arXiv subjects

Y. Kawazoe

Publications and source records attributed to Y. Kawazoe.

At least 19 recordsLinked to original sources

arXiv2020

The nano-structural inhomogeneity of dynamic hydrogen bond network of water

In the present study, water is considered as a dynamic network between molecules at distances not exceeding 3.2 angstroms. The instantaneous configurations obtained by using the molecular dynamics method have been sequentially analyzed, the mutual orientation of each molecule with its neighboring molecules has been studied and the interaction energy of each pair of neighbor molecules has been calculated. The majority of mutual orientation angles between molecules lie in the interval [10, 30] degrees. It has been shown that more than 85% of the molecular pairs in each instantaneous configuration form H-bonds and the H-bond network includes all water molecules in the temperature range 233-293 K. The number of H-bonds fluctuates near the mean value and increases with decreasing temperature, and the energy of the vast majority of such bonds is much higher than the thermal energy. The interaction energy of 80% of the H-bonding molecular pairs lies in the interval [-7; -4] kcal/mol. The interaction energy of pairs that do not satisfy the H-bond angle criterion lies in the interval [-5; 4] kcal/mol, and the number of such bonds does not exceed 15% and decreases with decreasing temperature. For the first time it was found that in each instantaneous configuration the H-bond network contains built-in nanometric structural heterogeneities formed by shorter H-bonds. The fraction of molecules involved in the structural heterogeneities increases from 40% to 60% with a temperature decrease from 293 K to 233 K. These heterogeneities have a finite lifetime, but are constantly present in the water. The number of large heterogeneities (containing more than 20 molecules) increases with decreasing temperature, and the number of small structural heterogeneities (less than 20) decreases.

cond-mat.mtrl-sci↗

arXiv2017

Tunable Band Gaps of In$_x$Ga$_{1-x}$N Alloys: From Bulk to Two-Dimensional Limit

Using first-principles calculations combined with a semi-empirical van der Waals dispersion correction, we have investigated structural parameters, mixing enthalpies, and band gaps of buckled and planar few-layer In$_x$Ga$_{1-x}$N alloys. We predict that the free-standing buckled phases are less stable than the planar ones. However, with hydrogen passivation, the buckled In$_x$Ga$_{1-x}$N alloys become more favorable. Their band gaps can be tuned from 6 eV to 1 eV with preservation of direct band gap and well-defined Bloch character, making them promising candidate materials for future light-emitting applications. Unlike their bulk counterparts, the phase separation could be suppressed in these two-dimensional systems due to reduced geometrical constraints. In contrast, the disordered planar thin films undergo severe lattice distortion, nearly losing the Bloch character for valence bands; whereas the ordered planar ones maintain the Bloch character yet with the highest mixing enthalpies.

cond-mat.mtrl-sci↗

arXiv2016

Role of Interlayer Coupling on the Evolution of Band Edges in Few-Layer Phosphorene

Using first-principles calculations, we have investigated the evolution of band-edges in few-layer phosphorene as a function of the number of P layers. Our results predict that monolayer phosphorene is an indirect band gap semiconductor and its valence band edge is extremely sensitive to strain. Its band gap could undergo an indirect-to-direct transition under a lattice expansion as small as 1% along zigzag direction. A semi-empirical interlayer coupling model is proposed, which can well reproduce the evolution of valence band-edges obtained by first-principles calculations. We conclude that the interlayer coupling plays a dominated role in the evolution of the band-edges via decreasing both band gap and carrier effective masses with the increase of phosphorene thickness. A scrutiny of the orbital-decomposed band structure provides a better understanding of the upward shift of valence band maximum surpassing that of conduction band minimum.

cond-mat.mtrl-sci↗

arXiv2015

Native point defects in few-layer phosphorene

Using hybrid density functional theory combined with a semiempirical van der Waals dispersion correction, we have investigated the structural and electronic properties of vacancies and self-interstitials in defective few-layer phosphorene. We find that both a vacancy and a self-interstitial defect are more stable in the outer layer than in the inner layer. The formation energy and transition energy of both a vacancy and a self-interstitial P defect decrease with increasing film thickness, mainly due to the upward shift of the host valence band maximum in reference to the vacuum level. Consequently, both vacancies and self-interstitials could act as shallow acceptors, and this well explains the experimentally observed p-type conductivity in few-layer phosphorene. On the other hand, since these native point defects have moderate formation energies and are stable in negatively charged states, they could also serve as electron compensating centers in n-type few-layer phosphorene.

