From more than 40 years ago, Professor Yamashita has been working on the nano-science of advanced metal complexes, which has involved the study of their magnetic properties, optical properties, electrical conductivities, etc. The important keywords for the projects are the followings: (1) inorganic-organic hybrid electronic structures, (2) nano-size (1-100 nm) and nano-space, (3) self-assembly and bottom-up, and (4) nonlinearity and quantum effects. So far, he has produced highly original and creative results, focusing on the nonlinearity and quantum effects among them.
Below, his major achievements are summarized.
1. Nano-wire Pd(III) Mott-insulator with strong electron correlations
In quasi-one-dimensional halogen bridged metal complexes (metal = Pt, Pd, and Ni), Pt and Pd complexes form M(II)-M(IV) mixed valence states due to strong electron-phonon interactions. On the other hand, Ni complexes are Ni(III) Mott-insulators due to strong electron correlations. In addition, Ni(III) complexes exhibit gigantic third-order optical nonlinearity effects. Pd(III) Mott-insulators are expected to show much larger third-order optical nonlinearity effects due to lower energy charge transfer bands. Professor Yamashita has synthesized Pd(III) Mott-insulators by shortening the Pd---Pd distances for the first time by: (1) applying chemical pressure via long alkyl chain counter anions, (2) employing cyclopentanediamine with a weaker ligand field, and (3) incorporating multiple hydrogen bonds between in-plane ligands and counter anions.
2. Direct observation of a spin soliton (nonlinear exciton) and its dynamics by using scanning tunneling microscopy
Trans-polyacetylene has a degenerated ground state, which shows spin soliton at the boundary region. In addition, M(II)-M(IV) (M = Pt and Pd) mixed valence compounds have degenerated ground states. Therefore, from a theoretical viewpoint, M(II)-M(IV) complexes should produce a spin soliton. Professor Yamashita has observed a spin soliton and its dynamics on a single crystal at room temperature by using scanning tunneling microscopy for the first time.
3. Kondo resonance and single-memory performance of single-molecule magnets by using scanning tunneling microscopy and scanning tunneling microscopy
Spintroncs, which is a key technology in this century, is based on the freedom of the charge and spin of electrons. Traditional spintronics with properties, such as giant magnetoresistance, tunneling magnetoresistance, etc., are composed of classical magnets or balk magnets and have faced to "Moore`s Limitation". In order to overcome "Moore`s Limitation", nano-size magnets, such as single-molecule magnets, should be used. One single-molecule magnet acts as one magnet (0-dimensional) as well as one bit of memory. Professor Yamashita has observed a Kondo resonance of double-decker type TbPc2 single-molecule magnets (Pc = phthalocyaninato) on Au(111) by using scanning tunneling microscopy for the first time. Usually, the twist angle between upper and lower Pc ligands is 45 degrees, at which a Kondo resonance is observed, whereas after electrons are injected into TbPc2, the twist angle changes to 30 degrees, at which no Kondo resonance is observed. The appearance and disappearance are reversibly controlled by electron injection. If a Kondo resonance can be considered to be one of memory, the single-molecule memory performance is realized.
4. Encapsulation of metallofullerene single-molecule magnets into single-walled carbon nanotubes
Single-walled carbon nanotubes exhibit a mixture of 80% semiconductor and 20% metallic properties. Professor Yamashita has encapsulated metallofullerene single-molecule magnets, DySc2N@C80, into single-walled carbon nanotubes to bring about magnetoresistance via the interactions between the conducting electron of the carbon nanotube and localized single-molecule magnets. After encapsulation, magnetic hysteresis increased one order of magnitude compared to that without encapsulation. This means that the encapsulation suppresses quantum tunneling in the ground state. By using Ni electrodes, a small magnetoresistance (~0.4 %) was observed.
5. Spin qubits in metal-organic framework (MOF) at room temperature
Classical bits are composed of 0 and 1, whereas spin qubits are composed of a superposition of 0 and 1. In order to realize quantum computers, spin qubits with long lifetimes of the superposition are needed. In order to suppress deformation, M-L stretching, lattice vibration, etc., Professor Yamashita synthesized a metal-organic framework (3-dimensional) containing a V(TPP) complex (0-dimensional). As a result, spin qubits and Rabi Nutation (Coherence) were observed even at room temperature.
Professor Yamashita has created a new scientific field, nano-science of advanced metal complexes, in which important properties, like nonlinearity and quantum effects, are investigated, over the last 40 years. His contributions have been rated highly worldwide among chemistry, physics, and materials sciences. For the reasons above, Professor Yamashita`s work is highly deserving of the Chemistry Society of Japan Award.