다중혈관 관상동맥 환자에서 y-문합을 이용하여 양쪽 내흉동맥만을 사용한 우회술의 조기 성적

[신간의 별자리x] 우리/미술, 그리고 ‘슬픔의 박물관’

We show that the pseudorelativistic physics of graphene near the Fermi level can be extended to three dimensional (3D) materials. Unlike in phase transitions from inversion symmetric topological to normal insulators, we show that particular space groups also allow 3D Dirac points as symmetry protected degeneracies. We provide criteria necessary to identify these groups and, as an example, present ab initio calculations of β-cristobalite BiO(2) which exhibits three Dirac points at the Fermi level. We find that β-cristobalite BiO(2) is metastable, so it can be physically realized as a 3D analog to graphene.

Dirac Semimetal in Three Dimensions

A novel crystal configuration of sandwiched S-Mo-Se structure (Janus SMoSe) at the monolayer limit has been synthesized and carefully characterized in this work. By controlled sulfurization of monolayer MoSe2 the top layer of selenium atoms are substituted by sulfur atoms while the bottom selenium layer remains intact. The peculiar structure of this new material is systematically investigated by Raman, photoluminescence and X-ray photoelectron spectroscopy and confirmed by transmission-electron microscopy and time-of-flight secondary ion mass spectrometry. Density-functional theory calculations are performed to better understand the Raman vibration modes and electronic structures of the Janus SMoSe monolayer, which are found to correlate well with corresponding experimental results. Finally, high basal plane hydrogen evolution reaction (HER) activity is discovered for the Janus monolayer and DFT calculation implies that the activity originates from the synergistic effect of the intrinsic defects and structural strain inherent in the Janus structure.

Janus Monolayer Transition Metal Dichalcogenides

We assess the performance of recent density functionals for the exchange-correlation energy of a nonmolecular solid, by applying accurate calculations with the GAUSSIAN, BAND, and VASP codes to a test set of 24 solid metals and nonmetals. The functionals tested are the modified Perdew-Burke-Ernzerhof generalized gradient approximation PBEsol GGA, the second-order GGA SOGGA, and the Armiento-Mattsson 2005 AM05 GGA. For completeness, we also test more standard functionals: the local density approximation, the original PBE GGA, and the Tao-Perdew-Staroverov-Scuseria meta-GGA. We find that the recent density functionals for solids reach a high accuracy for bulk properties lattice constant and bulk modulus. For the cohesive energy, PBE is better than PBEsol overall, as expected, but PBEsol is actually better for the alkali metals and alkali halides. For fair comparison of calculated and experimental results, we consider the zeropoint phonon and finite-temperature effects ignored by many workers. We show how GAUSSIAN basis sets and inaccurate experimental reference data may affect the rating of the quality of the functionals. The results show that PBEsol and AM05 perform somewhat differently from each other for alkali metal, alkaline-earth metal, and alkali halide crystals where the maximum value of the reduced density gradient is about 2, but perform very similarly for most of the other solids where it is often about 1. Our explanation for this is consistent with the importance of exchange-correlation nonlocality in regions of core-valence overlap.

https://dspace.mit.edu/openaccess-disseminate/1721.1/51869

Assessing the performance of recent density functionals for bulk solids

Monolayer SnP3 is a novel two-dimensional (2D) semiconductor material with high carrier mobility and large optical absorption coefficient, implying its potential applications in the photovoltaic and thermoelectric (TE) fields. Herein, we report on the TE properties of monolayer SnP3 utilizing first principles density functional theory (DFT) together with semiclassical Boltzmann transport theory. Results indicate that it exhibits a low lattice thermal conductivity of ∼4.97 W m-1 K-1 at room temperature, mainly originating from its small average acoustic group velocity (∼1.18 km s-1), large Gruneisen parameters (∼7.09), strong dipole-dipole interactions, and strong phonon-phonon scattering. A large in-plane charge transfer is observed, which results in a non-ignorable bipolar effect on the lattice thermal conductivity. The exhibited mixed mode between in-plane and out-of-plane vibrations enhances the complexity of the phonon phase space, which enhances the possibility of phonon scattering processes and results in suppression of thermal conductivity. A highly twofold degeneracy appearing at the K point gives a high Seebeck coefficient. Our calculated figure of merit (ZT) for optimal p-type doping at 500 K can approach 3.46 along the armchair direction, which is better than the theoretical value of 1.94 reported in the well-known TE material SnSe. Our studies here shed light on monolayer SnP3 in use as a TE material and supply insights to further optimize the TE properties in similar systems.

