Optical phonon modes and structure of ZnGa2Se4 and ZnGa2S4

We present the infrared and Raman study of the optical phonon modes of the defective compounds ZnGa₂Se₄ and ZnGa₂S₄. Most of the compounds have been found to crystallize in the thiogallate structure (defect chalcopyrite) with space group where all cations and vacancies are ordered. For some Zinc compounds a partially disordered cationic sublattice with various degrees of cation and vacancy statistical distribution, which lead to the higher symmetry (defect stannite), has been reported. For ZnGa₂Se₄ we have found three modes of A symmetry, showing Raman activity only. In addition, we have observed each five modes of B and E symmetry, showing infrared as well as Raman activity. The number of modes and their symmetry assignment, based on polarized measurements, clearly indicate space group for the investigated crystals of ZnGa₂Se₄.Regarding ZnGa₂S₄ we have found three modes exclusively showing Raman activity (2A⊕1B₁), and only eight modes showing infrared as well as Raman activity (3B₂⊕5E). The assignment of the modes has been derived by analyzing the spectral positions of the vibrational modes in comparison to a number of compounds. From the number and symmetry assignment of the optical phonon modes we confirm that ZnGa₂S₄ most likely crystallizes in space group .     

Temperature dependence of electron magnetic resonance and magnetization in NiO nanorods

For NiO nanorods of 5 nm diameter prepared by sol-gel technique, variations of the magnetization M with temperature T (5-370 K) and magnetic field H up to 55 kOe are reported. Also, temperature variations of the EMR (electron magnetic resonance) parameters (intensity I₀, linewidth ΔH and resonance field H_r) of an observed line due to uncompensated spins are followed for The M vs. H and T variations yield a blocking above which the data fits modified Langevin function with magnetic moment μ_p?1240 μ_B/particle. For the EMR line, I₀ decreases rapidly for T<T_B, and the line broadens and shifts to lower H with lowering T, following the lineshift δH_r=(ΔH)ⁿ with n?2.8. This is close to the value of n=3 expected for randomly oriented particles.     

Substantially low desorption barriers in recombinative desorption of deuterium from a Si(1 0 0) surface

We have studied desorption kinetics of deuterium molecules from a Si(1 0 0) surface by means of temperature-programmed desorption (TPD) spectra and isothermal desorptions.Three desorption components, denoted as β_1,A,β_1,B, and C, can be distinguished in semi-logarithmic plots of the TPD spectra.Their peak positions and intensities are strongly affected by the surface preparation methods employed, either with or without annealing to control the initial D coverage .Peak C appears at the leading edge of the TPD peak.It accounts for only about 5% of the TPD peak at and it diminishes rapidly with decreasing , vanishing at .In contrast, together the β_1,A and β_1,B peaks account for the whole TPD peak at any less than 1.0 ML. The maximum of the β_1,A peak is nearly constant at around the maximum temperature of the TPD peak.On the other hand, the β_1,B peak appears on the high-temperature side of the TPD peak and it systematically shifts to higher temperatures with decreasing .These results imply that first- and second-order kinetics are operating for the β_1,A and β_1,B desorptions, respectively.Isothermal desorption experiments confirm the above predicted kinetics for a limited region, namely .From the results for the rate curve analysis, the desorption barriers are evaluated to be 1.6 ± 0.1 eV and 1.8 ± 0.1 eV for the β_1,A and β_1,B desorptions, respectively.These values are substantially lower than the widely accepted value of ∼2.5 eV. To reproduce the measured TPD spectra we take the Arrhenius-type rate equation containing the first- and second-order rate terms for the β_1,A and β_1,B desorptions.The TPD spectra measured for can be reasonably fit with the proposed rate equation when the values given above for E_d,A and E_d,B are used. For , however, the TPD curves are not fit with the same values; rather, the best-fit curves require values for E_d,A and E_d,B larger than those given above. Combining the present kinetics results with those obtained by STM along with the studies, the β_1,A and β_1,B peaks may be attributed to desorption along the 2H path, while peak C may be attributed to desorption along the 4H path. The atomistic desorption mechanism as well as the energy relationship between the desorption barrier and isosteric heat of adsorption are discussed.     

Quantum critical behaviour in doped Y bB12 studied by ab initio calculations

The Kondo insulator Y bB₁₂ is known to undergo a transition to the metallic state with doping or under an external magnetic field. Within the virtual crystal approximation (VCA), we calculated the occupation of the Yb 4f and 5d shells, and , as a function of doping of Y bB₁₂ with the rare earths Tm and Lu. We found that exhibits an anomalous change at the critical concentration of the dopant, in agreement with experiment ( for Y b_1−xLu_xB₁₂ and for Y b_1−xTm_xB₁₂). We suggest that the critical behaviour seems to be strictly connected with the change of and in consequence the change of the Yb valency.     

