Coexistence of topological node surface and Dirac fermions in phonon-mediated superconductor YB2C2.

The interaction between nontrivial topology and superconductivity in condensed matter physics has attracted tremendous research interest as it could give rise to exotic phenomena. Herein, based on first-principles calculations, we investigate the electronic structures, mechanical properties, topological properties, dynamic stability, electron-phonon coupling (EPC), and superconducting properties of the synthesized real material YB2C2. It is a tetragonal structure with P4/mbm symmetry and exhibits excellent stability. The calculated electronic band structures reveal that a zero-dimension (0D) Dirac point and two-dimensional (2D) nodal surface coexist near the Fermi level. A spin-orbit coupling (SOC) Dirac point with the topological Fermi arc is observed on the (001) surface. These nodal surfaces are protected by a two-fold screw axis and time-reversal symmetry. Based on the Bardeen-Cooper-Schrieffer theory, the superconducting transition temperature (Tc) in the range 1.25-4.45 K with different Coulomb repulsion constant µ* for YB2C2 is estimated to be consistent with previous experimental results. In addition, the EPC is mainly from the coupling between the dx2-y2 and dz2 orbitals of the Y atom and low-energy phonon modes. The presence of superconductivity and nontrivial topological surface state in YB2C2 suggests that it may be a candidate material for topological superconductors.

Rational Design of Endohedral Superhalogens without Using Metal Cations and Electron Counting Rules.

Superhalogens, predicted 40 years ago, have attracted considerable attention due to their potential as building blocks of novel materials with various applications. While a large number of superhalogen clusters have been theoretically predicted and experimentally synthesized, they either require the use of a metal cation or electron counting rules. In particular, very rare endohedral cage clusters in defiance of the above requirements have been found to be superhalogens. In this work, motivated by recent experimental advances in endohedral cage clusters, we present a rational design principle for creating a new class of such superhalogens. Focusing on the chemical formula of A@Si20X20 (A = F, Cl, Br, I, BH4, BF4; X = H, F, Cl, Br, I, BO, CN, SCN, CH3), we use first-principles calculations to study 54 different clusters and show that these clusters possess electron affinities as high as 8.5 eV. Some of these clusters with X = BO and CN can even be stable as dianions, with large second electron affinity ∼2 eV. Similarly, Cl@C60 is found to be a superhalogen. This class of superhalogens is different from the conventional ones with chemical formula MXk+1, where X is a halogen and M is a cation with a formal +k oxidation state. Interestingly, the electron affinities of A@Si20X20 are almost independent of the central A moiety, but are guided by the functional group X. The potential of these endohedral superhalogens as electrolytes in Li-ion batteries is discussed.

Insights into the Microstructures and Energy Levels of Pr3+-Doped YAlO3 Scintillating Crystals.

Trivalent praseodymium (Pr3+)-doped materials have been extensively used in high-resolution laser spectroscopy, owing to their outstanding conversion efficiencies of plentiful transitions in the visible laser region. However, to clarify the microstructure and energy transfer mechanism of Pr3+-doped host crystals is a challenging topic. In this work, the stable structures of Pr3+-doped yttrium orthoaluminate (YAlO3) have been widely searched based on the CALYPSO method. A novel monoclinic structure with the Pm group symmetry is successfully identified. The Pr3+ impurity can precisely occupy the Y3+ position and get incorporated into the YAlO3 (YAP) host crystal with a Pr3+ concentration of 6.25%. The result of the electronic band structure reveals a 3.62 eV band gap, which suggests a semiconductor character of YAP:Pr. Using our developed well-established parametrization matrix diagonalization (WEPMD) method, we have systematically analyzed the energy level scheme and proposed a set of newly improved parameters. Additionally, the energy transfer mechanism of YAP:Pr is clarified by deciphering the numerical electric dipole and magnetic dipole transitions. The popular red emission at 653 nm is assigned to the transition 3P0 → 3F2, while the transition 3P0 → 3H4 with a large branching ratio is predicted to be a good laser channel. Many promising emission lines for laser actions are also obtained in the visible light region. Our results not only provide important insights into the energy transfer mechanisms of rare-earth ion-doped materials but also pave the way for the implementation of new types of laser devices.

Relative phase effect of nonsequential double ionization of molecules by counter-rotating two-color circularly polarized fields.

