1）Atomic-layer superconductors at surfaces and interfaces

1-1) Demonstration of superconductivity of surface atomic-layer materials

It is possible to create two-dimensional (2D) electron systems with high carrier densities at the surface/interface of semiconductors and insulators using surface adsorbates, charge transfer, and electric-field effect, etc. In addition, many surfaces are featured with unique crystalline structures having unknown physical properties. These systems have attracted extensive interest for a long time in terms of possible manifestation of superconductivity. However, many researchers have thought that superconductivity is difficult to realize in these atomically thin 2D systems, because fluctuation and disorder should suppress the phase transition in general.

We have created highly crystalline metal atomic layers on silicon substrates and have demonstrated for the first time that supercurrents can run through them over a macroscopic distance by direct transport measurements. The obtained transition temperature (T_c ~ 3 K) is nearly equal to the bulk value of indium, of which surface atomic layers are made. Furthermore, the critical supercurrent density is very large, comparable to those of practical bulk superconducting materials. This is surprising considering that the supercurrents are spatially confined within an atomic-scale thickness and numerous atomic steps on a surface could hinder their flows.

Recently, almost simultaneously as our finding, 2D superconductors have been discovered in various types of surface and interfaces, including one with a very high Tc. We are now working on the creation of new 2D superconductors with high Tc and/or high critical magnetic fields by utilizing the spin-orbit interaction and organic molecules and by applying our technique to different semiconductors. We also have a prospect of developing them into useful and practical materials.

1-2) STM observation of Josephson vortices

We have found that atomic steps ubiquitous on a semiconductor surface can work as Josephson junctions. When magnetic field is applied to a superconducting surface, superconducting vortices are created by the penetration of the field. Using a scanning tunneling microscope (STM), we have found that vortices trapped at atomic steps are strongly deformed and that the superconducting state is recovered at the vortex core that should have normal-like states. This originates from the discontinuity of the phase evolution at an atomic step, which is caused by the regulation of supercurrents at the Josephson junction. These anomalous vortices are called Josephson vortices.

The existence of the Josephson vortices has been recognized, and they play important roles in high-Tc superconductors, but this is the first time to directly observe their electronic states. This finding is important in terms of application, because Josephson junctions are the most basic component for superconducting devices. (This work is a collaboration with Y. Hasegawa Lab at ISSP, Tokyo University and the group of Hu PI at MANA, NIMS.)

2) Metal nanostructures and quantum states at semiconductor surfaces

Solid surfaces are essentially low-dimensional systems, and the application of state-of-the-art nanotechnology allows us to form nanomaterials and to observe their properties at atomic and molecular levels. We have successfully fabricated unique surface nanostructures and observed the electronic states that are governed by the quantum mechanics. For example, we have realized ultrathin silver films with one-dimensional (1D) periodic structures and electronic states based on atomic-scale templates on silicon surfaces. Electrons are strongly quantum-mechanically confined there.

We have also observed edge states of bismuth atomic layers on silicon surfaces, which originates from adsorbed molecules. Bismuth is a heavy element with a large spin-orbit interaction, and because of this, spins can be polarized due to the Rashba effect and topological effects even without magnetic field. Such atomic-layer materials are expected to be applied to spintronics devices.

3) Reconfigurable molecular motors self-assembled on a solid surface

In view of creating new 2D superconductors and quantum nanostructure on surfaces, one of the important basic techniques is to control organic molecules through the bottom-up approach. In particular, porphyrin-derivative molecules, which are well-known biomolecules, can be self-assembled into a supramolecule when functionalized with appropriate groups. We have accidently found that a supramolecular porphyrin dimer can be rotated on a solid surface.

When a tunneling current is injected into the dimer from a STM tip, the dimer rotates unidirectionally depending on its surface chirality. Furthermore, the chirality of the dimer can be inverted through an intra-dimer recombination in a pre-determined condition, thus enabling the inversion of the rotational direction at will.

Such a behavior is very similar to that of certain kinds of biomolecules, which are self-assembled and operate autonomously. It has been one of the ultimate goals of nanotechnology to assemble molecular motors on a substrate surface and to make a nanoscale machine. We think that our studies can also contribute to this type of research topics.