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Search Results: 1 - 10 of 2057 matches for " Atsushi Oshiyama "
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Structural stability and energy-gap modulation through atomic protrusion in freestanding bilayer silicene
Yuki Sakai,Atsushi Oshiyama
Physics , 2015, DOI: 10.1103/PhysRevB.91.201405
Abstract: We report on first-principles total-energy and phonon calculations that clarify structural stability and electronic properties of freestanding bilayer silicene. By extensive structural exploration, we reach all the stable structures reported before and find four new dynamically stable structures, including the structure with the largest cohesive energy. We find that atomic protrusion from the layer is the principal relaxation pattern which stabilizes bilayer silicene and determines the lateral periodicity. The hybrid-functional calculation shows that the most stable bilayer silicene is a semiconductor with the energy gap of 1.3 eV.
New Identification of Metallic Phases of In Atomic layers on Si(111) Surfaces
Kazuyuki Uchida,Atsushi Oshiyama
Physics , 2012, DOI: 10.1103/PhysRevB.87.165433
Abstract: We report first-principles calculations that clarify atomic structures and coverage of the metallic phases of In overlayers on Si (111) surfaces. Calculated energy bands and scanning tunneling microscopy images along with the obtained energetics of various phases reveal that the two metallic phases with the $\sqrt{7} \times \sqrt{3}$ periodicity observed experimentally are single and double In overlayers, as opposed to prevailing assignments.
Structural Tristability and Deep Dirac States in Bilayer Silicene on Ag(111) Surfaces
Zhi-Xin Guo,Atsushi Oshiyama
Physics , 2013, DOI: 10.1103/PhysRevB.89.155418
Abstract: We report on total-energy electronic-structure calculations in the density-functional theory performed for both monolayer and bilayer silicene on Ag(111) surfaces. The rt3 x rt3 structure observed experimentally and argued to be the monolayer silicene in the past [Chen et al., Phys. Rev. Lett. 110, 085504 (2013)] is identified as the bilayer silicene on the Ag(111) surface. The identification is based on our accurate density-functional calculations in which three approximations, the local density approximation, the generalized-gradient approximation, and the van-der-Waals-density-functional approximation, to the exchange-correlation energy have been carefully examined. We find that the structural tristability exists for the rt3 x rt3 bilayer silicene. The calculated energy barriers among the three stable structures are in the range of 7 - 9 meV per Si atom, indicating possible flip-flop motions among the three. We have found that the flip-flop motion between the two of the three structures produces the honeycomb structure in the STM images, whereas the motion among the three does the 1 x 1 structure. We have found that the electron states which effectively follow Dirac equation in the freestanding silicene couple with the substrate Ag orbitals due to the bond formation, and shift downwards deep in the valence bands. This feature is common to all the stable or metastable silicene layer on the Ag(111) substrate.
Crossover between Silicene and Ultra-Thin Si Atomic Layers on Ag(111) Surfaces
Zhi-Xin Guo,Atsushi Oshiyama
Physics , 2015, DOI: 10.1088/1367-2630/17/4/045028
Abstract: We report on total-energy electronic structure calculations in the density-functional theory performed for the ultra-thin atomic layers of Si on Ag(111) surfaces. We find several distinct stable silicene structures: $\sqrt{3}\times\sqrt{3}$, $3\times3$, $\sqrt{7}\times\sqrt{7}$ with the thickness of Si increasing from monolayer to quad-layer. The structural bistability and tristability of the multilayer silicene structures on Ag surfaces are obtained, where the calculated transition barriers infer the occurrence of the flip-flop motion at low temperature. The calculated STM images agree well with the experimental observations. We also find the stable existence of $2\times1$ $\pi$-bonded chain and $7\times7$ dimer-adatom-stacking fault Si(111)-surface structures on Ag(111), which clearly shows the crossover of silicene-silicon structures for the multilayer Si on Ag surfaces. We further find the absence of the Dirac states for multilayer silicene on Ag(111) due to the covalent interactions of silicene-Ag interface and Si-Si interlayer. Instead, we find a new state near Fermi level composed of $\pi$ orbitals locating on the surface layer of $\sqrt{3}\times\sqrt{3}$ multilayer silicene, which satisfies the hexagonal symmetry and exhibits the linear energy dispersion. By examining the electronic properties of $2\times1$ $\pi$-bonded chain structures, we find that the surface-related $\pi$ states of multilayer Si structures are robust on Ag surfaces.
