Ehlen, Niels
Hell, Martin
Marini, Giovanni
Hasdeo, Eddwi Hesky
Saito, Riichiro
Falke, Yannic
Goerbig, Mark Oliver
Di Santo, Giovanni
Petaccia, Luca
Profeta, Gianni
Grueneis, Alexander

A flat energy dispersion of electrons at the Fermi level of a material leads to instabilities in the electronic system and can drive phase transitions. Here we show that the flat band in graphene can be achieved by sandwiching a graphene monolayer by two cesium (Cs) layers. We investigate the flat band by a combination of angle-resolved photoemission spectroscopy experiment and the calculations. Our work highlights that charge transfer, zone folding of graphene bands, and the covalent bonding between C and Cs atoms are the origin of the flat energy band formation. Analysis of the Stoner criterion for the flat band suggests the presence of a ferromagnetic instability. The presented approach is an alternative route for obtaining flat band materials to twisting bilayer graphene which yields thermodynamically stable flat band materials in large areas.

Hung, Nguyen T.
Nugraha, Ahmad R. T.
Saito, Riichiro

We investigate the electromechanical properties of two-dimensional MoS2 monolayers with 1H, 1T, and 1T' 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' 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' are positive. By charge doping, the monolayer MoS2 shows a reversible 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. We also examine the stress generated in the MoS2 monolayers and we find that 1T and 1T' MoS2 monolayers have relatively better performance than 1H MoS2 monolayer. We argue that such excellent electromechanical performance originate from the electrical conductivity of the metallic 1T and semimetallic 1T' structures and also from their high Young's modulus of about 150-200 GPa.

The enhanced absorption probability of gigahertz and terahertz electromagnetic wave in monolayer graphene, sandwiched by the dielectric materials, up to 100% has been shown in our previous works by solving the classical Maxwell equation with transfer matrix method. In this paper, we show by a quantum description of the phenomena that the origin of the enhanced optical absorption is equivalent to the excitation of surface plasmon. The interaction between a photon with a surface plasmon is calculated, and the excitation probability of surface plasmon can be obtained by the Fermi golden rule, which can be compared with the optical spectrum that is calculated by the Maxwell equation. The calculated results show that both the intraband single-particle excitation of electron and collective excitation or surface plasmon contribute to optical absorption.

Hung, Nguyen T.
Nugraha, Ahmad R. T.
Saito, Riichiro

The search for new thermoelectric materials has been of great interest in recent years because thermoelectrics offers useful applications in next-generation vehicles that can directly convert waste heat to electricity. Twodimensional (2D) tetradymites with M2X3 compounds, in which M (Bi) and X (Te, Se, S) are a group-V metal and group-VI anion, respectivety, are theoretically investigated in this study. Their energy bands are characterized by small energy gaps, high group velocities, small effective masses, nonparabolic bands and multi-valleys convergence at near the center of the Brillouin zone, which are favorable conditions for high power factor with the optimum power factor values can be up to 0.20-0.25 W/mK(2) at room temperature. Moreover, the 2D M2X3 contains heavy atomic masses and high polarizability of some chemical bonds, leading to small group velocities of phonons and anharmonic phonon behavior that produce an intrinsic lattice thermal conductivity as low as similar to 1.5-2.0 W/mK at room temperature. We find that by mixtures of M and X atoms, such as Bi2Te2Se, the power factor further increases whereas the lattice thermal conductivity decreases. This design gives a high figure of merit of the p-type 2D Bi2Te2Se from 1.4 to 2.0 at operating temperature within 300 - 500 K.

We theoretically show that perfect circular dichroism (CD) occurs in the Haldane model in which the two-dimensional (2D) material absorbs only either left-handed or right-handed circularly polarized light. Perfect CD occuis in the phase diagram of the Haldane model when the zero-field quantum Hall conductivity has a nonzero value. The coincidence of the occurrence of perfect CD and zero-field quantum Hall effect is attributed to the fact that the effect of broken time-reversal symmetry is larger than the effect of broken inversion symmetry. On the other hand, valley polarization and perfect CD occur exclusively in the phase diagiam. Further, for the four regions of the phase diagram, pseudospin polarization occurs at the K and K' points in the hexagonal Brillouin zone with either the same sign or opposite sign for the K and K' points and fof the valence and conduction bands. This theoretical prediction may have an impact on search for a new optical device that selects circularly polarized light controlled by the electric field.

Nugraha, Ahmad R. T.
Hasdeo, Eddwi H.
Saito, Riichiro

The pulse-train technique within ultrafast pump-probe spectroscopy is theoretically investigated to excite 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 the integer multiple of the RBM phonon period, the RBM amplitude can be maintained while the amplitudes of the other modes are suppressed. As for the G-band, when we apply a repetition period of a half-integer multiple of the RBM period, the RBM can 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 the 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

Hung, Nguyen T.
Nugraha, Ahmad R. T.
Saito, Riichiro

Thermoelectric properties of monolayer indium selenide (InSe) are investigated by using Boltzmann transport theory and first-principles calculations as a function of Fermi energy and crystal orientation. We find that the maximum power factor of p-type (n-type) monolayer InSe can be as large as 0.049 (0.043) W/K-2 m at 300K in the armchair direction. The excellent thermoelectric performance of monolayer InSe is attributed to both its Seebeck coefficient and electrical conductivity. The large Seebeck coefficient originates from the moderate (about 2 eV) bandgap of monolayer InSe as an indirect gap semiconductor, while its large electrical conductivity is due to its unique two-dimensional density of states (DOS), which consists of an almost constant DOS near the conduction band bottom and a sharp peak near the valence band top. Published by AIP Publishing.

