New Superconducting Current Found Traveling Along the Outer Edges of a Superconductor

May 04, 2020 by Luke James

For the first time, scientists at Princeton University believe that they have spotted a superconducting current travelling along the edge of a material without straying into the middle.

A discovery that has eluded physicists for decades has reportedly been detected for the first time in a laboratory at Princeton University.

A team of physicists at the university found that superconducting currents were flowing along the exterior edge of a superconducting material. 


Superconducting Currents Detected Along the Exterior Edge of a Material

Nai Phuan Ong, the senior author of the team’s study, published in the journal Science on May 1, said, “Our motivating question was, what happens when the interior of the material is not an insulator but a superconductor? What novel features arise when superconductivity occurs in a topological material?”

To investigate superconductivity in topological materials, the team used a crystalline material, which features topological properties and is a superconductor under 100 milliKelvin (- 459 degrees Fahrenheit), called molybdenum ditelluride. 

Normally, superconducting currents, where electricity flows without losing energy, would permeate an entire material. However, in a thin sheet of molybdenum ditelluride which was chilled to near absolute zero, the interior and edge make up two superconductors that are distinct from one another. In the material, the tow superconductors are “basically ignoring each other,” added Ong. 


Topological Insulators 

The distinction between exterior and interior makes molybdenum ditelluride an example of a topological material. These materials exhibit behaviour that is closely tied to topology, a mathematical field, and can be used as topological insulators where electric currents can flow on the surface of a material but not the interior. 

Topological insulators are crystals with an insulating interior and a conducting surface. In contrast to conducting materials where electrons can hop from one atom to another, the electrons in insulators cannot move, however, topological insulators allow the movement of electrons on their conducting surface. 


Superconducting currents represented in a diagram.

Graphic illustrating superconductivity and its resistance to current flow. The jagged pattern in the diagram represents oscillation of the superconductivity which varies with the strength of an applied magnetic field. Image credited to Stephan Kim, Princeton University


Pushing the Superconducting State to Its Limit

Stephan Kim, a graduate student in electrical engineering, who conducted many of the project’s experiments, said, “Most of the experiments done so far have involved trying to ‘inject’ superconductivity into topological materials by putting the one material close to the other. What is different about our measurement is we did not inject superconductivity, and yet we were able to show the signatures of edge states.”

Initially, the team grew crystals in the lab and then cooled them down to a temperature where superconductivity occurs. Then, by applying a weak magnetic field to the crystal, the current displays oscillations as the magnetic field is increased. In their experiment, Kim and colleagues gradually increased the magnetic field on the material and measured how much they could increase it by before the superconducting state was lost, a value known as the ‘critical current.’ 

As the magnetic field grew, the critical current oscillated in a repeating pattern—a tell-tale sign of an edge superconductor. This oscillation is caused by the physics of superconductors in which electrons form Cooper pairs. The pairs act as a unified whole, all taking on the same quantum state or wave function. 


What Could This Mean for Quantum Computing? 

Molybdenum Ditelluride is a metal-like compound known as a Weyl semimetal. Due to its unusual properties, scientists believe that it could keep Majorana fermions, disturbances within a material that holds promise for better quantum computers. Computers based on quantum topology are expected to resist the jitter that can impair quantum calculations. 

The next big challenge for scientists is to take these Majorana fermions and make them into qubits, or individual computational units, which would be a huge leap forward towards practical quantum computing.

Theoretically, a qubit would be made of combinations of pairs of Majorana fermions, each of which would be separated from its partner. If one member of the pair is disrupted by noise errors, the other should remain unaffected and thereby preserve the integrity of the qubit, enabling it to correctly carry out a computation. 


The Difficulty with Developing Qubits

To date, semiconductor-based setups with Majorana fermions have been difficult to scale up. This is because a practical quantum computer requires thousands or millions of qubits, and these require growing very precise crystals of semiconducting material which are difficult to turn into high-quality superconductors. This is where topological insulators come in.