3v Symmetry Research Articles

Robust transport and topological valley refraction of fundamental symmetric lamb waves in perforated phononic crystal plates

Lamb waves can be divided into antisymmetric Lamb waves (type A) and symmetric Lamb waves (type S), which are widely used in health monitoring of engineering structures and nondestructive evaluation of materials. However, due to the fact that each mode of Lamb wave has the setting group velocity in a certain bandwidth range, it is easy to cause the aliasing of multi-mode Lamb waves, partly limiting the application scenarios of Lamb wave. In this paper, a hexagonal lattice meta-structure is designed and the zero-order symmetric (S0) Lamb wave in a finite thickness plate is successfully separated. By breaking the spatial C 3v symmetry, a valley topological phononic crystal (PnC) plate for the pure S0 mode is constructed, and its robust topological transports are demonstrated. Firstly, the valley topological phase transition of S0 mode is realized by changing the angle of scatterer in PnC plate and the valley edge state of S0 mode is obtained. Furthermore, by introducing structural mismatch, it is verified that the edge state of S0 mode is robust to various bends and defect. Finally, by changing the material property of the terminal, the valley refraction with different directionality is realized. The results offer a route to individually study the symmetric modes of Lamb waves, and have potential application prospects in rapid and accurate ultrasonic nondestructive testing based on Lamb waves.

A spin modulating device, tuned by the Fermi energy, in honeycomb-like substrates periodically stubbed with transition-metal-dichalkogenides

We investigate spin transport through graphene-like substrates stubbed vertically with transition-metal-dichalcogenides (TMDs). A tight-binding model is used based on a graphene-like Hamiltonian that includes different types of spin–orbit coupling (SOC) terms permitted by the C 3v symmetry group in TMDs/graphene-like heterostructures. The results show a spin modulation obtained by tuning the strength and sign of the Fermi energy E F and not by varying the SOC strength as is mainly the case of Datta and Das. The spin conductance is directly controlled by the value of E F . In addition, a perfect electron-spin modulation is obtained when a vertical strain is introduced. In this case, the spin conductance exhibits a strong energy dependence. The results may open the route to a combination of graphene-like substrates with TMD stubs and the development of spin-transistor devices controlled by the Fermi energy rather than the SOC strength.

Enhanced ferromagnetic ordering of a Mn trimer symmetrically and fully exposed on iridium-doped graphene

The structure and magnetism of a Mn trimer adsorbed on iridium-doped graphene are studied using density functional theory calculations. Our calculation results show that the Mn trimer prefers to locate on top of the Ir atom and forms a fully exposed high-symmetry configuration with large binding energy and hardness of rotation. The ferromagnetic ordering of the Mn trimer fully exposed the on iridium-doped graphene is enhanced five times compared to a free Mn trimer. Our study shows that the enhancement originates from the fixed long bond and the C 3v symmetry of the Mn trimer constrained by the iridium-doped graphene.

Atomic and electronic structures of p-type dopants in 4H-SiC

Using hybrid density functional calculation, we study the atomic and electronic structures of p-type dopants, B, Al and Ga, in 4H-SiC. For B, depending on the growth condition, it can occupy both Si and C sites. In contrast, Al and Ga on the C sites exhibit too high formation energy to exist in a significant amount. In 4H-SiC, there exist two types of Si sites in wurtzite-like and zincblende-like local coordination, respectively. Our calculations suggest that the dopant atoms have negligible preference occupying the two sites. In neutral charge state, all the dopants exhibit significant distortions from the structure in the negatively charged state. For most cases, our calculations yield three distorted structures, in which the most stable one has the dopant atom displaced along its bond with one of the surrounding equatorial Si or C atoms, lowering the C 3v symmetry to Cs symmetry (i.e., a mirror symmetry only). Among the three dopant elements, Al on Si sites exhibits overall the lowest formation energy and the shallowest acceptor level. Nevertheless, it is not a hydrogenic dopant with the acceptor level 0.12 eV above the valence band maximum based on calculation using a 400-atom supercell. Its corresponding defect state exhibits apparent localization along the [0001] direction, but it is relatively delocalized in the (0001) plane.

