Superconductivity and quantum Hall effect coexist in novel nitride material - Happy Boss

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Monday, 5 April 2021

Superconductivity and quantum Hall effect coexist in novel nitride material

Doctoral students Phillip Dang (left) and Reet Chaudhuri at the National High Magnetic Field Laboratory, where measurements were made on a material structure that concurrently has superconductivity and the quantum Hall effect. Credit: Jena-Xing Lab

Large magnetic fields are usually a killer for superconductivity, but researchers in the US have now fabricated a material that simultaneously superconducts and exhibits the quantum Hall effect – a phenomenon that requires a strong magnetic field. The new material, made from nanoscale layers of gallium nitride and niobium nitride, could be used for “topological” quantum computing and to make more energy-efficient electronic devices.

The quantum Hall effect occurs when a current passing along the length of a thin conducting sheet gives rise to an extremely precise voltage across opposite surfaces of the sheet. This voltage only occurs when a strong magnetic field is applied perpendicular to the sheet, and it is quantized – that is, it can only change in discrete steps. Another consequence is that electronic states on the surface of the two-dimensional (2D) sheet are said to be “topologically protected”. This protection arises because electrons in these “edge” states can only travel in one direction, and they also steer around imperfections or defects in the material without backscattering. Since backscattering is the main energy-dissipating process in electronic devices, such protected states could be useful components in next-generation energy-efficient devices.

 Another important benefit is that an edge electron with a certain momentum cannot scatter into a state with opposite momentum (or spin) since to do so it would have to flip its spin. Topologically-protected states might thus be ideal for quantum-computing applications in which defects usually destroy information carried in the spin state of electrons.

Topologically protected superconducting electron states

In recent years, researchers have been trying to create heterostructures – 2D semiconductors grown on superconductors – in which the states of the electrons that make up the superconducting current are also topologically protected. This supercurrent comes from electrons with opposite spins that have paired up and can move through the material without any resistance below a certain critical temperature. Materials that can accommodate such supercurrents are limited, however, because the magnetic field required to produce the quantum Hall effect destroys superconductivity – either by breaking up the electron pairs or by trying to make both electron spins align in the same direction.

Modified growth process

In the new work, researchers at Cornell University, the Naval Research Laboratory, and Qorvo Inc. engineered heterostructures from nanoscale layers of semiconducting gallium nitride (GaN) on superconducting niobium nitride (NbN). These two materials have similar crystal structures and chemical properties and previous work by the same team showed that they can be made into a layered heterostructure using a technique called epitaxial growth.

These materials are routinely employed in light-emitting diodes and transistors for products like smartphones and home lighting, and the team chose them in part because they are robust. However, they do contain more structural defects than other technologically-important materials like silicon. To reduce the number of defects, and thus create a higher-quality heterostructure, the researchers modified the growth process employed in their earlier study. This modification also allowed them to precisely engineer the position of the electrons in the GaN atop the NbN.

Narrow “window” of temperatures and magnetic fields

Using measurements of resistance versus applied gate voltage at temperatures of 390 mK, the researchers showed that superconductivity in the improved NbN layer could survive applied magnetic fields as high as 17.8 Tesla. Meanwhile, the improved GaN semiconductor was of high enough quality to exhibit the quantum Hall effect at lower applied magnetic fields of 15 T. “Both these improvements mean the quantum Hall effect and superconductivity can occur at the same time in the heterostructure over a certain ‘window’ of temperatures and magnetic fields (that is, below 1 K and between magnetic fields of 15 to 17.8 T),” study lead author Phillip Dang tells Physics World.

According to the team, the new GaN/NbN heterostructure could be used in quantum computing and low-temperature electronics.  Reporting their work in Science Advances, the researchers say they now plan to further investigate the interaction between superconductivity and the quantum Hall effect in this material.

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