Interstitial Channels that Control Band Gaps and Effective Masses in Tetrahedrally Bonded Semiconductors
Yu-ichiro Matsushita,Atsushi Oshiyama
Physics , 2014, DOI: 10.1103/PhysRevLett.112.136403
Abstract: We find that electron states at the bottom of the conduction bands of covalent semiconductors are distributed mainly in the interstitial channels and that this floating nature leads to the band-gap variation and the anisotropic effective masses in various polytypes of SiC. We find that the channel length, rather than the hexagonality prevailed in the past, is the decisive factor for the band-gap variation in the polytypes. We also find that the floating nature causes two-dimensional electron and hole systems at the interface of different SiC polytypes and even one-dimensional channels near the inclined SiC surface.
Spontaneous Appearance of Low-dimensional Magnetic Electron System on Semiconductor Nanostructures
Keisuke Sawada,Jun-Ichi Iwata,Atsushi Oshiyama
Physics , 2015,
Abstract: We find that spin-polarized ground states emerge in nanofacets which are self-organized on SiC (0001) surfaces. Our large-scale density-functional calculations reveal that the nanofacet formed by bunching of single bilayer steps generates peculiar carbon dangling bond states localized at but extended along step edges. The flat-band characteristics of those C states cause either ferromagnetic or anti-ferromagnetic chains on covalent semiconductors.
Comparative study of hybrid functionals applied to structural and electronic properties of semiconductors and insulators
Yu-ichiro Matsushita,Kazuma Nakamura,Atsushi Oshiyama
Physics , 2011, DOI: 10.1103/PhysRevB.84.075205
Abstract: We present a systematic study that clarifies validity and limitation of current hybrid functionals in density functional theory for structural and electronic properties of various semiconductors and insulators. The three hybrid functionals, PBE0 by Perdew, Ernzerhof, and Becke, HSE by Heyd, Sucseria, and Ernzerhof, and a long-range corrected (LC) functional, are implemented in a well-established plane-wave-basis-set scheme combined with norm-conserving pseudopotentials, thus enabling us to assess applicability of each functional on equal footing to the properties of the materials. The materials we have examined in this paper range from covalent to ionic materials as well as a rare-gas solid whose energy gaps determined by experiments are in the range of 0.6 eV - 14.2 eV: i.e., Ge, Si, BaTiO$_3$, $\beta$-GaN, diamond, MgO, NaCl, LiCl, Kr, and LiF. We find that the calculated bulk moduli by the hybrid functionals show better agreement with the experiments than the generalized gradient approximation (GGA) provides, whereas the calculated lattice constants by the hybrid functionals and GGA show comparable accuracy. The calculated energy band gaps and the valence-band widths for the ten prototype materials show substantial improvement using the hybrid functional compared with GGA. In particular, it is found that the band gaps of the ionic materials as well as the rare-gas solid are well reproduced by the LC-hybrid functional, whereas those of covalent materials are well described by the HSE functional. We also examine exchange effects due to short-range and long-range components of the Coulomb interaction and propose an optimum recipe to the short-range and long-range separation in treating the exchange energy.
