The term “Andreev states” is used to describe the energy levels present within a given material that exhibits both magnetic flux and channel transmission. In short, this electronic configuration has been found to allow non-superconducting materials to conduct a “supercurrent,” which eliminates the possibility of electron resistance from occurring within the material.
Andreev states are present within superconductors, which therefore account for their impressive ability to maintain their electricity conductance without the presence of such resistance.
In an effort to investigate the properties of exotic particles, particularly Majorana fermions, a group of researchers from the Massachusetts Institute of Technology (MIT) led by Landry Bretheau, sandwiched graphene between two superconductors and analyzed its electronic properties1.
While Andreev states spectroscopy analyses have only been performed in a few systems, such as silver wires, it has never before been considered as an important property in two-dimensional materials, such as graphene. As one of the most notable 2D materials, graphene, a single-atom thick sheet of carbon atoms, will exhibit a normal movement of electrons, enabling the team of physics to easily study the Andreev states present within the materials.
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Carbon, which exhibits an extended electron configuration that allows for researchers to more easily study the Andreev states within this material, particularly as it changes when in direct contact with superconductor materials.
By sandwiching a flake of graphene, which was retrieved by graphite exfoliation processes, between two aluminum superconductor electrodes, in which the final product was placed in a dilution refrigerator at an extremely low temperature of 20 millikelvin, which is an optimal temperature to allow for aluminum to function as a superconductor1.
Once the superconductor sandwich was brought to its optimal temperature range, the team of researchers applied various and changing magnetic fields to the structure, as well as an external voltage directly to the graphene present between the aluminum electrodes.
Through use of tunneling spectroscopy, a scientific research technique that allows researchers to measure the density of electronic states within an electrically conductive material, the MIT team studied the change in the density of the electronic states within the sandwiched graphene layer.
By applying such external forces to the semiconductor sandwich, the MIT researchers found that graphene’s electron configuration transitioned from its standard individual states, to the paired Andreev states, which was directly related to the applied magnetic field and applied voltages1.
By becoming more evident as the electron density of graphene increased, the rising presence of such states also correlated with a stronger supercurrent to be present and flowing between the electrodes. Therefore, by placing graphene between the aluminum electrodes, this remarkable material adapted the electronic properties of the semiconductors to exhibit its own superior conducting abilities.
This discovery has inspired the MIT group to potentially investigate how Majorana fermions may arise from Andreev states, and therefore play an important role in building powerful quantum computers in the near future. A fermion describes any type of subatomic particle, such as quarks and leptons that make up particles such as atoms and nuclei, all of which have their own corresponding antiparticles2.
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There are three types of fermions including Weyl, Dirac, and Majorana. Of particular interest in the area of quantum physics, Majorana fermions describe a fermion that is its own bound states that are present in hybrid superconductor-nanowire-superconductor structures3.
By presenting successful research that connects the impressive electronic influence of Andreev states within superconductor sandwiches, the group of MIT researchers are hopeful that their work will introduce new ways to study how topological phases of materials can be manipulated to develop advanced quantum computers that have practically eradicated error production within their systems.
- “Tunneling spectroscopy of Andreev states in graphene” L. Bretheau, J. Wang, et al. Nature Physics. (2017). DOI: 10.1038/nphys4110.
- “Fermions and bosons” – The Particle Adventure
- “Topological Superconductors, Majorana Fermions and Topological Quantum Computation” – University of Pennsylvania