Observation of the state of CH2 by optical–optical double resonance

We report the observation of levels in the state of CH₂ via optical–optical double resonance spectroscopy. Direct transitions between the lowest singlet state and the state are allowed by symmetry, but weak because they correspond to a two electron excitation in the single configuration approximation to the electronic wavefunction. The observed transitions involve sequential single photon absorptions at visible and near infrared wavelengths using state intermediate levels. Recent ab initio results (S.N. Yurchenko et al., J. Mol. Spectrosc. 208 (2001), 136) predicted the positions of some of the levels which are confirmed by the present results. The new spectra provide accurate energies for rotational levels in the , l = 0 level of the state.     

Theoretical structure and surface energy of the reconstructed {01.2} form of calcite (CaCO3) crystal

Two different reconstructions of the (01.2) face (Ca or CO₃ terminated) of calcite (CaCO₃) were studied: (i) R1 reconstruction: the outermost layer is based on the [0 1 0] × 1/3[2 1 1] rectangular mesh, which is symmetrical with respect to the c glide plane of the crystal, thus fulfilling the 2D symmetry of the face and (ii) R2 reconstruction: the outermost layer is based on a lozenge shaped mesh that does not respect the 2D symmetry of the face.The , , and slabs geometry optimizations of calcite (CaCO₃) were performed either at DFT level or by using empirical potentials; the results obtained with these two different calculation methodologies are in good agreement. With respect to their arrangement in the bulk, the CO₃ groups of the outermost layer are significantly rotated about the crystallographic a-axis and about the normal to the 01.2 plane; further, the thickness of the outermost layer is significantly lower than that of the underneath ones.The surfaces energies (γ) at 0 K, for relaxed and unrelaxed , , and faces, were determined either at DFT level or by using empirical potentials. Independently of the method of calculation employed, the stability order of the relaxed faces is < < < . Concerning the unrelaxed faces, whose energies were evaluated by using empirical potentials only, the stability order is instead < < < ; such different ordering shows the importance of geometry relaxation in the calculation of the surface energy. The values of the relaxed surface energies are , , and erg/cm².     

Elliptic quantum group , Hopf algebroid structure and elliptic hypergeometric series

We propose a new realization of the elliptic quantum group equipped with the H-Hopf algebroid structure on the basis of the elliptic algebra . The algebra has a constructive definition in terms of the Drinfeld generators of the quantum affine algebra and a Heisenberg algebra. This yields a systematic construction of both finite- and infinite-dimensional dynamical representations and their parallel structures to . In particular we give a classification theorem of the finite-dimensional irreducible pseudo-highest weight representations stated in terms of an elliptic analogue of the Drinfeld polynomials. We also investigate a structure of the tensor product of two evaluation representations and derive an elliptic analogue of the Clebsch–Gordan coefficients. We show that it is expressed by using the very-well-poised balanced elliptic hypergeometric series .     

Magnetostructural phase transitions of DyFe4Ge2: Part II Neutron diffraction

We present the temperature magnetic phase diagram of the compound DyFe₄Ge₂ determined from neutron diffraction data for the entire magnetically ordered regime. DyFe₄Ge₂ undergoes at a simultaneous structural and magnetic transition of second order (or weakly first order) followed by two subsequent isostructural first-order magnetic transitions at and T_ic1=28K:

The re-entrant lock-in magnetic phase is stable in the high-temperature range T_ic2–T_N and in the low-temperature range 1.5 K–T_ic1 while the incommensurately modulated magnetic phase is sandwiched in the intermediate range T_ic1–T_ic2 between the two commensurate phases. The wave vector q₂ has a temperature-dependent length with a minimum in the middle of the incommensurate range and corresponds to a multiaxial amplitude modulated phase. Symmetry analysis leads for both propagation vectors in Cmmm to a twofold and fourfold splitting of the tetragonal Dy 2b site and the Fe 8i sites, respectively. The low temperature and the phases correspond to 3D canted magnetic structures described by the irreducible representations (Irreps) Γ₂+Γ₃ while the high-temperature q₁ phase to 2D canted magnetic structures described by a single Irrep Γ₂. The T_ic2 transition is connected with reorientations of both Fe and Dy moments.     