Monolayer SnP3: an excellent p-type thermoelectric material

Through first-principles calculations, we report on the phonon-limited transport properties of two-dimensional (2D) hexagonal MSe (M = Ge, Sn, and Pb) compounds, which can be seen as a new family of 2D group-IV selenides established by the isovalent substitutions of germanium and tellurium in layered ${\mathrm{Ge}}_{4}{\mathrm{Se}}_{3}\mathrm{Te}$ phase [Angew. Chem. Inter. Edit. 56 10204 (2017)]. We find that 2D PbSe exhibits low values of sound velocity (800--2030 m/s), large $\mathrm{Gr}\stackrel{\ifmmode \ddot{}\else \&#34;{}\fi{}}{\mathrm{u}}\mathrm{neisen}$ parameters $(\ensuremath{\sim}1.93)$, low-lying optical modes $(\ensuremath{\sim}20.02\phantom{\rule{4pt}{0ex}}{\mathrm{cm}}^{\ensuremath{-}1})$, and strong optical-acoustic phonon coupling. These intrinsic properties mainly stem from strong phonon anharmonicity, which greatly suppress the phonon transport and therefore give rise to an ultralow thermal conductivity $(\ensuremath{\sim}0.26\phantom{\rule{4pt}{0ex}}\mathrm{W}\phantom{\rule{0.16em}{0ex}}{\mathrm{m}}^{\ensuremath{-}1}\phantom{\rule{0.16em}{0ex}}{\mathrm{K}}^{\ensuremath{-}1})$ for 2D PbSe at room temperature. Our studies may offer perspectives for applications of thermoelectricity and motivate further experimental efforts to synthesize MSe compounds.

First-principles calculations of the ultralow thermal conductivity in two-dimensional group-IV selenides

High-performance infrared (IR) nonlinear optical (NLO) materials with large laser damage thresholds (LDTs) are urgently needed because the current commercially available AgGaS2, AgGaSe2, and ZnGeP2 suffer their very low LDTs which shorten significantly their service lifetimes Here, a novel sulfide, Ba6Zn7Ga2S16 with a very wide band gap of 35 eV, has been discovered This compound crystallizes in the chiral trigonal R3 space group with a novel 3D framework that is constructed by ZnS4 tetrahedra, Zn3GaS10 supertetrahedra (a T2-type), and Zn3GaS10 quadri-tetrahedral clusters via vertex-sharing Such a novel structure exhibits desirable features which suggest a promising NLO material: phase-matchability (PM), good NLO efficiency (about half that of benchmark AgGaS2), and the highest LDT among PM chalcogenides (28 times that of benchmark AgGaS2) In addition, the density functional theory (DFT) calculations confirm its PM behavior and reveal that the second harmonic generation (SHG) origin is mainly ascribe

Ba6Zn7Ga2S16: A Wide Band Gap Sulfide with Phase-Matchable Infrared NLO Properties

Combining the first-principles density functional method and crystal structure prediction techniques, we report a series of hexagonal two-dimensional transition metal borides including Sc2B2, Ti2B2, V2B2, Cr2B2, Y2B2, Zr2B2, and Mo2B2. Their dynamic and thermal stabilities are testified by phonon and molecular dynamics simulations. We investigate the potential of the two-dimensional Ti2B2 monolayer as an anode material for Li-ion and Na-ion batteries. The Ti2B2 monolayer possesses high theoretical specific capacities of 456 and 342 mA h g-1 for Li and Na, respectively. The very high Li/Na diffusivity with an ultralow energy barrier of 0.017/0.008 eV indicates an excellent charge-discharge capability. In addition, good electronic conductivity during the whole lithiation process is found by electronic structure calculations. The very small change in volume after the adsorption of one, two, and three layers of Li and Na ions indicates that the Ti2B2 monolayer is robust. These results highlight the suitability of Ti2B2 monolayer as well as the other two-dimensional transition metal borides as excellent anode materials for both Li-ion and Na-ion batteries.

Hexagonal Ti2B2 monolayer: a promising anode material offering high rate capability for Li-ion and Na-ion batteries.

Infrared nonlinear optical (IR NLO) materials are the key materials in the modern laser frequency conversion technology that are widely applied in many fields. Yet, the major issue is lacking of high performance materials with large nonlinear susceptibility and high laser damage threshold, which are two correlated and contradictory parameters. Here, the novel dual ion substitution synergy leads to the discovery of Ba4MGa4Se10Cl2 (M = Zn, 1; Cd, 2; Mn, 3; Cu/Ga, 4; Co, 5; and Fe, 6), crystallizing in the tetragonal I4¯. Polycrystalline 1–4 exhibits very strong IR second harmonic generations (SHG). (1: 59×; 2: 52×; 4: 39×; and 3: 30× that of IR benchmark NLO material AgGaS2). Remarkably, the laser-induced damage threshold of 1 is 143.6 MW cm–2, roughly 17 times that of benchmark AgGaS2. These distinguish 1 as a new promising NLO material. In addition, 1, 2, and 4 illustrate the coexistence of NLO and photoluminescence properties. Especially, 3 demonstrate for the first time the coexistence of triple properties (NLO, photoluminescence, and magnetism) in a single crystal structure. The insights of the driving force of the synergetic substitution, SHG origin, and microscopic contributions of the building units open a new route to discovering novel multi-functional NLO materials.

Peng-Fei Liu

Papers

Monolayer SnP3: an excellent p-type thermoelectric material

First-principles calculations of the ultralow thermal conductivity in two-dimensional group-IV selenides

Ba6Zn7Ga2S16: A Wide Band Gap Sulfide with Phase-Matchable Infrared NLO Properties

Hexagonal Ti2B2 monolayer: a promising anode material offering high rate capability for Li-ion and Na-ion batteries.

Strong IR NLO Material Ba4MGa4Se10Cl2: Highly Improved Laser Damage Threshold via Dual Ion Substitution Synergy