Researchers detect supercurrent at edge of superconductor with topological spin

The superconductor the researchers studied is also a topological semi-metal, a material that comes with its own unusual electronic properties. The finding suggests ways to unlock a new era of “topological superconductivity” that could have value for quantum computing.

“As far as we know, this is the first observation of an edge supercurrent in any superconductor,” said Nai Phuan Ong, professor of physics at Princeton at Eugene Higgins and lead author of the study.

“Our motivating question was: What happens when the interior of the material is not an insulator but a superconductor?” Ong said. “What new features emerge when superconductivity occurs in a topological material?”

Although conventional superconductors already enjoy widespread use in magnetic resonance imaging (MRI) and long-distance transmission lines, new types of superconductivity may unlock the ability to overcome the limitations of our familiar technologies.

Researchers at Princeton and elsewhere have been exploring the connections between superconductivity and topological insulators, materials whose nonconforming electronic behaviors were the subject of the 2016 Nobel Prize in Physics for F. Duncan Haldane, professor of physics at Princeton Sherman Fairchild University. .

Topological insulators are crystals that have an insulating interior and a conductive surface, like a brownie wrapped in aluminum foil. When conducting materials, electrons can jump from one atom to another, allowing the electric current to flow. Insulators are materials in which electrons are trapped and cannot move. However, curiously, topological insulators allow the movement of electrons on their surface but not inside.

To explore superconductivity in topological materials, the researchers turned to a crystalline material called molybdenum ditelluride, which has topological properties and is also a superconductor once the temperature drops below 100 milli Kelvin frigid, which is -459 degrees Fahrenheit. .

“Most of the experiments done so far have been trying to ‘inject’ superconductivity into topological materials by placing one material very close to the other,” said Stephan Kim, a graduate student in electrical engineering, who conducted many of the experiments. “The difference from our measurement is that we did not inject superconductivity, and yet we were able to show the signatures of the edge states.”

The team first cultivated crystals in the lab and then cooled them to a temperature where superconductivity occurs. They then applied a weak magnetic field by measuring the current flow through the glass. They observed that a quantity called critical current shows oscillations, which appear as a sawtooth pattern, as the magnetic field increases.

Both the height of the oscillations and the frequency of the oscillations are consistent with predictions of how these fluctuations arise from the quantum behavior of electrons confined to the edges of materials.

Researchers have long known that superconductivity arises when electrons, which normally move randomly, come together in two to form Cooper pairs, which in a sense dance at the same rate. “A rough analogy is that a billion couples perform the same strictly written dance choreography,” said Ong.

The script the electrons follow is called the superconductor wave function, which can roughly be thought of as a stretched ribbon along the superconducting wire, Ong said. A slight twist of the wave function forces all Cooper pairs on a long wire to move at the same speed as a “superfluid”, in other words, acting as a single collection rather than as individual particles, flowing without produce heating.

If there are no turns along the tape, Ong said, all Cooper pairs are stationary and no current flows. If the researchers expose the superconductor to a weak magnetic field, this adds an additional contribution to the torsion that the researchers call magnetic flux, which, for very small particles like electrons, follows the rules of quantum mechanics.

The researchers anticipated that these two contributors to the number of turns, superfluid velocity and magnetic flux, work together to keep the number of turns as an exact integer, an integer such as 2, 3, or 4 instead of a 3.2 or a 3.7. They predicted that as the magnetic flux increased smoothly, the speed of the superfluid would increase in a sawtooth pattern as the speed of the superfluid adjusts to cancel the extra .2 or add .3 to get an exact number of turns.

The team measured the superfluid current while varying the magnetic flux and found that the sawtooth pattern was indeed visible.

In molybdenum dithelluride and other so-called Weyl semimetals, this Cooper electron pairing in the mass appears to induce a similar pairing at the edges.

The researchers noted that the reason why the edge supercurrent remains independent of the mass supercurrent is currently not well understood. Ong compared electrons that move collectively, also called condensates, to pools of liquid.

“According to classical expectations, one would expect two pools of fluids that are in direct contact to merge into one,” Ong said. “However, the experiment shows that the edge condensates remain different from those of most of the crystal.”

The research team speculates that the mechanism that prevents the two condensates from mixing is the topological protection inherited from the protected edge states in molybdenum ditelluride. The group hopes to apply the same experimental technique to search for edge supercurrents in other unconventional superconductors.

“There are probably a lot of them,” said Ong.

The study, “Evidence for an Edge Supercurrent in the Weyl MoTe2 Superconductor,” by Wudi Wang, Stephan Kim, Minhao Liu, F. A. Cevallos, Robert. J. Cava and Nai Phuan Ong, were published in the magazine. Science May 1, 2020.