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Capturing theevolution to novel superconductivity in graphene with insertedCa atoms

Ryota Akiyama, Assistant Professor, Department of Physics

Shuji Hasegawa, Professor, Department of Physics

Graphene is still hot. Graphene is expected to be a next-generation electronic device because it is composed only of carbon atoms, has a low environmental impact, and is 100 times easier for electrons to move in graphene than in Si. In 2018, a shocking announcement was made by a team at MIT that bilayers of graphene rotated by 1.1° from each other exhibit unique superconductivity. However, we had achieved graphene superconductivity earlier by intercalation of atoms.

When silicon carbide SiC crystals are heated to about 1,600°C, only the Si atoms evaporate from the surface, leaving the C atoms, which form a graphene layer. In 2016, he announced that bilayer graphene could be made in this way and that intercalating Ca between the layers would lead to superconductivity. Later, however, we discovered that even one layer of this graphene can be superconducting (with a transition temperature of about 5 K), raising the big question of where the Ca atoms fit in. Detailed analysis of the atomic structure and electronic bands revealed that, as shown in the figure (left), a layer of carbon called the "buffer layer" bonded to the SiC substrate under the monoatomic layer of graphene is transformed into graphene by Ca intercalation, resulting in a total of two layers of graphene, and that Ca is inserted between these layers to form When the layers are regularly lined up, superconductivity is generated. Graphene has an hourglass-shaped electronic band called the Dirac cone, which allows electrons to move at high speed. Furthermore, intercalation of Ca in graphene results in the appearance of new electron bands, which are found to be essential for the appearance of superconductivity.

Figure: (left) Intercalation of Ca atoms in graphene with regular periodicity (right) Relationship between the superconducting transition temperature and the normal-conducting electrical conductivity of the dome-shaped superconductor observed in this study.

According to the BCS theory, which is the basic theory of superconductivity, the higher the electron density, the higher the normal-conductive conductivity and the superconducting transition temperature should be. However, our experimental results are "dome-shaped" as shown in the figure (right), which does not fit the BCS theory. This suggests that a special state called the van Hove singularity exists in the electronic band of graphene, which may give rise to superconductivity in the form of chiral d-waves. This has only been predicted theoretically, but this is the first experimental evidence of such a phenomenon.

These results show that graphene has the potential to exhibit a wide variety of physical properties, and since it can be fabricated on SiC substrates, which are also attracting attention as power semiconductors, it offers us great dreams for new hybrid devices with superconductivity.

This work was published in ACS Nano 16, 3582 (2022) by H. Toyama et al.

* BCS theory (Bardeen Cooper Schrieffer: the first microscopic theory of superconductivity since its discovery in 1911; developed in 1957 by John Bardeen, Leon Neil Cooper, and John Robert The theory was proposed by John Bardeen, Leon Neil Cooper, and John Robert Schrieffer in 1957 and named BCS after the initials of their names.

(Press release, February 25, 2022)

Published in the July 2022 issue of Faculty of Science News

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