Detrending moving average algorithm: A closed-form approximation of the scaling law

The Hurst exponent H of long range correlated series can be estimated by means of the detrending moving average (DMA) method. The computational tool, on which the algorithm is based, is the generalized variance , with being the average over the moving window n and N the dimension of the stochastic series y(i). The ability to yield H relies on the property of to vary as n^2H over a wide range of scales [E. Alessio, A. Carbone, G. Castelli, V. Frappietro, Eur. J. Phys. B 27 (2002) 197]. Here, we give a closed form proof that is equivalent to C_Hn^2H and provide an explicit expression for C_H. We furthermore compare the values of C_H with those obtained by applying the DMA algorithm to artificial self-similar signals.     

Stretch-bend combination polyads in the ÃAu state of acetylene, C2H2

Rotational analyses are reported for a number of newly-discovered vibrational levels of the S₁-trans (ùA_u) state of C₂H₂. These levels are combinations where the Franck-Condon active and vibrational modes are excited together with the low-lying bending vibrations, and . The structures of the bands are complicated by strong a- and b-axis Coriolis coupling, as well as Darling-Dennison resonance for those bands that involve overtones of the bending vibrations. The most interesting result is the strong anharmonicity in the combinations of (trans bend, a_g) and (in-plane cis bend, b_u). This anharmonicity presumably represents the approach of the molecule to the trans-cis isomerization barrier, where ab initio results have predicted the transition state to be half-linear, corresponding to simultaneous excitation of and . The anharmonicity also causes difficulty in the least squares fitting of some of the polyads, because the simple model of Coriolis coupling and Darling-Dennison resonance starts to break down. The effective Darling-Dennison parameter, K₄₄₆₆, is found to increase rapidly with excitation of , while many small centrifugal distortion terms have had to be included in the least squares fits in order to reproduce the rotational structure correctly. Fermi resonances become important where the K-structures of different polyads overlap, as happens with the 2¹3¹B¹ and 3¹B³ polyads (B = 4 or 6). The aim of this work is to establish the detailed vibrational level structure of the S₁-trans state in order to search for possible S₁-cis (¹A₂) levels. This work, along with results from other workers, identifies at least one K sub-level of every single vibrational level expected up to a vibrational energy of 3500 cm⁻¹.     

Third and fourth order phase transitions: Exact solution for the Ising model on the Cayley tree

In this work we present the first exact solution of a system of interacting particles with phase transitions of order higher than two. The presented analytical derivation shows that the Ising model on the Cayley tree exhibits a line of third order phase transition points, between temperatures and , and a line of fourth order phase transitions between T_BP and ∞, where k_B is the Boltzmann constant, and J is the nearest-neighbor interaction parameter.     

Local magnetization study of the first-order superconducting transition in CeCoIn5

We report local magnetization measurements on the heavy fermion superconductor CeCoIn₅ using a micro-Hall probe. Below a critical point T₀ (), the local magnetization shows a clear jump at the superconducting transition temperatures for both H∥a and H∥c, indicating that the phase transition at the upper critical field H_c2 becomes a first-order phase transition. In addition, we observed an undershoot behavior of magnetization jump above (H∥c), which suggests a rapid change of textured superconducting structure with vortices and the nodal planes expected in the Fulde-Ferrell-Larkin-Ovchinnikov state.     

New heavy mesons as hadronic molecules

We discuss a possible interpretation of the , D_s1(2460), and B_s1(5778) mesons as hadronic molecules. Using an effective Lagrangian approach we calculate their weak, strong and radiative decays. The new impact of the molecular structure of these states is the presence of u(d) quarks in the K, D^(∗) and B^(∗) mesons which gives rise to the direct strong isospin-violating transitions and in addition to the modes generated by η−π⁰ mixing as was considered before in the literature.     

4-benzyl pyridinium dihydrogenmonophosphate C6H5CH2C5H4NHH2PO4 : Single-crystal structure and its conductivity in polycrystalline form

The salt 4-benzyl pyridinium dihydrogenmonophosphate is monoclinic P2₁/c with the following unit cell dimensions: ; ; ; and β=97.328(11). Also, , D_x=1.403, , F(000)=560; ; and R=0.0495 and R_w=0.0964 for 3733 independent reflections. The structure consists of infinite parallel two-dimensional planes built of H₂PO₄⁻ anions and C₆H₅CH₂C₅H₄NH⁺ cations mutually connected by strong O-H ?O and N-H ?O hydrogen bonding. There are no contacts other than the normal Van der Waals interactions between the layers. The conductivity relaxation parameters associated with some H⁺ conduction have been determined from an analysis of the spectrum measured in a wide temperature range.     

The millimetre-wave rotational spectrum of phenylacetylene

The room-temperature rotational spectrum of phenylacetylene (C₆H₅CCH), was studied at frequencies up to 340 GHz. Extensive new measurements, covering rotational transitions with quantum number values up to J=140 and K_a=59, allowed determination of precise spectroscopic constants for the ground state and for the lowest two excited vibrational states, v₂₄=1 and v₃₆=1. The two excited states belong to the lowest B₁ and B₂ symmetry normal modes and their rotational transitions are very strongly perturbed by a-axis Coriolis resonance. A successful fit of the resonance is reported, resulting in and , in good agreement with results of ab initio computations.