Dual-polarization topological phases and phase transition in magnetic photonic crystalline insulator

Two-dimensional topological photonic crystals have rapidly emerged as a recent and fascinating branch of photonic research. However, most of them were limited to a specific type of polarization, TE or TM polarization. Here, we explored the dual-polarization topological phases in two-dimensional magnetic photonic crystal (PC) which are composed of ferrite rod clusters in the plasma background. Under the perturbations of the bias magnetic field and/or the cluster distortion in the unit cell, the PC exhibited dual-polarization topological phases, including the quantum Hall (QH) phase, the higher-order quantum spin Hall (HO-QSH) phase and the conventional insulator (CI) phase. We studied the topological nature of these phases by the Wilson loop, Chern number, and unidirectional edge states. Intriguingly, we showed that the HO-QSH phases could present in PC of C 3v symmetry instead of being restricted to C 6v symmetry. The lower symmetry enlarges the gap in the edge states, which helps for the emergence of corner states. By continuously deforming the unit cell configuration, we demonstrated the phase transition in the system was dual-polarization. Our results extend the topological phases in the PCs and pave the way for the dual-polarization topological devices and their applications.

High-Spin Iron(VI), Low-Spin Ruthenium(VI), and Magnetically Bistable Osmium(VI) in Molecular Group 8 Nitrido Trifluorides NMF3.

Pseudo‐tetrahedral nitrido trifluorides N≡MF3 (M=Fe, Ru, Os) and square pyramidal nitrido tetrafluorides N≡MF4 (M=Ru, Os) were formed by free‐metal‐atom reactions with NF3 and subsequently isolated in solid neon at 5 K. Their IR spectra were recorded and analyzed aided by quantum‐chemical calculations. For a d2 electron configuration of the N≡MF3 compounds in C 3v symmetry, Hund's rule predict a high‐spin 3A2 ground state with two parallel spin electrons and two degenerate metal d(δ)‐orbitals. The corresponding high‐spin 3A2 ground state was, however, only found for N≡FeF3, the first experimentally verified neutral nitrido FeVI species. The valence‐isoelectronic N≡RuF3 and N≡OsF3 adopt different angular distorted singlet structures. For N≡RuF3, the triplet 3A2 state is only 5 kJ mol−1 higher in energy than the singlet 1A′ ground state, and the magnetically bistable molecular N≡OsF3 with two distorted near degenerate 1A′ and 3A“ electronic states were experimentally detected at 5 K in solid neon.

Tuning electronic phase in noncentrosymmetric quantum spin Hall insulators through physical stimuli

In this work, the perturbation-induced phase transitions in noncentrosymmetric quantum spin Hall insulators (QSHIs) are analytically addressed. In particular, the dilute charged impurity, the electric field, and the Zeeman splitting field are considered within the tight-binding Hamiltonian model, Green’s function approach, and the Born approximation. Following the C 3v symmetry breaking in the PbBiI compound as a representative QSHI, the band gap becomes larger via the electric field, while the system transits to the semimetallic phase via the dilute charged impurities and Zeeman field, modifying the degenerate states in the electronic density of states. While the coexistence of electric field and impurities demonstrate that the system backs to its initial semiconducting phase, the combined Zeeman field and impurities do not alter the robust semimetallic phase.

Indium gallium nitride quantum dots: consequence of random alloy fluctuations for polarization entangled photon emission

We analyze the potential of the c-plane InGaN/GaN quantum dots for polarization entangled photon emission by means of an atomistic many-body framework. Special attention is paid to the impact of random alloy fluctuations on the excitonic fine structure and the excitonic binding energy. Our calculations show that c-plane InGaN/GaN quantum dots are ideal candidates for high temperature entangled photon emission as long as the underlying C 3v -symmetry is preserved. However, when assuming random alloy fluctuations in the dot, our atomistic calculations reveal that while the large excitonic binding energies are only slightly affected, the C 3v symmetry is basically lost due to the alloy fluctuations. We find that this loss in symmetry significantly impacts the excitonic fine structure. The observed changes in fine structure and the accompanied light polarization characteristics have a detrimental effect for polarization entangled photon pair emission via the biexciton–exciton cascade. Here, we also discuss possible alternative schemes that benefit from the large excitonic binding energies, to enable non-classical light emission from c-plane InGaN/GaN quantum dots at elevated temperatures.

Strain Tunable Berry Curvature Dipole, Orbital Magnetization and Nonlinear Hall Effect in WSe2 Monolayer* *Supported by the National Key Research and Development Program of China (Grant Nos. 2018YFA0703703 and 2016YFA0300802), and the National Natural Science Foundation of China (Grant Nos. 91964201, 61825401, and 11774004).

