Scientists recently discovered novel quantum materials whose charge carriers exhibit ‘topological’ features. Charge carriers are particles that transport electrical charges through a material. Topology is the study of the rules behind how shapes behave when they change. For example, a doughnut shape will still have a hole if it changes continuously from round to square or if it is twisted or stretched. Unless we do something drastic, like cutting or tearing, the doughnut cannot be changed into another topological shape such as a sphere. These newly discovered topological features result in the charge’s transport not being affected by continuous transformations. Because of this “protection,” topological materials often show peculiar quantum states on their surfaces and edges. This study observed superconducting edge currents for what the researchers believe is the first time, in a material called molybdenum ditelluride. Surprisingly, and for reasons that are not yet well understood, these edge currents behave independently of the currents in the bulk of the material, away from the edges.
Topological materials are an important innovation. Their unique electronic properties derive directly from the wave function, a mathematical description of a material’s quantum state. These properties are said to be topologically protected. This means they are not affected by external forces such as disorder or impurities. Scientists believe that topological protection in superconductors may give rise to novel quantum states that remain coherent over long times and distances. This makes topological superconductors potentially useful for future quantum computers, which rely on quantum coherence to work.
The periodic behavior of the current that the scientists observed in the topological superconductor, molybdenum ditelluride, is the consequence of two quantum properties of a superconductor. The first property is a magnetic flux that penetrates the superconductor one quantum at a time (symbolized fo)). The second property is how the magnetic field imparts a phase to the wavefunction describing the superconducting electrons, which varies in space like a winding twist on a decorative ribbon. The phase winds by 2p every time the magnetic flux equals a multiple of the magnetic flux quantum (fo). Recent measurements not only revealed the predicted periodic oscillations arising from the bulk wavefunction phase changes, they also showed an additional oscillation that originates from currents on the sample edges. Unlike the bulk mode, the frequency of the edge oscillations increases with the area of the sample. The surface mode oscillations come from successive entry of a flux quantum into a closed loop defined by a supercurrent circulating on the crystal edges. These experimental results are the first observation of a supercurrent at the edge of a topological superconductor.
The dilution refrigerator experiments for this research were supported by the Department of Energy. The research was also supported by the U.S. Army Research Office. Two of the researchers were supported by the Gordon and Betty Moore Foundation’s Emergent Phenomena in Quantum Systems Initiative. The growth and characterization of crystals were performed with support from the National Science Foundation.
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