Relative phase effect of nonsequential double ionization (NSDI) of aligned molecules by counter-rotating two-color circularly polarized (TCCP) fields is investigated with a three-dimensional classical ensemble model. Numerical results show that NSDI yield in counter-rotating TCCP fields sensitively depends on the relative phase of the two components, which exhibits a sin-like behavior with the period of π/2. NSDI yield achieves its maximum at the relative phase π/8 and minimum at 3π/8. Back analysis indicates the recollision time and the return angle of the electron strongly depend on the relative phase of the two components, which results in the dominant emission direction of the electrons, is different for different relative phases. This indicates that the recollision process can be steered by changing the relative phase of the two components in counter-rotating TCCP laser fields. Meantime, it provides an avenue to obtain information about the recollision time and the return angle in the recollision process from the electron momentum distribution.

Nonsequential double ionization by co-rotating two-color circularly polarized laser fields.

Nonsequential double ionization (NSDI) of Ar in co-rotating two-color circularly polarized (TCCP) laser fields is investigated with a three-dimensional classical ensemble model. Our numerical results indicate that co-rotating TCCP fields can induce NSDI by recollision process, while the yield is an order of magnitude lower than counter-rotating case. NSDI yield in co-rotating TCCP fields strongly depends on field ratio of the two colors and achieves its maximum at a ratio of 2.4. In co-rotating TCCP fields, the short recollision trajectory with traveling time smaller than one cycle is dominant. Moreover, the recollision time in co-rotating TCCP laser fields depends on the field ratio, which is mapped to the electron momentum distribution. This provides anavenue to obtain information about recollision time and access the subcycle dynamics of the recollision process.

Deciphering the Microstructure and Energy-Level Splitting of Tm3+-Doped Yttrium Aluminum Garnet.

Thulium-doped yttrium aluminum garnet (Tm:YAG) is an important solid-state laser crystal. The energy-level splitting within it is still an unresolved problem. Here, we perform a theoretical study on the microstructure of Tm3+-doped YAG using the CALYPSO structure search method in conjunction with first-principles calculations. The calculated results show that the 4.16% doping concentration of Tm3+ impurity causes an obvious structural distortion of YAG crystal, forming an orthorhombic phase in C222 symmetry. On the basis of our developed WEPMD method, we obtain a new and complete set of free-ion and crystal field parameters by a good fit (with proper irreducible representations) to 69 observed energy levels and determine the exact energy-level splitting of Tm3+ in YAG. The calculated Stark levels and electric dipole transitions are in excellent agreement with the measured data and similar theoretical calculations. Some promising emission lines between 3F3, 3F2, 1D2, and 1I6 states are presented. These findings offer fundamental insights and practical tools for further exploration of the structural and electronic properties of other transition-metal-doped YAG crystal.

Intensity-dependent two-electron emission dynamics in nonsequential double ionization by counter-rotating two-color circularly polarized laser fields.

Nonsequential double ionization of helium in counter-rotating two-color circularly polarized laser fields is investigated with a three-dimensional classical ensemble model. At moderate intensity, the momentum distribution of the two electrons shows a maximum in the middle of each side of the triangle of the negative vector potential. At high intensity, the momentum distribution exhibits a double-triangle structure, which is attributed to the different values of the laser intensity where the two electrons are released after recollision. At low intensity, the momentum distribution shows a shift deviating from the middle of the side of the triangle of the negative vector potential. This is because the first electrons are emitted within a narrow time window after the field maximum. In addition, at low intensity, double-recollision events and NSDI originating from doubly excited states induced by recollision are prevalent.

Rational Design of Stable Dianions by Functionalizing Polycyclic Aromatic Hydrocarbons.

Using density functional theory, we have carried out a systematic study of the stability and electronic properties of neutral and multiply charged molecules Bn C10-n X8 (n=0, 1, 2; X=H, F, CN). Our main objective is to explore if the replacements of core C atoms and/or H atoms in naphthalene (C10 H8 ) can enhance the stability of their dianions. Indeed, we find that the dianions of Bn C10-n (CN)8 are more stable than their monoanions with energies of 0.61 eV, 0.57 eV, and 1.97 eV for n=0, 1, 2, respectively. In addition, polycyclic aromatic hydrocarbons become stable as dianions only when H atoms are substituted by more electronegative species. Thus, a rational design approach by tailoring composition and ligands can lead to a new class of organic molecules that are capable of carrying multiple charges.

Role of ligands in the stability of BnXn and CBn-1Xn (n = 5-10; X = H, F, CN) and their potential as building blocks of electrolytes in lithium ion batteries.