The electronic topology is generally related to the Berry curvature, which can induce the anomalous Hall effect in time-reversal symmetry breaking systems. Intrinsic monolayer transition metal dichalcogenides possesses two nonequivalent K and K′ valleys, having Berry curvatures with opposite signs, and thus vanishing anomalous Hall effect in this system. Here we report the experimental realization of asymmetrical distribution of Berry curvature in a single valley in monolayer WSe2 via applying uniaxial strain to break C 3v symmetry. As a result, although the Berry curvature itself is still opposite in K and K′ valleys, the two valleys would contribute equally to nonzero Berry curvature dipole. Upon applying electric field E , the emergent Berry curvature dipole D would lead to an out-of-plane orbital magnetization M ∝ D ⋅ E , which further induces an anomalous Hall effect with a linear response to E 2, known as nonlinear Hall effect. We show the strain modulated transport properties of nonlinear Hall effect in monolayer WSe2 with moderate hole-doping by gating. The second-harmonic Hall signals show quadratic dependence on electric field, and the corresponding orbital magnetization per current density M/J can reach as large as 60. In contrast to the conventional Rashba–Edelstein effect with in-plane spin polarization, such current-induced orbital magnetization is along the out-of-plane direction, thus promising for high-efficient electrical switching of perpendicular magnetization.

The Existence of Tricyanomethane.

Calcium tricyanomethanide reacts with hydrogen fluoride under formation of tricyanomethane and Ca(HF2)2. Tricyanomethane is stable below -40 °C and was characterized by IR, Raman, and NMR spectroscopy. The vibrational spectra were compared to the quantum-chemical frequencies at the PBE1PBE/6-311G(3df,3dp) level of theory and confirm the predicted C(3v) symmetry of the molecule with regular C-H (109.8 pm), C-C (146.7 pm), and C≡N (114.7 pm) bonds.

Two-site diamond-like point defects as new single-photon emitters

In this small review, we recall two promising candidates for biomarker nanosystems, in which a two-site defect embedded in a diamond-like lattice makes a single-photon source. The two candidates are the silicon-vacancy defect in diamond, and the carbon antisite-vacancy pair in 4H silicon carbide.These defects, which by symmetry resemble to the famous nitrogen-vacancy defect in diamond, bear an exact or nearly exact C 3v symmetry, giving them selection rules which lead their important magnetooptical properties. The embedding diamond-like crystal lattice not only determines the symmetry of two-site defects, but also ensure a nontoxic vehicle on which they reside; a definitive requirement against biomarker nanosystems.In the silicon-vacancy case, the size of the biomarker system is also an important feature. Nanoparticles of the embedding crystal do not exceed the size of molecular clusters, in order to be able to aid measuring all types of relevant biomolecular processes.

Structure and Other Molecular Properties of Actinide Trichlorides AnCl3 (An = Th–Cm)

The ground-state molecular properties of the trichlorides of light actinides (An = Th-Cm) have been predicted by state-of-the-art quantum chemical calculations. The ground electronic states have been determined by multireference calculations at the CASPT2 level including both scalar and spin-orbit relativistic effects. These studies supported the expected single-configuration character of ThCl3 and CmCl3 with their well-defined 6dσ/7s hybrid and 5f(7) configurations, respectively. In contrast, the intermediate actinides (PaCl3-AmCl3) with partly filled 5f shells have numerous very low-lying excited states and consequently a mixed character of the spin-orbit ground states. Apart from the planar ThCl3 the ground-state molecular geometries proved to be pyramidal with C(3v) symmetry. The gradually decreasing An-Cl bond distances reveal the actinide contraction known for the atomic and ionic radii of these actinide atoms. Other ground-state molecular properties as vibrational frequencies and natural charges have been obtained by density functional theory calculations using the B3LYP exchange-correlation functional in conjunction with small-core relativistic energy-consistent pseudopotentials for the actinides.

Do halogen and methyl substituents have electronic effects on the structures of simple disilanes? An experimental and theoretical study of the molecular structures of the series X3SiSiMe3 (X = H, F, Cl and Br)

The gas-phase molecular structures of a series of halogen-substituted disilanes [X3SiSiMe3 (X = H, F, Cl and Br)], 1,1,1-trimethyldisilane (H3SiSiMe3), 1,1,1-trifluoro-2,2,2-trimethyldisilane (F3SiSiMe3), 1,1,1-trichloro-2,2,2-trimethyldisilane (Cl3SiSiMe3) and 1,1,1-tribromo-2,2,2-trimethyldisilane (Br3SiSiMe3), have been determined in the gas phase by electron diffraction. Ab initio calculations at the HF and MP2 level were used to support the experimental investigation using the SARACEN method. All of the investigated structures were determined to adopt a staggered structure with C 3v symmetry. The effect of substitution on the Si–Si bond and the Si–Si–X bond angle was investigated and these results were compared to results obtained from a recent study of halogen-substituted disilanes [X3SiSiXMe2 (X = F, Cl, Br and I)] to consider the effect of the methyl groups on the substituted disilanes.