Ukhtary, Muhammad Shoufie
Nugraha, Ahmad R. T.
Saito, Riichiro

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 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.

The origin of valley polarization of the hexagonal lattice is analytically discussed by tight binding method as a function of energy band gap. When the energy gap decreases to zero, the intensity of optical absorption becomes sharp as a function of k near the K(or K') point in the hexagonal Brillouin zone, while the peak intensity at the K (or K') point keeps constant with decreasing the energy gap. When the dipole vector as a function of k can have both real and imaginary parts that are perpendicular to each other in the k space, the valley polarization occurs. When the dipole vector has only real values by selecting a proper phase of wave functions, the valley polarization does not occur. The degree of the valley polarization may show a discrete change that can be relaxed to a continuous change of the degree of valley polarization when we consider the life time of photo-excited carrier.

Hung, Nguyen T.
Nugraha, Ahmad R. T.
Yang, Teng
Saito, Riichiro

Thermoelectric (TE) materials, or materials that can generate an electrical energy from temperature gradient, are promising for renewable energy technology. One fundamental aspect in the TE research is the demand to maximize the TE power-factor, PF =3D S-2 sigma, by having as large Seebeck coefficient (S) and electrical conductivity (sigma) as possible. In the early 90s, Hicks and Dresselhaus proposed the PF enhancement by using low-dimensional materials, in which electrons are confined in certain directions and they move freely in the other directions. This quantum effect is known as the confinement length (L) effect, in which L is the thickness or diameter of the two-dimensional (2D) or one-dimensional materials, respectively. However, a key challenge is to understand the critical value of L, at which the PF can be significantly enhanced. Recently, we reevaluated the confinement theory of the low-dimensional materials to solve this issue. We showed that electrons are fully confined only when L is smaller than an intrinsic length ?, the so-called thermal de Broglie wavelength, which depends on the materials and can be experimentally measured. Monolayer 2D materials naturally satisfy the condition of L < ? since their confinement length is similar to 1 nm, while their thermal de Broglie wavelength is similar to 5-10 nm. Therefore, they could be a good candidate for TE materials. In this review article, we first review the TE materials with low dimensions. Then, we show the basic concept of the confinement effect and the consequence of such an effect. Finally, based on this effect, we turn our attention to the progress achieved recently in the TE properties of the 2D materials such as monolayer InSe, GaN electron gas, and SrTiO3 superlattices.

Huang, Shengxi
Tatsumi, Yuki
Ling, Xi
Guo, Huaihong
Wang, Ziqiang
Watson, Garrett
Puretzky, Alexander A.
Geohegan, David B.
Kong, Jing
Li, Ju
Yang, Teng
Saito, Riichiro
Dresselhaus, Mildred S.

Layered gallium telluride (GaTe) has attracted much attention recently, due to its extremely high photoresponsivity, short response time, and promising thermoelectric performance. Different from most commonly studied two-dimensional (2D) materials, GaTe has in-plane anisotropy and a low symmetry with the C-2h(3) space group. Investigating the in-plane optical anisotropy, including the electron photon and electron phonon interactions of GaTe is essential in realizing its applications in optoelectronics and thermoelectrics. In this work, the anisotropic light-matter interactions in the low-symmetry material GaTe are studied using anisotropic optical extinction and Raman spectroscopies as probes. Our polarized optical extinction spectroscopy reveals the weak anisotropy in optical extinction spectra for visible light of multilayer GaTe. Polarized Raman spectroscopy proves to be sensitive to the crystalline orientation of GaTe, and shows the intricate dependences of Raman anisotropy on flake thickness, photon and phonon energies. Such intricate dependences can be explained by theoretical analyses employing first-principles calculations and group theory. These studies are a crucial step toward the applications of GaTe especially in optoelectronics and thermoelectrics, and provide a general methodology for the study of the anisotropy of light-matter interactions in 2D layered materials with in-plane anisotropy.

We performed infrared absorption spectroscopy for alpha-RuCl3 with a two-dimensional honeycomb lattice of Ru3+ ions. In the mid-infrared range, we observed three absorption peaks corresponding to intra-atomic d-d transitions among Ru3+ ions in accordance with a previous report. In the far-infrared range, we observed conspicuous broad absorption structures near 600 cm(-1) in addition to several sharp single phonon peaks. Our first-principles calculations for the isolate sandwich model together with the factor group analysis revealed that the peak structures near 600 cm(-1) are absorptions related to a two-phonon process. Our results stimulate further study on the spin-lattice coupling in alpha-RuCl3 as well as the optical detection of Majorana excitations in Kitaev spin liquids.

Using first-principles calculations based on density functional theory, the energetics and electronic structure of domain boundaries of MoS2, in which the same polar edges face each other, are investigated. We find that the interface model with homoelemental bonds is not energetically preferred in this system. The domain boundaries have defect levels that have wide distributions inside the band gap of MoS2. The upshift (or downshift) of the MoS2 energy band occurs around the domain boundaries when the occupation number of electrons in the defect levels increases (or decreases). The charge transfer of electrons from the graphite substrate plays an important role in band bending, which is observed in the recent experiments by scanning tunneling microscopy/spectroscopy. (C) 2018 The Japan Society of Applied Physics.