Stabilizing small multiply charged negative ions in the gas phase has been of considerable interest in recent years. B12H122- is one of the most well-known dianions which is stable against auto-detachment of its second electron in the gas phase by 0.9 eV, whereas BnHn2- with n < 12 is unstable. Using density functional theory, we have examined systematically the role of ligands in stabilizing smaller mono- and di-anions of BnXn and CBn-1Xn (n = 5-10; X = H, F, CN). We show that the stability of the negative ions of these complexes increases with the electron affinity of the ligand and Bn(CN)n2- can even be stable against electron emission for n≥ 5. We also show that CBn-1(CN)n2- is stable against electron emission for n≥ 8, even though these moieties contain one electron more than needed to satisfy the Wade-Mingos rule. We have examined the potential of these stable negative ions as building blocks of electrolytes in Li-ion batteries. By calculating the binding energies between the CBn-1Xn1-,2- and Li+, we find that some of these clusters may even outperform CB11H12- as electrolytes in metal-ion batteries.

Recollision dynamics in nonsequential double ionization of atoms by long-wavelength pulses.

Recollision dynamics and electron correlation behavior are investigated for several long laser wavelengths (1200-3000 nm) in nonsequential double ionization (NSDI) of helium using three-dimensional classical ensembles. Numerical results show that for these long wavelengths NSDI events are mainly from the multiple-return trajectory which is different from the case of 800 nm. Moreover, with increasing laser wavelength NSDI events move from the diagonal to the two axes in the correlated electron momentum distributions, and finally form an experimentally observed prominent V-shaped structure [Phys. Rev. X 5, 021034 (2015)] in the first and third quadrants. Back analysis indicates that the asymmetric energy sharing between the two electrons at recollision is responsible for the formation of the prominent V-shaped structure of 3000 nm.

Origin of double-line structure in nonsequential double ionization by few-cycle laser pulses.

We investigate nonsequential double ionization (NSDI) of molecules by few-cycle laser pulses at the laser intensity of 1.2-1.5 × 10(14) W/cm(2) using the classical ensemble model. The same double-line structure as the lower intensity (1.0 × 10(14) W/cm(2)) is also observed in the correlated electron momentum spectra for 1.2-1.4 × 10(14) W/cm(2). However, in contrast to the lower intensity where NSDI proceeds only through the recollision-induced double excitation with subsequent ionization (RDESI) mechanism, here, the recollision-induced excitation with subsequent ionization (RESI) mechanism has a more significant contribution to NSDI. This indicates that RDESI is not necessary for the formation of the double-line structure and RESI can give rise to the same type of structure independently. Furthermore, we explore the ultrafast dynamics underlying the formation of the double-line structure in RESI.

Ab initio calculations of many-body interactions for compressed solid argon.

An investigation on many-body effects of solid argon at high pressure was conducted based on a many-body expansion of interaction energy. The three- and four-body terms in the expansion were calculated using the coupled-cluster method with single, double, and noniterative triple theory and incremental method, in which the configurations of argon trimers and tetramers were chosen as the same as those in the actual lattice. The four-body interactions in compressed solid argon were estimated for the first time, and the three-body interaction ab initio calculations were extended to a small distance. It shows that the four-body contribution is repulsive at high densities and effectively cancels the three-body lattice energy. The dimer potential plus three-body interaction can well reproduce the measurements of equation of state at pressure approximately lower than ∼60 GPa, when including the four-body effects extends the agreement up to the maximum experimental pressure of 114 GPa.

Formation and properties of iron-based magnetic superhalogens: a theoretical study.

In order to explore new magnetic superhalogens, we have systematically investigated the structures, electrophilic properties, stabilities, magnetic properties, and fragmentation channels of neutral and anionic Fe(m)F(n) (m = 1, 2; n = 1-7) clusters using density functional theory. Our results show that a maximum of six F atoms can be bound atomically to one Fe atom, and the Fe-Fe bonding is not preferred in Fe2F(n)(0/-) clusters. The computed electron affinities (EAs) indicate that FeF(n) with n ≥ 3 are superhalogens, while Fe2F(n) can be classified as superhalogens for n ≥ 5. To further understand their superhalogen characteristic, the natural population analysis charge distribution and the HOMOs of anionic clusters were also analyzed. When the extra negative charge and the content of HOMO are mainly located on F atoms, the clusters could be classified as superhalogens with EAs substantially surpass that of Cl. By calculating the binding energies per atom and the HOMO-LUMO gaps, FeF3, FeF4(-), Fe2F4, Fe2F5(-), and Fe2F7(-) clusters were found to have higher stabilities, corresponding to the Fe atoms that are attained at their favorite +2 and +3 oxidation states. Furthermore, we also predicted the most preferred fragmentation channel and product for all the ground state clusters. Even more striking is the fact that both neutral and anionic Fe(m)F(n) (m = 1, 2; n = 1-7) clusters carry large magnetic moments which mainly come from 3d orbital of iron atom.

