arXiv subjects

Kenji Shiraishi

Publications and source records attributed to Kenji Shiraishi.

17 recordsLinked to original sources

arXiv2025

Multistability of interstitial magnesium and its carrier recombined migration in gallium nitride

We present density-functional-theory calculations which provide a microscopic picture of the recombination-enhanced migration of interstitial Mg in GaN. We determine stable structures and migration pathways with accurate HSE approximation to the exchange-correlation energy, and also computed recombination rates using the obtained energy spectrum and wavefunctions. It is found that the migration between the most stable octahedral sites (Mg$_{\textrm{O}}$) via newly found interstitial complex structure shows the lowest migration energy in which one or two electrons are captured during the migration, that the most stable charge state of 2+ changes to 1+ or neutral, and that by this recombination of carriers the migration barrier is significantly reduced. Starting from Mg$_{\textrm{O}}^{2+}$, Mg captures an electron becoming the 1+ charge state and overcomes the barrier of 1.65 eV, much reduced from 2.23 eV in case of the migration with the 2+ charge state kept. Moreover, further electron capture is realized accompanied by substantial structural relaxation, thus Mg becoming neutral. Detailed HSE calculations for this second capture show that the migration barrier is 1.55 eV, thus clarifying the important role of the carrier recombination for Mg migration in GaN. These findings are corroborated by the present quantitative calculations of recombination rates based on electronic Hamiltonian constructed from our DFT-obtained energy spectrum. The timescale of the recombination is clarified to be in or under the timescale of the migration with typical electron density and the enhancement is expected to be significant.

cond-mat.mtrl-sci↗

arXiv2024

First-Principles Study of Recombination-Enhanced Migration of an Interstitial Magnesium in Gallium Nitride

The stable and metastable configurations of interstitial Mg in GaN and its migration energy barriers are studied from first-principles calculations. In addition to the conventional octahedral (O, global energy minimum) and tetrahedral (T, metastable) interstitial sites, we discover two new metastable interstitial complexes with formation energy lower than or close to that of T configuration but higher than O. Except for Mg at O site which only has +2 charge state, all other configurations also permit charge states +1 or 0. The minimum migration energy barrier for Mg$^{++}$ between O sites is found to be 1.95 eV. We further find that, when Fermi energy is close to the conduction band, the migration between O sites via metastable configurations occur through a recombination-enhanced mechanism in which the charge state changes from +2 at O site to 0 at metastable sites by consecutive capture of two electrons during the migration. This process greatly reduces the migration energy barrier to as low as 1.47 eV. This value is consistent with experiments, and we also discuss the role of intrinsic defects in the migration of Mg.

cond-mat.mtrl-sci↗

arXiv2022

Beyond ab initio reaction simulator: an application to GaN metalorganic vapor phase epitaxy

To develop a quantitative reaction simulator, data assimilation was performed using high-resolution time-of-flight mass spectrometry (TOF-MS) data applied to GaN metalorganic vapor phase epitaxy system. Incorporating ab initio knowledge into the optimization successfully reproduces not only the concentration of CH$_4$ (an impurity precursor) as an objective variable but also known reaction pathways. The simulation results show significant production of GaH$_3$, a precursor of GaN, which has been difficult to detect in TOF-MS experiments. Our proposed approach is expected to be applicable to other applied physics fields that require quantitative prediction that goes beyond ab initio reaction rates.

cond-mat.mtrl-sci↗

arXiv2020

Negative-charge-storing mechanism of potassium-ion electrets used for vibration-powered generators: Microscopic study of a-SiO2 with and without potassium atoms

A potassium-ion electret, which is a key element of vibration-powered microelectromechanical generators, can store negative charge almost permanently. However, the mechanism by which this negative charge is stored is still unclear. We theoretically study the atomic and electronic structures of amorphous silica (a-SiO2) with and without potassium atoms using first-principles molecular-dynamics calculations. Our calculations show that a fivefold-coordinated Si atom with five Si-O bonds (an SiO5 structure) is the characteristic local structure of a-SiO2 with potassium atomsm, which becomes negatively charged and remains so even after removal of the potassium atoms. These results indicate that this SiO5 structure is the physical origin of the robust negative charge observed in potassium-ion electrets. We also find that the SiO5 structure has a Raman peak at 1000cm-1.

