Imagine you are walking and encounter a barrier, such as a hill or a wall. The only way to make it to the other side is to climb all the way up and over it. Yet what if you had the same superpowers as quantum particles?
The strange laws of quantum mechanics allow particles to sometimes bust through barriers like they are not there, even if the particles cannot climb over whatever is in their path. But the challenge of tunneling through these barriers increases as the roadblocks get taller, making it so that fewer particles can break through. A special case of quantum tunneling called Klein tunneling, however, changes the game. It effectively makes barriers transparent, opening up portals that allow particles to pass through, even when incredibly tall walls stand in their way.
Nearly 100 years ago, Swedish physicist Oskar Klein first predicted this phenomenon. Yet until recently, scientists had seen very limited signs of it. In a study published in Nature on June 19, an interdisciplinary team of researchers present direct evidence of Klein tunneling.
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Accidental Discovery
The study is not the first to directly observe this effect. “Klein tunneling has been pretty well demonstrated in graphene,” a carbon-based material, says David Goldhaber-Gordon, a physicist at Stanford University who was not involved with the study. Before that finding, “people had not really thought about” searching for experimental evidence of Klein tunneling and had “put it on a shelf,” says Ichiro Takeuchi, a materials scientist and engineer at the University of Maryland, College Park, and senior author of the new study. “The results of the current work are more direct,” however, than those found in the graphene research, says Boris Nadgorny, a physicist at Wayne State University in Detroit, who was not involved with the study. The researchers also used an “ingeniously designed experimental setup,” he says.
“I would call this groundbreaking,” says Teun Klapwijk, a quantum nanoscientist at the Kavli Institute of Nanoscience at the Delft University of Technology in the Netherlands who was not involved in the research, “because it is a phenomenon that one might on paper expect to occur… but one has to hit on an experimental system which convincingly shows the phenomenon.” This particular experiment stands out as “a clear case of independent experimental exploration and thinking,” he adds.
The finding is perhaps even more stunning because the researchers did not set out to observe this phenomenon in action. “This project stemmed out of our research on topological insulators,” says Johnpierre Paglione, a physicist at the University of Maryland and a co-author of the study. Topological insulators are strange materials with insulated interiors but conductive surfaces.
For the past several years, he and his colleagues have studied a material called samarium hexaboride and worked to show that it is a topological insulator. They were looking for signs that samarium hexaboride exhibits quantum behavior, an important aspect of proving that a material is, indeed, a topological insulator.
Perfect Conductance
The researchers put a thin film of the samarium hexaboride on top of another compound that, at low temperatures, becomes a superconductor—a material that can conduct electricity without resistance. When they cooled everything down to just a few degrees above absolute zero (–273.15 degrees Celsius), the second material became a superconductor and, through their close proximity, so did the metallic surface of the samarium hexaboride. The scientists then touched a tiny metal tip to the surface of the samarium hexaboride and studied how electrons passed into the second material.
At every boundary between a metal and a superconductor, a special type of reflection called Andreev reflection occurs, which stems from the fact that electrons in superconductors exist only in pairs. Like two people in a three-legged race, when one electron hops from the metal to the superconductor, it must bring a “buddy” with it. Because charges must be balanced in the system, however, a positively charged “hole”—essentially, the absence of an electron where there otherwise should be one—must jump back from the superconductor to the metal.
Researchers account for the electron and “hole” movement by measuring the system's conductance. If every electron that attempted to hop into the superconductor succeeds, the conductance would double. However, this doesn't usually happen, because in most cases, some of the electrons do not have enough energy to make the jump. The less energetic electrons reflect off the boundary between the metal and the superconductor, making the system's conductance more than 100 percent but less than doubled.
To the researchers' shock, perfectly doubled conductance happened in their samarium hexaboride experiment. The team approached Victor Galitski, a theoretical physicist at the University of Maryland, with the odd results, which held up on repeated trials. He suggested that Klein tunneling allowed all of the electrons to burrow through the physical interface between the two materials. Another conservation law pertaining to the spin of electrons prevents electrons that lack the energy to jump the barrier to simply go back where they came from, so they “have to tunnel through,” he says, leading to perfectly doubled conductance.
“These exciting results experimentally demonstrate perfect Andreev reflection between a normal point contact”—the tiny metal tip—“and the proximity-induced superconductor in the topological insulator, samarium hexaboride,” Nadgorny says. “The study links these unexpected and elegant results with the absence of normal [electron] scattering, one of the key manifestations of Klein tunneling,” he adds.
Now that researchers have demonstrated this quantum quirk, they hope to use their findings to improve conventional computer components or even create materials for the quantum devices of tomorrow, Paglione says. Harnessing electrons' tunneling powers could help design “perfect transistors” or even solve problems with the junctions in quantum computers, he says. Yet Klapwijk cautions that the “route to applications is much more complex” than it often appears.