Haruko Toyama, 3rd Year Doctoral Student, Department of Physics

Ryota Akiyama (Assistant Professor, Department of Physics)

Yoshihiro Endo (Doctoral student (at that time), Department of Physics) ^*1

Shuji Hasegawa (Professor, Department of Physics)

Kiyoshi Ichinokura (Assistant Professor, Tokyo Institute of Technology)

Toru Hirahara, Associate Professor, Tokyo Institute of Technology

Takuji Iimori (Technical Staff, Institute for Solid State Physics)

Fumio Komori (then Professor, Institute for Solid State Physics) ^*2

Key points of the presentation

• Silicon carbide (SiC) ^(*1 ) We discovered that calcium (Ca) atoms penetrate beneath a single atomic layer of graphene, which is formed on a semiconductor crystal substrate, to produce superconductivity and clarified its mechanism.
• The observed superconducting properties are peculiar and cannot be explained by the conventionally assumed model, and may involve the electronic state expected for unconventional superconductivity ^{(Note 2)}.
• This research presents a new field of superconductivity involving Dirac particles and is expected to be applied to novel devices in which an extremely thin "Dirac superconductor" is fused with a SiC power device on the same chip.

Summary of the presentation

A research group led by Graduate Student Haruko Toyama, Assistant Professor Ryota Akiyama, Graduate Student (at that time) Yoshihiro Endo, and Professor Shuji Hasegawa of the Department of Physics, Graduate School of Science, the University of Tokyo, in collaboration with Assistant Professor Sei Ichinokura, Associate Professor Toru Hirahara, Department of Physics, Graduate School of Science, the University of Tokyo, Technical Staff Takuji Iimori and Professor Fumio Komori of the Institute for Solid State Physics, the University of Tokyo, has recently They have discovered that a sample made by fabricating a single atomic layer of graphene on the surface of a silicon carbide (SiC) crystal substrate, a semiconductor used in power devices, and then depositing calcium (Ca) on top of it (depositing atoms in a vacuum) and heating it to treat it, exhibits superconductivity when cooled. This superconductivity was found to be caused by the direct bonding of Ca atoms on the SiC crystal surface, which transformed monoatomic graphene into bilayer graphene, and then Ca atoms penetrated into the interlayer of the bilayer graphene. In other words, the change in the interface between graphene and the SiC crystal substrate plays an important role in the development of superconductivity. The results also suggest that not only the usual metallic electronic state, which has been believed to be the origin of superconductivity, but also the electronic state of the Dirac particle in graphene and the "van Hove singularity ^{"(Note 3)}, an unusual electronic state that has attracted attention in recent years as the source of unconventional superconductivity, are involved in superconductivity. The result suggests that the Van Hove singularity is also involved in superconductivity. This result means that graphene 2-dimensional superconductors can be fabricated on SiC substrates, which have been widely studied as power devices, and is expected to be applied to new "superconducting devices". This research was published in ACS Nano, a journal of the American Chemical Society.

Publication details

Graphene is a two-dimensional sheet-like material consisting of carbon atoms arranged in a hexagonal shape and only one atom thin. Electrons flowing through graphene behave according to the Dirac equation, which incorporates the theory of relativity, and move extremely fast compared to electrons in ordinary semiconductors such as silicon. Since its experimental discovery in 2004, not only basic research but also applied research to use it as a next-generation high-speed device has been actively conducted. In this study, we succeeded in inducing superconductivity while taking advantage of the properties of graphene, and clarified the mechanism of superconductivity. Since superconductors have zero electrical resistance, they have the interesting and important property of allowing electricity to flow without energy loss, and are expected to be used in a variety of applications such as long-distance power transmission, high-speed and environmentally friendly linear motor cars, and quantum computers. As a means of inducing superconductivity in graphene, the group reported that the insertion of Ca atoms in bilayer graphene grown on semiconducting SiC crystal substrates leads to superconductivity ^{(Note 4)}. (Note 4) Whether or not superconductivity is achieved depends largely on the interlayer distance of graphene after atom insertion, the arrangement density and valence of the inserted atoms, and other factors. However, the position of the Ca atom and the electronic state that causes superconductivity have not yet been elucidated.

In this study, we discovered for the first time that superconductivity occurs when Ca atoms are deposited on the thinnest "monoatomic layer" of graphene and heated. According to previous studies, at least two layers of graphene were considered necessary to obtain a structure with metal atoms inserted between graphene layers. However, in the case of single-layer graphene on SiC substrate, as in this study, the interface between graphene and substrate plays an important role, where the atomic structure is changed, so that even a "single-layer" graphene is eventually transformed into a bilayer graphene and superconductivity is achieved by insertion of Ca atoms between the layers of bilayer graphene. The insertion of Ca atoms between the layers of bilayer graphene is responsible for the superconductivity (Fig. 1).

Figure 1: (a) Schematic diagram of the atomic structure change during the Ca atom insertion process (cross-sectional view from the side). The leftmost is a single layer of graphene on a SiC substrate in the starting state. When Ca is inserted into the graphene, it is transformed into bilayer graphene, and Ca atoms are inserted between the layers of the bilayer graphene, resulting in "Ca-inserted superconducting bilayer graphene". (b,c) Temperature dependence of the two-dimensional resistivity (sheet resistance) of superconducting Ca-inserted bilayer graphene under various planar magnetic fields, and planar magnetic field dependence at various temperatures. It is systematically shown that the more the magnetic field is applied or the temperature is increased, the more the superconductivity breaks down.

