Reduction Of Symmetry Research Articles

Gold nanocups with multimodal plasmon resonance for quantum-dot random lasing

Reduction of symmetry in plasmonic nanostructures may lead to great opportunities in controlling their photophysics of extinction, local field, light response, and plasmonic interactions. With their strong light scattering and unique modulation of field distribution, plasmonic nanocups hold great potential for applications in random laser, three-dimensional optical nanoantenna, and optical coherence tomography imaging. Here we demonstrate a novel and robust solution-phase synthesis approach to gold nanocups, which combines templating and seeded growth techniques to precisely control the size, morphology, and composition of the nanocups. Ripening is the key strategy in this wet-chemistry process to ensure primary anisotropic growth of gold within the cup-shaped space provided by each template, consisting of a silica nanosphere and a flexible resorcinol-formaldehyde resin shell. The resulting plasmonic nanocups possess three types of plasmon resonance modes, including axial, transverse S, and transverse P modes, determined by the polarization of the incident light. This robust and scalable synthesis allows the production of gold nanocups with high quality and efficiency, opening the door to unique photophysical properties and many potential applications, as demonstrated by their excellent performance in plasmonic quantum dot random lasing.

Anisotropic Second-Harmonic Generation Induced by Reduction of In-Plane Symmetry in 2D Materials with Strain Engineering.

Strain engineering is an attractive method to induce and control anisotropy for polarized optoelectronic applications with two-dimensional (2D) materials. Herein, we have investigated the nonlinear optical coefficient dispersion relationship and the second-harmonic generation (SHG) pattern evolution under the uniaxial strains for graphene, WS2, GaSe, and In2Se3 monolayers. The uniaxial strain can break the in-plane symmetry of 2D materials, leading to both trade-off breaking of the nonlinear coefficient and new emergent nonlinear coefficients. In such a case, a classical sixfold ϕ-dependent SHG pattern is transformed into a distorted sixfold SHG pattern under the strain. Due to the lattice symmetry breaking and the uneven charge density distribution in strained 2D materials, the SHG patterns also depend on the excitation photon energy. The results could give a guide for the SHG pattern analysis in experiments, suggesting strain engineering on 2D materials for the tunable anisotropy in polarized and flexible nonlinear optical devices.

Potential energy surfaces of a stacked dimer of benzene and its radical cation: what remains and what appears.

A stepwise qualitative consideration of the reduction in symmetry for the highly symmetrical "right sandwich" and "twisted sandwich" structures of the stacked benzene dimer caused by the pseudo-Jahn-Teller effect made it possible to construct a scheme of the potential energy surface (PES) of this dimer. Thirty-six equivalent structures of minimum energy are ordered on this extremely flat surface, transforming into each other in an almost barrier-free manner. There are two kinds of these transformations, both of which are pseudorotation. The transformation pathways inherent in this neutral dimer are also characteristic of its radical cation (RC). Structural transformations of the RC are inextricably linked with changes in its electronic state since its stationary structures relate to different electronic states. (C6H6)2+˙ exists in the form of two orbital isomers, each of which "pseudorotates" on its own area of the PES. During the pseudorotation, the SOMO distribution on the fragments of (C6H6)2+˙ changes, which is identical to what occurs during the pseudorotation of the Jahn-Teller benzene RC. These areas are connected by pairwise interconversions of their minimum energy structures. The interconversion of the orbital isomers is a bypassing of conical intersections between corresponding electronic states, which occurs through a synchronized pseudorotation of the fragments. The conclusions of the qualitative consideration and the results of quantum chemical calculations of various levels performed for (C6H6)2+˙ are in full agreement with each other. The revealed features of the structure of the PES of the two reference systems in studying the intermolecular and ion-molecule interactions are the basis for considering their more complex analogs, primarily their less symmetrical ones.

Diffraction Features from (101¯4) Calcite Twins Mimicking Crystallographic Ordering

During phase transitions the ordering of cations and/or anions along specific crystallographic directions can take place. As a result, extra reflections may occur in diffraction patterns, which can indicate cell doubling and the reduction of the crystallographic symmetry. However, similar features may also arise from twinning. Here the nanostructures of a glendonite, a calcite (CaCO3) pseudomorph after ikaite (CaCO3·6H2O), from Victoria Cave (Russia) were studied using transmission electron microscopy (TEM). This paper demonstrates the occurrence of extra reflections at positions halfway between the Bragg reflections of calcite in 0kl electron diffraction patterns and the doubling of d104 spacings (corresponding to 2∙3.03 Å) in high-resolution TEM images. Interestingly, these diffraction features match with the so-called carbonate c-type reflections, which are associated with Mg and Ca ordering, a phenomenon that cannot occur in pure calcite. TEM and crystallographic analysis suggests that, in fact, (101¯4) calcite twins and the orientation change of CO3 groups across the twin interface are responsible for the extra reflections.

