arXiv subjects

Riichiro Saito

Publications and source records attributed to Riichiro Saito.

At least 37 records · Page 2Linked to original sources

arXiv2018

Universal curve of optimum thermoelectric figures of merit for bulk and low-dimensional semiconductors

Analytical formulas for thermoelectric figure of merit and power factor are derived based on the one-band model. We find that there is a direct relationship between the optimum figures of merit and the optimum power factors of semiconductors despite of the fact that the two quantities are generally given by different values of chemical potentials. By introducing a dimensionless parameter consisting of optimum power factor and lattice thermal conductivity (without electronic thermal conductivity), it is possible to unify optimum figures of merit of both bulk and low-dimensional semiconductors into a single universal curve that covers lots of materials with different dimensionalities.

cond-mat.mtrl-sci↗

arXiv2017

Two-dimensional MoS$_2$ electromechanical actuators

We investigate electromechanical properties of two-dimensional MoS$_2$ monolayers in the 1H, 1T, and 1T$^\prime$ structures as a function of charge doping by using density functional theory. We find isotropic elastic moduli in the 1H and 1T structures, while the 1T$^\prime$ structure exhibits an anisotropic elastic modulus. Moreover, the 1T structure is shown to have a negative Poisson's ratio, while Poisson's ratios of the 1H and 1T$^\prime$ are positive. By charge doping, the monolayer MoS$_2$ shows a reversibly strain and work density per cycle ranging from -0.68% to 2.67% and from 4.4 to 36.9 MJ/m$^3$, respectively, making them suitable for applications in electromechanical actuators. Stress generated is also examined in this work and we find that 1T and 1T$^\prime$ MoS$_2$ monolayers relatively have better performance than 1H MoS$_2$ monolayer. We argue that such excellent electromechanical performance originate from the electrical conductivity of the metallic 1T and semimetallic 1T$^\prime$ structures high Young's modulus of about $150-200$ GPa.

cond-mat.mtrl-sci↗

arXiv2017

Spontaneous antiferromagnetic order and strain effect on electronic properties of $α$-graphyne

Using hybrid exchange-correlation functional in ab initio density functional theory calculations, we study magnetic properties and strain effect on the electronic properties of $α$-graphyne monolayer. We find that a spontaneous antiferromagnetic (AF) ordering occurs with energy band gap ($\sim$ 0.5 eV) in the equilibrated $α$-graphyne. Bi-axial tensile strain enhances the stability of AF state as well as the staggered spin moment and value of the energy gap. The antiferromagnetic semiconductor phase is quite robust against moderate carrier filling with threshold carrier density up to 1.7$\times$10$^{14}$ electrons/cm$^2$ to destabilize the phase. The spontaneous AF ordering and strain effect in $α$-graphyne can be well described by the framework of the Hubbard model. Our study shows that it is essential to consider the electronic correlation effect properly in $α$-graphyne and may pave an avenue for exploring magnetic ordering in other carbon allotropes with mixed hybridization of s and p orbitals.

physics.comp-ph↗

arXiv2017

Three-dimensional carbon Archimedean lattices for high-performance electromechanical actuators

We propose three-dimensional carbon (3D-C) structures based on the Archimedean lattices (ALs) by combining $sp^2$ bonding in the polygon edges and $sp^3$ bonding in the polygon vertices. By first-principles calculations, four types of 3D-C ALs: (4, $8^2$), (3, $12^2$), ($6^3$), and ($4^4$) 3D-Cs are predicted to be stable both dynamically and mechanically among 11 possible ALs. Depending on the index of ALs, the 3D-C ALs show distinctive electronic properties: the (4, $8^2$) 3D-C is an indirect bandgap semiconductor, the (3, $12^2$) 3D-C is semimetal, while the ($6^3$) and ($4^4$) 3D-Cs are metals. Considering the structural deformation due to the changes in their electronic energy bands, we discuss the electromechanical properties of the 3D-C ALs as a function of charge doping. We find a semiconductor-to-metal and semimetallic-to-semiconductor transitions in the (4, $8^2$) and (3, $12^2$) 3D-Cs as a function of charge doping, respectively. Moreover, the (3, $12^2$) 3D-C exhibits a $sp^2$-$sp^3$ phase transformation at high charge doping, which leads to a huge 30% irreversible strain, while the reversible strain in the (4, $8^2$) 3D-C is up to 9%, and thus they are quite promising for electromechanical actuators.

