-quantum classical world.” It can be very odd to see quantum behavior, but it is actually quite possible. It is real.” Professor Cava stated that “the use of experiments to probe quantum behavior of materials” was essential to broaden our understanding of fundamental physical property and eventual exploitation. Further research using these materials may provide additional insights into superconductivity in copper-containing materials called “cuprates”. These materials are also known as “high-temperature SUPERCONDUCTORS”, as they operate at higher temperatures than other superconductors, like those in MRI machines. Many theorists including Nobel laureate Professor Philip Anderson have suggested that spinons might be carriers of heat currents in quantum systems such as this one. Spinons exist in theory. Although the team claims to have seen a spinon, they do not believe that it has been observed. Professor Ong and his colleagues think that their research could help in this direction. This work is a step in the right direction for the hunt for the spinon. He said that we have already seen the tracks. This study was supported by the U.S. National Science Foundation and the Army Research Office, as well as the U.S. Department of Energy’s Division of Basic Energy Sciences. Citation: Max Hirschberger, Jason W. Krizan, R. J. Cava, N. P. Ong. Science. Published April 3, DOI: 10.1126/science. 1257340.