cond-mat.mtrl-sci↗

arXiv2014

Sources of n-type conductivity in GaInO3

Using hybrid density functional theory, we investigated formation energies and transition energies of possible donor-like defects in GaInO3, with the aim of exploring the sources of the experimentally observed n-type conductivity in this material. We predicted that O vacancies are deep donors; interstitial Ga and In are shallow donors but with rather high formation energies (>2.5 eV). Thus these intrinsic defects cannot cause high levels of n-type conductivity. However, ubiquitous H impurities existing in samples can act as shallow donors. As for extrinsic dopants, substitutional Sn and Ge are shown to act as effective donor dopants and can give rise to highly n-type conductive GaInO3; while substitutional N behaviors as a compensating center. Our results provide a consistent explanation of experimental observations.

cond-mat.mtrl-sci↗

arXiv2013

Roles of oxygen vacancies on ferromagnetism in Ni doped In2O3: A hybrid functional study

The roles of oxygen vacancies on the electronic and magnetic properties of Ni doped In$_2$O$_3$ have been studied by first-principles calculations based on hybrid functional theory. Our results predict that the Ni-doped In$_2$O$_3$ system displays a ferromagnetic semiconducting character. However, the presence of oxygen vacancies results in antiferromagnetic coupling between the neighboring Ni pair bridged by an oxygen vacancy. The antiferromagnetic coupling is found to arise from the predominant role of superexchange due to the strong Ni 3d-O 2p hybridization. Consequently, the oxygen vacancies play a key role in the lower saturation magnetization of Ni:In$_2$O$_3$ polycrystalline sample, as observed in experiments.

cond-mat.mtrl-sci↗

arXiv2011

Functionalized Nanofullerenes for Hydrogen Storage: A Theoretical Perspective

The increase in threats from global warming due to the consumption of fossil fuels requires our planet to adopt new strategies to harness the inexhaustible sources of energy. Hydrogen is an energy carrier which holds tremendous promise as a new renewable and clean energy option. Hydrogen is a convenient, safe, versatile fuel source that can be easily converted to a desired form of energy without releasing harmful emissions. However, no materials was found satisfy the desired goals and hence there is hunt for new materials that can store hydrogen reversibly at ambient conditions. In this chapter, we discuss and compare various nanofullerene materials proposed theoretically as storage medium for hydrogen. Doping of transition elements leads to clustering which reduces the gravimetric density of hydrogen, while doping of alkali and alkali-earth metals on the nanocage materials, such as carborides, boronitride, and boron cages, were stabilized by the charger transfer from the dopant to the nanocage. Further, the alkali or alkali-earth elements exist with a charge, which are found to be responsible for the higher uptake of hydrogen, through a dipole- dipole and change-induced dipole interaction. The binding energies of hydrogen on these systems were found to be in the range of 0.1 eV to 0.2 eV, which are ideal for the practical applications in a reversible system.

cond-mat.mtrl-sci↗

arXiv2011

DFT Perspective of Hydrogen Storage on Porous Materials

In this chapter, the physisorption of hydrogen molecules in porous materials as possible hydrogen storage systems has been reviewed. Owing to the weak interaction between H2 molecules and the adsorbent, high storage capacities are typically reached only at cryogenic temperature. Different classes of porous materials possessing different structure and composition have been designed for hydrogen storage applications using computational methods and especially with the aid of DFT methods. The adsorption energies for hydrogen in different porous materials have been increases by the doping of light weight alkali and alkali earth metals. Ab initio molecular dynamics has been carried out to know the stability of the newly functionalized materials. GCMC methods have been employed to know the gravimetric and volumetric uptake percentage of the newly functionalized materials. Therefore, the combined approach provides a better understanding and designing new materials to operate at near room temperature for the reversible hydrogen storage application.