cond-mat.mtrl-sci↗

arXiv2017

Negative refraction in Weyl semimetals

We theoretically propose that Weyl semimetals may exhibit negative refraction at some frequencies close to the plasmon frequency, allowing transverse magnetic (TM) electromagnetic waves with frequencies smaller than the plasmon frequency to propagate in the Weyl semimetals. The idea is justified by the calculation of reflection spectra, in which \textit{negative} refractive index at such frequencies gives physically correct spectra. In this case, a TM electromagnetic wave incident to the surface of the Weyl semimetal will be bent with a negative angle of refraction. We argue that the negative refractive index at the specified frequencies of the electromagnetic wave is required to conserve the energy of the wave, in which the incident energy should propagate away from the point of incidence.

physics.optics↗

arXiv2017

Hidden symmetries in $N$-layer dielectric stacks

The optical properties of a multilayer system of dielectric media with arbitrary $N$ layers is investigated. Each layer is one of two dielectric media, with thickness one-quarter the wavelength of light in that medium, corresponding to a central frequency. Using the transfer matrix method, the transmittance $T$ is calculated for all possible $2^N$ sequences for small $N$. Unexpectedly, it is found that instead of $2^N$ different values of $T$ at the central frequency ($T_0$), there are either $(N/2+1)$ or $(N+1)$ discrete values of $T_0$ for even or odd $N$, respectively. We explain the high degeneracy in the $T_0$ values by defining new symmetry operations that do not change $T_0$. Analytical formulae were derived for the $T_0$ values and their degeneracy as functions of $N$ and an integer parameter for each sequence we call "charge". Additionally, the bandwidth of the transmission spectra at $f_0$ is investigated, revealing some asymptotic behavior at large $N$.

physics.optics↗

arXiv2017

Charge-induced electrochemical actuation of armchair carbon nanotube bundles

The effects of charge doping on the structural deformation and on the electronic structure of armchair single wall carbon nanotube (SWNT) bundles are investigated through first-principles calculations. In particular, we select a (6; 6) SWNT as an example and we calculate a mechanical deformation in the SWNT bundles as a function of gate voltage, which could serve as a basis of the electromechanical actuators in an artificial muscle. We find that the magnitudes of the actuation responses such as strain and stress of the (6; 6) SWNT bundle in the case of hole doping are substantially larger than those of electron doping. The (6; 6) SWNT bundle also exhibits a low-symmetry and opens an energy band gap of about 0.41 eV around the charge neutral condition, which allows a semiconductor-to-metal transition in the electron-doping regime when the relative shift of the Fermi energy goes up to 0.60 eV, above which the Young modulus increases.

cond-mat.mtrl-sci↗

arXiv2016

Selective coherent phonon mode generation in single wall carbon nanotubes

The pulse-train technique within ultrafast pump-probe spectroscopy is theoretically investigated to enhance a specific coherent phonon mode while suppressing the other phonon modes generated in single wall carbon nanotubes (SWNTs). In particular, we focus on the selectivity of the radial breathing mode (RBM) and the G-band for a given SWNT. We find that if the repetition period of the pulse train matches with integer multiple of the RBM phonon period, the RBM amplitude could be kept while the amplitudes of the other modes are suppressed. As for the G-band, when we apply a repetition period of half-integer multiple of the RBM period, the RBM could be suppressed because of destructive interference, while the G-band still survives. It is also possible to keep the G-band and suppress the RBM by applying a repetition period that matches with integer multiple of the G-band phonon period. However, in this case we have to use a large number of laser pulses having a property of "magic ratio" of the G-band and RBM periods.

cond-mat.mes-hall↗

arXiv2016

Phonon-assisted indirect transitions in angle-resolved photoemission spectra of graphite and graphene