Phase stability, mechanical properties, hardness, and possible reactive routing of chromium triboride from first-principle investigations.

The first-principles calculations are employed to provide a fundamental understanding of the structural features and relative stability, mechanical and electronic properties, and possible reactive route for chromium triboride. The predicted new phase of CrB3 belongs to the rhombohedral phase with R-3m symmetry and it transforms into a hexagonal phase with P-6m2 symmetry at 64 GPa. The mechanical and thermodynamic stabilities of CrB3 are verified by the calculated elastic constants and formation enthalpies. Also, the full phonon dispersion calculations confirm the dynamic stability of predicted CrB3. Considering the role of metallic contributions, the calculated hardness values from our semiempirical method for rhombohedral and hexagonal phases are 23.8 GPa and 22.1 GPa, respectively. In addition, the large shear moduli, Young's moduli, low Poisson's ratios, and small B∕G ratios indicate that they are potential hard materials. Relative enthalpy calculations with respect to possible constituents are also investigated to assess the prospects for phase formation and an attempt at high-pressure synthesis is suggested to obtain chromium triboride.

Synthesis of CoNi-layered double hydroxide on graphene oxide as adsorbent and construction of detection method for taste and odor compounds in smelling water.

Taste and odor (T&O) compounds are important water pollutant, some of which are toxic. The relevant studies are all expand upon the well-known T&O compounds but for the unknown odors in smelling water. In this work, a method combining purge and trap with gas chromatograph-mass spectrometer (PT-GC/MS) and disperse solid-phase extraction with gas chromatograph (GC) was first proposed to detect T&O compounds in unknown odorous water accurately. Firstly, PT-GC/MS was used for a qualitative test on unknown odors in smelling water and determine the analytes. The hollow CoNi-layered double hydroxide (LDH) on graphene oxide (GO) was then used as a composite adsorbent to pretreat the water, in which the GO provided large specific surface, and the LDH worked as a confinement cavity to enhance capture and retention capacity for volatile organic compounds (VOCs). According to the properties of T&O compounds determined by PT-GC/MS in water, a corresponding GC method was established for accurately quantitative analysis. In this paper, five T&O compounds were detected simultaneously, including dimethyl sulfide, meistylene, N, N-dimethylbenzylamine, 2, 4-dimethylbenzaldehyde and 2, 4-di-tert-butylphenol. Extraction parameters were optimized, including ratio of desorption solvent, amount of adsorbent, pH value, etc. Under the optimal conditions, the detection limits for analysis were 1.14 µg/L to 3.07 mg/L. The satisfactory recoveries were 94-98%. Furthermore, two optimal determination outcomes of odor waters from different places support the practicability of the method, which is expected to be widely used in the detection of unknown odors in smelling water.

Colorimetric sensor array based on CoOOH nanoflakes for rapid discrimination of antioxidants in food.

The identification of synthetic antioxidants has considerable significance in food safety. Here, we described the development of a colorimetric sensor array for rapid detection of eight antioxidants in food through the redox reaction between CoOOH and antioxidants in the presence of colorimetric signal indicators. The CoOOH nanoflakes exhibited high catalytic oxidation activity and can independently catalyze oxidation signal indicators showing different colors. The color reaction was inhibited to different degrees in the presence of antioxidants, which resulted in distinct signal response patterns for their discrimination. The method showed good linearity in the range from 50 to 1000 nM for butylated hydroxytoluene (BHT), butylhydroxyanisole (BHA), propyl gallate (PG) and tert-butyl hydroquinone (TBHQ). Moreover, different proportions of antioxidants were located in the middle pattern of each single antioxidant, and showed certain linear relationships among different concentration ratios. Finally, the proposed colorimetric sensor array was used for practical applications where TBHQ and BHT were detected in biscuits and sausages, and BHA and PG were detected in fried pork kebabs, respectively. The results were further confirmed by high-performance liquid chromatography, which demonstrated the great potential of the colorimetry sensor array for practical applications.