cond-mat.mtrl-sci↗

arXiv2009

Polarization Induced Switching Effect in Graphene Nanoribbon Edge-Defect Junction

With nonequilibrium Green's function approach combined with density functional theory, we perform an ab initio calculation to investigate transport properties of graphene nanoribbon junctions self-consistently. Tight-binding approximation is applied to model the zigzag graphene nanoribbon (ZGNR) electrodes, and its validity is confirmed by comparison with GAUSSIAN03 PBC calculation of the same system. The origin of abnormal jump points usually appearing in the transmission spectrum is explained with the detailed tight-binding ZGNR band structure. Transport property of an edge defect ZGNR junction is investigated, and the tunable tunneling current can be sensitively controlled by transverse electric fields.

cond-mat.mes-hall↗

arXiv2007

Ferroelectric Phase Transitions in Ultra-thin Films of BaTiO3

We present molecular dynamics simulations of a realistic model of an ultrathin film of BaTiO$_3$ sandwiched between short-circuited electrodes to determine and understand effects of film thickness, epitaxial strain and the nature of electrodes on its ferroelectric phase transitions as a function of temperature. We determine a full epitaxial strain-temperature phase diagram in the presence of perfect electrodes. Even with the vanishing depolarization field, we find that ferroelectric phase transitions to states with in-plane and out-of-plane components of polarization exhibit dependence on thickness; it arises from the interactions of local dipoles with their electrostatic images in the presence of electrodes. Secondly, in the presence of relatively bad metal electrodes which only partly compensate the surface charges and depolarization field, a qualitatively different phase with stripe-like domains is stabilized at low temperature.

cond-mat.mtrl-sci↗

arXiv2006

Electronic structures and optical properties of layered perovskites Sr_2MO_4 (M=Ti, V, Cr, and Mn): An ab initio study

A series of layered perovskites Sr$_2$$M$O$_4$ ($M$=Ti, V, Cr, and Mn) is studied by $ab $ $initio$ calculations within generalized gradient approximation (GGA) and GGA+$U$ schemes. The total energies in different magnetic configurations, including the nonmagnetic, ferromagnetic, the layered antiferromagnetic with alternating ferromagnetic plane and the staggered in-plane antiferromagnetic (AFM-II) order, are calculated. It is found that Sr$_2$TiO$_4$ is always a nonmagnetic band insulator. For Sr$_2$MnO$_4$, both GGA and GGA+$U$ calculations show that the insulating AFM-II state has the lowest total energy among all the considered configurations. For $M$=V and Cr, the GGA is not enough to give out the insulating AFM-II states and including the on-site electron-electron correlation effect $U$ is necessary and efficient. The AFM-II state will have the lowest total energy in both cases when $U$ is larger than a critical value. Further, the optical conductivity spectra are calculated and compared with the experimental measurements to show how well the ground state is described within the GGA or GGA+$U$. The results indicate that $U$ is overestimated in Sr$_2$VO$_4$ and Sr$_2$CrO$_4$. To make up such a deficiency of GGA+$U$, the contributions from proper changes in the ligand field, acting cooperatively with $U$, are discussed and shown to be efficient in Sr$_2$CrO$_4$.

cond-mat.str-el↗

arXiv2006

Calculations of spin induced transport in ferromagnets

Based on first-principles density functional calculations, a general approach for determining and analyzing the degree of spin polarization (P) in ferromagnets is presented. The approach employs the so-called tetrahedron method to evaluate the Fermi surface integrations of P in both ballistic and diffusive regimes. The validity of the method is examined by comparing the calculated P values for Fe and Ni with the experiment. The method is shown to yield highly accurate results with minimal computational effort. Within our approach, it is also possible to systematically analyze the contributions of various types of electronic states to the spin induced transport. As a case study, the transport properties of the soft-ferromagnet CeMnNi4 are investigated in order to explain the origin of the existing difference between the experimental and theoretical values of P in this intermetallic compound.

cond-mat.mtrl-sci↗

arXiv2006

Ab initio study of single molecular transistor modulated by gate-bias

We use a self-consistent method to study the current of the single molecular transistor modulated by the transverse gate-bias in the level of the first-principles calculations. The numerical results show that both the polyacene-dithiol molecules and the fused-ring oligothiophene molecules are the potential high-frequency molecular transistor controlled by the transverse field. The long molecules of the polyacene-dithiol or the fused-ring thiophene are in favor of realizing the gate-bias controlled molecular transistor. The theoretical results suggest the related experiments.