Observation of the quantum Hall effect in cleaved InAs surfaces

We have performed the in-plane magnetotransport measurements on the two-dimensional electron gas at the cleaved p-InAs (1 1 0) surface by deposition of Ag. The surface electron density N_s is determined from the Hall coefficient at . The coverage dependence of N_s is well explained by the assumption that each adsorbed Ag atom denotes one electron into InAs until the surface Fermi level reaches the adsorbate-induced donor level. The electron mobility μ is about and does not show a clear dependence on the coverage over . In the high-magnetic field regime of B>1/μ, Shubnikov–de Hass oscillations were observed. A beating pattern due to the strong spin–orbit interaction appears for high N_s. For lower N_s of , an apparent quantum Hall plateau for ν=4 and vanishing of the longitudinal resistivity were observed around .     

Korringa-like nuclear spin–lattice relaxation in a 2DES at

Via a resistively detected NMR technique, the nuclear spin–lattice relaxation time T₁ of ⁷¹Ga has been measured in a GaAs/AlGaAs heterostructure containing two weakly coupled 2D electron systems (2DES) at low temperatures, each at Landau level filling . Incomplete electronic spin polarization, which has been reported previously for low density 2DESs at , should facilitate hyperfine-coupled nuclear spin relaxation owing to the presence of both electron spin states at the Fermi level. Composite fermion theory suggests a Korringa-law temperature dependence: T₁T=constant is expected for temperatures . Our measurements show that for temperatures in the range , T₁ rises less rapidly with falling temperature than this law predicts. This may suggest the existence of alternate nuclear spin relaxation mechanisms in this system. Also, our data allows for an estimate of the composite fermion mass.     

Absolute photoionization cross sections with ultra-high energy resolution for Ar, Kr, Xe and N2 in inner-shell ionization regions

The high-resolution absolute photoionization cross sections for Ar, Kr, Xe and N₂ in the inner-shell ionization region have been measured using a multi-electrode ion chamber and monochromatized synchrotron radiation. The energy ranges of the incident photons for the target gases were as follows: Ar: 242–252 eV (2p Rydberg excitation), Kr: 1650–1770 eV (near the 2p ionization thresholds), Xe: 665–720 eV (near the 3d ionization thresholds) and 880–1010 eV (near the 3p ionization thresholds), N₂: 400–425 eV (N 1s excitation and ionization). It is the first time to measure the absolute ionization cross sections of Ar, Kr, Xe and N₂ over the present energy ranges with the energy resolution of over 10,000. The natural lifetime widths of , , and resonances for Ar, resonance for Xe, and resonance for N₂ have been obtained based on the cross sections determined. The ionization energies into the Ar⁺ (), Ar⁺ () and Xe⁺ () ionic states are also determined using the Rydberg formula.     

Symmetry-adapted tight-binding calculations of the totally symmetric A1 phonons of single-walled carbon nanotubes and their resonant Raman intensity

The atomistic calculations of the physical properties of perfect single-walled carbon nanotubes based on the use of the translational symmetry of the nanotubes face increasing computational difficulties for most of the presently synthesized nanotubes with up to a few thousand atoms in the unit cell. This difficulty can be circumvented by use of the helical symmetry of the nanotubes and a two-atom unit cell. We present the results of such symmetry-adapted tight-binding calculations of the totally symmetric A₁ phonons (the RBM and the G-band modes) and their resonant Raman intensity for several hundred nanotubes.In particular, we show that (1) the frequencies and the resonant Raman intensity of the RBM and the G-band modes show diameter and chirality dependence and family patterns, (2) the strong electron– phonon interactions in metallic nanotubes lead to Kohn anomalies at the zone center, (3) the G-band consists of a subband due to phonons of semiconducting tubes centered at 1593 cm⁻¹, a subband of phonons at 1570 cm⁻¹, and a subband of phonons of metallic tubes at 1540 cm⁻¹. The latter prediction confirms previous theoretical results but disagrees with the commonly adopted assignment of the G-band features.     

Structural and electronic behavior of Sr2GdRuO6 complex perovskite

We report experimental and theoretical study of crystallographic lattice and electronic structure of Sr₂GdRuO₆ complex perovskite, which is used as precursor in the fabrication process of superconducting ruthenocuprate RuSr₂GdCu₂O₈. Samples were produced by the standard solid state reaction. Rietveld refinement of experimental X-ray diffraction patterns shows that material crystallizes in a monoclinic structure, which belongs to the P2₁/n (#14) space group, with lattice parameters , , , and tilt angle β=90.258. Calculations of electronic structure were performed by the density functional theory. The exchange and correlation potentials were included through the LDA+U approximation. Density of states (DOS) study was carried out considering the two spin polarizations. Results show Gd are majority responsible for the magnetic character in this material, but Ru contribution is also relevant because d-orbital is closer to Fermi level. Theoretical results evidence that Sr₂GdRuO₆ material behaves as a magnetic semiconductor, with 20μ_B effective magnetic moment.     