Stability, electronic structure, and magnetic moment of vanadium phthalocyanine grafted to the Au(1 1 1) surface

The studies of electronic and magnetic properties of V-Pc molecule adsorbed onto Au(111) surface are based on ab-initio calculations in the framework of density functional theory. We compute adsorption energies, investigate interaction mechanisms between constituents of the hybrid system consisting of V-Pc molecule and Au surface, and determine geometry changes in the system, particularly in the grafted molecule. We find out that the energetically most stable configuration of the V-Pc/Au(111) occurs when V-Pc is grafted to the Au surface’s fcc site, which leads to the reduction of the point group symmetry of the hybrid system in comparison to the free standing V-Pc molecule. Further, our studies reveal that the electronic structure and magnetic properties of the V-Pc change significantly after adsorption to the Au(111). Generally, these studies shed light on physical mechanisms of the V-Pc adsorption to metallic surfaces and open up new prospects for design of novel spintronic devices.

New compound Sm2Ru3Sn5 with a structure derived from Ru3Sn7

Abstract Ternary intermetallic compound Sm2Ru3Sn5 was synthesized in the system Sm-Ru-Sn by arc-melting and annealing at 600 °C in the field with high content of Sn. Its crystal structure was determined using single crystal X-ray diffraction data (at 240 K). The compound crystallizes in cubic system with space group I 4 ‾ 3m (No. 217), unit cell parameter is a = 9.4606 (8) Å, Z = 4, Pearson symbol c/40. The intermetallic compound Sm2Ru3Sn5 represents an ordered version of the centrosymmetric Ru3Sn7 structure (space group Im 3 ‾ m), in which 16f Sn-filled crystallographic site is split into two 8c sites, each of which is solely occupied of one sort of atoms – Sn or Sm. The occupation of these two 8c sites leads to a reduction of symmetry due to the removal of the inversion center.

Realizing the Intrinsic Anisotropic Growth of 1T′ ReS2 on Selected Au(101) Substrate toward Large‐Scale Single Crystal Fabrication

AbstractLow‐symmetry 2D materials with strong in‐plane anisotropy are ideal platforms for building multifunctional optoelectronic devices. However, the random orientations and easy formation of multidomain structures lead to the single‐crystal synthesis of these materials remains a big challenge. Herein, for the first time, the orientation‐controlled synthesis of ReS2, a typical low‐symmetry 2D material, is explored via interface engineering based on the strong interaction between the material and Au substrates with different symmetries. It is revealed that the lattice orientation and growth behavior of ReS2 are closely relevant to the lattice symmetry of Au facets. Single crystal ReS2 domains with two and even one orientations are acquired on the four‐fold symmetry Au(001) facet and the two‐fold symmetry Au(101) facet, respectively. Combined with density functional theory calculations, it is demonstrated that the synergy of ultra‐strong ReS2‐Au interfacial coupling and reduction of symmetry of Au facet is critical to realizing its intrinsic anisotropic growth. Furthermore, great enhancement of electrical and photoelectrical performances are acquired on the well‐aligned single crystal ReS2 device. The progress achieved in this work provides significant guidance for the controllable synthesis of wafer‐scale single crystals of low‐symmetry 2D materials for their practical device applications.

Toroidic and antitoroidic orders in hexagonal arrays of dielectric trimers: Magnetic group approach

Herein, we investigate the symmetry-protected toroidal dipole resonances and conditions of their excitation in a new type of electromagnetic metamaterials. These metamaterials are all-dielectric planar periodic arrays of dielectric disks disposed on a dielectric substrate. The elementary building blocks of the array are trimers which are distributed in hexagonal unit supercells. The highest geometrical symmetry of the unit supercell is C6v. The analysis is fulfilled by using the representation theory of groups with the application of the magnetic group theory, which is a new approach in solving such problems. We have shown that to get access to the toroidal supermodes of the array, the unit supercell symmetry must be broken twice: firstly, the C3v symmetry of the trimer, and secondly, the C6v symmetry of the unit supercell needs to be reduced. Selection rules for the symmetric and antisymmetric orders of the toroidal dipole moments in the arrays are defined. In particular, we have shown that with the reduction of the unit supercell symmetry to the C2v group, the array exhibits the toroidal dipole resonance with an antitoroidic order. The arrays with the lower Cs symmetry can provide the resonances with both toroidic and antitoroidic orders. It is also shown that these arrays are always polarization sensitive. Full-wave simulations and experiments confirm the theoretical predictions. The suggested metamaterials can provide an enhanced light-matter interaction due to the spatially and temporally confined light in resonant systems with very high quality factors.

