Yuji Suwa

Gate-Induced Band Ferromagnetism in an Organic Polymer

We propose that a chain of five-membered rings (polyaminotriazole) should be ferromagnetic with an appropriate doping that is envisaged to be feasible with a field-effect transistor structure. The ferromagnetism is confirmed by a spin density functional calculation, which also shows that ferromagnetism survives the Peierls instability. We explain the magnetism in terms of the Mielke and Tasaki flatband ferromagnetism with the Hubbard model. This opens a new possibility of band ferromagnetism in purely organic polymers.

New Theoretical View for High Temperature Superconductivity

A new theoretical explanation is given for the superconducting state in copper oxides. The key feature of the state is the following. A coherent motion of itinerant holes from low-spin to high-spin states in the presence of a relatively long-range antiferromagnetic ordering of the Cu localized spins creates a superconducting state. Based on this state, it is suggested that the attractive interaction between two holes in different energy bands with the same dispersion mediated by Jahn-Teller-type optic phonons is primarily responsible for superconductivity.

Stimulated emission of near-infrared radiation by current injection into silicon (100) quantum well

We describe the observation of stimulated emissions by current injections into a silicon quantum well. The device consists of a free standing membrane with a distributed feedback resonant cavity fabricated by state-of-the-art silicon processes. The emission spectra have multimode structures peaked in the near-infrared region above the submilliampere threshold currents at room temperatures. Consequently, electronics and photonics should be able to be converged on chips by using silicon quantum well laser diodes.

Group IV light sources to enable the convergence of photonics and electronics

Group IV lasers are expected to revolutionize chip-to-chip optical communications in terms of cost, scalability, yield, and compatibility to the existing infrastructure of silicon industries for mass production. Here, we review the current state-of-the-art developments of silicon and germanium light sources towards monolithic integration. Quantum confinement of electrons and holes in nano-structures has been the primary route for light emission from silicon, and we can use advanced silicon technologies using top-down patterning processes to fabricate these nano-structures, including fin-type vertical multiple quantum wells. Moreover, the electromagnetic environment can also be manipulated in a photonic crystal nano-cavity to enhance the efficiency of light extraction and emission by the Purcell effect. Germanium is also widely investigated as an active material in Group IV photonics, and novel epitaxial growth technologies are being developed to make a high quality germanium layer on a silicon substrate. To develop a practical germanium laser, various technologies are employed for tensile-stress engineering and high electron doping to compensate the indirect valleys in the conduction band. These challenges are aiming to contribute towards the convergence of electronics and photonics on a silicon chip.

Stimulated emission of near-infrared radiation in silicon fin light-emitting diode

We propose top-down processes to make silicon multiple quantum wells called fins for a light-emitting diode. The silicon fins are formed vertically to a substrate and embedded in a Si3N4 waveguide. By current injections into silicon fins, we have observed stimulated emission spectra peaked at the wavelengths corresponding to the periodic structures of fins. The near-field mode profiles obtained at the edge of the waveguide qualitatively agreed with theoretical calculations. It has been turned out that both transverse-electric and transverse-magnetic fields can contribute to the optical gain.

Observation of optical gain in ultra-thin silicon resonant cavity light-emitting diode

We have observed net optical gain by current injections to ultra-thin Si embedded in a resonant optical cavity. The cavity consists of a dielectric waveguide fabricated by CMOS and MEMS process. The photoluminescence (PL) spectra show narrow resonances peaked at the designed wavelength, and the electroluminescence (EL) intensity increases super-linearly with currents. The comparisons with first principle calculations suggest that the optical gain is originated from intrinsic material properties of ultra-thin Si due to quantum confinements.

Identification of boron clusters in silicon crystal by B1s core-level X-ray photoelectron spectroscopy: A first-principles study

We carried out a comprehensive study on the B1s core-level X-ray photoelectron spectroscopy (XPS) binding energies for B clusters in crystalline Si using a first-principles calculation with careful evaluation of the local potential boundary condition for the model system, where convergence within 0.1 eV was confirmed for the supercell size. For ion-implanted samples, we identified experimental peaks due to B clusters and threefold B as icosahedral B12 and 〈001〉B-Si defects, respectively. For as-doped samples prepared by plasma doping, it was found that the calculated XPS binding energies for complexes of vacancies and B atoms were consistent with the experimental spectra.

Reduced Density of Missing-Dimer Vacancies on Tungsten-Contaminated Si(100)-(2×n) Surface by Hydrogen Termination

A Si(100) surface with missing-dimer vacancies forming (2×n) phase was prepared by tungsten deposition and the morphological change was observed by scanning tunneling microscopy when the surface was terminated by hydrogen. The density of dimer vacancies was significantly reduced by the hydrogen termination, suggesting that the density of subsurface W atoms decreased. We discuss the mechanism of this morphological change based on the traditional theory of chemisorption-induced surface segregation and on the energetic instability of W atoms buried in the subsurface of the hydrogen-terminated Si surface.

Spin-density-functional study of the organic polymer dimethylaminopyrrole: A realization of the organic periodic Anderson model

While the periodic Anderson model (PAM) has been recognized as a good model for various heavy f-electron systems, here we design a purely organic polymer whose low-energy physics can be captured by PAM. By means of the spin density functional calculation, we show that polymer of dimethylaminopyrrole is a candidate, where its ground state can indeed be magnetic depending on the doping. We discuss the factors favoring ferromagnetic ground state.

First principles study of isotope effect in hydrogen-bonded K3H(SO4)2: I - stable structures

Abstract The first principles calculations for K 3 H(SO 4 ) 2 (KHS) system are performed in order to determine its stable structure in the presence of the hydrogen or the deuterium in its hydrogen bond, and to discuss the origin of the large isotope effect in the KHS system. As a result, a reasonable value of the antiferroelectric interaction energy is obtained. It is also found that the position of the hydrogen is closer to the center of the hydrogen bond than that of the deuterium, based on the calculated results of the proton-position dependence of the oxygen–oxygen distance and on the experimental fact that the oxygen-oxygen distance in K 3 D(SO 4 ) 2 (DKHS) is larger than that in KHS.