Synthesis and field-emission properties of the tungsten oxide nanowire arrays

High-density, uniformly distributed and quasi-aligned tungsten oxide nanowire arrays have been synthesized by a conventional thermal evaporation approach on indium tin oxide (ITO) coated glass substrates without any catalysts. The temperature of the substrate was . The tungsten oxide nanowires are single crystalline with growth direction of [0 1 0]. For commercial applications, field emission properties of tungsten oxide nanowires were studied under a poor vacuum at room temperature. The electron field-emission turn-on field (E_to), defined as the macroscopic field required to produce a current density of , is about . The performance reveals that the tungsten oxide nanowire arrays can be served as a good candidate for commercial application in field-emission displays.     

Structure of light hypernuclei

The structure of light hypernuclei with strangeness S=−1 and −2 is investigated with the microscopic cluster model and the Gaussian expansion method (GEM). We emphasize that the cluster picture as well as the mean-field picture is invaluable to understand the structure of Λ hypernuclei, Σ hypernuclei and double Λ hypernuclei. A variety of aspects of Λ hypernuclei is demonstrated through a systematic study of p-shell hypernuclei (,, , , , , ) and sd-shell ones (, ): for example, the appearance of genuine hypernuclear states with new spatial symmetry which cannot be seen in ordinary nuclei, the glue-like role of the Λ particle which shrinks the size of nuclear core and thus reduces the B(E2) value, and the halo and skin structures in and etc. The typical light hypernucleus is thoroughly investigated, including its production, structure and decay. Precise three-body and four-body calculations of , and using GEM provide important information on the spin structure of the underlying ΛN interaction, by comparing with recent experimental data from γ-ray hypernuclear spectroscopy. The Λ–Σ coupling effect is studied in and . The binding mechanism of is discussed together with the possible existence of , emphasizing the fact that the study of is useful for extracting information on the ΣN interaction differing from that from . A systematic study of double-Λ hypernuclei, constrained by the NAGARA data () within a four-body cluster model indicates that the recently observed Demachi–Yanagi event can be interpreted as the 2⁺ state of . The effect of hyperon mixing in and is investigated using one-boson-exchange potentials and quark-cluster-model interactions for the S=−2 sector. A close relation between nuclear deep hole states and hypernuclei is discussed, emphasizing the selection rule for fragmentation of the s-hole in light nuclei, which is promising for understanding the production mechanism of double-Λ and twin-Λ hypernuclei via Ξ-atomic capture.     

Direct observation of tunneling emission to determine localization energies in self-organized In(Ga)As quantum dots

We report direct observation of tunneling emission of electrons and holes from In(Ga)As/GaAs QDs in time resolved capacitance spectroscopy. From the dependence of the tunneling time constant on the external electric field the important entire localization energies of electron and holes in In(Ga)As QDs are determined with high accuracy. The results yield electron and hole localization energies of and , respectively, which is in excellent agreement with 8-band k·p theory.     

Three equations of state for solids considering thermal effect

 The Einstein model to consider thermal effect in universal equations of state (UEOS) is modified. It is proposed that the zero-point vibration term should be deleted in a thermal UEOS, and the parameters cannot be directly taken as experimental data at a reference temperature, V_R, B_R, and , but their values at absolute zero temperature, V₀, B₀, and . An approach is proposed to solve V₀, B₀, and from V_R, B_R, and . The approaches are applied to three typical universal EOSs, including the Baonza, mGLJ and Morse EOSs. The numerical results show that the solved values of parameters are almost identical for different EOSs. And the thermo-physical properties predicted through different EOSs are almost identical at zero- and low-pressure conditions, once the same approach and input experimental data are used to solve the parameters. It is concluded that the prediction of thermo-physical properties at zero- and low-pressure conditions cannot be taken as the criteria to judge the applicability of a universal EOS.     