Absence of Dirac Electrons in Silicene on Ag (111) Surfaces
Zhi-Xin Guo,Shinnosuke Furuya,Jun-ichi Iwata,Atsushi Oshiyama
Physics , 2012, DOI: 10.7566/JPSJ.82.063714
Abstract: We report first-principles calculations that clarify stability and electronic structures of silicene on Ag(111) surfaces. We find that several stable structures exist for silicene/Ag(111), exhibiting a variety of images of scanning tunneling microscopy. We also find that Dirac electrons are {\em absent} near Fermi energy in all the stable structures due to buckling of the Si monolayer and mixing between Si and Ag orbitals. We instead propose that either BN substrate or hydrogen processing of Si surface is a good candidate to preserve Dirac electrons in silicene.
Structural Evolution and Optoelectronic Applications of Multilayer Silicene
Zhi-Xin Guo,Yue-Yu Zhang,Hongjun Xiang,Xin-Gao Gong,Atsushi Oshiyama
Physics , 2015, DOI: 10.1103/PhysRevB.92.201413
Abstract: Despite the recent progress on two-dimensional multilayer materials (2DMM) with weak interlayer interactions, the investigation on 2DMM with strong interlayer interactions is far from its sufficiency. Here we report on first-principles calculations that clarify the structural evolution and optoelectronic properties of such a 2DMM, multilayer silicene. With our newly developed global optimization algorithm, we discover the existence of rich dynamically stable multilayer silicene phases, the stability of which is closely related to the extent of sp3 hybridization that can be evaluated by the average bonds and effective bond angles. The stable Si(111) surface structures are obtained when the silicene thickness gets up to four, showing the critical thickness for the structural evolution. We also find that the multilayer silicene with pi-bonded surfaces present outstanding optoelectronic properties for the solar cells and optical fiber communications due to the incorporation of sp2-type bonds in the sp3-type bonds dominated system. This study is helpful to complete the picture of structure and related property evolution of 2DMM with strong interlayer interactions.
Introduction of Music Therapy Incorporated into Cognitive Remediation: A New Approach to Cognitive Dysfunction in Psychiatric Disorders and a Preliminary Report on Its Effects in Schizophrenia  [PDF]
Naoko Kosugi, Chiaki Oshiyama, Naoki Kodama, Shin-Ichi Niwa
Open Journal of Psychiatry (OJPsych) , 2019, DOI: 10.4236/ojpsych.2019.91003
Abstract: Aim: Cognitive dysfunction and negative symptoms interfere social participation in patients with schizophrenia. Cognitive remediation has proven effective for cognitive deficits. While there are few effective treatments for the negative symptoms of schizophrenia, music therapy is expected to ameliorate negative symptoms. With the goal of improving both cognitive dysfunction and negative symptoms, we originally developed a program of Music Therapy incorporated into Cognitive Remediation (MTCR). We introduce the MTCR program and our preliminary results of conducting MTCR in patients with schizophrenia. Methods: The program was based on the NEAR (neuropsychological educational approach to cognitive remediation) program. The MTCR program uses instrumental performances and ensemble vocal performances to train cognitive functions, which consists of 30 sessions in total, with each session being 60 minutes long (45 minutes of music, 15 minutes of verbal session). The participants attended sessions semiweekly in groups of fewer than 12 members. We measured the participants’ symptoms twice, before the intervention and after the final session. For both measurements, we used the BACS and BPRS. Thirty-six individuals (26 men, 10 women; average age of 42.4 years) with schizophrenia participated in this study. They participated in an average of 28.2 sessions. Results: BACS total scores (p < 0.05), “verbal fluency” (p < 0.01) and “attention” (p < 0.02) subtests improved significantly. Overall BPRS scores were essentially unchanged, but significant improvement was observed in the “emotional withdrawal” (p < 0.05) and “blunted affect” (p < 0.07). “Hostility” (p < 0.05), “bizarre behavior” (p < 0.01), “mannerisms and posturing” (p < 0.01), and “disorientation” (p < 0.03) also improved significantly. Conclusions: The present study demonstrated that MTCR program can improve both cognitive dysfunction and negative symptoms of patients with schizophrenia, which implies that it can also potentially facilitate the social participation of such patients.
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