Indirect transitions of electrons in graphene and graphite are investigated by means of angle-resolved photoemission spectroscopy (ARPES) with several different incident photon energies and light polarizations. The theoretical calculations of the indirect transition for graphene and graphite are compared with the experimental measurements for highly-oriented pyrolytic graphite and a single-crystal of graphite. The dispersion relations for the transverse optical (TO) and the out-of-plane longitudinal acoustic (ZA) phonon modes of graphite and the TO phonon mode of graphene can be extracted from the inelastic ARPES intensity. We find that the TO phonon mode for $k$ points along the $Γ$-$K$ and $K$-$M$-$K'$ directions in the Brillouin zone can be observed in the ARPES spectra of graphite and graphene by using a photon energy of about 11.1 eV. The relevant mechanism in the ARPES process for this case is the resonant indirect transition. On the other hand, the ZA phonon mode of graphite can be observed by using a photon energy of about 6.3 eV through a nonresonant indirect transition, while the ZA phonon mode of graphene within the same mechanism should not be observed.

cond-mat.mtrl-sci↗

arXiv2016

Broadband Transverse Electric Surface Wave in Silicene

Transverse electric (TE) surface wave in silicine is theoretically investigated. The TE surface wave in silicene is found to exhibit better characteristics compared with that in graphene, in terms of a broader frequency range and more confinement to the surface which originate from the buckled structure of silicene. We found that even undoped silicene can support the TE surface wave. We expect to obtain the similar characteristics of the TE surface wave in other two-dimensional materials that have slightly buckled honeycomb lattice.

cond-mat.mes-hall↗

arXiv2016

Quantum effects in the thermoelectric power factor of low-dimensional semiconductors

We theoretically investigate the interplay between the confinement length $L$ and the thermal de Broglie wavelength $Λ$ to optimize the thermoelectric power factor of semiconducting materials. An analytical formula for the power factor is derived based on the one-band model assuming nondegenerate semiconductors to describe quantum effects on the power factor of the low dimensional semiconductors. The power factor is enhanced for one- and two-dimensional semiconductors when $L$ is smaller than $Λ$ of the semiconductors. In this case, the low-dimensional semiconductors having $L$ smaller than their $Λ$ will give a better thermoelectric performance compared to their bulk counterpart. On the other hand, when $L$ is larger than $Λ$, bulk semiconductors may give a higher power factor compared to the lower dimensional ones.

cond-mat.mes-hall↗

arXiv2016

Fermi energy dependence of first- and second-order Raman spectra in graphene: Kohn anomaly and quantum interference effect

Intensity of the first- and the second-order Raman spectra are calculated as a function of the Fermi energy. We show that the Kohn anomaly effect, i.e., phonon frequency renormalization, in the first-order Raman spectra originates from the phonon renormalization by the interband electron-hole excitation, whereas in the second-order Raman spectra, a competition between the interband and intraband electron-hole excitations takes place. By this calculation, we confirm the presence of different dispersive behaviors of the Raman peak frequency as a function of the Fermi energy for the first- and the second-order Raman spectra, as observed in experiments. Moreover, the calculated results of the Raman intensity sensitively depend on the Fermi energy for both the first- and the second-order Raman spectra. These results thus also show the importance of quantum interference effect phenomena.

cond-mat.mes-hall↗

arXiv2016

Intrinsic strength and failure behaviors of ultra-small single-walled carbon nanotubes

The intrinsic mechanical strength of single-walled carbon nanotubes (SWNTs) within the diameter range of 0.3-0.8 nm has been studied based on ab initio density functional theory calculations. In contrast to predicting "smaller is stronger and more elastic" in nanomaterials, the strength of the SWNTs is significantly reduced when decreasing the tube diameter. The results obtained show that the Young`s modulus E significantly reduced in the ultra-small SWNTs with the diameter less than 0.4 nm originates from their very large curvature effect, while it is a constant of about 1.0 TPa, and independent of the diameter and chiral index for the large tube. We find that the Poisson`s ratio, ideal strength and ideal strain are dependent on the diameter and chiral index. Furthermore, the relations between E and ideal strength indicate that Griffith`s estimate of brittle fracture could break down in the smallest (2, 2) nanotube, with the breaking strength of 15% of E. Our results provide important insights into intrinsic mechanical behavior of ultra-small SWNTs under their curvature effect.