Effect of Charge Injection on the Conducting Filament of Valence Change Anatase TiO2 Resistive Random Access Memory Device.

The recent observation of stable quantized conductance in anatase TiO2 resistive random access memory (ReRAM) devices opens up a new pathway toward the realization of brain-inspired neuromorphic computing devices. Herein, for the first time, ab initio calculations are implemented to understand the resistive switching phenomena in anatase TiO2. Oxygen vacancy configurations with different charge states are studied to gain insight into the ON and OFF states of ReRAM devices. Among the trivacancy configurations, the Vo+ state is observed to induce highly dispersed defect states within the bandgap forming a charge density channel where the carriers behave as free electrons leading to the formation of a conducting filament (CF). On the contrary, the breakdown of the CF is noticed by the removal of an oxygen vacancy from the trivacancy configuration. In this OFF state, the defect state carriers are found to be highly localized. In addition, we have also investigated the effect of charge injection on the crystal field symmetry of the CF. The reduction of symmetry due to the trivacancy configuration lowers the eg manifold energy, whereas the divacancy configuration lowers the t2g manifold energy.

Higher-form symmetries of 6d and 5d theories

We describe general methods for determining higher-form symmetry groups of known 5d and 6d superconformal field theories (SCFTs), and 6d little string theories (LSTs). The 6d theories can be described as supersymmetric gauge theories in 6d which include both ordinary non-abelian 1-form gauge fields and also abelian 2-form gauge fields. Similarly, the 5d theories can also be often described as supersymmetric non-abelian gauge theories in 5d. Naively, the 1-form symmetry of these 6d and 5d theories is captured by those elements of the center of ordinary gauge group which leave the matter content of the gauge theory invariant. However, an interesting subtlety is presented by the fact that some massive BPS excitations, which includes the BPS instantons, are charged under the center of the gauge group, thus resulting in a further reduction of the 1-form symmetry. We use the geometric construction of these theories in M/F-theory to determine the charges of these BPS excitations under the center. We also provide an independent algorithm for the determination of 1-form symmetry for 5d theories that admit a generalized toric construction (i.e. a 5-brane web construction). The 2-form symmetry group of 6d theories, on the other hand, is captured by those elements of the center of the abelian 2-form gauge group that leave all the massive BPS string excitations invariant, which is much more straightforward to compute as it is encoded in the Green-Schwarz coupling associated to the 6d theory.

ДОСЛІДЖЕННЯ ПОРУШЕННЯ РІВНОВАГИ ЕНЕРГІЇ СТОХАСТИЧНИХ ПРОЦЕСІВ І СИСТЕМ З МЕТОЮ ПІДВИЩЕННЯ ЇХ ЕНЕРГОЕФЕКТИВНОСТІ

Purpose. The results are based on the study of the paradox of disturbed energy balance and the introduction of the virial theorem on the initial state of equilibrium energy and the formalized physical mechanism of energy accumulation due to the violation of its equilibrium. Methodology. According to the results of the analysis of the mathematical apparatus of the model of energy accumulation dynamics on the basis of the energy paradox of the disturbed lever equilibrium, the previously undescribed property of the probability of occurrence of random events is revealed. This property is related to the concept of entropy in its information and energy interpretations, and this makes it possible to establish information criteria for assessing the energy performance of processes and systems of different nature, and vice versa. Results. A positive result of qualitative assessment of general opportunities to increase energy efficiency of stochastic processes and systems by violating their energy balance was obtained by creating conditions to improve energy efficiency of information transmission systems by quality assessment, which combines information and energy performance indicators based on the concept of entropy, namely - the signal/noise ratio. Originality. The previously described property of the probability of occurrence of random events, which quantitatively describes the relationship of information and energy performance of stochastic processes and systems. Practical value. The set of obtained results and previous researches testifies that the property of probability of dynamics of random events revealed by results of the analysis of mathematical device of model of dynamics of accumulation of energy, is display of the fundamental principle of imbalance, and provides a way of management of indicators and criteria increasing the energy efficiency of deterministic and stochastic processes and systems of different physical nature. Based on the identified property of the probability of random events, a method of controlling the probability of occurrence of random events is proposed, which is that to double the increase in the difference of probabilities of opposite random events it is enough to increase the probability of only one of these events. W. Pauli's hypothesis about the reduction of symmetry serves in favor of the concept of energy imbalance. There is good reason to believe that, regardless of whether Pauli considered the reduction of symmetry by division at the level of the particle or not, the nature and mechanism of the reduction of symmetry reflects the paradox of energies of disturbed lever equilibrium. Figures 14, references 11.