Type I quasi-phase-matched blue second harmonic generation with different polarizations in periodically poled LiNbO3

First-order type I quasi-phase-matched (QPM) blue second-harmonic generation was demonstrated in periodically poled LiNbO₃ with period of 14.5 μm using d₃₁. 52 μJ of harmonic blue light at 0.473 μm was generated pumped by 114 μJ 35 ps pulse laser at 0.946 μm at 150 °C with a conversion efficiency of 45.6%. The average conversion efficiencies of 41.3% and 19% were also obtained at 150 °C, respectively, in the conventional first- and third-order QPM blue second-harmonic generation at 0.473 μm. The temperature acceptance bandwidths of 20 mm length periodically poled LiNbO₃ with first-order grating periods of 14.5 and 4.5 μm are 2.0 and 0.9 °C, respectively. The larger acceptance bandwidths and grating period for than those for enhance the frequency conversion efficiency, which shows the polarization dependence of quasi-phase matching.     

Mn2 CoSb compound: Structural, electronic, transport and magnetic properties

High-ordered Mn₂CoSb compound has been synthesized successfully by a melt-spinning technique. The band structure calculation shows that Mn₂CoSb is a true half-metallic ferromagnet characterized by an indirect Γ–X band gap of about 0.4007 eV around the Fermi level for minority-spin electrons. The calculated magnetic moment is per formula unit, which is close to the experimental value of . The electronic resistivity shows a power-law T^1.33 temperature dependence at low temperature. The T^1.5 dependence of the magnetization was observed at low temperature, which is expected from Bloch’s law.     

The observation of hot bands in the 288 nm system of the FeCl2 radical

The 288 nm band system of FeCl₂ has been recorded with a sample produced in a warmed, free-jet expansion at moderately high resolution (with a linewidth of 0.28 cm⁻¹). Under these conditions, several hot bands are observed involving excitation of the symmetric and anti-symmetric stretching vibrations. The wavenumbers determined as a result for FeCl₂ in its ground ⁵Δ_g,4 state are and . No hot, sequence bands in the bending vibration were observed. The most likely explanation is that the wavenumber for ν₂ is essentially the same in both the electronic states involved (88 cm⁻¹). Additional strong hot bands are observed that are unrelated to the previously assigned electronic transitions; they appear to emanate from a low-lying electronic state of FeCl₂.     

Widely tunable high power OPO based on a periodically poled MgO doped lithium niobate crystal

An optical parametric oscillator (OPO) based on a highly MgO doped periodically poled lithium niobate (PPMgLN) crystal was experimentally demonstrated and the result is presented in this report. The PPMgLN wafer was fabricated from a MgO doped (with 6 mol% doping concentration) lithium niobate crystal by means of high voltage pulse trigged domain reversal technique and has 20 domain reversal periods from 27.8 to with a step of between the neighbor periods. An acousto-optic (AO) Q-switched Nd³⁺:YVO₄ laser was used as the pumping laser. A maximum laser output power of 4.8 W has been achieved for the OPO when the pumping power is 10.8 W and it corresponds to an optic-optic conversion efficiency of 44%. By shifting the PPMgLN wafer, the periods of the domain structure on the PPMgLN wafer can be changed, thus enabling a wide spectral tuning range of the laser output from 1.42 to (for the signal light) and from 2.76 to (for the idler light).     

Double-quantum NMR spectroscopy of P species submitted to very large CSAs

We introduce an original pulse sequence, , which is a block super-cycled sequence employing as basic element a π pulse sandwiched by ‘window’ intervals. This homonuclear dipolar recoupling method allows the efficient excitation of double-quantum coherences between spin-1/2 nuclei submitted to very large chemical shift anisotropy. We demonstrate that this technique can be employed in double-quantum ↔ single-quantum ³¹P homonuclear correlation experiment at high magnetic field (B₀  14 T) and high MAS frequencies (ν_R  30 kHz). The performances of are compared to those of the double-quantum recoupling methods, such as BABA and bracketed fp-RFDR, which were already employed at fast MAS rates. The sequence displays a higher robustness to CSA and offset than the other existing techniques.     

Boundary Lax pairs for the Toda field theories

Based on the recent formulation of a general scheme to construct boundary Lax pairs, we develop this systematic construction for the affine Toda field theories (ATFT). We work out explicitly the first two models of the hierarchy, i.e. the sine-Gordon () and the models. The Toda theory is the first non-trivial example of the hierarchy that exhibits two distinct types of boundary conditions. We provide here novel expressions of boundary Lax pairs associated to both types of boundary conditions.