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Welcome to the Surface Chemistry Lab., KUCHEM. We are studying the chemistry and physics of the solid surface from a vewpoint of the basic science. Our research interest includes structural and electronic properties of low-dimensional materials formed on solid surfaces, elementary surface chemical reactions, and physics and chemistry of single molecules, isolated or as well-defined small clusters.

The solid surface is an interface between a solid and the vacuum (or, gas and liquid phases). Thus, because of the broken translational symmetry, the surface often behaves as a “contamination” in the samples of the researches in the field of bulk solid state physics and chemistry. Here however is an alternative view. The broken symmetry at surfaces leads to intrigueing phenomena, which often are totally unexpected and shall provide a breakthrough in materials science. For example, a metal that is non-magnetic as a three-dimensional body can have an asymmetry in the electron spin structure at surfaces. The inversion asymmetry at surfaces causes a coupling between electron spin and spacial momenta of conduction electrons. While the electrons have no net spin polarization, their energy is split in the reciprocal space. This is called the Rashba-Bychkov effect (or, Rashba effect) and has long been known for two-dimensional elactron gases at semiconductor heterojunctions. Two research groups including ours reported in 2007 that peculiar surface systems exhibit Rashba-type spin splitting on the order of 1 eV, which is an order of magnitude larger than the previous “world record” and indicates a possible application of the “giant Rashba effect” at surfaces to novel spintronic devices. Our recent activity include related research themes. (See publications.)

We are also interested in phase transition phenomena at surfaces. In low-dimensional systems such as surfaces, a variety of types of phese transitions are driven by electron-electron and electron-phonon interactions. Recently we found a previously unknown type of the Peierls transition, which is driven by electron-phonon interaction in metals with low-dimensional electronic structure and is sometimes called charge-density-wave (CDW) phase transition. The new type is associated with strong electron-phonon coupling and long spacial coherence, yielding a transition mechanism totally different from those previously studied. We also found a new type of order-disorder transition, which is driven by the vacancy configurational entropy associated with the monomer-tetramer structural conversion of adatoms. We are studying these phase transitions by precision structure analysis/critical scattering by surface X-ray diffraction as well as detailed analysis of valence electronic structure bu means of angle-resolved photoemission and first-principles electronic structure calculation. (Butsuri 63, 178-186 (2008); Surf. Sci. Rep. 61, 283-302 (2006).)

The surface is a nuisance also for researchers in the field of gas- and liquid-phase reactions, as it catalyses chemical reactions much more efficiently than in homogenious phases. This, on the other hand, suggests that the surface serves an important field for chemical reactions, both in industry as well as in our daily life. Recent progress in new research field such as fuel cell and hydrogen storage also highlights the importance of the elementary surface processes, which often play key roles in complicated overall processes. We utilize electron energy-loss spectroscopy, which is a high-sensitivity surface vibrational spectroscopy, to study elementary surface chemical processes. In particular, we are interested in the dynamic and microscopic behaviour of hydrogen at metal surfaces.

Recent progress in nanoscience and nanotechnology is fascinating. We in the Surface Chemistry Lab. are carrying out “single-molecule science” at surfaces, which may be an extreme of the nanoscience at this time. We isolate a single small molecule such as water and a well-defined clusters of such molecules on the surface, carry out vibrational and electronic spectroscopy, control the motion and reaction of the molecule by exciting peculiar vibrational modes by inelastic tunneling, and even study the electric conductivity of a single molecule and well-defined clusters.
Our most recent activity revealed hydrogen-bond exchange dynamics governed by quantum tunneling of hydrogen in isolated water dimers on an ultralow-temperature metal surface. (Phys. Rev.Lett. 100, 166101 (2008).)