Ab initio investigation of hydrogen-based high T c superconductor that is stable under ambient environment

We report an ab initio investigation of a hydrogen-based high-T c superconductor candidate—crystalized C4H4 in the cubic-gauche (cg-C4H4) structure, with the symmetry of space group I213. We find the cg-C4H4 structure to be stable under ambient environment; and the evaluation of the electron-phonon coupling strength indicates that the heavily doped cg-C4H4 can generate superconductivity with a T c ∼ 72 K. The high frequency vibrational modes of hydrogen atoms are found to play an important role in the total electron-phonon interaction strength, and the reduction of structural symmetry compared with graphene further enhances the electron–phonon coupling of the carbon framework. Our investigation illustrates a BCS route to realizing the hydrogen-based high T c superconductivity.

Modeling the atomic structure in the vicinity of the spherical voids and calculation of void growth rate anisotropy in bcc iron and tungsten

We study the influence of the atomic structure in the vicinity of voids on their growth rate anisotropy. In the first part, we model the atomic structure in the vicinity of nanovoids in α-Fe and W using the advanced Molecular Statics method. In the second part, we use the earlier obtained equations that taking the influence of elastic fields into account to calculate the shifting rate of the void surface elements, and to evaluate the components of the strain tensor we use the atomic structure modeling results from the first part. The calculations are performed for voids of several sizes at certain oversaturation’s in a wide temperature range. The simulation results for the mentioned metals with a bcc structure show that displacements of atoms located along the crystallographic directions of the <100>, <110>, <111> types in the vicinity of the voids are significantly different, and this anisotropy of atom displacements leads to a reduction of spherical symmetry for the shifting rate of the surface elements. As the result, the initially spherical void shape becomes faceted.

Reduced 4D oscillators and orbital elements in Keplerian systems: Cushman–Deprit coordinates

We study the reduction and regularization processes of perturbed Keplerian systems from an astronomical point of view. Our approach connects axially symmetric perturbed 4-DOF oscillators with Keplerian systems, including the case of rectilinear solutions. This is done through a preliminary reduction recently studied by the authors. Then, the reduction program continues by removing the Keplerian energy. For each value of the semi-major axis, we explain the astronomical meaning of the sextuples defining the orbit space $$\mathbb {S}^2\times \mathbb {S}^2$$ and its connection with the orbital elements. More precisely, we present alternative sextuple coordinates for the set of bounded Keplerian orbits that ‘separate’ the node of the orbital plane from the argument of perigee giving the Laplace vector in that plane. Still, the reduction of the axial symmetry defined by the third component of the angular momentum is performed. For the thrice reduced space $$\varGamma _{0,L,H}$$ we propose the Cushman–Deprit coordinates, a variant to the set given by Cushman. The main feature of these variables is that they are all with the same dimensions, which is convenient for the normalization procedure. As an application of the proposed scheme, we study the spatial lunar problem.

Experimental investigation of Kramers–Kronig relations in chiral metasurfaces with reduced rotational symmetry

We examine the Kramers–Kronig relations between the circular dichroism and the optical rotation dispersion in the k-space obtained from a chiral metasurface with a reduced rotational symmetry. We operate a leakage-radiation microscopy system to probe the near-field plasmonic modes and measure the polarization effects of the grating. By using our system we were able to capture and analyze the plasmonic modes at both air/gold and gold/glass interfaces. The k-space mapping of the chirality parameters allows us to fully analyze the optical activity of the metasurface. We experimentally find that the reduction in rotational symmetry affects the locality condition which unavoidably leads to the deviation from the Kramers–Kronig relation.

Edge Influence on Charge Carrier Localization and Lifetime in CH3NH3PbBr3 Perovskite: Ab Initio Quantum Dynamics Simulation.

The distribution of charge carriers in metal halide perovskites draws strong interest from the solar cell community, with experiments demonstrating that edges of various microstructures can improve material performance. This is rather surprising because edges and grain boundaries are often viewed as the main source of charge traps. We demonstrate by ab initio quantum dynamics simulations that edges of the CH3NH3PbBr3 perovskite create shallow trap states that mix well with the valence and conduction bands of the bulk and therefore support mobile charge carriers. Charges are steered to the edges energetically, facilitating dissociation of photo-generated excitons into free carriers. The edge-driven charge separation extends carrier lifetimes because of decreased overlap of the electron and hole wave functions, which leads to reduction of the nonadiabatic coupling responsible for nonradiative electron-hole recombination. Reduction of spatial symmetry near the edges activates additional vibrational modes that accelerate coherence loss within the electronic subsystem, further extending carrier lifetimes. Enhanced atomic motions at edges increase fluctuations of edge energy levels, enhancing mixing with band states and improving charge mobility. The simulations contribute to the atomistic understanding of the unusual properties of metal halide perovskites, generating the fundamental knowledge needed to design high-performance optoelectronic devices.

