Scientists have measured the subatomic intricacies of an exotic phenomenon first predicted more than 60 years ago. This so-called van Vleck magnetism is the key to harnessing the quantum quirks of topological insulators—hybrid materials that are both conducting and insulating—and could lead to unprecedented electronics.
“Our experiment is the first to show conclusive evidence of van Vleck magnetism, which mediates the magnetic properties of topological insulators,” said MIT and Brookhaven Lab Ph.D. student Mingda Li, lead author on the study. “Synthesis and characterization techniques have finally caught up to seminal theoretical work, and we are thrilled to have performed this groundbreaking research.” The collaboration—including the U.S. Department of Energy’s Brookhaven National Laboratory, MIT, and Pennsylvania State University—used cutting-edge electron microscopy facilities at Brookhaven Lab to pinpoint this never-before-seen behavior. The results were published online April 9, 2015, in the journal Physical Review Letters.
Tunable topological insulators could lay the foundation for new generations of spintronics, quantum computers, and ultra-efficient semiconductor devices (see sidebar).
Van Vleck’s volleyball
Classical materials tend to conduct electricity or insulate against it—think rubber versus copper. Topological insulators, however, live in both worlds: the bulk is insulating, but the surface is highly conductive. The relationship between these competing qualities introduces strange phenomena, especially in the surface electrons.
“The surface electrons—called Dirac electrons—exhibit the light-like mobility and extreme stability that enables so many exciting potential applications,” Li said. “But these electrons cannot be controlled directly. That’s where van Vleck magnetism comes in, to induce and harness Dirac electrons.”
Imagine an endless game of volleyball between perfectly matched opponents. Now replace the players with magnetic ions and the ball with a free electron—that interplay mirrors magnetism in traditional semiconductors. Interrupting the game or shifting the behavior of that free electron, which is key to semiconductor applications, is a relatively simple task.
In topological insulators, however, that volleyball game never gets going. The magnetic action is contained within a single crystal structure—no back-and-forth and no free electrons. This subtle, intra-atomic magnetism behaves like a lone player engaging in a virtual volley. In fact, a rogue volleyball (free electron) would ruin the game.
“Those all-important outer electrons can only be influenced through the topological insulator’s core electrons,” Li said. “The outer electrons can ‘feel’ the effect of energy or magnetic fields on the core. That conversation between core and shell is mediated by van Vleck magnetism.”
John Hasbrouck van Vleck, considered the father of modern magnetism, won the 1977 Nobel Prize in Physics for his quantum revisions of magnetism theory. His groundbreaking work included predicting this internal magnetism, which has been notoriously difficult to detect—until now.
Congratualtions Mingda!!
Full article: http://www.bnl.gov/newsroom/news.php?a=25598
In addition, these findings have been featured on a number of other science news media websites:
- http://www.sciencedaily.com/releases/2015/04/150413091641.htm
- http://phys.org/news/2015-04-long-sought-magnetic-mechanism-exotic-hybrid.html
- http://esciencenews.com/sources/science.daily/2015/04/13/long.sought.magnetic.mechanism.observed.exotic.hybrid.materials
- http://www.microscopy-analysis.com/editorials/editorial-listings/eels-confirms-elusive-magnetism