The Boron Conundrum: Which Principles Underlie the Formation of Large Hollow Boron Cages?

Extensive optimisation calculations are performed for the B(80) isomers in order to find out which principles underlie the formation of large hollow boron cages. Our analysis shows that the most stable isomers contain triangular B(10) or rhombohedral B(16) building blocks. The lowest-energy isomer has C(3v) symmetry and is characterised by a belt of three interconnected B(16) units and two separate B(10) units. At the B3LYP/6-31G(d) level of theory, this newly discovered isomer is 2.29, 1.48, and 0.54 eV below the leapfrog B(80) of Szwacki et al., the T(h) -B(80) of Wang, and the D(3d) -B(80) of Pochet et al., respectively. Our C(3v) isomer is therefore identified as the most stable hollow cage isomer of B(80) presently known. Its HOMO-LUMO gap of 1.6 eV approaches that of the leapfrog B(80). The leapfrog principle still remains a reliable scheme for producing boron cages with larger HOMO-LUMO gaps, whereas the thermodynamically most stable B(80) cages are formed when all pentagonal faces are capped. We show that large hollow cages of boron retain a preference for fullerene frames. The additional capping is in accordance with the following rules: preference for capping of pentagonal faces, formation of B(10) and/or B(16) units, homogeneous distribution of the hexagonal caps, and hole density approaching 1/9. Although our most stable B(80) isomer still remains higher in energy than the B(80) core-shell structure, we show that by applying the bonding principles to larger structures it is possible to construct boron cages with higher stabilisation energy per boron atom than the core-shell structure; a prototypical example is B(160). This clearly shows the continuous competition between the two suggested construction schemes, namely, the formation of multiple-shell structures and hollow cages.

Communication: A six-dimensional state-to-state quantum dynamics study of the H + CH4 → H2 + CH3 reaction (J = 0)

We report a quantum state-to-state reaction dynamics study for the title reaction. The calculation was based on an approximation that we introduced to the eight-dimensional model for the X + YCZ(3) → XY + CZ(3) type of reactions that restricts the non-reacting CZ(3) group in C(3V) symmetry proposed by Palma and Clary [J. Chem. Phys. 112, 1859 (2000)], by assuming that the CZ(3) group can rotate freely with respect to its C(3V) symmetry axis. With the CH bond length in group fixed at its equilibrium distance, the degree of freedom included in the calculation was reduced to six. Our calculation shows that the six-dimensional treatment can produce reaction probabilities essentially indistinguishable from the seven-dimensional (with CH bond length fixed in the original eight-dimensional model) results. The product vibrational/rotational state distributions and product energy partitioning information are presented for ground initial rovibrational state with the total angular momentum J = 0.

Analytical contribution of NAD 2D‐NMR spectroscopy in polypeptide mesophases to the investigation of triglycerides

In this work, we report and discuss on the use and limitations of the natural abundance deuterium two-dimensional NMR spectroscopy in polypeptide chiral and achiral aligning media in the studies of homogenous triglycerides at 14.1 T. As illustrative examples, two triglycerides with short and long alkyl chains were investigated: the 1,3-di(butanoyloxy)propan-2-yl butanoate or tributyrin (TB) and the 1,3-di(tetradecanoyloxy)propan-2-yl tetradecanoate or trimyristin (TM). If both flexible compounds are theoretically of C(s) symmetry on average, according to the Altmann's definition (Proc. Roy. Soc., 1967, A298, 184.), the analysis of spectral data in terms of enantiotopic and diastereotopic discriminations shows noticeable differences related to their orientational ordering behavior inside the mesophases. Although from NMR analysis viewpoint, TB behaves as a C(s) symmetry molecule as expected, the NMR results obtained for TM suggest a behavior that could be formally predicted for a C(3v) symmetry molecule on average. This conclusion was nicely supported by the comparison with the tri-n-propylorthoformate, a real C(3v) symmetry solute on average on the NMR timescale. This difference of effective orientational behavior could originate from the difference of size and shape between lateral and central alkyl chains of the solute molecule.