cond-mat.mtrl-sci↗

arXiv2006

Electronic Structure and Optical Properties of the Co-doped Anatase TiO$_{2}$ Studied from First Principles

The Co-doped anatase TiO$_{2}$, a recently discovered room-temperature ferromagnetic insulator, has been studied by the first-principles calculations in the pseudo-potential plane-wave formalism within the local-spin-density approximation (LSDA), supplemented by the full-potential linear augmented plane wave (FP-LAPW) method. Emphasis is placed on the dependence of its electronic structures and linear optical properties on the Co-doping concentration and oxygen vacancy in the system in order to pursue the origin of its ferromagnetism. In the case of substitutional doping of Co for Ti, our calculated results are well consistent with the experimental data, showing that Co is in its low spin state. Also, it is shown that the oxygen vacancy enhances the ferromagnetism and has larger effect on both the electronic structure and optical properties than the Co-doping concentration only.

cond-mat.mtrl-sci↗

arXiv2006

First-principles study of phenyl ethylene oligomers as current-switch

We use a self-consistent method to study the distinct current-switch of $2^{'}$-amino-4-ethynylphenyl-4'-ethynylphenyl-5'-nitro-1-benzenethiol, from the first-principles calculations. The numerical results are in accord with the early experiment [Reed et al., Sci. Am. \textbf{282}, 86 (2000)]. To further investigate the transport mechanism, we calculate the switching behavior of p-terphenyl with the rotations of the middle ring as well. We also study the effect of hydrogen atom substituting one ending sulfur atom on the transport and find that the asymmetry of I-V curves appears and the switch effect still lies in both the positive and negative bias range.

cond-mat.mes-hall↗

arXiv2006

Unusual Length Dependence of Conductance of Some Oligomers

Recent experiment found a quantum length dependence of oligothiophene molecule conductance at low bias [Xu et al., nano Lett. \textbf{5}, 1491 (2005)], the long molecule has large conductance. By means of a first-principles method we obtain both the quantum length dependence of conductance at low bias and the classical length dependence of conductance at high bias region for oligothiophene. In between there is an oscillated conductance behavior. The transport behaviors are determined by the distinct electronic structures of the molecular compounds. The various conductance length dependence may appear for the organic compounds. Our further investigation finds that the classical conductance length dependence in polyphenanthrene dithiolates and another unusual conductance length dependence in polyacene ditholates: the quantum length dependence of conductance is at the high bias and the classical length dependence of conductance is at the low bias.

cond-mat.mes-hall↗

arXiv2005

Local Energy Gap in Deformed Carbon Nanotubes

The effects of graphite surface geometrical deformation on the dynamics of conducting electrons are investigated theoretically. The analysis is performed within the framework of a deformation-induced gauge field and corresponding deformation-induced magnetic field. It is shown that the latter gives a local energy gap along the axis of a deformed nanotube. We compare our energy gap results with experimental data on energy gaps in nanotubes and peapods. We also discuss the mixing of two Fermi points and construct a general model of low energy dynamics, including a short-range deformation of the graphite sheet. This model is equivalent to the Weyl equation in {\it U}(1) Abelian and {\it SU}(2) non-Abelian deformation-induced gauge fields.

cond-mat.mes-hall↗

arXiv2005

Controlling edge states of zigzag carbon nanotubes by the Aharonov-Bohm flux

It has been known theoretically that localized states exist around zigzag edges of a graphite ribbon and of a carbon nanotube, whose energy eigenvalues are located between conduction and valence bands. We found that in metallic single-walled zigzag carbon nanotubes two of the localized states become critical, and that their localization length is sensitive to the mean curvature of a tube and can be controlled by the Aharonov-Bohm flux. The curvature induced mini-gap closes by the relatively weak magnetic field. Conductance measurement in the presence of the Aharonov-Bohm flux can give information about the curvature effect and the critical states.

cond-mat.mes-hall↗