Synthesis and evaluation of UiO-66@MIP towards norfloxacin in water.

Norfloxacin (NOX), a kind of quinolone antibiotic, is widely used in disease treatment and the control of human and livestock products. Due to overuse, norfloxacin has become a common organic pollutant in water. We combine the high specific surface area and high stability of metal-organic frameworks with the high selectivity of molecularly imprinted polymers. By grafting a carbon-carbon double bond on the surface of UiO-66-NH2, a molecularly imprinted layer is formed on the surface of UiO-66-NH2 upon free radical polymerization. The saturated adsorption capacity of UiO-66@MIP reaches 58.01 mg g-1. UiO-66@MIP exhibits high adsorption performance in real water samples and its recoveries range from 96.7% to 98.3%, which demonstrates a higher adsorption capacity and recovery than other molecularly imprinted materials and has potential applications in the removal of norfloxacin in real life.

The geometrical structure and electronic properties of trivalent Ho3+ doped Y2O3 crystals: a first-principles study.

Trivalent rare-earth holmium ion (Ho3+) doped yttrium oxide (Y2O3) has attracted great research interest owing to its unique optoelectronic properties and excellent performances in many new-type laser devices. But the crystal structures of the Ho3+-doped Y2O3 system (Y2O3 : Ho) are still unclear. Here, we have carried out a first-principle study on the structural evolution of the trivalent Ho3+ doped Y2O3 by using the CALYPSO structure search method. The results indicate that the lowest-energy structure of Ho3+-doped Y2O3 possesses a standardized monoclinic P2 phase. It is found that the doped Ho3+ ion are likely to occupy the sites of Y3+ in the host crystal lattice, forming the [HoO6]9- local structure with C 2 site symmetry. Electronic structure calculations reveal that the band gap value of Ho3+-doped Y2O3 is approximately 4.27 eV, suggesting the insulating character of Y2O3 : Ho system. These findings could provide fundamental insights to understand the atomic interactions in crystals as well as the information of electronic properties for other rare-earth-doped materials.

Rotational design of BP/XY2 (X = Mo, W; Y = S, Se) composites for overall photocatalytic water-splitting.

Photocatalytic water-splitting for hydrogen generation is a promising way to solve the energy crisis, yet the design of efficient photocatalysts is still a challenge. By utilization of first principles calculations, we predict the photocatalytic properties of monolayer boron phosphide (BP) based BP/XY2 (X = Mo, W; Y = S, Se) composites of different rotated configurations. Our results suggest that the BP/XY2 composites can be stably formed, and the narrowed bandgaps ensure these composites are suitable for absorbing visible light. The bandgaps and band edge positions are slightly affected by the rotation angles. The BP/MoS2, BP/MoSe2, and BP/WSe2 are type II heterostructures. Furthermore, the transferred charge from BP to XY2 layers leads to the formation of electric fields, which efficiently separate the photoinduced carriers. The band alignments of BP/MoS2, BP/MoS2, BP/MoSe2, and BP/WSe2 satisfy the requirements of overall water-splitting within the pH scope of 3.6-7.9, 6.8-7.9, 4.0-8.0, and 8.7-8.8. This work will provide valuable insight for designing efficient water-splitting photocatalysts.

Strain-Tunable Visible-Light-Responsive Photocatalytic Properties of Two-Dimensional CdS/g-C3N4: A Hybrid Density Functional Study.

By means of a hybrid density functional, we comprehensively investigate the energetic, electronic, optical properties, and band edge alignments of two-dimensional (2D) CdS/g-C 3 N 4 heterostructures by considering the effect of biaxial strain and pH value, so as to improve the photocatalytic activity. The results reveal that a CdS monolayer weakly contacts with g-C 3 N 4 , forming a type II van der Waals (vdW) heterostructure. The narrow bandgap makes CdS/g-C 3 N 4 suitable for absorbing visible light and the induced built-in electric field between the interface promotes the effective separation of photogenerated carriers. Through applying the biaxial strain, the interface adhesion energy, bandgap, and band edge positions, in contrast with water, redox levels of CdS/g-C 3 N 4 can be obviously adjusted. Especially, the pH of electrolyte also significantly influences the photocatalytic performance of CdS/g-C 3 N 4 . When pH is smaller than 6.5, the band edge alignments of CdS/g-C 3 N 4 are thermodynamically beneficial for oxygen and hydrogen generation. Our findings offer a theoretical basis to develop g-C 3 N 4 -based water-splitting photocatalysts.