cond-mat.mtrl-sci↗

arXiv2020

Screw dislocation that converts p-type GaN to n-type: Microscopic study on the Mg condensation and the leakage current in p-n diodes

Recent experiments suggest that Mg condensation at threading dislocations induce current leakage, leading to degradation of GaN-based power devices. To study this issue, we perform first-principles total-energy electronic-structure calculations for various Mg and dislocation complexes. We find that threading screw dislocations (TSDs) indeed attract Mg impurities, and that the electronic levels in the energy gap induced by the dislocations are elevated towards the conduction band as the Mg impurity approaches the dislocation line, indicating that the Mg-TSD complex is a donor. The formation of the Mg-TSD complex is unequivocally evidenced by our atom probe tomography in which Mg condensation and diffusion through [0001] screw dislocations is observed in p-n diodes. These findings provide a novel picture that the Mg being a p-type impurity in GaN diffuses toward the TSD and then locally forms an n-type region. The appearance of this region along the TSD results the reverse leakage current.

cond-mat.mtrl-sci↗

arXiv2018

Influence of edge magnetization and electric fields on zigzag silicene, germanene and stanene nanoribbons

Using a multi-orbital tight-binding model, we have studied the edge states of zigzag silicene, germanene, and stanene nanoribbons (ZSiNRs, ZGeNRs and ZSnNRs, respectively) in the presence of the Coulomb interaction and a vertical electric field. The resulting edge states have non-linear energy dispersions due to multi-orbital effects, and the nanoribbons show induced magnetization at the edges. Owing to this non-linear dispersion, ZSiNRs, ZGeNRs and ZSnNRs may not provide superior performance in field effect transistors, as has been proposed from single-orbital tight-binding model calculations. We propose an effective low-energy model that describes the edge states of ZSiNRs, ZGeNRs, and ZSnNRs. We demonstrate that the edge states of ZGeNR and ZSnNR show anti-crossing of bands with opposite spins, even if only out-of-plane edge magnetization is present. The ability to tune the spin polarizations of the edge states by applying an electric field points to future opportunities to fabricate silicene, germanene and stanene nanoribbons as spintronics devices.

cond-mat.mes-hall↗

arXiv2018

Market based embedded Real Time Operation for Distributed Resources and Flexibility

We build upon previous work out of UC Berkeley's energy, controls, and applications laboratory (eCal) that developed a model for price prediction of the energy day-ahead market (DAM) and a stochastic load scheduling for distributed energy resources (DER) with DAM based objective\cite{Travacca}. In similar fashion to the work of Travacca et al., in this project we take the standpoint of a DER aggregator pooling a large number of electricity consumers - each of which have an electric vehicle and solar PV panels - to bid their pooled energy resources into the electricity markets. The primary contribution of this project is the optimization of an aggregated load schedule for participation in the California Independent System Operator (CAISO) real time (15-minute) electricity market. The goal of the aggregator is to optimally manage its pool of resources, particularly the flexible resources, in order to minimize its cost in the real time market. We achieves this through the use of a model predictive control scheme. A critical difference between the prior work in \cite{Travacca} is that the structure of the optimization problem is drastically different. Based upon our review of the current and public literature, no similar approaches exist. The main objective of this project were building methods. Nevertheless, to illustrate a simulation with 100 prosumers was realized. The results should therefore be taken with a grain of salt. We find that the Real Time operation does not substantially decrease or increase the total cost the aggregator faces in the RT market, but this is probably due to parameters that need further tuning and data, that need better processing.

