Scientists at the University of Maryland have captured the most direct evidence to date by a quantum quirk that allows particles to tunnel through a barrier that it is not even there. The result mentioned in the June 20, 2019 edition of the journal Nature can enable engineers to design more uniform components for future quantum computers, quantum sensors and other devices.
The new trial is an observation of Klein tunneling, a particular case of a more common quantum phenomenon. In the quantum world, tunneling allows particles such as electrons to pass through a barrier, although they do not have enough energy to actually climb over it. A higher barrier usually makes it harder and allows fewer particles to pass through.
Small tunneling occurs when the barrier becomes completely transparent, opening a portal that particles can cross regardless of the barrier height. Scientists and engineers from the UMD Center for Nanophysics and Advanced Materials (CNAM), the Joint Quantum Institute (JQI) and The Condensed Matter Theory Center (CMTC), with appointments in the UMD Materials Science and Engineering Department and the Department of Physics, have made the most compelling measurements yet of the effect.
"Klein tunneling was originally a relativistic effect that was first predicted almost a hundred years ago," said Ichiro Takeuchi, professor of materials science and technology (MSE) at UMD and the senior author of the new study. "Until recently, you couldn't see it."
It was almost impossible to gather evidence of Klein tunneling, where it was first predicted – the world of high-energy quantum particles moved close to the speed of light. But in recent decades, scientists have discovered that some of the rules for fast-flowing quantum particles also apply to the relatively weak particles that travel near the surface of some unusual materials.
Such a material – which researchers used in the new study is samarium hexaboride (SmB6), a substance that becomes a topological insulator at low temperatures. In a normal insulator such as wood, rubber or air, electrons are trapped, unable to move even when the voltage is applied. Thus, unlike their free roaming comrades in a metal wire, electrons in an insulator cannot perform a current.
Topological insulators such as SmB6 behave like hybrid materials. At low enough temperatures, the interior of SmB6 is an insulator, but the surface is metallic and gives electrons some freedom to move around. In addition, the direction the electrons move is locked to an inner quantum property called spin that can be oriented up or down. Electrons moving to the right will always have their spinning pointing up, for example, and electrons moving to the left will have their spin pointing down.
SmB6's metallic surface would not have been enough to see Klein tunneling. It turned out that Takeuchi and colleagues needed to transform the surface of SmB6 into a superconductor – a material that can conduct electric current without any resistance.
To turn SmB6 into a superconductor, they laid a thin film on top of a layer of yttrium hexaboride (YB6). When the entire assembly was cooled to just a few degrees above absolute zero, YB6 became a superconductor and because of its proximity, the metal surface of SmB6 also became a superconductor.
It was a "piece of serendipity" that SmB6 and its yttrium-exchanged relatively shared the same crystal structure, says Johnpierre Paglione, a professor of physics at UMD, the director of CNAM and a co-author of the research paper. "The interdisciplinary team we have, however, was one of the keys to this success. With topological physics experts, thin-film synthesis, spectroscopy, and theoretical understanding really got us to this point," Paglione adds.
The combination showed that the right mix to observe Klein tunneling. By contacting a small metal tip with the top of the SmB6, the team measured the transport of electrons from the tip to the superconductor. They observed a perfectly doubled conductance – a measurement of how the current through a material changes, as the voltage across it is varied.
"When we first observed the doubling, I didn't think so," Takeuchi said. "After all, it's an unusual observation, so I asked my postdoc Seunghun Lee and researcher Xiaohang Zhang to go back and do the experiment again."
When Takeuchi and his experimental colleagues convinced that the measurements were correct, did not originally understand the source of the doubled conductance. Then they began to look for an explanation. UMD's Victor Galitski, a JQI Fellow, a professor of physics and a member of the CMTC, suggested that Klein tunneling could be involved.
"First, it was just a hunch," Galitski says. "But over time we became more convinced that the Klein scenario could actually be the root cause of the observations."
Valentin Stanev, an associate researcher in the MSE and a researcher at JQI, took Galitski's hunch and prepared a cautious theory of how Klein tunneling could appear in the SmB6 system – ultimately making predictions that matched the experimental data well.
The theory suggested that Klein tunneling manifests itself in this system as a perfect form of Andreev reflection, an effect present at all boundaries between a metal and a superconductor. Andreev reflection can occur when an electron from the metal jumps on a superconductor. Within the superconductor, electrons are forced to live in pairs, so when an electron jumps on, it gathers a buddy.
To balance the electric charge before and after the hop, a particle with the opposite charge – which scientists call a hole – must reflect back into the metal. This is characterized by Andreev's reflection: an electron enters, a hole comes back. And since a hole in one direction carries the same current as an electron moving in the opposite direction, this whole process doubles the overall conductance signature of Klein tunneling through a cross of a metal and a topological superconductor.
In conventional crossings between a metal and a superconductor, there are always some electrons that do not jump. They spread out of the border, reduce the amount of Andreev reflection and prevent a precise doubling of the conductance.
But because the electrons in the surface of SmB6 have their direction of movement bound to their spin, electrons near the border cannot jump back – meaning they will always go directly into the superconductor.
"Klein tunneling was also seen in the graph," Takeuchi says. "But here, because it is a superconductor, I want to say that the effect is more spectacular. You get this exact doubling and a complete cancellation of the spread, and there is no analogue of that in the graphene experiment."
Junctions between superconductors and other materials are ingredients in some proposed quantum computers, as well as in precision sensors. The path in these components has always been that each junction is a little different, Takeuchi says, requiring endless tuning and calibration to achieve the best performance. But with Klein tunneling in SmB6, researchers can finally oppose this irregularity.
"In electronics, the device's device is scattered number one enemy," Takeuchi says. "Here is a phenomenon that gets rid of variability."
The research paper, "Perfect Andreev reflection due to the Klein paradox in a topological superconducting state", Seunghun Lee, Valentin Stanev, Xiaohang Zhang, Drew Stasak, Jack Flowers, Joshua S. Higgins, Sheng Dai, Thomas Blum, Xiaoqing Pan, Victor M. Yakovenko, Johnpierre Paglione, Richard L. Greene, Victor Galitski and Ichiro Takeuchi, were published in the journal Nature on June 20, 2019.
Settlement of the debate: Solution of the electronic surface states of samarium hexaboride
Perfect Andreev reflection because of Klein's paradox in a topological superconducting state, Nature (2019). DOI: 10.1038 / s41586-019-1305-1, https://www.nature.com/articles/s41586-019-1305-1
Perfect quantum portal emerging from exotic interface (2019, June 19)
June 19, 2019
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