This reveals the overall atomic and electronic structure of "Ca-inserted superconducting graphene," which was not clear before. In this system, electrons are supplied to graphene from the interface between graphene and SiC substrate in addition to electrons supplied by the inserted Ca atoms, indicating that the effect of the interface, which has not been paid attention to so far, is very important for superconductivity in this system. The interface effect, which has not been paid attention to so far, is very important for the superconductivity in this system.

Furthermore, it was also revealed that the superconductivity is "unusual". In the conventional superconductivity theory, BCS theory ^{(Note 5)}, which assumes free electron-like behavior, the superconducting transition temperature increases monotonically as the electrical conductivity of the material increases. However, in this system, the superconducting transition temperature decreases at a certain point as the electrical conductivity increases, which is a peculiar behavior that cannot be explained by conventional theories (Fig. 2).

Figure 2: Relationship between the electrical conductivity (normal conductivity) at normal conduction (when superconductivity breaks down) and the superconducting transition temperature. The red area is the superconducting phase and the light blue-green area is the normal-conductive phase. The red arrows indicate the peculiar behavior that the superconducting transition temperature decreases while the normal-conductive conductivity increases.

This indicates that a special electronic structure called the van Hove singularity may be involved. The electronic state is predicted to have a variety of exotic properties and is predicted to cause "unconventional superconductivity," which is awaiting demonstration. The present results are the first experimental demonstration of the specificity of superconductivity in Ca-inserted graphene.

In this study, the mechanism of two-dimensional superconductivity in graphene, a promising next-generation electronic material, was clarified from both the atomic and electronic structure aspects. Furthermore, we have found specific guidelines for exploring new graphene superconductors with higher superconducting transition temperatures, paving the way for the search for unconventional superconductivity using graphene as a stage. In addition, from the perspective of reducing the environmental burden for a sustainable society (SDGs), we will take advantage of our ability to construct graphene 2-dimensional superconductor chips on SiC substrates, a next-generation material that is being actively studied for applications to realize power devices that can significantly reduce transmission energy loss and are smaller and lighter in weight, This is expected to contribute to the development of novel high-speed atomic layer devices that are the thinnest and most energy-efficient.

This research was supported by JSPS Research Fellowship, Grant-in-Aid for Scientific Research (A) No. 20H00342, Grant-in-Aid for Basic Research (B) No. 20H02616, Grant-in-Aid for Basic Research (A) No. 18H03877, Young Scientist Research No. 19K15443, Young Scientist Research No. 21K14533 and Grant-in-Aid for Young Scientists No. 20J 11972.

Journals

Journal name	ACS Nano
Title of paper	Two-dimensional superconductivity of the Ca-intercalated graphene on SiC: vital role of the interface between monolayer graphene and the substrate
Authors	Haruko Toyama*, Ryota Akiyama*, Satoru Ichinokura, Mizuki Hashizume, Takushi Iimori, Yukihiro Endo, Rei Hobara, Tomohiro Matsui, Kentaro Horii, Shunsuke Sato, Toru Hirahara, Fumio Komori, Shuji Hasegawa
DOI number	10.1021/acsnano.1c11161

Terminology

1 Silicon carbide (SiC, silicon carbide )

A compound semiconductor crystal composed of silicon (Si) and carbon (C) atoms. Compared to conventional Si semiconductors, it is an attractive semiconductor material with the advantages of reduced power loss, smaller size, and lighter weight. In this research, graphene is fabricated by heating this substrate to desorb Si atoms from its surface. ↑up

Note 2 Unconventional superconductivity

A novel superconductivity that deviates from the typical BCS theory and arises from a different expression mechanism from the BCS theory ^{(Note 5)}. The copper oxide high-Tc superconductor, a very important discovery in the history of superconductivity research, is an example of such a superconductor. The Nobel Prize in Physics was awarded in 1987. ↑up

Note 3 van Hove singularity (van Hove singularity, vHs)

A singularity in which the density of states (number of electron seats) is so large that its involvement is theoretically predicted to induce unconventional superconductivity (chiral d-wave superconductivity). Because of the presence of vHs in the electronic structure of graphene, theoretical calculations suggest the possibility of unconventional superconductivity in the graphene setting, and experimental reports of such superconductivity are awaited. ↑up

Note 4 February 4, 2016 Press Release.

'Success in making graphene superconducting' https://www.s.u-tokyo.ac.jp/ja/press/2016/4597/ ↑

Note 5 BCS Theory

The most basic theory to explain the mechanism of superconductivity, proposed by Bardeen, Cooper, Schrieffer, and others in the U.S. in 1957. Normally, electrons repel each other due to their negative charge, but the vibration of the atoms that make up the crystal acts like a glue, causing the two electrons to feel an attraction toward each other and flow through the material as a "pair of electrons. The theory is that the energy gain/loss between the two electrons cancels each other out, resulting in current flow with zero electrical resistance, for which the three were awarded the Nobel Prize in Physics in 1972. ↑up