An eight-dimensional quantum mechanical Hamiltonian for X + YCZ3 system and its applications to H + CH4 reaction

An eight-dimensional quantum mechanical Hamiltonian has been proposed based on Palma and Clary's model in which the non-reacting CZ(3) group keeps a C(3v) symmetry in the X + YCZ(3) ↔ XY + CZ(3) reaction J. Palma and D. C. Clary [J. Chem. Phys. 112, 1859 (2000)]. By transforming the original cartesian coordinate system (x, s) into a scaled polar coordinate system (q, γ), the vibrational Hamiltonian of CZ(3) group is expressed in a simple form with a clear physical picture. This Hamiltonian is used to investigate the H + CH(4) → H(2) + CH(3) reaction on the Jordan-Gilbert potential energy surface. The total reaction probabilities are calculated for the initial ground state, and umbrella, bending, symmetric, and asymmetric stretching excited states of CH(4) with total angular momentum J = 0. The integral cross sections for the reaction are also studied for these initial vibrational states with a centrifugal-sudden approximation. The total integral cross sections for the asymmetric stretching vibrational excited state are in good agreement with the experimental observations. The results also showed the difference of dynamical behavior between reactions from symmetric and asymmetric stretching excited states. The thermal rate constants are calculated for the temperature range T = 250-2000 K and compared with the experimental and other theoretical results.

AsH3 ultraviolet photochemistry: An ab initio view

Multireference configuration interaction calculations have been carried out for low-lying electronic states of AsH(3). Bending potentials for the nine lowest states of AsH(3) are obtained in C(3v) symmetry for As-H distances fixed at the ground state equilibrium value of 2.850 a(0), as well as for the minimum energy path constrained to R(1) = R(2) = R(3). The calculated equilibrium geometry and bond energy for the X (1)A(1) ground state agree very well with the previous experimental and theoretical data. It is shown that the lowest excited singlet state belongs to the (1)A(1) symmetry (in C(3v)), in contradiction to the previous calculations. This state is characterized by a planar equilibrium geometry. Asymmetric stretch potential energy surface (PES) cuts along the H(2)As-H recoil coordinate (at R(1) = R(2) = 2.850 a(0), θ = 123.9° and 90°) for numerous excited states and two-dimensional PESs for the X and à states up to the dissociation limits are obtained for the first time. The à (1)A(1), B(1)E-X (1)A(1) transition moments are calculated as well and used together with the PES data for the analysis of possible photodecay channels of arsine in its first absorption band.

Highly Fluorinated Aryl-Substituted Tris(indazolyl)borate Thallium Complexes: Diverse Regiochemistry at the B–N Bond

The synthesis and characterization (mainly by (19)F NMR and X-ray diffraction) of highly fluorinated aryl-4,5,6,7-tetrafluoroindazoles and their corresponding thallium hydrotris(indazolyl)borate complexes are reported [aryl = phenyl, pentafluorophenyl, 3,5-dimethylphenyl, 3,5-bis(trifluoromethyl)phenyl]. Thanks to N-H···N hydrogen bonds, the indazoles crystallize as dimers that pack differently depending on the nature of the aryl group. The thallium hydrotris(indazolyl)borate complexes Tl[Fn-Tp(4Bo,3aryl)] resulting from the reaction of aryl-4,5,6,7-tetrafluoroindazoles [aryl = phenyl, 3,5-dimethylphenyl, 3,5-bis(trifluoromethyl)phenyl] with thallium borohydride adopt overall C(3v) symmetry with the indazolyl groups bound to boron via their N-1 nitrogen in a conventional manner. When the perfluorinated pentaphenyl-4,5,6,7-tetrafluoroindazole is reacted with thallium borohydride, a single regioisomer of C(s) symmetry having one indazolyl ring bound to boron via its N-2 nitrogen, TlHB(3-pentafluorophenyl-4,5,6,7-tetrafluoroindazol-1-yl)(2)(3-pentafluorophenyl-4,5,6,7-tetrafluoroindazol-2-yl) Tl[F27-Tp((4Bo,3C6F5)*)], is obtained for the first time. Surprisingly, the perfluorinated dihydrobis(indazolyl)borate complex Tl[F(18)-Bp(3Bo,3C6F5)], an intermediate on the way to the hydrotris(indazolyl)borate complex, has C(s) symmetry with two indazolyl rings bound to boron via N-2. The distortion of the coordination sphere around Tl and the arrangement of the complexes in the crystal are discussed.

Electronic spectroscopy of jet-cooled YbNH3

We report the first spectroscopic study of a complex consisting of a rare earth atom in combination with ammonia. Using two-color resonance-enhanced multiphoton ionization (REMPI) spectroscopy, the lowest energy electronic transition of YbNH(3) has been found in the near-infrared. The spectrum arises from a spin-forbidden transition between the (1)A(1) ground electronic state and the lowest (3)E excited electronic state. The transition is metal centered and approximately correlates with the Yb 6s6p (3)P ← 6s(2) (1)S transition. The observation of clear spin-orbit structure in the spectrum confirms the C(3v) symmetry of YbNH(3). Vibrational structure is also observed in the REMPI spectrum, which is dominated by excitation of the Yb-N stretching vibration.