cond-mat.mtrl-sci↗

arXiv2015

Diameter dependence of thermoelectric power of semiconducting carbon nanotubes

We calculate the thermoelectric power (or thermopower) of many semiconducting single wall carbon nanotubes (s-SWNTs) within a diameter range 0.5-1.5 nm by using the Boltzmann transport formalism combined with an extended tight-binding model. We find that the thermopower of s-SWNTs increases as the tube diameter decreases. For some s-SWNTs with diameters less than 0.6 nm, the thermopower can reach a value larger than 2000 μV/K at room temperature, which is about 6 to 10 times larger than that found in commonly used thermoelectric materials. The large thermopower values may be attributed to the one-dimensionality of the nanotubes and to the presence of large band gaps of the small-diameter s-SWNTs. We derive an analytical formula to reproduce the numerical calculation of the thermopower and we find that the thermopower of a given s-SWNT is directly related with its band gap. The formula also explains the shape of the thermopower as a function of tube diameter, which looks similar to the shape of the so-called Kataura plot of the band gap dependence on tube diameter.

cond-mat.mes-hall↗

arXiv2015

Double resonance Raman modes in mono- and few-layer MoTe$_2$

We study the second-order Raman process of mono- and few-layer MoTe$_2$, by combining {\em ab initio} density functional perturbation calculations with experimental Raman spectroscopy using 532, 633 and 785 nm excitation lasers. The calculated electronic band structure and the density of states show that the electron-photon resonance process occurs at the high-symmetry M point in the Brillouin zone, where a strong optical absorption occurs by a logarithmic Van-Hove singularity. Double resonance Raman scattering with inter-valley electron-phonon coupling connects two of the three inequivalent M points in the Brillouin zone, giving rise to second-order Raman peaks due to the M point phonons. The predicted frequencies of the second-order Raman peaks agree with the observed peak positions that cannot be assigned in terms of a first-order process. Our study attempts to supply a basic understanding of the second-order Raman process occurring in transition metal di-chalcogenides (TMDs) and may provide additional information both on the lattice dynamics and optical processes especially for TMDs with small energy band gaps such as MoTe$_2$ or at high laser excitation energy.

cond-mat.mtrl-sci↗

arXiv2014

Breit-Wigner-Fano lineshapes in Raman spectra of graphene

Excitation of electron-hole pairs in the vicinity of the Dirac cone by the Coulomb interaction gives rise to an asymmetric Breit-Wigner-Fano lineshape in the phonon Raman spectra in graphene. This asymmetric lineshape appears due to the interference effect between the phonon spectra and the electron-hole pair excitation spectra. The calculated Breit-Wigner-Fano asymmetric factor 1/qBWF as a function of the Fermi energy shows a V-shaped curve with a minimum value at the charge neutrality point and gives good agreement with the experimental result.

cond-mat.mes-hall↗

arXiv2013

Excitonic effects on coherent phonon dynamics in single wall carbon nanotubes

We discuss how excitons can affect the generation of coherent radial breathing modes in ultrafast spectroscopy of single wall carbon nanotubes. Photoexcited excitons can be localized spatially and give rise to a spatially distributed driving force in real space which involves many phonon wavevectors of the exciton-phonon interaction. The equation of motion for the coherent phonons is modeled phenomenologically by the Klein-Gordon equation, which we solve for the oscillation amplitudes as a function of space and time. By averaging the calculated amplitudes per nanotube length, we obtain time-dependent coherent phonon amplitudes that resemble the homogeneous oscillations that are observed in some pump-probe experiments. We interpret this result to mean that the experiments are only able to see a spatial average of coherent phonon oscillations over the wavelength of light in carbon nanotubes and the microscopic details are averaged out. Our interpretation is justified by calculating the time-dependent absorption spectra resulting from the macroscopic atomic displacements induced by the coherent phonon oscillations. The calculated coherent phonon spectra including excitonic effects show the experimentally observed symmetric peaks at the nanotube transition energies in contrast to the asymmetric peaks that would be obtained if excitonic effects were not included.

cond-mat.mtrl-sci↗

arXiv2013

Fano resonance in Raman scattering of graphene

Fano resonances and their strong doping dependence are observed in Raman scattering of single-layer graphene (SLG). As the Fermi level is varied by a back-gate bias, the Raman G band of SLG exhibits an asymmetric line shape near the charge neutrality point as a manifestation of a Fano resonance, whereas the line shape is symmetric when the graphene sample is electron or hole doped. However, the G band of bilayer graphene (BLG) does not exhibit any Fano resonance regardless of doping. The observed Fano resonance can be interpreted as interferences between the phonon and excitonic many-body spectra in SLG. The absence of a Fano resonance in the Raman G band of BLG can be explained in the same framework since excitonic interactions are not expected in BLG.

cond-mat.mes-hall↗