Graphene as a nanoelectromechanical reference piezoresistor

Motivated by the recent prediction of anisotropy in piezoresistance of ballistic graphene along longitudinal and transverse directions, we investigate the angular gauge factor of graphene in the ballistic and diffusive regimes using highly efficient quantum transport models. It is shown that the angular guage factor in both ballistic and diffusive graphene between $0^{\circ}$ to $90^{\circ}$ bears a sinusoidal relation with a periodicity of $\pi$ due to the reduction of six-fold symmetry into a two-fold symmetry as a result of applied strain. The angular gauge factor is zero at critical angles $20^{\circ}$ and $56^{\circ}$ in ballistic and diffusive regimes respectively. Based on these findings, we propose a graphene based ballistic nano-sensor which can be used as a reference piezoresistor in a Wheatstone bridge read-out technique. The reference sensors proposed here are unsusceptible to inherent residual strain present in strain sensors and unwanted strain generated by the vapours in explosives detection. The theoretical models developed in this paper can be applied to explore similar applications in other 2D-Dirac materials. The proposals made here potentially pave the way for implementation of NEMS strain sensors based on the principle of ballistic transport, which will eventually replace MEMS piezoresistance sensors with a decrease in feature size. The presence of strain insenstive ``critical angle'' in graphene may be useful in flexible wearable electronics also.

An XAS study of the local structure in Czochralski-grown optical scheelite-type sodium-gadolinium molybdates

The local environment of Mo6+ ions in the structures of scheelite-type crystals (space group I41/a) with the initial compositions (Na1/2Gd1/2)MoO4 (NGM 1:1) and (Na2/7Gd4/7□1/7)MoO4 (NGM 1:2; □ - vacancies) was determined by X-ray absorption spectroscopy (XAS) for the first time. Different coordination environments of Na1+ and Gd3+ ions caused primarily by the difference in their formal charges have been established based on the comparison of Mo–Gd and Mo–Na interatomic distances. According to resent results and structural data known for the (Na1-хREx)MO4 crystals (RE3+ = rare-earth cations), the character of distribution of different Na1+ and RE3+ polyhedra over NGM crystal structures is influenced by the sizes and electronegativities of Na1+, RE3+, and М6+ (M6+ = Mo, W) ions, the Na1+:RE3+ ratio, and vacancies in the M site. Disordered distribution of Na1+ and RE3+ polyhedra in NGM occurs without changing the symmetry of crystals (space group I41/a) and ordered one is accompanied by reduction of crystal symmetry.

Hydrostatic contraction and anisotropic contraction effects on oxygen molecule nanorods

We study the effects of both hydrostatic and anisotropic contractions on the molecular condensation of oxygen molecules (O2) physisorbed to nanosized pores, termed “O2 nanorods”, through the magnetization measurements. Multisteps of O2 solidification accompany the reduction in structural symmetry with decreasing temperature, such that the structural change by external stress varies the stability of O2 solidification. For initial pore diameters of D = 8.5, 14.5, and 24.0 nm, anisotropic compression for nanorods (preferential compression along radial direction of the pores) occurred, and molecule solidification is suppressed at the lower temperature side compared with that under hydrostatic compression. For the smallest D = 6.5 nm, a hydrostatic contraction almost occurred, and the high adsorption capability enabled the detection of both the melting transition and change in crystal structure within the β phase, in addition to α–β and β–γ transitions.

Terahertz Photogalvanics in Twisted Bilayer Graphene Close to the Second Magic Angle.

We report on the observation of photogalvanic effects in tBLG with a twist angle of 0.6°. We show that excitation of the tBLG bulk causes a photocurrent, whose sign and magnitude are controlled by the orientation of the radiation electric field and the photon helicity. The observed photocurrent provides evidence for the reduction of the point group symmetry in low twist-angle tBLG to the lowest possible one. The developed theory shows that the current is formed by asymmetric scattering in gyrotropic tBLG. We also detected the photogalvanic current formed in the vicinity of the edges. For both bulk and edge photocurrents, we demonstrate the emergence of pronounced oscillations upon variation of the gate voltage. The gate voltages associated with the oscillations correlate with peaks in resistance measurements. These are well explained by interband transitions between a multitude of isolated bands in tBLG.