eess.SY↗

arXiv2018

Dust Coagulation Regulated by Turbulent Clustering in Protoplanetary Disks

The coagulation of dust particles is a key process in planetesimal formation. However, the radial drift and bouncing barriers are not completely resolved, especially for silicate dust. Since the collision velocities of dust particles are regulated by turbulence in a protoplanetary disk, the turbulent clustering should be properly treated. To that end, direct numerical simulations (DNSs) of the Navier Stokes equations are requisite. In a series of papers, Pan & Padoan used a DNS with the Reynolds number Re~1000. Here, we perform DNSs with up to Re=16100, which allow us to track the motion of particles with Stokes numbers of 0.01<~St<~0.2 in the inertial range. By the DNSs, we confirm that the rms relative velocity of particle pairs is smaller by more than a factor of two, compared to those by Ormel & Cuzzi (2007). The distributions of the radial relative velocities are highly non-Gaussian. The results are almost consistent with those by Pan & Padoan or Pan et al. at low-Re. Also, we find that the sticking rates for equal-sized particles are much higher than those for different-sized particles. Even in the strong-turbulence case with alpha-viscosity of 10^{-2}, the sticking rates are as high as >~50% and the bouncing probabilities are as low as ~10% for equal-sized particles of St<~0.01. Thus, the turbulent clustering plays a significant role for the growth of cm-sized compact aggregates (pebbles) and also enhances the solid abundance, which may lead to the streaming instability in a disk.

astro-ph.EP↗

arXiv2016

Edge states of hydrogen terminated monolayer materials: silicene, germanene and stanene ribbons

We investigate the energy dispersion of the edge states in zigzag silicene, germanene and stanene nanoribbons with and without hydrogen termination based on a multi-orbital tight-binding model. Since the low buckled structures are crucial for these materials, both the $π$ and $σ$ orbitals have a strong influence on the edge states, different from the case for graphene nanoribbons. The obtained dispersion of helical edge states is nonlinear, similar to that obtained by first-principles calculations. On the other hand, the dispersion derived from the single-orbital tight-binding model is always linear. Therefore, we find that the non-linearity comes from the multi-orbital effects, and accurate results cannot be obtained by the single-orbital model but can be obtained by the multi-orbital tight-binding model. We show that the multi-orbital model is essential for correctly understanding the dispersion of the edge states in tetragen nanoribbons with a low buckled geometry.

cond-mat.mes-hall↗

arXiv2014

Semiclassical Klein Tunneling and Valley Hall Effect in Graphene

We study the dynamics of semiclassical electrons in (gapped) graphene in two complementary limits, i.e. in the Klein tunneling and valley Hall effect regimes, by scattering wave packets off armchair step potentials and by exposing wave packets to a uniform electric field, respectively. Our numerical wave packet simulation goes beyond semiclassical analytical approximations and standard Klein tunneling treatments and allows to study intra- and intervalley scattering processes. We find distinct Klein tunneling characteristics for low and tall steps, which include unusual Berry curvature induced side shifts of the scattered wave packet trajectories. In the presence of a uniform field, our simulations capture the semiclassical valley Hall effect which manifests in the form of laterally shifted Bloch oscillations. Such anomalous trajectory corrections can be relevant for Klein tunneling experiments and electron optics devices. We present detailed simulation results.

cond-mat.mes-hall↗

arXiv2013

Interacting Electron Wave Packet Dynamics in a Two-dimensional Nanochannel

Classical and quantum dynamics are important limits for the understanding of the transport characteristics of interacting electrons in nanodevices. Here we apply an intermediate semiclassical approach to investigate the dynamics of two interacting electrons in a planar nanochannel as a function of Coulomb repulsion and electric field. We find that charge is mostly redistributed to the channel edges and that an electric field enhances the particle-like character of electrons. These results may have significant implications for the design and study of future nanodevices.

cond-mat.mes-hall↗

arXiv2012

Intrinsic Origin of Negative Fixed Charge in Wet Oxidation for Silicon Carbide

We demonstrate on the basis of first-principles calculations that the formation of carbonate-like moiety in SiO$_2$ could be the intrinsic origin of negative fixed charge in SiC thermal oxidation. We find that two possible origins for the negative fixed charges are O-lone-pair state and a negatively charged CO$_3$ ion in SiO$_2$. Such CO$_3$ ion is able to be formed as a result of the existence of residual C atoms in SiO$_2$, which are expected to be emitted from the interface between SiC and SiO$_2$, and the incorporation of H atoms during wet oxidation.

cond-mat.mtrl-sci↗

arXiv2008

Importance of Initial Electronic State in Two-Dimensional Electron System for Electron Transport from Two-Dimensional Electron Gas to Quantum Dot

We have theoretically investigated the time-evolution of electron transport from a two-dimensional electron system (2DES) to a quantum dot (QD). We clearly showed that the coherent electron transport is remarkably modified depending on the initial electronic state in the 2DES. The electron transport from the 2DES to the QD is strongly enhanced when the initial state of electron in the 2DES is localized below the QD. We indicate that these features should be obtained in other nano-structure-electrode coupled systems, and the electron transport between the systems having completely different geometrical conditions is significantly affected by the initial spatial distribution of the electron density.

cond-mat.mes-hall↗

arXiv2002

Superconductivity in quantum-dot superlattices composed of quantum wire networks

Based on calculations using the local density approximation, we propose quantum wire networks with square and plaquette type lattice structures that form quantum dot superlattices. These artificial structures are well described by the Hubbard model. Numerical analysis reveals a superconducting ground state with transition temperatures $T_c$ of up to 90 mK for the plaquette, which is more than double the value of 40 mK for the square lattice type and is sufficiently high to allow for the experimental observation of superconductivity.

cond-mat.mes-hall↗

arXiv2001

Flat-band ferromagnetism in quantum dot superlattices

Possibility of flat-band ferromagnetism in quantum dot arrays is theoretically discussed. By using a quantum dot as a building block, quantum dot superlattices are possible. We consider dot arrays on Lieb and kagome lattices known to exhibit flat band ferromagnetism. By performing an exact diagonalization of the Hubbard Hamiltonian, we calculate the energy difference between the ferromagnetic ground state and the paramagnetic excited state, and discuss the stability of the ferromagnetism against the second nearest neighbor transfer. We calculate the dot-size dependence of the energy difference in a dot model and estimate the transition temperature of the ferromagnetic-paramagnetic transition which is found to be accessible within the present fabrication technology. We point out advantages of semiconductor ferromagnets and suggest other interesting possibilities of electronic properties in quantum dot superlattices.

cond-mat.str-el↗

arXiv2001

Magnetic field effects on two-dimensional Kagome lattices

Magnetic field effects on single-particle energy bands (Hofstadter butterfly), Hall conductance, flat-band ferromagnetism, and magnetoresistance of two-dimensional Kagome lattices are studied. The flat-band ferromagnetism is shown to be broken as the flat-band has finite dispersion in the magnetic field. A metal-insulator transition induced by the magnetic field (giant negative magnetoresistance) is predicted. In the half-filled flat band, the ferromagnetic-paramagnetic transition and the metal-insulator one occur simultaneously at a magnetic field for strongly interacting electrons. All of the important magnetic fields effects should be observable in mesoscopic systems such as quantum dot superlattices.

cond-mat.str-el↗

arXiv2001

Design of a semiconductor ferromagnet in a quantum dot artificial crystal

We present the theoretical design of quantum dot (QD) artificial ferromagnetic crystals. The electronic structure calculations based on local spin density approximation (LSDA) show that our designed QD artificial crystal from a structure comprising the crossing 0.104-micrometer-wide InAs quantum wires (an effective Kagome lattice) has flat band characteristics. Our examined QD artificial crystal has the ferromagnetic ground state when the flat band is half-filled, even though it contains no magnetic elements. The ferromagnetic and the paramagnetic state can be freely switched by changing the electron filling via a gate voltage.

cond-mat.mtrl-sci↗